1.3. Structure and Function of Ecosystem

STRUCTURE AND FUNCTION OF ECOSYSTEM

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Introduction to Ecology and Ecosystem

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Classification of Ecosystem

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Attributes of Ecosystem

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Structure of Ecosystem

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Function of Ecosystem

INTRODUCTION

We had earlier defined ecology and ecosystem. To enhance our understanding  of ecology and ecosystem, we shall have a small review of these terms.

Ecology comes from the greek words oikos (house or place where one lives) and logos(study of). It can be surmised as the Study of the “House” in which We Live. This term was introduced by Ernst Haeckl in 1869. Ecology can be defined more specifically specifically as the study of the interactions between organisms organisms and the nonliving components of their environment. ecology involves collecting information about organisms and their environment, looking for patterns, and seeking to explain these patterns. The Biosphere is composed composed of smaller units called ecosystems. ecosystems. An ecosystem includes all the organisms and the nonliving environment that are found in a particul particular ar place. place. Ecosys Ecosystems tems can can be as large or as small small as we decide. decide. Any area you decide to Study can be considered an Ecosystem. For example you may choose to study your back yard, a multi storeyed apartment, a mountain range or a forest/zoo. Any of these would be considered an Ecosystem. In an ecosystem, there are various levels of organisation. The simples simplestt level level of organ organiza izatio tion n in Ecosy Ecosyst stem em is that that of the orga organis nism. m. An organism refers to a particular organism in an ecosystem, say cat, dog etc. A population includes all the members of the same organism that live in one place at one time. All the different populations that live in a particular area make up a comm commun unity ity.. The The phys physic ical al loca locati tion on of a Comm Commun unit ity y is call called ed the the habi habita tat. t. Ecos Ecosys yste tem m is in turn turn a leve levell of orga organi nisa sati tion on and and has has one one high higher er leve levell of  organisation organisation called biosphere. The photograph on the next page derived from a foreign ecology book would clearly illustrate the various levels of organisation.

The diversity of an ecosystem is a measure of the number of different species there, and how common common each species is. Ecosystems Ecosystems are very complex. complex. They can contain hundreds or even thousands of interacting species. Each organism or species in the community has a role or profession in that community and in ecology this is the organism’s niche.

CLASSIFICATION OF ECOSYSTEM

An ecosystem can be classified as below

ECOSYSTEM

NATU ATUR RAL ECO COSY SYS STE TEM M

TERRESTRIAL ECOSYSTEM Forests Grasslands Deserts

ARTI AR TIFI FICI CIAL AL ECO COSY SYS STE TEM M

AQUATIC ECOSYSTEM Fresh Waters Marine Waters

There are further classifications in the above chart, but for a beginner level, it is enough to concentrate on these areas. Also the study of artificial ecosystem is not the scope scope of an environ environmen mental tal scienti scientist. st. The environ environmen mentali talists sts deal with natura naturall creat creation ions s and and mana manage geme ment nt only only.. Moreov Moreover er the syste system m in artif artific icial ial ecosystem does not offer much to study. Therefore we are more interested in natural ecosystem and don’t consider artificial ecosystem APPROACH TO ECOSYSTEM

With an ecosystem ecosystem comprising comprising of large number number of speices, it would would seem and is imprac impractic tical al to study the interac interactio tion n of each each orga organis nism m with with anoth another er,,

It is

impossib impossible le to approac approach h an ecosyst ecosystem em by studyin studying g the individu individual al organism organism – enviro environm nmen entt relati relations onship hip.. There Therefor fore e we study study an ecosy ecosyst stem em follow following ing an wholesome approach. We stud study y the the ecos ecosys yste tems ms by stud studyi ying ng the the two two aspe aspect cts s (att (attri ribu bute tes) s) of an ecosystem. They are (1) Structure or Architectural Process

(2) Function or Working Process

STRUCTURE OF AN ECOSYSTEM

By Architecture or Structure of an Ecosystem, we mean 

the composition of biological community including species, numbers, biomass, life history and distribution in space, etc.

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the quantity and distribution of non living materials like nutrients, water etc.

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the conditions of existence such as temperature, light etc.

An ecosystem possesses both living components and biotic factors and nonliving or abiotic factors. The The nonl nonliv ivin ing g fact factor ors, s, call called ed abio abioti tic c fact factor ors, s, are are phys physic ical al and and chem chemic ical al characteristics of the environment. They include solar energy (amount of sun light), oxygen, CO 2, water, temperature, humidity, ph, and availability of nitrogen. The living components of the environment are called Biotic Factors. They include all the the Livin Living g Thing Things s that that affe affect ct an orga organis nism. m. Biot Biotic ic Comp Compon onen ents ts are are often often categorized as Producers, Consumers, and Decomposer. The structure of an ecosystem can be represented as below

ECOSYSTEM

ABIO AB IOTI TIC C CO COMP MPON ONEN ENTS TS CLIMATIC FACTORS

EDAPHIC FACTORS

E.g. Rain Light Wind Temp.

E.g. Soil Minerals Oxygen Topography

BIO BI OTI TIC C CO COMP MPO ONEN ENTS TS

PRODUCERS

CONSUMERS

DECOMPOSERS

also known as autotrophs, they produce energy

also known as heterotrophs, they consume and transfer  energy

better known as reducers or  saptrotrophs recycle energy

FUNCTION OF AN ECOSYSTEM

The function of an ecosystem is a broad, vast and often confused topic. The function of an ecosystem can be best studied by understanding the history of  ecological studies. The function of an ecosystem can be studied under the three heads. 1. Troph rophic ic Leve Levell Intera Interacti ction on 2. Ecol Ecologi ogical cal Succe Success ssion ion 3. Biog Biogeo eoch chem emis istry try

Trophic Level Interaction deals with how the members of an ecosystem are connected based on nutritional needs. Ecological Succession deals with the chan change ges s in feat featur ures es/m /mem embe bers rs of an ecos ecosy ystem stem over over a peri period od of time time.. Bioge Biogeoch ochem emist istry ry is focuss focussed ed upon upon the cyclin cycling g of essen essentia tiall mater material ials s in an ecosystem.   As we would would be discus discussin sing g about about ecolog ecologica icall success succession ion and bio geo chemistry in the future chapters, we shall confine to Trophic level interaction alone in this chapter. chapter. For examination examination purposes, the student may also stop with explaining the constituents of trophic level interaction.

Trophic Level Interaction was developed by zoologist Charles Elton. It deals with who eats who and is eaten by whom in an ecosystem. The study of  trophic level interaction in an ecosystem gives us an idea about the energy flow through the ecosystem. The trophic level interaction involves three concepts namely 1. Food Chain 2. Food Web 3. Ecol Ecologi ogical cal Pyra Pyramid mids s

FOOD CHAIN In an ecosystem ecosystem one can observe the transfer or flow of energy from one trophic level to other in succession. A trophic level can be defined as the number of links by which it is separated from the producer, producer, or as the wh position of the organism

in the food chain. The patterns of eating and being eaten forms a linear chain called food chain which can always be traced back to the producers. Thus, primary producers trap radiant energy of sun and transfer that to chemical or  potential energy of organic compounds such as carbohydrates, proteins and fats.

When a herbivore animal eats a plant (or when bacteria decompose it) and these organic compounds are oxidized, the energy liberated is just equal to the amount of energy used in synthesizing synthesizing the substances (first law of thermodynamics), thermodynamics), but some of the energy is heat and not useful energy (second law of  thermodynamics). If this animal, in rum, is eaten by another one, along with transfer of energy from a herbivore to carnivore a further decrease in useful energy occurs as the second animal (carnivore) oxidizes the organic substances substances of the first first (herb (herbivo ivore re or omni omnivor vore) e) to libera liberate te ener energy gy to synth synthesi esize ze its own cellular constituents. Such transfer of energy from organism to organism sustains the ecosystem and when energy is transferred from individual to individual in a particular community, as in a pond or a lake or a river, we come across the food chains. The number of steps in a food chain are always restricted to four or five, since the energy available decreases with each step. Many direct or indirect methods arc employed to study food chain relationships in nature. They include gut content analysis, use of radioactive isotopes, precipitin test, etc.

In nature, basically two types of food chains arc recognized—grazing food chain and detritus food chain. 1. Grazing food chain. chain . This type of food chain (Fig. 9.4) starts from the living green plants, goes to grazing herbivores and on to the carnivores. Ecosystems with such type of food chain are directly dependent on an influx of solar radiation. Thus, this type of food chain depends on autotrophic energy capture and the movement of this energy to herbivores. Most of the ecosystems in nature follow this type of food chain. These chains are very significant from energy standpoint. standpoint. The phytoplanktons phytoplanktons -> zooplanktons zooplanktons -» fish sequence sequence or the grasses -> rabbit -> fox sequence arc the examples of grazing food chain. Further the producer -> herbivore herbivore -> carnivor carnivore e chain chain is a predator predator chain. chain. Parasit Parasitic ic chains chains also exist wherein smaller organisms consume larger ones without outright killing as the case of the predators. 2. Detritus food chain. chain . The organic wastes, exudates and dead matter derived from the grazing food chain are generally termed detritus. The energy contained in this detritus in not lost to the ecosystem as a whole; rather it serves as the source of energy for a group of organisms (dctritivorcs (dctritivorcs that are separate separate from the grazing food chain, and generally termed as the detritus food chain (Fig. 9.5). Tl\e detritus food chain represents an exceedingly important component in the energy (low of an ecosystem. Indeed in some ecosystems, considerably more en-ergy en-ergy flows through through the detritu detritus s food chain than through through the grazing grazing food chain chain.. In the the detri detritus tus food food chain chain the the energ energy y flow flow remai remains ns as a contin continuo uous us passage rather than as a stepwise flow between discrete entities. The organisms of the detritus food chain are many and include algae, bacteria, slime molds, actinomycetes, fungi. Protozoa, insects mites. Crustacea, centipedes, molluscs, rotifers, annelid worms, nematodes and some vertebrates. Some species are highly specific in their food requirements and some can eat almost anything. detritus itself. Signific Significanc ance e of food chain chain : The food chain chain studies/ studies/help help under stand the feeding relationshi relationships ps and the interaction interaction between between organisms organisms in in anv ecosystem. ecosystem. They They also also help help us to appre apprecia ciate te the the energy energy flow mecha mecha-- nism nism and and matte matter  r  circulation in eco- system, and understand the movement of toxic substances in the eco-system and the problem of biological magnification FOOD WEB In nature simple ood chains occur rarely The same organism may operate in the ecosystem ecosystem at more than than one trophic trophic level i.e i.e it may derive derive its food from more

than one source. Even the same organism may be eaten by several organisms of  a higher trophic level or an organism may feed upon several different organisms of a lower trophic level. usually the kind of food changes with the age of the organism organism and the food availabili availability ty.. Thus in a given given ecosyst ecosystem em various various food food chains are linked together and interested each other to form a complex network called food Web. Generally food webs are not too complex. As more and more spec specie ies s are are invo involv lved ed in a web web the the conn connec ecta tanc nce e fall falls. s. Expe Expect ct in inse insect ct communities, omnivores are scare and when they occur they usually feed on species in adjacent trophic levels. Within habitats, food webs arc rarely broken up into discrete compartments. The number of species of predators in a food web typi-cally exceeds the number of species of prey by an aver-age of 1.3 predator  species per prey species.

ECOLOGICAL PYRAMIDS In the success successive ive steps steps of grazing grazing food chain-p chain-photo hotosyn synthet thetic ic autotro autotrophs, phs, herbivorous herbivorous heterotrophs, heterotrophs, carnivores heterotrophs, heterotrophs, decay bactcria-the number  and mass of the or-ganisms in each step is lim-ited by the amount of en-ergy available. Since some energy is lost as heat, in each transformation the steps become progressively progressively smaller near the top. This relationship relationship is sometimes sometimes called ecological pyramid. The ecological pyramids represent represent the trophic structure and also also troph trophic ic functi function on of the the ecosy ecosyst stem em.. In many many ecolo ecologic gical al pyra pyrami mids, ds, the producer form the base and the successive trophic levels make up the apex. The ecological pyramids may be of following three kinds: 1. Pyramid of number: It depicts depicts the number number of individu individual al organis organisms ms at different trophic levels of food chain. This pyramid was advanced by Charles Elto Elton n (192 (1927) 7),, who who poin pointe ted d out out the the grea greatt diff differ eren ence ce in the the numb number er of the the organisms involved in each step of the food chain. The animals at the lower end (base (base of pyram pyramid id)) of the chain chain arc arc the the most most abun abunda dant. nt. Succe Success ssive ive links links of  carnivores decrease rapidly in number until there are very few carnivores at the top. The pyramid of number ignores the biomass of organisms and it also docs not indicate the energy transferred transferred or the use of energy by the groups in vol ved. The lake ecosystem provides a typical example for pyramid of number. 2. Pyramid of biomass: biomass : The biomass of the members of the food chain present at any one time forms the pyramid of the biomass. Pyramid of biomass indicates decrease of biomass in each trophical level from base to apex. For  example, the total biomass of the producers ingested by herbivores is more than the total biomass of the herbivores in an ecosystem. Likewise, the total biomass of the the prim primar ary y carn carniv ivor ores es (or (or seco second ndary ary cons consum umer er)) will will be less less man man the the herbivores and so on. 3. Pyramid of energy: energy : When production is considered in terms of energy, the pyramid indicates not only the amount of energy flow at each level, but more important, the actual role the various organisms play in the transfer of energy. The base upon which the pyramid of energy is constructed is the quantity of  organisms produced per unit lime, or in other words, the rate at which food material passes through the food chain. Some organisms may have a small biomass, but the total energy they jssimilate and pass on, may be considerably greater than that of organisms organisms with a much larger biomass. Energy pyramids pyramids are

always slopping because less energy is transferred from each level than was paid into it. In cases such as in open water communities communities the producers have less bulk than consumers but the energy they store and pass on must be greater than that of the next level. Otherwise the biomass that producers support could not be grea greate terr than than that that of the the prod produc ucer ers s them themse selv lves es.. This This high high ener energy gy flow flow is maintained by a rapid turn over of individual plankton, rather than an increase of  total mass.

Pyramid of Bio Mass

Pyramid of Numbers

Pyramid of Energy