EE-Unit-II Atmospheric Composition

The atmosphere contains many gases, most in small amounts, including some pollutants and greenhouse gases.  The most abundant gas in the atmosphere is nitrogen, with oxygen second.  Argon, an inert gas, is the third most abundant gas in the atmosphere.

Why do I care? The composition of the atmosphere, among other things, determines its ability to transmit sunlight and trap infrared light, leading to potentially long-term changes in climate.

The atmosphere is concentrated at the earth’s surface and rapidly thins as you move upward, blending with space at roughly 100 miles above sea level. The atmosphere is actually very thin compared to the size of the earth, equivalent in thickness to a piece of paper laid over a beach ball.  However, it is responsible for keeping our earth habitable and for producing weather.

Graphs of the overall atmospheric concentration and the relative percentages of trace gases.
Figure A

The atmosphere is composed of a mix of several different gases in differing amounts.  The permanent gases whose percentages do not change from day to day are nitrogen, oxygen and argon.  Nitrogen accounts for 78% of the atmosphere, oxygen 21% and argon 0.9%.  Gases like carbon dioxide, nitrous oxides, methane, and ozone are trace gases that account for about a tenth of one percent of the atmosphere.  Water vapor is unique in that its concentration varies from 0-4% of the atmosphere depending on where you are and what time of the day it is.  In the cold, dry artic regions water vapor usually accounts for less than 1% of the atmosphere, while in humid, tropical regions water vapor can account for almost 4% of the atmosphere.  Water vapor content is very important in predicting weather.

Greenhouse gases whose percentages vary daily, seasonally, and annually have physical and chemical properties which make them interact with solar radiation and infrared light (heat) given off from the earth to affect the energy balance of the globe.  This is why scientists are watching the observed increase in greenhouse gases like carbon dioxide and methane carefully, because even though they are small in amount, they can strongly affect the global energy balance and temperature over time.

EE-Unit-I Forest ecosystem

The entire assemblage of organisms (trees, shrubs, herbs, bacteria, fungi, and animals,including people) together with their environmental substrate (the surrounding air, soil, water,organic debris, and rocks), interacting inside a defined boundary. Forests and woodlandsoccupy about 38% of the Earth’s surface, and they are more productive and have greaterbiodiversity than other types of terrestrial vegetation. Forests grow in a wide variety of climates,from steamy tropical rainforests to frigid arctic mountain slopes, and from arid interiormountains to windy rain-drenched coastlines. The type of forest in a given place results from acomplex of factors, including frequency and type of disturbances, seed sources, soils, slopeand aspect, climate, seasonal patterns of rainfall, insects and pathogens, and history of humaninfluence.

Ecosystem concept

Often forest ecosystems are studied in watersheds draining to a monitored stream: thestructure is then defined in vertical and horizontal dimensions. Usually the canopy of the tallesttrees forms the upper ecosystem boundary, and plants with the deepest roots form the lowerboundary. The horizontal structure is usually described by how individual trees, shrubs, herbs,and openings or gaps are distributed. Wildlife ecologists study the relation of stand andlandscape patterns to habitat conditions for animals.

Woody trees and shrubs are unique in their ability to extend their branches and foliage skywardand to capture carbon dioxide and most of the incoming photosynthetically active solarradiation. Some light is reflected back to the atmosphere and some passes through leaves tothe ground (infrared light). High rates of photosynthesis require lots of water, and many woodyplants have deep and extensive root systems that tap stored ground water between rainstorms. Root systems of most plants are greatly extended through a relation between plantsand fungi, called mycorrhizal symbiosis.

The biomass of a forest is defined here as the mass of living plants, normally expressed as dryweight per unit area. Biomass production is the rate at which biomass is accrued per unit areaover a fixed interval, usually one year. If the forest is used to grow timber crops, productionmeasures focus on the biomass or volume of commercial trees. Likewise, if wildlife populationsare the focus of management, managers may choose to measure biomass or numbers ofindividual animals. Ecologists interested in the general responses of forest ecosystems,however, try to measure net primary production (Npp), usually expressed as gross primaryproduction (Gpp) minus the respiration of autotrophs (Ra).

Another response commonly of interest is net ecosystem production (NEP), usually expressed as where Rh is respiration of heterotrophs.

Productivity is the change in production over multiple years. Monitoring productivity isespecially important in managed forests. Changes in forest productivity can be detected only over very long periods.

Forested ecosystems have great effect on the cycling of carbon, water, and nutrients, andthese effects are important in understanding long-term productivity. Cycling of carbon, oxygen,and hydrogen are dominated by photosynthesis, respiration, and decomposition, but they arealso affected by other processes. Forests control the hydrologic cycle in important ways.Photosynthesis requires much more water than is required in its products. Water is lost back tothe atmosphere (transpiration), and water on leaf and branch surfaces also evaporates underwarm and windy conditions. Water not taken up or evaporated flows into the soil and eventuallyappears in streams, rivers, and oceans where it can be reevaporated and moved back overland, completing the cycle.

Forest plants and animals alter soil characteristics, for example, by adding organic matter,which generally increases the rate at which water infiltrates and is retained. Nutrient elementscycle differently from water and from each other.

Elements such as phosphorus, calcium, and magnesium are released from primary minerals inrocks through chemical weathering. Elements incorporated into biomass are returned to the soilwith litterfall and root death; these elements become part of soil organic matter and aremineralized by decomposers or become a component of secondary minerals. All elements canleave ecosystems through erosion of particles and then be transported to the oceans anddeposited as sediment. Deeply buried sediments undergo intense pressure and heat thatreforms primary minerals. Volcanoes and plate tectonic movements eventually distribute thesenew minerals back to land.

Nitrogen is the most common gas in the atmosphere. Only certain bacteria can form a specialenzyme (nitrogenase) which breaks apart N2 and combines with photosynthates to form aminoacids and proteins. In nature, free-living N2-fixing microbes and a few plants that can harborN2-fixing bacteria in root nodules play important roles controlling the long-term productivity offorests limited by nitrogen supply. Bacteria that convert ammonium ion (NH+4) to NO–3(nitrifiers) and bacteria that convert NO–3 back to N2 (denitrifiers) are important in nitrogen cycling as well.

Changes in the plant species of a forest over 10 to 100 years or more are referred to assuccession. Changes in forest structure are called stand development; changes in composition,structure, and function are called ecosystem development. Simplified models of succession and development have been created and largely abandoned because the inherent complexity of the interacting forces makes model predictions inaccurate.

Streams

One of the products of an undisturbed forest is water of high quality flowing in streams. Theecological integrity of the stream is a reflection of the forested watershed that it drains. Whenthe forest is disturbed (for example, by cutting or fire), the stream ecosystem will also bealtered. Forest streams are altered by any practices or chemical input that alter forestvegetation, by the introduction of exotic species, and by the construction of roads that increasesediment delivery to streams.

Vertebrates

Forest animals are the consumers in forest ecosystems. They influence the flow of energy andcycling of nutrients through systems, as well as the structure and composition of forests,through their feeding behavior and the disturbances that they create. In turn, their abundanceand diversity is influenced by the structure and composition of the forest and the intensity,frequency, size, and pattern of disturbances that occur in forests. Forest vertebrates make upless than 1% of the biomass in most forests, yet they can play important functional roles inforest systems.

Invertebrates

Invertebrates are major components of forest ecosystems, affecting virtually all forestprocesses and uses. Many species are recognized as important pollinators and seeddispersers that ensure plant reproduction. Even so-called pests may be instrumental inmaintaining ecosystem processes critical to soil fertility, plant productivity, and forest health.

Invertebrates affect forests primarily through the processes of herbivory and decomposition.They are also involved in the regulation of plant growth, survival, and reproduction; forestdiversity; and nutrient cycling. Typically, invertebrate effects on ecosystem structure andfunction are modest compared to the more conspicuous effects of plants and fungi. However,invertebrates can have effects disproportionate to their numbers or biomass.

Changes in population size also affect the ecological roles of invertebrates. For example, smallpopulations of invertebrates that feed on plants may maintain low rates of foliage turnover andnutrient cycling, with little effect on plant growth or survival, whereas large populations candefoliate entire trees, alter forest structure, and contribute a large amount of plant material andnutrients to the forest floor. Different life stages also may represent different roles. Immaturebutterflies and moths are defoliators, whereas the adults often are important pollinators.

Microorganisms

Microorganisms, including bacteria, fungi, and protists, are the most numerous and the mostdiverse of the life forms that make up any forest ecosystem. The structure and functioning offorests are dependent on microbial interactions. Four processes are particularly important:nitrogen fixation, decomposition and nutrient cycling, pathogenesis, and mutualistic symbiosis.

Nitrogen fixation is crucial to forest function. While atmospheric nitrogen is abundant, it isunavailable to trees or other plants unless fixed, that is, converted to ammonia (NH4), by eithersymbiotic or free-living soil bacteria.

Most microorganisms are saprophytic decomposers, gaining carbon from the dead remains ofother plants or animals. In the process of their growth and death, they release nutrients fromthe forest litter, making them available once again for the growth of plants. Their roles incarbon, nitrogen, and phosphorus cycling are particularly important. Fungi are generally mostimportant in acid soils beneath conifer forests, while bacteria are more important in soils with ahigher pH. Bacteria often are the last scavengers in the food web and in turn serve as food to ahost of microarthropods.

Microorganisms reduce the mass of forest litter and, in the process, contribute significantly tothe structure and fertility of soils as the organic residue is incorporated.

Some bacteria and many fungi are plant pathogens, obtaining their nutrients from living plants.Some are opportunists, successful as saprophytes, but capable of killing weakened orwounded plant tissues. Others require a living host, often preferring the most vigorous trees inthe forest. Pathogenic fungi usually specialize on roots or stems or leaves, on one species orgenus of trees.

Pathogenic fungi are important parts of all natural forest ecosystems. The forest trees evolvedwith the fungi, and have effective means of defense and escape, reducing the frequency ofinfection and slowing the rates of tissue death and tree mortality. However, trees are killed, andthe composition and structure of the forest is shaped in large part by pathogens.

Pathogens remove weak or poorly adapted organisms from the forest, thus maintaining thefitness of the population. Decay fungi that kill parts of trees or rot the heartwood of living treescreate an essential habitat for cavity-nesting birds and the other animals dependent on hollowtrees.

By killing trees, pathogens create light gaps in the forest canopy. The size and rate of light gapformation and the relative susceptibility of the tree species present on the site determine theecological consequences of mortality. Forest succession is often advanced as shade-toleranttrees are released in small gaps. Gaps allow the growth of herbaceous plants in the island oflight, creating habitat and food diversity for animals within the forest. In many forests,pathogens are the most important gap formers and the principal determinants of structure and succession in the long intervals between stand-replacing disturbances such as wildfires orhurricanes.

The fungus roots of trees, and indeed most plants, represent an intimate physical andphysiological association of particular fungi and their hosts. Mycorrhizae are the products oflong coevolution between fungus and plant, resulting in mutual dependency. Mycorrhizae areparticularly important to trees because they enhance the uptake of phosphorus from soils.Mycorrhizal fungi greatly extend the absorptive surface of roots through the network of externalhyphae.

EE-Unit-I Pond Ecosystem

A pond, a large earth depression where water collects, often has a serene, shallow depth composition to it. The ponds shallowness allows sunlight to penetrate to the bottom, which allows plants to grow. Pond plants either grow entirely underwater or partially on the surface. A minority of plants will also grow along the pond’s edge. Ponds eventually turn into a large plot of soil if left untreated by intervention. Ponds will support a large variety of animal and plant life, such as birds, crayfish, small fish, frogs, insects, turtles, protozoa, algae, and lily pads. Ponds usually regulate the same water temperature ranging from the water’s surface to the bottom. Ponds may freeze solid in colder climates.

A pond ecosystem, a basic unit in ecology formed from the cohabitation of plants, animals, microorganisms, and a surrounding environment, refers to a community of freshwater organisms largely dependent on each of the surviving species to maintain a life cycle. Ponds shallow water bodies barely reach 12 to 15 feet in-depth and allow the sun to penetrate to its bottom, allowing freshwater plants to grow. A pond ecosystem consists of algae, fungi, microorganisms, plants, and various fish, which may fall into three distinct classifications: producer, consumer, and decomposer. The pond’s natural cycle begins with the producers and then to the consumers before ending with the decomposers.

A pond’s ecosystem consists of abiotic environmental factors and biotic communities of organisms. Abiotic environmental factors of a pond’s ecosystem include temperature, flow, and salinity. The percentage of dissolved oxygen levels in a water body determines what kind of organisms will grow there. After all, fish need dissolved oxygen in order to survive; however, anaerobic bacteria will not thrive in an ecosystem pumped with dissolved oxygen. A water body’s salinity may also determine the different species present. For instance, marine organisms tolerate salinity, while freshwater organisms will not thrive when exposed to salt. In fact, freshwater ecosystems often have plant species present which will absorb salts that are dangerous for freshwater organisms.

A pond ecosystem consists of four habitats, including the shore, surface film, open water, and bottom water. The shore, depending on its rocky, sandy, or muddy composition, lures in various organisms. For instance, rocky shores may now allow plants to grow, while muddy or sandy shores attract grasses, algae, earthworms, snails, protozoa, insects, small fish, and microorganisms. The pond’s surface breeds excellent ground for water striders, marsh traders, free-floating organisms, and organisms that can walk on the surface of water. An open-water habitat permits sizable fish, plankton, phytoplankton, and zooplankton to grow. Phytoplankton includes a large variety of algae, while zooplankton refers to insect larvae, rotifers, small crustaceans and invertebrates. Fish feed on plankton, or tiny organisms. The bottom-water habitat varies depending upon the pond’s depth. Shallow ponds with sandy bottoms provide a nesting environment for earthworms, snails, and insects. Deep-ended ponds have muddy bottoms, which allow various microorganisms, such as flatworms, rat-tailed maggots, and dragonfly nymphs to reproduce and survive.

A pond’s ecosystem food chain has three basic trophic levels. The first trophic level represents the producer and autotrophs, such as phytoplankton and plants. Producers prepare their own food with the energy emitted from the sun through a process known as photosynthesis. The second trophic level consists of herbivores, such as insects, crustaceans, and invertebrates that inhabit the pond and consume the plants. The third trophic level comprises of carnivores, such as various sizes of fish, which feed on both the plants and herbivores atop the first and second trophic levels. Saprotrophic organisms, also known as decomposers located on the bottom of the food chain, help decompose dead organic matter, which further breaks down into carbon dioxide and essential nutrients, such as nitrogen, phosphorus, and magnesium. These nutrients supply the necessary life force for the first trophic level organisms to produce food for the second trophic organisms, which results in the perpetual flow of energy in the pond’s ecosystem.

EE-Unit-I Grassland ecosystem

A biological community that contains few trees or shrubs, is characterized by mixedherbaceous (nonwoody) vegetation cover, and is dominated by grasses or grasslike plants.Mixtures of trees and grasslands occur as savannas at transition zones with forests or whererainfall is marginal for trees. About 1.2 × 108 mi2 (4.6 × 107 km2) of the Earth’s surface iscovered with grasslands, which make up about 32% of the plant cover of the world. In NorthAmerica, grasslands include the Great Plains, which extend from southern Texas into Canada.The European meadows cross the subcontinent, and the Eurasian steppe ranges from Hungaryeastward through Russia to Mongolia; the pampas cover much of the interior of Argentina andUruguay. Vast and varied savannas and velds can be found in central and southern Africa and throughout much of Australia.

Grasslands occur in regions that are too dry for forests but that have sufficient soil water tosupport a closed herbaceous plant canopy that is lacking in deserts. Thus, temperategrasslands usually develop in areas with 10–40 in. (25–100 cm) of annual precipitation,although tropical grasslands may receive up to 60 in. (150 cm). Grasslands are found primarilyon plains or rolling topography in the interiors of great land masses, and from sea level toelevations of nearly 16,400 ft (5000 m) in the Andes. Because of their continental location theyexperience large differences in seasonal climate and wide ranges in diurnal conditions. Ingeneral, there is at least one dry season during the year, and drought conditions occurperiodically.

Significant portions of the world’s grasslands have been modified by grazing or tillage or havebeen converted to other uses. The most fertile and productive soils in the world have developedunder grassland, and in many cases the natural species have been replaced by cultivatedgrasses (cereals).

Different kinds of grasslands develop within continents, and their classification is based onsimilarity of dominant vegetation, presence or absence of specific dominant species, orprevailing climate conditions.

The climate of grasslands is one of daily and seasonal extremes. Deep winter cold does notpreclude grasslands since they occur in some of the coldest regions of the world. However, thesuccess of grasslands in the Mediterranean climate shows that marked summer drought is notprohibitive either. In North America, the rainfall gradient decreases from an annual precipitationof about 40 in. (100 cm) along the eastern border of the tallgrass prairie at the deciduous forestto only about 8 in. (20 cm) in the shortgrass prairies at the foothills of the Rocky Mountains. Asimilar pattern exists in Europe. Growing-season length is determined by temperature in thenorth latitudes and by available soil moisture in many regions, especially those adjacent todeserts. Plants are frequently subjected to hot and dry weather conditions, which are oftenexacerbated by windy conditions that increase transpirational water loss from the plant leaves.

Soils of mesic temperate grasslands are usually deep, about 3 ft (1 m), are neutral to basic,have high amounts of organic matter, contain large amounts of exchangeable bases, and arehighly fertile, with well-developed profiles. The soils are rich because rainfall is inadequate forexcessive leaching of minerals and because plant roots produce large amounts of organicmaterial. With less rainfall, grassland soils are shallow, contain less organic matter, frequentlyare lighter colored, and may be more basic. Tropical and subtropical soils are highly leached,have lower amounts of organic material because of rapid decomposition and more leachingfrom higher rainfall, and are frequently red to yellow.

Grassland soils are dry throughout the profile for a portion of the year. Because of their densefibrous root system in the upper layers of the soil, grasses are better adapted than trees tomake use of light rainfall showers during the growing season. When compared with forest soils,grassland soils are generally subjected to higher temperatures, greater evaporation, periodic drought, and more transpiration per unit of total plant biomass.

Throughout the year, flowering plants bloom in the grasslands with moderate precipitation, andflowers bloom after rainfall in the drier grasslands. With increasing aridity and temperature,grasslands tend to become less diverse in the number of species; they support more warm-season species; the complexity of the vegetation decreases; the total above-ground and below-ground production decreases; but the ratio of above-ground to below-ground biomass becomessmaller.

There are many more invertebrate species than any other taxonomic group in the grasslandecosystem. Invertebrates play several roles in the ecosystem. For example, many areherbivorous, and eat leaves and stems, whereas others feed on the roots of plants.Earthworms process organic matter into small fragments that decompose rapidly, scarab beetles process animal dung on the soil surface, flies feed on plants and are pests to cattle,and many species of invertebrates are predaceous and feed on other invertebrates. Soil nematodes, small non arthropod invertebrates, include forms that are herbivorous, predaceous,or saprophagous, feeding on decaying organic matter.

Most of the reptiles and amphibians in grassland ecosystems are predators. Relatively few birdspecies inhabit the grassland ecosystem, although many more species are found in theflooding pampas of Argentina than in the dry grasslands of the western United States. Theirrole in the grassland ecosystem involves consumption of seeds, invertebrates, and vertebrates;seed dispersal; and scavenging of dead animals.

Small mammals of the North American grassland include moles, shrews, gophers, groundsquirrels, and various species of mice. Among intermediate-size animals are the opossum, fox,coyote, badger, skunk, rabbit, and prairie dog; large animals include various types of deer andelk. The most characteristic large mammal species of the North American grassland is thebison, although many of these animals were eliminated in the late 1800s. Mammals includeboth ruminant (pronghorns) and nonruminant (prairie dogs) herbivores, omnivores (opossum),and predators (wolves).

Except for large mammals and birds, the animals found in the grassland ecosystem undergorelatively large population variations from year to year. These variations, some of which arecyclical and others more episodic, are not entirely understood and may extend over severalyears. Many depend upon predator–prey relationships, parasite or disease dynamics, orweather conditions that influence the organisms themselves or the availability of food, water,and shelter.

Within the grassland ecosystem are enormous numbers of very small organisms, includingbacteria, fungi, algae, and viruses. From a systems perspective, the hundreds of species of bacteria and fungi are particularly important because they decompose organic material,releasing carbon dioxide and other gases into the atmosphere and making nutrients available for recycling. Bacteria and some algae also capture nitrogen from the atmosphere and fix it into forms available to plants.

Much of the grassland ecosystem has been burned naturally, probably from fires sparked bylightning. Human inhabitants have also routinely started fires intentionally to remove predatorsand undesirable insects, to improve the condition of the rangeland, and to reduce cover forpredators and enemies; or unintentionally. Thus, grasslands have evolved under the influences of grazing and periodic burning, and the species have adapted to withstand these conditions. If burning or grazing is coupled with drought, however, the grassland will sustain damage that may require long periods of time for recovery by successional processes.

EE-Unit-I Nutrients Cycling

Inorganic nutrients occur in limited quantities and their loss to an ecosystem or retention and re-use is of great importance. The cycles of chemical elements in an ecosystem are known as nutrient cycles. If there is no loss to the ecosystem the cycle is said to be a ‘perfect cycle’ and if loss does occur the cycle is said to be ‘imperfect’. The decomposers play an important role in these cycles because they break down dead organisms and make the nutrient components available once more to other organisms.

The carbon and nitrogen cycle are two such cycles.

    • The Carbon Cycle
      All organic compounds contain carbon and the most important sources of all inorganic carbon is carbon dioxide in the atmosphere.
  • carbon dioxide is taken up by autotrophic organisms during photosynthesis and the carbon is incorporated into carbohydrates and other compounds , such as proteins and fats;
  • consumers (heterotrophic organisms) feed on plants, and their bodies assimilate carbon compounds derived from the plants;
  • all organisms, including plants, release carbon dioxide during respiration as a by product. (Fermentation releases of carbon dioxide);
  • when autotrophic and heterotrophic organisms die or lose body parts such as leaves, carbon dioxide is released as a result of decomposition;
  • combustion of dead animal and plant material also releases carbon dioxide;
  • under high pressures, dead plants and animals are carbonized, forming fossil-fuels, such as coal and crude-oil. These release carbon dioxide during combustion.

 

A Diagrammatic representation of the Carbon Cycle

 

  • The Nitrogen cycle
    Nitrogen is an element essential in all organisms, occurring in proteins and other nitrogenous compounds, e.g. nucleic acids. Although organisms live in nitrogen-rich environments (78% of the atmosphere is nitrogen) the gaseous forms of nitrogen can only be used by certain organisms. Free nitrogen must first be fixed into a useable form.
  • free nitrogen in the atmosphere is mainly fixed by two groups of bacteria, nl. Azotobacter and Clostridium. The nitrogen is then used to manufacture proteins in their bodies, when they die, their proteins are broken down by decomposers (mainly bacteria and other micro-organisms), and converted into ammonia (blue-green algae, cyanobacteria, can also be use free nitrogen from the atmosphere);
  • during electrical changes in the atmosphere(e.g. lightning), free nitrogen is fixed (combined) finally forming nitrate;
  • nitrates are taken up by plants which use them to manufacture proteins;
  • animals (herbivores) eat plants and convert plant proteins to animal proteins, while carnivores obtain their plant proteins by indirect means (by eating herbivores);
  • when plants and animals die, the proteins in their bodies are broken down into ammonia by decomposers. The process is known as ammonification;
  • ammonia is converted to nitrites by nitrite bacteria (Nitrosomonas and Nitrosococcus). Nitrites are again converted to nitrates by nitrate bacteria (Nitrobacter )This process is known as nitrification;
  • different types of bacteria are also able to break down nitrates, nitrites and ammonia which results in the release of nitrogen. This process is known asdenitrification.

 

A Diagrammatic Representation of the Nitrogen Cycle.

Water Cycle

Earth’s water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.

The Water Cycle: Graphic showing the movement of water through the water cycle.

Global water distribution

For an estimated explanation of where Earth’s water exists, look at the chart below. By now, you know that the water cycle describes the movement of Earth’s water, so realize that the chart and table below represent the presence of Earth’s water at a single point in time. If you check back in a thousand or million years, no doubt these numbers will be different!

Notice how of the world’s total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday.

Barcharts of the distribution of water on Earth

Source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources (Oxford University Press, New York).

Where is Earth’s water?

For a detailed explanation of where Earth’s water is, look at the data table below. Notice how of the world’s total water supply of about 333 million cubic miles (1,386 million cubic kilometers) of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Thus, rivers and lakes that supply surface water for human uses only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 0.007 percent of total water, yet rivers are the source of most of the water people use.

One Estimate of Global Water Distribution
(Numbers are rounded)
Water source Water volume, in cubic miles Water volume, in cubic kilometers Percent of
freshwater		 Percent of
total water
Oceans, Seas, & Bays 321,000,000 1,338,000,000 — 96.5
Ice caps, Glaciers, & Permanent Snow 5,773,000 24,064,000 68.7 1.74
Groundwater 5,614,000 23,400,000 — 1.69
    Fresh 2,526,000 10,530,000 30.1   0.76
    Saline 3,088,000 12,870,000 —   0.93
Soil Moisture 3,959 16,500 0.05 0.001
Ground Ice & Permafrost 71,970 300,000 0.86 0.022
Lakes 42,320 176,400 — 0.013
    Fresh 21,830 91,000 0.26 0.007
    Saline 20,490 85,400 — 0.006
Atmosphere 3,095 12,900 0.04 0.001
Swamp Water 2,752 11,470 0.03 0.0008
Rivers 509 2,120 0.006 0.0002
Biological Water 269 1,120 0.003 0.0001
Source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources (Oxford University Press, New York).

Oxygen Cycle

Oxygen, like carbon and hydrogen, is a basic element of life. In addition, in the form of O3, ozone, it provides protection of life by filtering out the sun’s UV rays as they enter the stratosphere. In addition to constituting about 20% of the atmosphere, oxygen is ubiquitous. It also occurs in combination as oxides in the Earth’s crust and mantle, and as water in the oceans.

Early in the evolution of the Earth, oxygen is believed to have been released from water vapor by UV radiation and accumulated in the atmosphere as the hydrogen escaped into the earth’s gravity. Later, photosynthesis became a source of oxygen. Oxygen is also released as organic carbon in CHO, and gets buried in sediments. The role of oxygen in life is describe in the unit onBiological Systems.

Figure O1. The Oxygen Cycle

Oxygen is highly reactive. A colorless, odorless gas at ordinary temperatures, it turns to a bluish liquid at -183° C. Burning or combustion is essentially oxidation, or combination with atmospheric oxygen. Figure O1 shows a very broad overview of oxygen cycling in nature. The environment of oxygen in numerous reactions make it hard to present a complete picture.

Oxygen is vital to us in many ways (beside the most obvious–for breathing). Water can dissolve oxygen and it is this dissolved oxygen that supports aquatic life. Oxygen is also needed for the decomposition of organic waste. Wastes from living organisms are “biodegradable” because there are aerobic bacteria that convert organic waste materials into stable inorganic materials. If enough oxygen is not available for these bacteria, for example, because of enormous quantities of wastes in a body of water, they die and anaerobic bacteria that do not need oxygen take over. These bacteria change waste material into H2S and other poisonous and foul-smelling substances. For this reason, the content of biodegradable substances in waste waters is expressed by a special index called “biological oxygen demand” (BOD), representing the amount of oxygen needed by aerobic bacteria to decompose the waste. 

 

EE-Unit-I Energy Flow in an Ecosystem

Nearly all of the energy that drives ecosystems ultimately comes from the sun. Solar energy, which is an abiotic factor, by the way, enters the ecosystem through the process of photosynthesis. You can learn more than you want to know about this process in the unit on photosynthesis. Or, you could just chat with your local botanist. Everyone has one, right? The organisms in an ecosystem that capture the sun’s electromagnetic energy and convert it into chemical energy are called producers. Not to be confused with these Producers.

The name is appropriate because producers make the carbon-based molecules, usually carbohydrates, that the rest of the organisms in the ecosystem, including you, consume. Producers include all of the green plants and some bacteria and algae. Every living thing on Earth literally owes its life to the producers. The next time you see a plant, it wouldn’t be a bad idea for you to thank it for its services…which, as you will learn in other units, go way beyond just supplying you with food.

After a producer has captured the sun’s energy and used it to grow yummy plant parts, other organisms come along and greedily gobble it up. These primary consumers, as they are called, exclusively feed on producers. If these consumers are human, we call them vegetarians. Otherwise, they are known as herbivores.

Primary consumers only obtain a fraction of the total solar energy—about 10%—captured by the producers they eat. The other 90% is used by the producer for growth, reproduction, and survival, or it is lost as heat. You can probably see where this is going. Primary consumers are eaten by secondary consumers. An example would be birds that eat bugs that eat leaves.Secondary consumers are eaten by tertiary consumers. Cats that eat birds that eat bugs that eat leaves, for instance.

At each level, called a trophic level, about 90% of the energy is lost. What a shame. So, if a plant captures 1000 calories of solar energy, a bug that eats the plant will only obtain 100 calories of energy. A chicken that eats the bug will only obtain 10 calories, and a human that eats the chicken will only obtain 1 calorie of the original 1000 calories of solar energy captured by the plant. When you think about this way, it would take 100 1000-calorie plants—those would be enormo plants, by the way—to produce a single 100-calorie piece of free-range chicken. You are now recalling all of the plants you have ever forgotten to water in your life and feeling really, really terrible about it, aren’t you?

The relationships among producers, primary consumers, secondary consumers, and tertiary consumers is usually drawn as a pyramid, known as an energy pyramid, with producers at the bottom and tertiary consumers at the top. You can see from the example above why producers are at the bottom of this pyramid. It takes a lot of producers for higher-trophic-level consumers, like humans, to obtain the energy they need to grow and reproduce.

This is the answer to the great mystery as to why there are so many plants on Earth. We will even spell it out for you because it is so important to understand: there are so many plants on Earth because energy flow through ecosystems is inefficient. Only 10% of the energy in one trophic level is ever passed to the next. So, there you have it. We hope you feel fulfilled.

In addition to energy pyramid diagrams, ecosystem ecologists sometimes depict the relationship between trophic groups in a linear way, with arrows pointing from one organism to another. If there is only one producer, one primary consumer, one secondary consumer, and one tertiary consumer, this linear diagram is called a food chain. Food chains help ecologists and students visualize the interactions between organisms in an ecosystem. As always seems to be the case, it isn’t ever that simple. In fact, trophic interactions among organisms in an ecosystem are often really complex. It’s rare that an ecosystem only has one species at each trophic level. Usually, there are multiple producers that are eaten by multiple primary consumers. Some consumers eat different kinds of producers. Likewise, secondary consumers sometimes eat producers as well as primary consumers. These are known as omnivores.

These complex relationships are often depicted—if they can be figured out, that is—in a diagram called a food web. These diagrams can become messy indeed, depending on the size of the ecosystem and the number of interactions among trophic groups. If you like puzzles and biology, though, ecosystem ecology is the field for you.

Ecologists use food webs to better understand the intricate workings of the ecosystems they study. Understanding exactly who is eating whom can provide valuable information for conservation biologists as well. Such knowledge can aid in restoration efforts, species recovery projects, and preservation efforts, just to name a few instances. In any case, uncovering food webs goes a long way to understanding the first half of an ecosystem, the community.

Brain Snack

Most of our energy comes from domesticated animals and plants, but we are not the only organisms on the planet that farm. Insects, such as the fungal ants, feeding leaf clippings to a special symbiotic fungus and protect it from invasive pathogens. The ants tendto  their fungus just as humans tend to their gardens. You can watch an ant colony tend to their fungus in real time here.

EE-Unit-I Trophic levels

Trophic levels are the feeding position in a food chain such as primary producers, herbivore, primary carnivore, etc. Green plants form the first trophic level, the producers. Herbivores form the second trophic level, while carnivores form the third and even the fourth trophic levels. In this section we will discuss what is meant by food chains, food webs and ecological pyramids.

    • Food Chains.
      The feeding of one organism upon another in a sequence of food transfers is known as a food chain. Another definition is the chain of transfer of energy (which typically comes from the sun) from one organism to another. A simple food chain is like the following:

      rose plant — aphids — beetle — chameleon — hawk.

      In this food chain, the rose plant is the primary producer. The aphids are the primary consumers because they suck the juice from the rose plant. The beetle is the primary carnivore because it eats the aphids. The chameleon, a secondary carnivore, eats the beetle. The hawk is the tertiary carnivore because it eats the secondary carnivore, the chameleon. The hawk eventually dies and its remains are broken down by decay-causing bacteria and fungi.
      Except in deep-sea hydrothermal ecosystems, all food chains start with photosynthesis and will end with decay.

    • Food Webs
      In an ecosystem there are many different food chains and many of these are cross-linked to form a food web. Ultimately all plants and animals in an ecosystem are part of this complex food web.

  • Ecological Pyramids
    Trophic levels and the energy flow from one level to the next, can be graphically depicted using an ecological pyramid. Three types of ecological pyramids can usually be distinguished namely:

      • Number pyramid.
        It is easily understood that many grass plants are needed to feed fewer snails on which, in turn, even fewer chickens would be able to feed. This in turn requires only a few people to eat the chickens that ate the snails. The Number pyramid shows the number of organisms in each trophic level and does not take into consideration the size of the organisms and over-emphasizes the importance of small organisms. In a pyramid of numbers the higher up one moves, so each consecutive layer or level contains fewer organisms than the level below it.

        The Number Pyramid

      • Biomass pyramid.
        This pyramid indicates the total mass of the organisms in each trophic level. The size of the organism is over-emphasized and it can happen that the mass of level 2 is greater than that of level 1, because the productivity of level 1 is not taken into consideration. Thus an enormous mass of grass is required to support a smaller mass of buck, which in turn would support a smaller mass of lions.

        The Biomass Pyramid

    • Energy pyramid.
      The Energy pyramid indicates the total amount of energy present in each trophic level. It also shows the loss of energy from one trophic level to the next. An energy pyramid shows clearly that the energy transfer from one trophic level to the next is accompanied by a decrease due to waste and the conversion of potential energy into kinetic energy and heat energy. The energy pyramid is more widely used than the others because comparisons can be made between trophic levels of different ecosystem. It is, however, more difficult to compile an energy pyramid than it is compile the other types of pyramids.

The Energy Pyramid

EE-Unit-I Food Chain,Food Web

Every organism needs to obtain energy in order to live. For example, plants get energy from the sun, some animals eat plants, and some animals eat other animals.

A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. A food chain starts with the primary energy source, usually the sun or boiling-hot deep sea vents. The next link in the chain is an organism that make its own food from the primary energy source — an example is photosynthetic plants that make their own food from sunlight (using a process calledphotosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs or primary producers.

Food web

Next come organisms that eat the autotrophs; these organisms are called herbivoresor primary consumers — an example is a rabbit that eats grass.

The next link in the chain is animals that eat herbivores – these are called secondary consumers — an example is a snake that eat rabbits.

In turn, these animals are eaten by larger predators — an example is an owl that eats snakes.

The tertiary consumers are are eaten by quaternary consumers — an example is a hawk that eats owls. Each food chain end with a top predator, and animal with no natural enemies (like an alligator, hawk, or polar bear).

The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step. A network of many food chains is called a food web.

Trophic Levels:
The trophic level of an organism is the position it holds in a food chain.

  1. Primary producers (organisms that make their own food from sunlight and/or chemical energy from deep sea vents) are the base of every food chain – these organisms are called autotrophs.
  2. Primary consumers are animals that eat primary producers; they are also called herbivores (plant-eaters).
  3. Secondary consumers eat primary consumers. They are carnivores (meat-eaters) and omnivores (animals that eat both animals and plants).
  4. Tertiary consumers eat secondary consumers.
  5. Quaternary consumers eat tertiary consumers.
  6. Food chains “end” with top predators, animals that have little or no natural enemies.

When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.

Some organisms’ position in the food chain can vary as their diet differs. For example, when a bear eats berries, the bear is functioning as a primary consumer. When a bear eats a plant-eating rodent, the bear is functioning as a secondary consumer. When the bear eats salmon, the bear is functioning as a tertiary consumer (this is because salmon is a secondary consumer, since salmon eat herring that eat zooplankton that eat phytoplankton, that make their own energy from sunlight). Think about how people’s place in the food chain varies – often within a single meal.

Food webNumbers of Organisms:
In any food web, energy is lost each time one organism eats another. Because of this, there have to be many more plants than there are plant-eaters. There are more autotrophs than heterotrophs, and more plant-eaters than meat-eaters. Although there is intense competition between animals, there is also an interdependence. When one species goes extinct, it can affect an entire chain of other species and have unpredictable consequences.

Equilibrium
As the number of carnivores in a community increases, they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other’s population. A similar equilibrium exists between plants and plant-eaters.

EE-Unit-I An Ecosystem and the Different Components of It

An ecosystem is a biological environment consisting of all the living organisms in a particular area as well as the non living component such as air, water, soil, and sunlight. It is the basic functional unit as it includes both the organism and its environment each influencing the properties of the other and are necessary for the survival and maintenance of life. The entire group of organisms inhabiting a particular ecosystem is called as a community. An ecosystem consists of four components and any recognizable unit of nature can be considered an ecosystem if it includes these four components.

The four components are:
1. Non living environment (abiotic component). These include air, water, soil, sunlight, and basic elements or components of the environment. The non living components enter the body of the living organisms, take part in various metabolic activities and then return back to the environment.
It can be further divided into
a. Climatic, which includes physical factors such as temperature, relative humidity etc.
b. Inorganic substances such as water, carbon, nitrogen, sulphur, phosphorus, etc.that helps in transporting and regulating the materials in the ecosystem.
c. Organic substances such as protein, carbohydrates, lipids etc. which largely form the living body that connects the biotic and abiotic components.

2. Producers or energy transducers (biotic component). These convert solar energy into chemical energy with the help of inorganic substances such as water and carbon dioxide and organic substances such as enzymes. The producers are autotrophic or self nourishing organisms and they are mostly green plants, which posses the green pigment called chlorophyll that converts solar energy into chemical energy particles. These include grasses, shrubs, plants, trees, phytoplankton, sea weeds etc.

3. Consumers (biotic component). These depend on other organisms for their nutrition. They are also called heterotrophs or other nurishing. Depending upon their food habits they may be classified into
a. Herbivores or plant eaters such as zooplankton, insects, rabbits, squirrels, deer, cattle, elephant etc.
b. Carnivores or flesh eaters such as praying mantis, snakes, leopard, tiger, lion etc.

4. Decomposers (biotic component). These are heterotrophic organisms which depend upon dead organic matter for their food. They break down complex organic matter like cellulose, hemicllulose, chitin etc. that are found on plant and animal bodies, into simple substances. These are mainly microorganisms such as bacteria, actinomycetes and fungi. There are also decomposer organisms such as the invertebrate animals like protozoa and oligochaetes such as earthworms, enchytraeid worms, etc. which uses dead organic matter for their food.

EE-Unit-I Ecosystem and its Components

An ecosystem consists of the biological community that occurs in some locale, and the physical and chemical factors that make up its non-living or abiotic environment. There are many examples of ecosystems — a pond, a forest, an estuary, a grassland. The boundaries are not fixed in any objective way, although sometimes they seem obvious, as with the shoreline of a small pond. Usually the boundaries of an ecosystem are chosen for practical reasons having to do with the goals of the particular study.

The study of ecosystems mainly consists of the study of certain processes that link the living, or biotic, components to the non-living, or abiotic, components. Energy transformations andbiogeochemical cycling are the main processes that comprise the field of ecosystem ecology. As we learned earlier, ecology generally is defined as the interactions of organisms with one another and with the environment in which they occur. We can study ecology at the level of the individual, the population, the community, and the ecosystem.

Studies of individuals are concerned mostly about physiology, reproduction, development or behavior, and studies of populations usually focus on the habitat and resource needs of individual species, their group behaviors, population growth, and what limits their abundance or causes extinction. Studies of communities examine how populations of many species interact with one another, such as predators and their prey, or competitors that share common needs or resources.

In ecosystem ecology we put all of this together and, insofar as we can, we try to understand how the system operates as a whole. This means that, rather than worrying mainly about particular species, we try to focus on major functional aspects of the system. These functional aspects include such things as the amount of energy that is produced by photosynthesis, how energy or materials flow along the many steps in a food chain, or what controls the rate of decomposition of materials or the rate at which nutrients are recycled in the system.

Components of an Ecosystem

You are already familiar with the parts of an ecosystem. You have learned about climate and soils from past lectures. From this course and from general knowledge, you have a basic understanding of the diversity of plants and animals, and how plants and animals and microbes obtain water, nutrients, and food. We can clarify the parts of an ecosystem by listing them under the headings “abiotic” and “biotic”.

ABIOTIC COMPONENTS
BIOTIC COMPONENTS
Sunlight Primary producers
Temperature Herbivores
Precipitation Carnivores
Water or moisture Omnivores
Soil or water chemistry (e.g., P, NH4+) Detritivores
etc. etc.
 

By and large, this set of environmental factors is important almost everywhere, in all ecosystems.

Usually, biological communities include the “functional groupings” shown above. A functional group is a biological category composed of organisms that perform mostly the same kind of function in the system; for example, all the photosynthetic plants or primary producers form a functional group. Membership in the functional group does not depend very much on who the actual players (species) happen to be, only on what function they perform in the ecosystem.

Processes of Ecosystems

This figure with the plants, zebra, lion, and so forth illustrates the two main ideas about how ecosystems function: ecosystems have energy flows and ecosystems cycle materials. These two processes are linked, but they are not quite the same (see Figure 1).

Figure 1. Energy flows and material cycles.

Energy enters the biological system as light energy, or photons, is transformed into chemical energy in organic molecules by cellular processes including photosynthesis and respiration, and ultimately is converted to heat energy. This energy is dissipated, meaning it is lost to the system as heat; once it is lost it cannot be recycled.  Without the continued input of solar energy, biological systems would quickly shut down. Thus the earth is an open system with respect to energy.

Elements such as carbon, nitrogen, or phosphorus enter living organisms in a variety of ways. Plants obtain elements from the surrounding atmosphere, water, or soils. Animals may also obtain elements directly from the physical environment, but usually they obtain these mainly as a consequence of consuming other organisms. These materials are transformed biochemically within the bodies of organisms, but sooner or later, due to excretion or decomposition, they are returned to an inorganic state. Often bacteria complete this process, through the process called decomposition or mineralization (see previous lecture on microbes).

During decomposition these materials are not destroyed or lost, so the earth is a closed systemwith respect to elements (with the exception of a meteorite entering the system now and then). The elements are cycled endlessly between their biotic and abiotic states within ecosystems. Those elements whose supply tends to limit biological activity are called nutrients.

The Transformation of Energy

The transformations of energy in an ecosystem begin first with the input of energy from the sun. Energy from the sun is captured by the process of photosynthesis. Carbon dioxide is combined with hydrogen (derived from the splitting of water molecules) to produce carbohydrates (CHO). Energy is stored in the high energy bonds of adenosine triphosphate, or ATP (see lecture on photosynthesis).

The prophet Isaah said “all flesh is grass”, earning him the title of first ecologist, because virtually all energy available to organisms originates in plants. Because it is the first step in the production of energy for living things, it is called primary production (click here for a primer on photosynthesis). Herbivores obtain their energy by consuming plants or plant products,carnivores eat herbivores, and detritivores consume the droppings and carcasses of us all.

Figure 2 portrays a simple food chain, in which energy from the sun, captured by plant photosynthesis, flows fromtrophic level to trophic level via the food chain. A trophic level is composed of organisms that make a living in the same way, that is they are all primary producers (plants),primary consumers (herbivores) or secondary consumers (carnivores). Dead tissue and waste products are produced at all levels. Scavengers, detritivores, and decomposers collectively account for the use of all such “waste” — consumers of carcasses and fallen leaves may be other animals, such as crows and beetles, but ultimately it is the microbes that finish the job of decomposition. Not surprisingly, the amount of primary production varies a great deal from place to place, due to differences in the amount of solar radiation and the availability of nutrients and water.

For reasons that we will explore more fully in subsequent lectures, energy transfer through the food chain is inefficient. This means that less energy is available at the herbivore level than at the primary producer level, less yet at the carnivore level, and so on. The result is a pyramid of energy, with important implications for understanding the quantity of life that can be supported.

Usually when we think of food chains we visualize green plants, herbivores, and so on. These are referred to asgrazer food chains, because living plants are directly consumed. In many circumstances the principal energy input is not green plants but dead organic matter. These are called detritus food chains. Examples include the forest floor or a woodland stream in a forested area, a salt marsh, and most obviously, the ocean floor in very deep areas where all sunlight is extinguished 1000’s of meters above. In subsequent lectures we shall return to these important issues concerning energy flow.

 Finally, although we have been talking about food chains, in reality the organization of biological systems is much more complicated than can be represented by a simple “chain”. There are many food links and chains in an ecosystem, and we refer to all of these linkages as a food web. Food webs can be very complicated, where it appears that “everything is connected to everything else”, and it is important to understand what are the most important linkages in any particular food web.

Biogeochemistry

How can we study which of these linkages in a food web are most important? One obvious way is to study the flow of energy or the cycling of elements. For example, the cycling of elements is controlled in part by organisms, which store or transform elements, and in part by the chemistry and geology of the natural world. The term Biogeochemistry is defined as the study of how living systems influence, and are controlled by, the geology and chemistry of the earth. Thus biogeochemistry encompasses many aspects of the abiotic and biotic world that we live in.

There are several main principles and tools that biogeochemists use to study earth systems. Most of the major environmental problems that we face in our world toady can be analyzed using biogeochemical principles and tools. These problems include global warming, acid rain, environmental pollution, and increasing greenhouse gases. The principles and tools that we use can be broken down into 3 major components: element ratios, mass balance, and element cycling.

1. Element ratios

In biological systems, we refer to important elements as “conservative”. These elements are often nutrients. By “conservative” we mean that an organism can change only slightly the amount of these elements in their tissues if they are to remain in good health. It is easiest to think of these conservative elements in relation to other important elements in the organism. For example, in healthy algae the elements C, N, P, and Fe have the following ratio, called the Redfield ratioafter the oceanographer who discovered it:

C : N : P : Fe = 106 : 16 : 1 : 0.01

Once we know these ratios, we can compare them to the ratios that we measure in a sample of algae to determine if the algae are lacking in one of these limiting nutrients.

2. Mass Balance

Another important tool that biogeochemists use is a simple mass balance equation to describe the state of a system. The system could be a snake, a tree, a lake, or the entire globe. Using a mass balance approach we can determine whether the system is changing and how fast it is changing. The equation is:

NET CHANGE = INPUT + OUTPUT + INTERNAL CHANGE

In this equation the net change in the system from one time period to another is determined by what the inputs are, what the outputs are, and what the internal change in the system was. The example given in class is of the acidification of a lake, considering the inputs and outputs and internal change of acid in the lake.

3. Element Cycling

Element cycling describes where and how fast elements move in a system. There are two general classes of systems that we can analyze, as mentioned above: closed and open systems.

A closed system refers to a system where the inputs and outputs are negligible compared to the internal changes. Examples of such systems would include a bottle, or our entire globe. There are two ways we can describe the cycling of materials within this closed system, either by looking at the rate of movement or at the pathways of movement.

  1. Rate = number of cycles / time * as rate increases, productivity increases
  2. Pathways-important because of different reactions that may occur

In an open system there are inputs and outputs as well as the internal cycling. Thus we can describe the rates of movement and the pathways, just as we did for the closed system, but we can also define a new concept called the residence time. The residence time indicates how long on average an element remains within the system before leaving the system.

  1. Rate
  2. Pathways
  3. Residence time, Rt

Rt = total amount of matter / output rate of matter

(Note that the “units” in this calculation must cancel properly)

Controls on Ecosystem Function

Now that we have learned something about how ecosystems are put together and how materials and energy flow through ecosystems, we can better address the question of “what controls ecosystem function”? There are two dominant theories of the control of ecosystems. The first, called bottom-up control, states that it is the nutrient supply to the primary producers that ultimately controls how ecosystems function. If the nutrient supply is increased, the resulting increase in production of autotrophs is propagated through the food web and all of the other trophic levels will respond to the increased availability of food (energy and materials will cycle faster). 

The second theory, called top-down control, states that predation and grazing by higher trophic levels on lower trophic levels ultimately controls ecosystem function. For example, if you have an increase in predators, that increase will result in fewer grazers, and that decrease in grazers will result in turn in more primary producers because fewer of them are being eaten by the grazers. Thus the control of population numbers and overall productivity “cascades” from the top levels of the food chain down to the bottom trophic levels.

So, which theory is correct? Well, as is often the case when there is a clear dichotomy to choose from, the answer lies somewhere in the middle. There is evidence from many ecosystem studies that BOTH controls are operating to some degree, but that NEITHER control is complete. For example, the “top-down” effect is often very strong at trophic levels near to the top predators, but the control weakens as you move further down the food chain. Similarly, the “bottom-up” effect of adding nutrients usually stimulates primary production, but the stimulation of secondary production further up the food chain is less strong or is absent.

Thus we find that both of these controls are operating in any system at any time, and we must understand the relative importance of each control in order to help us to predict how an ecosystem will behave or change under different circumstances, such as in the face of a changing climate.

The Geography of Ecosystems

There are many different ecosystems: rain forests and tundra, coral reefs and ponds, grasslands and deserts. Climate differences from place to place largely determine the types of ecosystems we see. How terrestrial ecosystems appear to us is influenced mainly by the dominant vegetation.

The word “biome” is used to describe a major vegetation type such as tropical rain forest, grassland, tundra, etc., extending over a large geographic area (Figure 3). It is never used for aquatic systems, such as ponds or coral reefs. It always refers to a vegetation category that is dominant over a very large geographic scale, and so is somewhat broader than an ecosystem.

Figure 3: The distribution of biomes.

We can draw upon previous lectures to remember that temperature and rainfall patterns for a region are distinctive. Every place on earth gets the same total number of hours of sunlight each year, but not the same amount of heat. The sun’s rays strike low latitudes directly but high latitudes obliquely. This uneven distribution of heat sets up not just temperature differences, but global wind and ocean currents that in turn have a great deal to do with where rainfall occurs. Add in the cooling effects of elevation and the effects of land masses on temperature and rainfall, and we get a complicated global pattern of climate.

A schematic view of the earth shows that, complicated though climate may be, many aspects are predictable (Figure 4). High solar energy striking near the equator ensures nearly constant high temperatures and high rates of evaporation and plant transpiration. Warm air rises, cools, and sheds its moisture, creating just the conditions for a tropical rain forest. Contrast the stable temperature but varying rainfall of a site in Panama with the relatively constant precipitation but seasonally changing temperature of a site in New York State. Every location has a rainfall- temperature graph that is typical of a broader region.

Figure 4. Climate patterns affect biome distributions.

We can draw upon plant physiology to know that certain plants are distinctive of certain climates, creating the vegetation appearance that we call biomes. Note how well the distribution of biomes plots on the distribution of climates (Figure 5). Note also that some climates are impossible, at least on our planet. High precipitation is not possible at low temperatures — there is not enough solar energy to power the water cycle, and most water is frozen and thus biologically unavailable throughout the year. The high tundra is as much a desert as is the Sahara.


Figure 5. The distribution of biomes related to temperature and precipitation.

   

  • Ecosystems are made up of abiotic (non-living, environmental) and biotic components, and these basic components are important to nearly all types of ecosystems.  Ecosystem Ecology looks at energy transformations and biogeochemical cycling within ecosystems.
  • Energy is continually input into an ecosystem in the form of light energy, and some energy is lost with each transfer to a higher trophic level. Nutrients, on the other hand, are recycled within an ecosystem, and their supply normally limits biological activity.  So, “energy flows, elements cycle”.
  • Energy is moved through an ecosystem via a food web, which is made up of interlocking food chains. Energy is first captured by photosynthesis (primary production). The amount of primary production determines the amount of energy available to higher trophic levels.
  • The study of how chemical elements cycle through an ecosystem is termed biogeochemistry. A biogeochemical cycle can be expressed as a set of stores (pools) and transfers, and can be studied using the concepts of “stoichiometry”, “mass balance”, and “residence time”.
  • Ecosystem function is controlled mainly by two processes, “top-down” and “bottom-up” controls.
  • A biome is a major vegetation type extending over a large area. Biome distributions are determined largely by temperature and precipitation patterns on the Earth’s surface.