CHAPTER. 3

Environmental Science : Ecosystem

INTRODUCTION

No life exists in a vacuum. Materials and forces which constitutes its environment and from which it must derive its needs surround every living organism. Thus, for its survival, a plant, an animal, or a microbe cannot remain completely aloof in a shell. Instead, it requires from its environment a supply of energy, a supply of materials, and a removal of waste products.

For various basic requirements, each living organism has to depend and also to interact with different nonliving or abiotic and living or biotic components or the environment.

1. Abiotic

The abiotic environmental components include basic inorganic elements and compounds such as water and carbon dioxide, calcium and oxygen, carbonates and phosphates besides such physical factors as soil, rainfall, temperature, moisture, winds, currents, and solar radiation with its concomitants of light and heat.

2. Biotic

The biotic environmental factors comprise plants, animals, and microbes; They interact in a fundamentally energy-dependent fashion. In the words of Helena Curtis “The scientific study of the interactions of organisms with their physical environment and with each other, is called ecology”. According to Herreid II “It mainly concerns with the directive influences of abiotic and biotic environmental factors over the growth, distribution behaviour and survival of organisms.

Ecology Defined

(1) Ernst Haeckel (1866) defined ecology “as the body of knowledge concerning the economy of nature-the investigation of the total relations of animal to its inorganic and organic environment.

(2) Frederick Clements (1916) considered ecology to be “the science of community.

(3) British ecologist Charles Elton (1927) defined ecology as “the scientific natural history concerned with the sociology and economics of animals.”

(4) Taylor (1936) defines ecology as “the science of the relations of all organisms to all their environments.”

(5) Taylor (1936) defined ecology as “the science of the relations of all organisms to all their environments.”

(6) Allee (1949), considered ecology as “the science of inter-relations batwing living organisms and their environment, including both the physical and biotic environments, and emphasizing inter-species as well as intra-species relations.

(7) G.L. Clarke (1954) defined ecology as “the study of inter-relations of plants and animals with their environment which may include the influences of other plants and animals present as well as those of the physical features.”

(8) Woodbury (1955) regarded ecology as “the science which in investigates organisms in relation to their environment: a philosophy in which the world of life is interpreted in terms of natural processes.

(9) A. Macfadyen (1957) defined ecology as “ a science, which concerns itself with the inter-relationships of living organisms, plants and animals, and their environments.”

(10) S.C. Kendeigh (1961, 1974) defined ecology as “the study of animals and plants in their relation to each other and to their environment.” Certain modern ecologists have provided somewhat broader definitions of ecology.

(11) M.E. Clark (1973) considers ecology as “a study of ecosystems of the totality of the reciprocal interactions between living organisms and their physical surroundings.

(12) Pinaka (1973) defined ecology as “the scientific study of the relationships of loving organisms with each other and with their environments.” He adds that “it is the science of biological interactions among individuals, populations, and communities; and it is also the science of ecosystems-the inter-relations of biotic communities with their non-living environments.

(13) R.L. Smith (1977), considers ecology as “a multidisciplinary science which deals with the organism and its place to live and which focuses on the ecosystems.”

ECO-SYSTEM

At present ecological studies are made at Eco-system level. At this level the units of study are quite large. This approach has the view that living organisms and their non-living environment are inseparably interrelated and interact with each other. A.G. Tansley (in 1935) defined the Eco-system as ‘the system resulting from the integrations of all the loving and non-living actors of the environment’. Thus he regarded the Eco-systems as including not only the organism complex but also the whole complex of physical factors forming the environment.

HISTORICAL BACKGROUND

The idea of Eco-system is quite an old one. We find in literature some such parallel terms as (i) biocoenosis (Karl Mobius, 1977), (ii) microcosm (S.A. Forbes, 1887),

(i) Geobiocoenosis (V.V. Doduchaev, 1846-1903); G.F. Morozov; see Sukachev, 1944),

(ii) hlocoen (Frienderichs, 1930), (v) biosystem (Thienemann, 1939), (vi) bioenert body (Vernadsky, 1994), and ecosom etc. use for such ecological systems.

The terms ecosystems is most preferred, where ‘eco’ implies the environment, and ‘system’ implies an interacting, inter-dependent complex.

In this way, it can be said that any unit that includes all the organisms i.e. the communities in a given area, interact with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycle (i.e. exchange of materials between living and non-living components) within the system, is known as an ecological system or eco-system.

Eco-system may be visualized as 3-dimensional cutouts from the ecosphere. All primary and secondary producers composing the ecosystem are its essential elements. The unique feature of eco-systems is the maintenance of their chemical state and of their environment.

Thus an eco-system is an integrated unit, consisting of interacting plants and animals whose survival depends upon the maintenance of abiotic i.e. physicochemical environment and gradients such as moisture, wind and solar radiation with its concomitants of light and heat, as well as biotic structures and functions. The integrated unit may or may not be isolated but it must have definable limits within which there are integrated functions. The physiologists study various functions in individual plants or animals, but the ecologists study them at the eco-system level. A real ecologist endeavors for maintaining holistic or eco-system perspective of the process being studied by him.

ASPECTS OF ECO-SYSTEM

The eco-system can be defined as any spatial or organizational unit including living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts. The eco-system can be studied from either structural or functional aspects.

1. Structural Aspect

The structural aspects of ecosystem include a description of the arrangement, types and numbers of species and their life histories, along with a description of the physical features of the environment.

2. Functional

The functional aspects of the ecosystem include the flow of energy and the cycling of nutrients.

Habitat

The non-living part of the eco-system includes different kinds of habitats such as air, water and land, and a variety of abiotic factors. Habitat can be defined as the natural abode or locality of an animal, plant or person. It includes all features of the environment in a given locality. For example, water is used as habitat by aquatic organisms and it comprises three major categories-marine, brackish and freshwater habitats. Each of these categories. may be subdivided into smaller unit, such a freshwater habitat may exist as a large lake, a pond, a puddle, a river or a stream.

The land is used as a habitat for numerous terrestrial organisms. It includes many major categories of landmasses, which are called biomes. Biomes are distinct large areas of earth inclusive of flora and fauna, e.g. deserts, prairie, tropical forests, etc. Soil is also used as a habitat by a variety of microbes, plants and animals.

Abiotic Factors

Among the main abiotic factors of the ecosystem are included the follwing:

(1) The climatic factors as solar radiation, temperature, wind, water currents, rainfall.

(2) The physical factors as light, fire, pressure, geomagnetism,

(3) Chemical factors as acidity, salinity and the availability of inorganic nutrients needed by plants.

Biotic or Biological Factors

The biological (biotic) factors of ecosystem include all the living organisms-plants, animals, bacteria and viruses. Each kind of living organism found in an ecosystem is given the name a species. A species includes individuals which have the following features:

(1) They are genetically alike.

(2) They are capable of freely inter-breeding and producing fertile offsprings.

Relationships

In an ecosystem, there exist various relationships between species. The relationship may be as under:

(1) Effects

Two species may have any of the following kind of effects:

(i) They may have a negative effect upon one another (competition).

(ii) They may have a neutral effect (neutralism).

(iii) They may have beneficial effect (protoco-operation and mutualism).

(2) Other kinds of Relationship

The species may aggregate, or separate, or show a random relationship to one another.

Population

A population is a group of inter-acting individuals, usually of the same species, in a definable space. In this way we can speak of population of deer on an island, and the population of fishes in a pond. A balance between two aspects determines the size of a population of any given species:

(i) Its reproductive potential,

(ii) Its environmental resistance.

In this way population size is determined by the relative number of organisms added to or removed from the group as under:

(i) Addition

Recruitment into the population is a function of birth rate and immigration rate.

(ii) Removal

Loss from the population is a function of death rate and emigration.

Factors Regulating Population

Following factors does population regulation:

(i) Physical attributes of the environment (e.g. climate),

(ii) Food (quantity and quality),

(iii) Disease (host-parasite relationships).

(iv) Predation,

(v) Competition (inter-specific and intra-specific).

An ecosystem contains numerous populations of different species of plants, animals and microbes; all of them interact with one another as a community and with the physical environment as well. A community or biotic community, thus, consists of the population of plants and animals living together in a particular place.

Division of Ecosystem

The ecosystem can be divided, from the energetic view point into three types of organisms: producers, consumers, and reducers. These can be explained as under:

(1) Producer

Photosynthetic algae, plants and bacteria are the producers of the ecosystem; all other organisms depend upon them directly or indirectly for food.

(2) Consumers

Consumers are herbivorous, carnivorous, and omnivorous animals; they eat the organic matter produced by other organisms.

(3) Reducers

Reducers are heterotrophic organisms like animals; they are fungi and bacterial that decompose dead organic matter.

FOOD CHAINS OF FOOD WEB

Species are related by their feeding behaviour in food chains or food webs. There are two basic types of food chains as under-

(i) The consumer food chain includes the sequence of energy flow from producer+herbivore+carnivore+reducer;

(ii) The detritus food chain pypasses the consumers, going from producer+reducer.

Basic Theme of Ecosystems

(1) Relationship

The first and foremost theme of an ecosystem in that everything is somehow or other related to everything else, the relationships include interlocking functioning of organisms

among themselves besides with their environment. Biocoenosis and bioecocoenois are roughly equivalent to community and ecosystem respectively. Biotopes are the physical environment in which such communities exist. According to Lamotte (1969), it is this network of multiple interactions that permits us to define the ecosystem completely. Many ecologists regard Interdependence as the first basic theme of ecology. Ecosystem includes interacting and interdependent components that are open and linked to each other.

(2) Limitation

The second basis theme is Limitation which means that limits are ubiquitous and that no individual or species goes on growing indefinitely. Various species control and limit their own growth in response to overcrowding or other environmental signals and the total numbers keep pace with the resources available.

(3) Complexity

Complexity is a third characteristic of any eco-system. The three-dimensional interactions of the various constituent elements of an ecosystem are highly complex and often beyond the comprehension on the human brain.

GENERAL CHARACTERISTICS OF AN ECO-SYSTEM

According to Smith following are the general characteristics of eco-system.

(1) The ecosystem is a major structural and functional unit of ecology.

(2) The structure of an eco-system is related to its species diversity; as such the more complex ecosystem has high species diversity.

(3) The relative amount of energy required to maintain an ecosystem depends on its structure. The more complex the structure, the lesser the energy it requires to maintain itself.

(4) The function of the ecosystem is related to energy flow in material cycling through and within the system.

(5) Ecosystems mature by passing from less complex to more complex states. Early stages of such succession have an excess of potential energy. Later (mature) stages have less energy accumulation.

(6) Both the environment and the energy fixation in any given ecosystem are limited. They cannot be exceeded in any way without causing serious undesirable effect.

(7) Alterations in the environments represent selective pressures upon the population to which it must adjust. Organisms, which fail to adjust to the changed environment, must vanish.

To conclude the eco-system is an integrated unit or zone of variable size, it comprises vegetation, fauna, microbes and the environment. Most ecosystems process a well-defined soil, climate, flora and fauna and their own potential for adaptation, change and tolerance. The functioning of any ecosystem involves a series of cycles. These cycles are driven by energy flow, the energy being the solar energy.

STRUCTURE OF ECO-SYSTEMS

Meaning of Structure

By structure of an eco-system we mean as under.

(i) The composition of biological community including species, numbers, biomass, life history and distribution in space etc.

(ii) The quantity and distribution of the non-living materials, such as nutrients, water

etc.

(iii) Structure of an ecosystem the range, or gradient of conditions of existence, such as temperature.

Natural And Function of Structure of Eco-system

The structure of an ecosystem is in fact, a description of the species of organisms that are present, including information on their life histories, population and distribution in space. It guides us to know who’s who in the ecosystem. It also includes descriptive information on the non-living features of ecosystem give us information about the range of climatic conditions that prevail in the area. From structural point of view all ecosystems consist of following two basic components:

1. Abiotic Substances (Non-Living Components)

The Abiotic substances include basic inorganic and organic compounds of the environment or habitat of the organism.

(a) Inorganic Components: The inorganic components of an ecosystem are as undercarbon dioxide, water, nitrogen, calcium, and phosphate. All of these are involved in matter cycles (biogeochemical cycles).

(b) Organic Components: The organic components of an ecosystem are proteins, carbohydrates; lipids and amino acids, all of these are synthesized by the biota (flora and fauna) of an ecosystem and are reached to ecosystem as their wastes, dead remains, etc.

(3) Biotic Substances (Living Components):

This is indeed the trophic structure of any ecosystem, where living organisms are distinguished on the basis of their nutritional relationships. From this trophic (nutritional) standpoint, an ecosystem has two components:

(a) Autotrophic Component of Producers

These are the components in which fixation of light energy use of simple inorganic substances and build up of complex substance predominate.

(i) The component is constituted mainly by green plants, including photosynthetic bacteria.

(ii) To some lesser extent, chemosynthetic microbes also contribute to the build up of organic matter.

(iii) Members of the autotrophic component are known as eco-system producers because they capture energy from non-organic sources, especially light, and store some of the energy in the form of chemical bonds, for the later use.

(iv) Algae of various types are the most important producers of aquatic eco-systems, although in estuaries and marshes, grasses may be important as producers.

(v) Terrestrial ecosystems have trees, herbs, grasses, and mosses that contribute with varying importance to the production of the eco-systems.

(b) Heterotrophic Component or Consumers

These are the components in which utilization; rearrangement and decomposition of complex materials predominate. The organisms involved are known as consumers, as they consume autotrophic organisms like bacterial and algae for their nutrition, the amount of energy that the producers capture, sets the limit on the availability of energy for the ecosystem. Thus, when a green plant captures a certain amount of energy from sunlight, it is said to produce the energy for the ecosystem. The consumers are further categorized as:

(i) Macroconsumers

Marcoconsumers are the consumers, which in a order as they occur in a food chain are, herbivores, carnivores (or omnivores).

(a) Herbivores are also known as primary consumers.

(b) Secondary and tertiary consumers, if preset, are carnivores of omnivores. They all phagotrophs that include mainly animals that ingest other organic and particulate organic matter.

(ii) Microconsumers

These are popularly known as decomposers. They are saprotrophs (=osmotrophs) they include mainly bacteria, actinomycetes and fungi. They breakdown complex compounds of dead or living protoplasm, they absorb some of the decomposition or breakdown products. Besides, they release inorganic nutrients in environment, making them available again to autotrophs.

The biotic component of any ecosystem may be thought of as the functional kingdom of nature. The reason is, they are based on the type of nutrition and the energy source used. The trophic structure of an ecosystem is one kind of producer consumer arrangement, where each “food” level is known as trophic level.

Standing Corp

The amount of living material in different trophic levels or in a component population is known as the standing corp. This term applies to both, plants as well as animals. The standing crop may be expressed in terms

(i) Number of organisms per unit area,

(ii) Biomass i.e.organism mass in unit area, we can measure it as living weight, dry weight, ash-free dry weight of carbon weight, or calories or any other convenient unit suitable.

Decomposers

In the absence of decomposers, no ecosystem could function long. In their absence, dead organisms would pile up without rotting, as would waste products, It would not be long before and an essential element, phosphorus, for example, would be first in short supply and then gone altogether, the reason is the dead corpses littering the landscape would be hoarding the entire supply. The decomposers tear apart organisms and in their metabolic processes release to the environment atoms and molecules that can be reused again by autotrophic point of view. Instead they are important from the material (nutrient) point of view. Energy cannot be recycled, but matter can be. Hence it is necessary to feed Energy into ecosystem to keep up with the dissipation of heat or the increase in entrophy. Matter must be recycled again and again by an ecological process called biogeochemical cycle.

An Illustration

The Structure of ecosystem can be illustrated as under with the help of ponds example.

1. Abiotic Part

The abiotic or non-living parts of a freshwater pond include the follwing:

(i) Water,

(ii) Dissolved oxygen,

(iii) Carbon Dioxide,

(iv) Inorganic salts such as phosphates, nitrates and chlorides of sodium, potassium, and calcium

(v) A multitude of organic compounds such as amino acids, humic acids, etc. according to the functions of the organisms, i.e., their contribution towards keeping the ecosystem operating as a stable, interacting whole.

(a) Produces

In a freshwater pond there are two types of producers,

(i) First are the larger plants growing along the shore or floating in shallow, water,

(ii) Second are the microscopic floating plants, most of which are algae,

These tiny plants are collectively referred to as phytoplankton. They are usually not visible. They are visible only when they are present in great abundance and given the water a greenish tinge. Phytoplanktons are more significant as food producers for the freshwater pond ecosystem than are the more readily visible plants.

(b) Consumers

Among the macro consumers or phagotrophas of pond ecosystems include insects and insect larvae, Crustaces, fish and perhaps some freshwater clams.

(i) Primary Consumers: Primary consumers such as zooplankton (animal plankton) are found near the surface of water. Likewise benthos (bottom forms) are the plant eaters (herbivores).

(ii) Secondary consumers: The secondary consumers are the carnivores that eat the primary consumers. There might be some tertiary consumers that eat the carnivores (secondary consumers).

Saprotrophs

The ecosystem is completed by saprotrophs or decomposer organisms such as bacteria, flagellage protozoans and fungi, They break down the organic compounds of cells from dead producer and consumer organisms in any of these ways-

(i) Into small organic molecules, which they utilize themselves, or

(ii) Inorganic substances that can be used as raw materials by green plants.

ECOLOGICAL PYRAMIDS

The main characteristic of each type of Ecosystem in Trophic structure, i.e. the interaction of food chain and the size metabolism relationship between the linearly arranged various biotic components of an ecosystem. We can show the trophic structure and function at successive trophic levels, as under:-

Producers--------->Herbivores---->Carnivores

It may be known by means of ecological pyramids. In this pyramid the first or producer level constitutes the base of the pyramid. The successive levels, the three make the apex. Ecological pyramids are of three general types as under:

(i) Pyramid of numbers: It shows the number of individual organisms at each level,

(ii) Pyramid of energy: It shows the rate of energy flow and/or productivity at successive trophic levels.

(iii) Pyramid of energy: It shows the rate of energy flow and/or productivity at successive trophic levels.

The first two pyramids

That is the pyramid of numbers and biomass may be upright or inverted. It depends upon the nature of the food chain in the particular ecosystem, However, the pyramids of energy are always upright.

A brief description of these pyramids is as under:

1. Pyramids of numbers

The pyramids of numbers show the relationship between producers, herbivores and carnivores at successive trophic levels in terms or their numbers.

(i) In a grassland the producers, which are mainly grasses, are always maximum in number.

(ii) This number shows a decrease towards apex, the reason is obvious, number than the grasses.

(iii) The secondary consumers, snakes and lizards are less in number than the rabbits and mice.

(iv) In the top (tertiary) consumers hawks or other birds, are least in number.

In this way the pyramid becomes upright. In a pond ecosystem, also the pyramid is upright as under:

(i) The producers, which are mainly the phyto-planktons as algae, bacteria etc. are maximum in number;

(ii) The herbivores, which are smaller fish; rotifers etc are less in number than the producers;

(iii) The secondary consumers (carnivores), such as small fish which eat up each other, water beetles etc. are less in number than the herbivores;

(iv) Finally, the top (tertiary) consumers, the bigger fish are least in number.

However, the case is not so in a forest eco-system. There the pyramid of numbers is somewhat different in shape:—

(i) Producer, here the producers, are mainly large-sized trees, they are less in number, and form the base of the pyramid.

(ii) The herbivores, which are the fruit-eating birds, elephants, deer etc. are more in number than the producers.

(iii) Thereafter there is a gradual decrease in the number of successive carnivores.

In this way the pyramid is made again upright. However, in a parasites food chain the pyramids are inverted. This is for the reason that a single plant may support the growth of many herbivores. In its turn, each herbivore may provide nutrition to several parasites, which support many hyperparasites. Consequently from the producer towards consumers, there is a reverse position. In other words the number of organisms gradually shows an increase, making the pyramid inverted in shape.

2. Pyramids of biomass

The pyramids of biomass are comparatively more fundamentalism; as the reason is they instead of geometric factor; show the quantitative relationships of the standing crops. The pyramids of biomass in different types of ecosystem may be compared as under:

In grassland and forest there is generally a gradual decrease in biomass of organisms at successive levels from the producers to the top carnivores. In this way, the pyramids are upright. However, in a pond the producers are small organisms, their biomass is least, and this value gradually shows an increase towards the apex of the pyramid and the pyramids are made inverted in shape.

3. Pyramid of energy

The energy pyramid gives the best picture of overall nature of the ecosystem. Here, number and weight of organisms at any level depends on the rate at which food is being produced. If we compare the pyramid of energy with the pyramids of numbers and biomass, which are pictures of the standing situations (organisms present at any moment), the pyramid of energy is a picture of the rates of passage of food mass through the food chain. It is always upright in shape.

FUNCTION OF AN ECO-SYSTEM

For a fuller understanding of ecosystems a fuller understanding of their functions besides their structures is essential. The function of ecosystems includes, the process how an eco-system works or operates in normal condition.

From the operational viewpoint, the living and non-living components of ecosystem are interwoven into the fabric of nature. Hence their separation from each other becomes

practically very much difficult. The producers, green plants, fix radiant energy and with the help of minerals (C, O, N, P, L, Ca, Mg, Zn, Fe etc.) taken from their soil and aerial environment (nutrient pool) they build up complex prefer to call the green plants as converters or transducers because in their opinion the terms ‘producer’ form an energy viewpoint which is somewhat misleading. They contend that green plants produce carbohydrates and not energy and since they convert or transducer radiant energy into chemical form, they must be better called the converters or transducers. However, the term’ producer’ is so widely used that it is preferred to retain it as such.

While considering the function of an ecosystem, we describe the flow of energy and the cycling of nutrients. In other words, we are interested in things like how much sunlight plants trap in a year, how much plant material is eaten by herbivores, and how many herbivores carnivores eat.

Functions of Eco-system

The functions of Ecosystem are as under:

1. Transformation of Solar Energy into Food Energy

The solar radiation is major source of energy in the ecosystem. It is the basic input of energy entering the ecosystem. The green plants receive it. And is converted into heat energy. It is lost from the ecosystem to the atmosphere through plant communities. It is only a small proportion of radiant solar energy that is used by plant to make food through the process of photosynthesis. Green plants transform a part of solar energy into food energy or chemical energy. The green plants to develop their tissues use this energy. It is stored in the primary producers at the bottom of trophic levels. The chemical energy, which is stored at rapid level one, becomes the source of energy to the herbivorous animals at trophic level two of the food chain. Some portion energy is lost from trophic level one through respiration and some portion is transfereed to plant-eating animals at trophic level two.

2. The Circulation of elements through Energy Flow

It is seen that in the various biotic components of the ecosystem the energy flow is the main driving force of nutrient circulation. The organic and inorganic substances are moved reversibly through various closed system of cycles in the biosphere, atmosphere, hydrosphere and lithosphere. This activity is done in such a way that total mass of these substances remains almost the same and is always available to biotic communities.

3. The Conversion of Elements into Inorganic Flow

The organic elements of plants and animals are released in the under mentioned ways:

(i) Decomposition of leaf falls from the plants dead plants and animals by decomposers and their conversion into soluble inorganic form.

(ii) Burning of vegetation by lighting, accidental forest fire or deliberate action of man. When burnt, the portions of organic matter are released to the atmosphere and these again fall down, under the impact of precipitation, on the ground. Then they become soluble inorganic form of element to join soil storage, some portions in the form of ashes are decomposed by bacterial activities.

(iii) The waste materials released by animals are decomposed by bacteria. They find their way in soluble inorganic form to soil storage.

4. The Growth and Development of Plants

In the biogeochemical cycles are included the uptake of nutrients of inorganic elements by the plants through their roots. The nutrients are derived from the soil where these inorganic elements are stored. The decomposition of leaves, plants and animals and their conversion into soluble inorganic form are stored into soil contributing to the growth and development of plants. Decompositions are converged into some elements. These elements are easily used in development of plant tissues and plant growth by biochemical processes, mainly photosynthesis.

5. Productivity of ecosystem

The productivity of an ecosystem refers to the rate of production i.e. the amount of organic matter, which is accumulated in any unit time. Productivity is of the following types:

(1) Primary productivity: It is associated with the producers which are autotrophic, Most of these are photosynthetic, Thus, they are, to a much lesser extent the chemosynthetic micro organisms. These are the green plants, higher saprophytes as well as lower forms, the phytoplankton’s and some photosynthetic bacteria. We can define Primary productivity as “the rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producers.” Primary productivity is further distinguished as:

Gross primary productivity: Gross Primary Productivity is the rate of storage of organic matter in plant tissues in excess of the respiratory utilization by plants during the measurement period. This is, thus, the rate of increases of biomass. In this way, net primary productivity refers to balance between gross photosynthesis and respiration and other plant losses as death etc.

(2) Secondary productivity: These are the rates of energy storage at consumers level. Since consumers only utilize food materials (already produced) in their respiration, simply covering the food matters to different tissues by an overall process. The secondary productivity is not divided into ‘gross’ and ‘net’ amount.

(3) Net Productivity: Net productivity refers to the rate of storage of organic matter not used by the heterotrophs (consumer) i.e. equivalent to net primary production minus consumption by the heterotrophs during the unit period. It is thus the rate of increase of biomass of the primary producers, which has been left over by the consumers.

(4) Stability of Ecosystem: The stability of ecosystems refers to the balance between production and consumption of each element in the ecosystem. In other words, balance between input and output of energy and normal functioning of different biogeochemical cycles and stable conditions of equilibrium as under:-

(i) The Equilibrium Model: The equilibrium model states that an ecosystem, always tends towards stability. As soon as the community of an ecosystem is disturbed due to external environmental change, it quickly returns to original state where as.

(ii) The non-equilibrium model: The non-equilibrium model states that an ecosystem stability is rarely attained because disturbances caused by frequent external environmental change do not allow to develop ordered state of species assemblages in an ecosystem.

DECOMPOSERS

In this world all living organisms require a constant supply of nutrients for growth. The death and decomposition of plants and animals, with release of nutrients constitutes an essential link in the maintenance of nutrient cycles. When an organism dies, an initial period of rapid leaching takes place and populations of macromolecules. The dead organism is disintegrated beyond recognition. Enzymic action breaks down the disintegrating parts of the litter. Animals invade and either eat the rapidly recolonized by micro- organisms, and the litter biomass decreases. It becomes simpler in structure and chemical composition.

Process of Decomposition

The process of decomposition involves three interrelated components, viz.

(i) Leaching (ii) Catabolism,

(iii) Comminution.

1. Leaching

Leaching is a physical phenomenon operating soon-after litter fall. Soluble matter is removed from detritus by the action of water. Sometime over 20% of the total nitrogen content of litter maybe leached off.

2. Catabolism

The process in a plant or animal by which living tissue is changed into waste products.

3. Comminution

Comminution to make small to reduce to power or minute particles. Comminution means the reduction in particle size of detritus. During the course of feeding, the decomposer animals community detritus physically. And utilize the energy and nutrients for their own growth (secondary production). In due course, the decomposers themselves die and contribute to the detritus.

Function of Decomposition

The two major functions of decomposition within ecosystems are as under:-

(1) The mineralization of essential elements,

(2) The formation of soil organic matter to inorganic form.

The formation of soil organic matter in nature is a slow process. The decomposition of any piece of plant detritus may take hundreds of years to complete. However, some residues of decomposition within this period do contribute to the formation of soil organic matter.

Community of Decomposer Organisms

The community of decomposer organisms includes several bacteria, fungi, protests and invertebrates. The different species in such a community function in an integrated manner. For example, a fungus decomposes plant litter and is eaten by an animal. Upon death, bacteria decompose the animal, and protozoa may eat the bacteria.

Fungi and bacteria are the principal organisms that break down organic matter. Certain protozoa, nematodes, annelids and arthopods strongly influence their functioning (i.e. of fungi and bacteria) due to their feeling activities. Microarthopod fauna, comprising mainly of oribatid mites besides other mites and collembolans, are abundant in most forest, grassland and desert ecosystem.

Most of these micro-arthropods are predominantly fungal-feeders. They can do as under:

(1) They can decompose substrata.

(2) They can decrease substrata’s mass by leaching soluble intercellular components.

(3) They can do so by oxidation.

(4) They can physically cut in into smaller fragments.

Increased mineralization of nitrogen, phosphorus and potassium has been reported to be mediated by microarhropods in several studies.

In the same way, the interactions of micro-arthropods with soil fungi are also quite important in nutrient cycling. Studies of this aspect are made in mycorrhizal fungi and themicro-arthropods which feed upon these fungi:

(1) It is found that Mycorrhizal pump massive amounts of nutrients form detritus and represent a sizable nutrient reservoir themselves.

(2) The orbited mites and other micro-arthropods feed on myocardial fungi they act like herbivorous pests, and can alter nutrient relations/cycling in terrestrial ecosystems.

DECOMPOSERS WITH VARYING RELATIONS

Some decomposer organism’s cannot be assigned a rigid or fixed position in the food web. Their trophic relations can vary from time to time.

1. Nectroph: Some decomposers are nectrophs. They cause rapid death of the food source because they have a short-term exploitation of living organism. Nectrophs include may plant parasitic microbes as well as some herbivores, predators, and microtrophs (organisms which feed on living bacteria and fungi.)

2. Biotrophs: On the other hand biotopes resort to a long-term exploitation of their living food resource. For example, root-feeding nematodes and aphids, obligate plant parasites, e.g., and mycorhizae and root nodules, etc.

3. Saprotophs: The apostrophes utilize food already dead, and most of the decomposers belong to this category.

Decomposers occupying different trophic levels

There are some such organisms causing decompositions as can occupy various trophic levels under different conditions. For instance the root parasites like Fusarium and Thizoctonia are necrotrophs, which often show a saprotrophic tendency. In the same way, the predators (foxes and kites) sometime behave as saprotrophs. Biotrophs sometime act as necrotrophs or as saprotrophs.

Soul Invertebrates And Termites

There are some soil invertebrates e.g. earthworms and collembolans distribute organic matter throughout the soil whereas others e.g. termites and ants, concentrate it at localized sites around or near the royal chamber or in mounds. The following table shows the estimated activities of major groups of soil animals.

ENERGY-ITS FLOW IN ECOSYSTEM

Energy-Defined

Energy can be defined as the capacity to do work, whether that work be on a gross scale as raising mountains and moving air masses over continents, or on a small scale such as transmitting a nerve impulse from one cell to another.

Kinds of Energy

There are two kinds of energy, potential and kinetic. They can be explained as under:-

1. Potential Energy

Potential energy is energy at rest. It is capable and available for work.

2. Kinetic Energy

Kinetic energy is due to motion, and results in work. Work that results from the expenditure of energy can be of two kinds:

(1) It can store energy (as potential energy).

(2) It can order matter without storing energy.

3. Laws of Thermodynamics

The expenditure and storage of energy is described by two laws of thermodynamics:-

(i) Law of conservation of energy: The law of conservation of energy states that energy is neither created nor destroyed. It may change forms, pass from one place to another, or act upon matter in various ways. In this process no gain or loss in total energy occurs. Energy is simply transferred from one form or place to another. Two Reactions

There may be either of the two reactions:

1. Exothermic Reaction

When wood is burnt the potential energy present in the molecules of wood equals the kinetic energy released, and heat is evolved to the surroundings. This is an exothermic reaction.

2. Endothermic Reaction

In an endothermic reaction, energy from the surrounding may be paid into a reaction. For example, in photosynthesis, the molecules of the products store more energy than the reactants. The extra energy is acquired from the sunlight yet there is no gain or loss in total energy.

(ii) Law of Decrease in Energy: The second law of thermodynamics states that on the transformation of from one kind to another, there is an increase in entropy and a decrease in the amount of useful energy. In this way, when coal in burned in a boiler to produce steam, some of the energy creates steam that performs work, but part of the energy is dispersed as heat to the surrounding air.

Three Sources of Energy

Three sources of energy account for all the work of the ecosystem. These sources are gravitation. Internal forces within the earth and solar radiation. The last one is significant

for ecosystem. The solar radiation, which originates from sun is the source of energy for life and is what sets the ecosystem, besides other natural system.

Energy Flow

Due to unidirectional flow of energy, the behaviour of energy in ecosystem is called Energy Flow. From the energetics point of view, energy flow is explained as under:

(i) The efficiency of the producers in absorption and conversion of solar energy.

(ii) The use of the above said converted chemical form of energy by the consumers.

(iii) The total input of energy in form of food and its efficiency of assimilation.

(iv) The loss caused through respiration, heat, excretion etc.

(v) The gross, net production.

Single Channel Energy Model

Lindemann (1942) was the first to propose the community energetics approach or the trophic-dynamic model) to ecology, which enables an investigator to compare the relative rates at which different kinds concerning energy flow through forest ecosystems by the application of this kind of approach, e.g. by comparing ratios of leaf fall to litter deposition on the forest floor. His conclusion was that the rates of leaf production are higher and those of litter accumulation lower, in the tropics than at higher latitudes.

The following conclusion can be drawn from the above figure:

(1) Out of the total incoming solar radiation (118,872 g cal/cm2 /yr), 118,761 gcal/cm2/yr remain unutilized. In this way, the gross production (net production plus respiration) by autotrophs comes to be 111 gcal/cm2/yr with an efficiency of energy capture of0.10 per sent.

(1) Again 21 per cent of this energy, or 23 gcal/cm2/yr (show on the bottom as respiration) is consumed in metabolic reactions of autotrophs for their growth, development, maintenance and reproduction.

(2) 15 gcl/cm2/yr are consumed by herbivores that graze of feed on autographs-this figure amounts to 17 per cent of net autotroph production.

(3) Decomposition is 3 gcal/cm2/yr which amount to be 3, 4 per cent of net production.

(4) The remainder of the plant material, 70 gcal/cm2/yr of 79.5 per cent production, is not utilised. It becomes part of the accumulating sediments. Apparently much more energy is available for herbivory than is consumed.

We may conclude the following conclusions

(1) Various pathways of loss are equivalent to and account for total energy capture of the autotrophs i.e. gross production.

(2) The three upper ‘fates’ i.e. decomposition, herbivory and not utilized collectively are equivalent to net production.

(3) Of the total energy which is incorporated at the herbivory level, i.e. 15/ gcal/cm2yr, 30 percent of 4.5 gcal/cm2/yr is used in metabolic reactions.

(4) In this way more energy is lost via respiration by herbivores (30 percent) than by autotrophs (21 percent),

(5) Considerble energy is available for the carnivores, namely 10.5 gcal/cm2/yr or 70-per cent. It is not entirely utilized, merely 3.0 gcal/cm2/or 28.6 per cent of net production passes to the carnivores. This utilization of resources is evidently more efficient than the one, which occurs at autotroph-herbivore transfer level.

(6) At the carnivore level the consumption in metabolic activity is about percent of the carnivores energy intake.

(7) The remainder becomes part of the un-utilized sediments;

(1) There is Noe-way Street along which energy moves (unidirectional flow of energy.

(a) The energy that is captured by the autotrophs does not revert back to solar input.

(b) The energy which passes does not pass back to the autotrophs. It moves progressively through the various trophic levels. As such, it is no longer available to the previous level. Since there is one-way flow of energy, the system would collapse in case the primary source, the sun, were cut off.

(2) Secondly, progressive decrease in energy level is seen at each trophic level. This decrease is accounted as under:

(i) By the energy dissipated as heat in metabolic activities.

(ii) Measured here as respiration coupled with unutilized energy. Below is a figure after Epodum (1963).

This is a simplified energy flow diagram

(1) The diagram depicts three trophic levels. Boxes numbered 1, 2, 3 in a leaner food chain exhibit these.

(2) L. shows total energy input (3000).

(3) LA shows light absorbed by plant cover (1500).

(4) P.G. shows gross Primary production.

(5) A shows total assimilation.

(6) Pn shows net primary production.

(7) P shows secondary (consumer) production.

(8) Nu shows energy not used (stored or exported).

(9) NA shows energy not assimilated by consumers (egested).

(10) R shows respiration.

Some more elucidation of the figure is as under:

(1) The ‘boxes’ represent the trophic levels

(2) The ‘pipes’ depict the energy flow in and out of each level.

Energy inflows balance outflows

The first law of thermodynamics requires it. The energy transfer is accompanied by dispersion of energy into unavailable heat (i.e. respiration). The second law requires it.

It is very simplified energy flow model of three trophic levels

Apparently the energy flows is greatly reduced at each successive trophic level from producers to herbivores and then to carnivores. It is reflected that at each transfer of energy from one level to another, major part of energy is lost as heat or other form. The energy flow is reduced successively. We may consider it in either term as under:

(1) In terms of total flow (i.e. total energy input and total assimilation).

(2) In terms of secondary production and respiration components.

In this way of the 3,000 Kcal of total light, which falls upon the green plants, approximately 50 per cent (1500 Kcal) is absorbed. Only 1 per cent (15 Kcal) of it is converted at first trophic level. Thus net primary production comes to be at 15 Kcal. Secondary productivity (P2 and P3 in the diagram) is about 10 percent at successive consumer trophic levels in other words at the levels of herbivores and the carnivores. However, efficiency may be sometimes higher as 20 per cent, at the carnivore level as shown (or P3=0.3 Kcal) in the diagram.

It may be concluded from the above studies as under:

(1) There is a successive reduction in energy flow at successive trophic levels. Thus shorter the food chain, greater would be the available food energy. The reason is with an increase in the length of food chain, there is a corresponding more loss of energy.

(2) With a reduction in energy flow (shown as ‘pipes’ in the diagram) at each successive trophic level, there is also a corresponding decrease in standing crop or biomass (shown as ‘boxes’ in the diagram). However, it does not mean that there exists any correlation between the biomass and energy. Indeed energy as taken here represents rate functions or production rates. The relationships between biomass and content are not fixed. They may differ according to the situations. For example, one gram of an algae (lesser biomass) may be equal to many grams (more biomass) of a forest tree leaves as the rate of production of the algae is higher than that of tree leaves.

Y-shaped energy flow model-Two channel energy flow model

Following the example of Lindeman, several authors described energy flow modes for different kinds of ecosystems. Two illustrations are here:

(1) Teal (1957) prepared an energy flow diagram of Root Spring in U.S.A.

(2) H.T. Odum (1957) prepared energy flow model for Silver Springs, Florida, U.S.A.

(3) 30, 810 Kcal/m2 y remained for net production.

In model given by Teal (1957) for Root Springs, most of the energy rich material eaten by heterotrophs entered the systems as plant debris. On the other hand in the model given by H.T. Odum (1957) for Silver Spring, most of the heterotroph’s food in food chain was produced by green with in some systems heterotrophs consume living plants while in others they feel on dead plant parts (detritus).

(1) In Root Springs, the chain began with dead plant parts.

(2) In Silver Springs the chain began with live plant parts.

On the basis of the studies E.P. Odum pointed out that in nature there are present two basic food chains in any system:

(1) The grazing food chain beginning with green plant base going to herbivores and then to carnivores, and

(2) The detritus food chain beginning with dead organic matter acted by microbes, then passing to detritivores and their consumers (predators).

The figure given below present one of the first published energy flow models as pioneered by H.T. Odum in 1956.

The above figure illustrates energy flow in a community with a large import and smaller export of organic matter.

P indicates gross primary production; PN indicates net primary production. P2.

P2...............P5 indicate secondary production at the shown levels.

Gross Primary production GPP = Total photosynthetic C fixation Autotrophic Respiration, RA = GPP-NPP

Net primary Production, NPP-RA

Heterotrophic Respiration, RH = respiration of consumers and decomposers. Ecosystem Production, NEP = GPP-RE

The three major steps in energy flow correspond to:—

(a) Exploitation efficiency,

(b) Assimilation efficiency,

The product of the assimilation net production efficiencies gives gross production efficiency i.e. by the fraction of the eaten material eventually transformed into consumer biomass. The whole food web may be taken to be the product of the gross production efficiency and the exploitation efficiency. The various kinds of energetic efficiencies can be defined as under:

Exploitation efficiency = Ingestion of food/prey production. Assimilation efficiency = Assimilation/ingestion;

Net production efficiency = Production/assimilation.

Ecological efficiency = Exploitation efficiency × Assimilation efficiency × Net

production efficiency;

= Consumer production/prey production;

= Production/ingestion.

Gross production = Production/ingestion.

In animals, rate of production appears to depend on body mass. Per unit body mass, small animals are found more productive than big animals. Again invertebrates are less productive than mammals. Molluscs, annelids, isopods, and insects are invertebrates of intermediates size between copepods and echinoids.

Some conclusions regarding energy flow in the ecosystem are as under:

There is no quantitative relationship between the production of a certain trophic level and the production of the next lower trophic level (both in calorific terms) except for the very high or very low values of the former. This applies to the “phytoplankton-filter feeders” and as well as “filtrators-invertebrate predators” trophic links in the plankton food chain.

The utilization of primary production in pelagic zone often depends on the nature of dominant species of producers and consumers. In a system containing phyto-planktonic algae-macroconsumers effective utilization occurs mostly via grazing. In the case of larger algae and smaller consumers, primary production is mainly utilized via bacterial detritus medium.

The energy transfer efficiency from the filtrator’s trophic level to their invertebrate predators is often higher than from phytoplankton to filtrators.

ECOLOGICAL SUCCESSION-MEANING AND TYPES

Meaning of Succession

Biotic communities are not static. Instead they change through time. This change can be understood on several levels. The simplest level is the growth, interaction and death of individual organisms as they pass through their life cycles, affected by the cycles of seasons and other natural phenomena. Some other levels of community change act over longer time

spans and that account for much larger changes in community composition and structure. These include ecological succession and community evolution.

It is evident from the above said that the term succession denotes a sequence of changes in the species composition of a community, which is generally associated with a sequence of changes in its structural and functional properties. The term is generally used for temporal sequence (in terms of years, decades or centuries) of vegetation on a site; although only short term changes can be observed directly and the long term ones are inferred from spatial sequences.

The changes associated with succession are usually progressive or directional. This fact enables one to predict which species are likely to replace other in the course of a succession. Sucession tends to continue until the species combinations best suited to the regional climate and the particular site are established.

Historical Background

The basic idea of succession was in the beginning forwarded by Anon Kerner (1863) in his book “Plant Life of the Danube Basin” during the description of the regeneration of a swamp forest. The term ecological succession was first of all used by Hult (1885) in the study of communities of Southern Sweden. H.C. Cowles held that communities are not static but dynamic. This changed understanding be visualized as an orderly, directional and predictable phenomenon. It was added that succession is autogenic i.e. regulated by biotic interactions within the community. The central foundation of the classical theory was that early communities alter the environment to their detriment and favour later successional communities. It was revealed by the later studies that allogenesis was perhaps more common and dominant than autogenesis; allogenesis means the control of community dynamics by factors originating outside the community boundaries.

The succession of animals on these dunes was studied by ‘Shelford (1913). Later on, Olson (1958) restudies the ecosystem development on these dunes and has given us an updated information about it. Federick Clements (1907-1936) elaborates the principles and theory of succession. He proposed the monoclimax hypotesis of succession. During the later years certain other hypotheses were proposed by various ecologists to explain the nature of climax communities: for example, polyclimx hypothesis by Braun-Blanquet (1932) and Tansley (1939): climax pattern hypothesis by Whittaker (1953), Mac intosh (1958) and Sellack (1960): and stored energy theory of information theory by Fosberg (1965, 1967) and Odum (1969).

Odum (1969) defined succession in terms of 3 parameters, viz.:

(1) Succession is an orderly process of reasonable directional and fairly predictable community development;

(2) Succession results from modification of the physical environment by a community,

i.e. succession is largely community controlled.

(3) Succession culminates in a stabilized ecosystem in which maximum biomass and symbiotic function between organisms are maintained per unit of available energy flow. Whittaker (1975), held that through the course of succession community production, height and mass, species-diversity, relative stability, and soil depth and differentiation generally all tend to increase. The culminating point of succession is a climax community of relatively stable species composition and steady-state function, It is adapted to its habitat. It is permanent in its habitat if it is not disturbed.

Illustrations

Ecological succession can be explained with the help of illustrations as under: -

1. Lake

When a lake fills with silt it changes gradually from a deep to a shallow lake of pond, then to a marsh, and beyond this, in some cases, to a dry-land forest.

2. Crop field

When a crop field is deserted or a forest is severely burned over, it is just like a plot of bare ground and a series of plant communities grow up there and replace on another - firest annual weeds, then perennial weeds and grasses, then shrubs, and trees-until a forest ends the development.

In this way, ecological succession is an orderly and progressive replacement of one community by another until a relatively stable community, called the climax community, occupies the area.

(1) In the first example the principal cause of the change in the community was physical process-the filling in of the lake with silt.

(2) In the second example, a principal cause was the growth of plants on an existing soil.

Development

Ecological succession develops as under:

1. Pioneers

The first organisms to become established in an ecosystem undergoing succession are called pioneers; the stable community that ends the succession is termed the climax community.

2. Sere

The whole series of communities which are involved in the ecological succession at a given area. For example, from grass to shrub to forest, and which terminates in a final stable climax community, is called as sere.

3. Seral stage

Each of the changes that take place is a seral stage.

4. Community

Each seral stage is a community, although temporary, with its own characteristics. It may remain for a very short time or for many years.

Classification of Seres

Seres are sometimes classified according to the predominant force that is bringing them about. These forces are biotic, climatic, physiographic, and geologic. Their resultant seres are commonly called bioseres, cliseres, eoseres and geoseres.

Types of Succession

The succession may be of the following two types:

1. Primary Succession

Primary Succession is the process of species colonization and replacement in which the environment is initially virtually free of life. In the other words the process starts with base rose or sand dune or river delta or glacial debris and it ends when climax is reached. The sere involved in primary succession is called presere.

2. Secondary Succession

Secondary succession is the process of change that occurs after an ecosystem is disrupted but not totally obliterated. In this situation, organic matter and some organisms from the original community will remain; thus the successional process does not start from scratch. As a result, secondary succession is more rapid than primary. It is seen in areas burned by fire or cut by farmers for cultivation. The sere involved in secondary succession is called subsere.

Types of Succession

The primary and secondary successions may be of three types. The classification is on the basis of the moisture contents:

(a) Hydrach of Hydrosere

The succession when starts in the aquatic environment such as ponds, lakes, streams, swamps, bogs, etc. is called hydrach or hydrosere.

(b) Mesarch

The succession when begins in and area, where adequate moisture is present, is called mesarch.

The succession when starts in xeric or dry habitat having minimum amounts of moisture, such as dry deserts, rocks, etc. is called xerach. A temporary community in an ecological succession on dry as sterile habitat is called xerosere. It may be of three types as under:-

(1) Iithosere-succession initiating on sand;

(2) Psammosere-succession initiating on sand;

(3) Halosere-succession starting on saline water or soil.

Autogenic Community

Autogenic community is the succession progressing entirely as a result of interactions of the organisms and their environment (i.e. “driving force” is internal to the community) for example succession on sand dunes.

Allegonic Community

Allegonic community is the succession moving under the influence of external factors, as input of nutrients, succession in a small pond or bog.

Autotrophic And Heterotrophic Succession

Sometimes, succession is classified as autotrophic and heterotrophic on the basis of community metabolism:

(1) Autotrophic succession is characterized by early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment. In it the energy flow is maintained indefinitely.

(2) Heterotrophic succession is characterized by early dominance of heterotrophs, such as bacteria, actinomycetes, fungi and animals. This sort of succession begins in a predominantly organic environment and there is a progressive decline in the energy content.

Serule

The miniature succession of micro-organic environment and different types of fungi on the fallen logs of the decaying wood, tree bark, etc. is called serule.

Drury and Nisbet (1973) classified succession into three main types:

(a) Category I includes many classical types of secondary succession and some primary successions. It involves temporal sequences on one site with climate and physiography mostly remaining stable.

(b) Category II includes many primary successions (especially those in ponds and lakes) and a few secondary successions. In this, temporal sequences on site with the local environment changes under the influence of such external factors as climate, erosion, drainage, nutrient inputs, etc.

(c) Category III includes those changes, which take place over long (geological) time scale, and cover spatial sequences on adjacent sites.

Common attributes of Ecological Succession

Some Common attributes of ecological succession are as under:

SUCCESSION: GENERAL PROCESS, CLIMAX

General Process

The process of succession being with a bare area or nudation formed by several reasons, such as volcanic eruption, landslide, following sequential steps.

1. Nudation

The process of succession begins with a bare area or nudation formed by several reasons, such as volcanic eruption, Landslide, flooding, erosion, deposite, fire, disease, or other catastrophic agency. Man also may be reason of formation of new lifeless bare areas for example, walls, stone quarrying, burning, digging, flooding large land areas under reservoirs, etc.

2. Invasion

The invasion means the arrival of the reproductive bodies or propagules of various organisms and their settlement in the new or bare area. Plants are the first invaders (pioneers) in any area the animals depend on them for food. The invasion includes the following three steps:

(a) Dispersal or migration: The seeds, spores or other propagules of the species reach the bare area through air, water or animals.

(b) Ecesis: Ecesis is the successful establishment of migrated plant species into the new area. It includes germination of seeds or propagules, growth of seedlings and starting of reproduction by adult plants.

(c) Aggregation: In this stage, the successful immigrant individuals of a species increase their number by reproduction and aggregate in large population in the area. As a result individuals of the species come close to one another.

Breaking

Tuesday 17 May 2022

Ecosystem Chapter 3 Environmental science