UNIT 17 AND FUNCTION - I

Structure 17.1 Introduction Objectives 17.2 Ecosystem 17.3 Component of Ecosystem Abiotic Components Biotic Components Trophic Levels Bioaccumulat~onand B~omagnification 17.4 Pyramids Pyramid of Numbers Pyra~n~dof Pyramid of'Energy 17.5 Energy of Ecosystem Flow of Energy in An Ecosystem Law of Thermodynamics 17.6 I'roductivity Secondary Production 17.7 Homeostasis System Feedback Mechanism Ecosystem Homeostasis 1 7.8 and Succession in Terrestrial Community Succession in Aquatic General Characteristic of Succession 1 7.9 Ecosystem and Human Use 1 7.1 0 and Human Intervention 1 7.1 1 Summary 1 7.12 Terminal Questions

17.1 INTRODUCTION

We all live in a community, consisting of plants, animals, and deconiposers that interact with one another and with their environments and are interdependent on one another for survival. The biotic components consist of producers and consumers. When sitting on dinner table you must be enjoying your non-vegetarian dish, this dish, is a part of a food chain that feed on one another. The abiotic components contribute substances needed for the ecosystem to function. Ecosystem is maintained through , the energy flow is unidirectional from sun to producers. Ecosystems are therefore systems in which there is a regulated transfer of energy and an orderly, controlled cycling of nutrients. An ecosystem self regulates or self maintains itself through homeostatis. They, over a period of time, with changing climate, slowly have modified into new ecosystems through a process called succession. In recent times human beings have negatively intervened in the working of ecosystem through introducing new species, predator-control practices, habitat destruction and pesticide use (biomagnifications) all of which have had unanticipated changes in ecosystem.

Objectives After studying this unit you will be able to:

identify the abiotic and biotic environmental factors, appreciate the relationship of in food chain and food web, idcntify the tropliic levels occupied by and , and Function - recognise the value of using a pyramid of number, pyramid of biomass or pyramid of energy, explain the laws of thermodynamics, recognise-the importance of primary and secondary productive in an ecosysteln, describe the process of succession in terrestrial and , describe various examples of human interventions towards destruction of natural ecosystems.

17.2 ECOSYSTEM

Each is constituted bv several ecosvstenls. For exam~le,~~. the .~~ - ~ - ~ biome- .- --.. of - - ~ I J -~ Rajasthan is cllaracterised by arid conditions, sandy terrain, cacti and succulent plants Animals found there are lizards, snakes, camels.

A subdivision of biome such as a pond is called an ecological system or ecosystem. The word ecosystem; \was I The vario~~skinds of organisms that inhabit an ecosystem form its populations. The coined by Prof. Arthur Tansley in 1935, l'he term, 'population' has many uses and meanings in other fields of study. In , 'a prefix 'eco' means population is a group of potentially interbreeding individuals that occur together environment. in space and time'. The individuals comprising a population are members of the same species. I Popi~latiollsof plants and animals in the ecosystem do not function independently of each other. They are always influencing each other and organising themselves into communities and have fcrnctional relationship with their external environment. 1

An ecosystem is defined as, any unit (a biosystem) that includes all the organisms that function toeether (the biotic community) in a given area, interacting with the u ", - - physical environment (abiotic component) so that flow of energy leads to clearly defined biotic structures and cycling of materials between living and non-living parts and which is self regulatory based on feed-back information about the population, and the limiting factors which control the living and non-living components. (Fig.17.1)

Abnosphere

Soil and water

Carbon dioxide1 ~utrink and &Carbon diox~de

Fig.17.1: Simplified biosphere. The different organisms of our biosphere cooperate in similar ways to create a functioning ecosystem.

The definition of ecosystem as you can see involves the interaction between living and non-living components of an ecosystem and input, transfer, storage and output of energy as well as cycling of essential materials through the system. Each of these 1 Ecology and Environment processes is energy dependent. As a result of these complex interactions, the ecosystem has to adjust to these changes to attain a state of equilibrium. Figure 17.2, illustrates this beautifully. Ecosystems differ greatly in composition, in the number and kinds of species, in the kinds and relative proportions of non-biological constituents and in the degree of variations in time and space. A Radiant . WY*".."".""'.'."'. I

Fig. 17.2: Schematic representation of an ecosystem. The dotted lines represent the boundary of the system. The three major components are the producers, the consumers, and the abiotic elements: inactive or dead organic matter, the matrix, nutrients in solution in aquatic ecosystems, sediments, and so on. The arrows indicate interactions within the system and with the environment.

17.3 COMPONENTS OF ECOSYSTEM

Any complete definition of an ecosystem includes the biotic as well as the abiotic components and the interaction between the two. I 17.3.1 Abiotic Components One of the important components of the ecosystem is abiotic or non-living components about which you have already read in section 16.2 (Refer to table 16.1 as well). h he physical or abiotic components are the non-living parts of the earth such as soil, water, air, and light energy etc. It also involves a large number of chemicals like oxygen, nitrogen etc. and chemical as well as physical processes including volcanoes, earthaiakes. floods. forest fires. climates. and weather conditions. There are numerous chemical processes, but the most important ones include the carbon, nitrogen and phosphorous cycles. These physical and chemical processes are the result of the physical characteristics of the earth: air, moisture, light and heat, and the various chemical reactions. While each of these abiotic factors may be studied by itself, each of these factors influences and are in turn is influenced by all the other factors.

Abiotic factors are usually the most important determinants of where and how well an exists in its environment. Although these factors interact with each other, usually there is one single factor which serves to limits the range of an organism. That single factor is called the .

Let us now discuss some of the important abiotic factors:

i) Light: Light energy is necessary for green plants to cany on . All animals are directly or indirectly dependent on the food substance produced by green plants. The intensity, duration and wavelength (color) of light are Ecosystem - structure important factors that regulate the life activities of many living things. Light and Function - 1 from the sun (solar energy) is the ultimate source of energy for all living things. The availability of light energy differs greatly on different parts of the earth. Precipitation: Precipitation in the form of fog, rain, sleet, snow, and hail is one of the most important abiotic factors. Most organisms depend on some form of precipitation, either directly or indirectly, from underground. The amount of precipitation differs, depending where on earth you are. iii) Humidity and Water: Moisture in the air is very necessary for many plants and animals to function properly. Some animals are, active only at night when the humidity is higher. Aquatic are subject to changes in chemical and gas content and to fluctuations of depth. Water holes in the Everglades of Florida and the Savanna of Africa are essential for the existence of native wildlife. Temperature: Many living things cany on their life activities at temperatures between 30 1" F and 185' F. Some organisms are able to exist at much lower temperatures. The daily and seasonal temperature changes often act as limiting factors and determine the number and kind of organisms present in a region. Temperature patterns vary with latitudes and altitudes of the earth. Substrate: This is defined as the base of material on which an organism lives. The type of soil, for example, is a limiting factor for the vegetation, which in turn, may be a determinant for the animal life capable of living in the habitat. 'The type of soil will determine such factors as pH as also the amount and type of minerals present. vi) Atmospheric gases: Oxygen and are generally not limiting factors for terrestrial organisms. These two gases are abundant in our atmosphere. The atmospheric gases can be limiting factors for aquatic organisms. vii) Altitude: Precipitation and temperature both vary with elevation. Usually precipitation increases with elevation although at extreme elevations it may decrease. Temperature usually decreases 2-3 degrees per thousand feet. viii) Latitude: As one moves north or south of the equator, the angle of the sun generally decreases, which results in a decrease in the average temperature. ix) Topography: Landforms like mountains, valleys, basins, cliffs, etc. may encourage, restrict or isolate organisms. x) Seasonal changes: Because of the tilt of the earth on its axis, the angle of solar radiation varies during the year as one travels from the equator. It produces pronounced changes in the weather during the year, giving rise to seasons like winter, spring, summer, and autumn. xi) Weather: is the combination of temperature, humidity, precipitation, wind, cloudiness and other atmospheric conditions at a specific place and time and has profound effect on organism. xii) Climate: is the long-term average pattern of weather and influences the vegetation and organisms of a place.

It is in this abiotic background that biotic organisms i.e. plants, animals and microbes interact.

17.3.2 Biotic Components The biological or biotic components (Fig. 17.3) of an ecosystem interact in an abiotic background and include: Food refers to complex organic compounds such as i) Organisms, basically green plants, certain bacteria and algae, that in the carbohydrates, proteins and presence of sunlight can synthesise their own food from simple inorganic fats. Green plants first substances. Organisms that are able to manufacture their own food are called produce simple carbohydrates like glucose and later various or primary producers. complex carbohydrates, fats and proteins. :ology and Environment ii) All other organisms that are unable to make their own food but deperrd on other organisms for food to meet their energy needs for survival are called or phagotrophs or consumers.

Among consumers, animals such as goat, cow, deer, rabbit and insects, which eat green plants, are called primary consumers or herbivores. Organisms which eat a , like a bird that eats grasshoppers are called secondary consumers. Organisms, which eat secondary consumers, are called tertiary consumers. While the primary consumers are herbivores, the secondary and tertiary consumers are carnivores. Animals like lions and vultures, which are not killed or eaten by other animals, are top carnivores.

Primary Green plants producers

Herbivores

Secondary consumers

Bacteria and Fungi

Fig. 17.3: Biotic members of the ecosystem and their position in the .

lents of decomposing Secondary and tertiary consumers may be i) predators, which hunt, capture and kill ic matter is called their prey, ii) carrion feeders which feed on corpses, iii) parasites that are smaller than IS. the host, and live on or inside the host on which they feed while the host is alive. The parasites depend on the metabolism of their host for their food supply. iv) there are some animals which have flexible food habits, as they eat both plants (therefore are herbivores) and animals (so are carnivores). They are called . We (humans) are good examples of an . Both the consumers and producers complete their life cycles and new generation of the Ecosystem - Structure population develop while the old ones die out. You must be wondering what happens to and Function - 1 the dead. There is a continuous breaking up or of the dead organic matter everywhere in the ecosystem and there is a continuous cycling of materials. Certain fungi and bacteria, which are responsible for the decomposition, are called or saprotrophs or reducers. Most of the saprotrophs are microscopic and they are all heterotrophic in . The role of decomposers is very special and important. Certain decomposers are also called . Some animals such as earthworms, soil inhabiting nematodes and arthropods are also feeders and are called detrivores. They also contribute to the breaking down of organic matter. Water, carbon dioxide, phosphates and a number of organic compounds axe largely the by- products generated through activity of organisms on dead organisms.

SAQ 1 i) Define environment and ecosystem. ii) Describe and discuss the function of various abiotic and biotic (define these term/components of an ecosystem. iii) Describe how abiotic factors affect biotic factors in an ecosystem. Give examples. iv) What are decomposers and how do they affect the recycling of materials in an ecosystem. v) Define primary producers, consumers, decomposers and detrivores, also give an example of each, and also draw them.

17.3.3 Trophic Levels You are now aware that an ecosystem is considered as a basic unit, where individuals belonging to a complex natural community obtain their food from plants through one, two, three or four steps and accordingly these steps are known as the first, second, Humans, being omnivores, third and fourth trophic (Trophe = nourishment) levels or food levels. (Fig. 17.4). may belong to more than one Let us see the trophic levels to which autotrophs and different types of heterotrophs trophic level. belong to:

Green plants (producers); trophic level I - Autotrophs Herbivores (primary consumers); trophic level 11 - Heterotrophs Carnivores (secondary consumers); trophic level I11 - Heterotrophs Carnivores (tertiary consumers); trophic level 1V - Heterotrophs Top carnivores (quarternary consumers); trophic level V - Heterotrophs

Thus energy flows through the trophic levels: from producers to subsequent trophic levels. This energy always flows from lower (producer) to higher (herbivore, etc.) trophic level. It never flows in the reverse direction that is from carnivores to herbivores to producers. Furthermore there is a loss of some energy in the form of unusable heat at each trophic level so that energy level decreases from the first trophic level upwards. As a result there are usually four or five trophic levels and seldom more than six because beyond that very little energy is left to support any organism.

The study of trophic levels gives us an idea about the energy transformation in an ecosystem. Furthermore it provides a useful conceptual basis to include all organisms that share the same general mode of feeding into one group which together are said to belong to the same trophic IeveI. This indicates that organisms belonging to the same trophic level obtain food through the same number of steps from the producer. Ecology and Environment

Primary

Fig.17.4: The organization of an ecosystem into different tophic levels on the basis of how they obtain energy.

Trophic levels are numbered according to the number of steps an organism is away from the source of food or energy, that is the producer. (see also Fig. 17.5)

swrpyh as heat

mwement

Ingestion

Fig. 17.5: Energy, use by consumers - Energy ingested in food is either digested and assimilated or passed through and eliminated in faeces. The assimilated energy is used for various functions of the body like respiration and movement, or stored and used for the growth of new tissues or excreted. When the organism dies the energy stored in tissues is used by the decomposers. Only the stored materials are available to organisms at the next trophic level.

17.3.4 Food Chain You now know from the previous section that organisms in the ecosystem are related through feeding or trophic levels, that is one organism becomes food for the other. A Each link in the food chain can also be sequence of organisms that feed on one another, form a food chain as depicted in called trophic level. Fig. 17.6. The arrows in the figure denote the direction and movement of nutrients and energy from producer to consumer. Similar to the trophic levels and for the same Ecosystem - Structure reasons the links or steps in a food chain are usually to four or five. and Function -I

Fig.17.6: A food chain in a pond.

Some animals eat only one kind of food and therefore, are members of a single food chain. Other animals eat different kinds of food, so they are not only members of different food chains but may also occupy different positions in different food chains Humans are at the top of and trophic levels, thus ensuring the survival of their species. An animal may be a a number of food chains. primary consumer in one chain, eating plants but a secondary or tertiary consumer in other chains, eating herbivorous animals or other carnivores. (See also Fig.17.3)

Since human beings can do nothing about increasing the amount of light energy and very little about the efficiency of energy transfer, they can only shorten the food chain, to get energy i.e., by eating the primary producers - plants, rather than animals.

In highly populated countries, people tend to be vegetarians because then the food chain is the shortest and a given area of land can in this way support larger number of people. Suppose that a farmer has a crop of wheat and vegetables. He can eat it directly or feed it to his goats and then eat the goats. Figure 17.7 shows that a large number of people can be supported on a vegetarian diet as compared to a non- vegetarian diet on a given piece of land. Thus the sun's energy is used most efficiently if people are vegetarians.

Fig. 17.7: The relative efficiency of vegetarian and non-vegetarian diets. a) In a vegetarian diet 25,000 calories support 10 people. b) In the same time 25,000 calories of plant matter support only one person who eats meat.

Disturbing a food chain or web In a well balanced food chain (or web) a certain mass of green plants grows continuously and supports a fairly constant and very large number of primary consumers, which in turn are eaten by a fairly constant number but lesser number of secondary consumers and so on. However, this balance is disturbed if one species is removed from food chain for example we have a food chain such as given below:

Alga -+ waterflea -+ stickleback (small fish) pike (big fish) + 4 5 Ecology and Environment If farmers remove the pike from the top of food chain then there will initially population explosion of the stikle back. However, these will soon eat all the waterfleas and die of starvation, leaving the algae to grow unchecked and choke up the pond.

If a food web has only a few links then the effect of removing one species on the remaining organisms can be severe but if a food web has many links, then removal of one species may not have such a drastic effect.

Types of food chains In nature, two main types of food chains have been distinguished:

i) Grazing Food Chain: The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain. This food chain begins from green plants at the base and the primary cansumer is a herbivore, for example: Grass + grasshopper + bird + hawk or falcon. In a community of organisms in a shallow sea, ii) Detritus Food Chain: The food chain starts from dead organic matter of about 30% of the total decaying animals and plant bodies to the micro-organisms and then to detritus energy flows via detritus feeding organisms called detrivores or decomposers then to herbivores and then chains. In a forest with a large biomass of plants and to predators. (Fig. 17.8) a relatively small biomass of animals even larger Litter + springtail (insect) + small spiders (carnivore) portion orenergy flow may be via detritus pathways. For example, forest floors and small streams receive a rain of leaves and other bits of material that the small animals use as a food source. The small pieces of organic matter, such as broken leaves, faeces, and body parts are known as detritus. The slugs, snails, earthworms, insects and small animals that eat detritus are often called detrivores. The detrivores break the leaves and other organic material and thus making them available to still other organisms as a food source. The fungi and bacteria are also eaten by other detritus feeders. Some biologists believe that enormous amount of energy flow through detritus food chains.

CqanicmaMal Fig.17.8: Detritus food chain.

/ All food webs begin with The distinction between the two food chains mentioned above is the source of energy autotrophs and end with for the first level consumers. In the grazing food chain the primary source of energy is decomposers. living plant biomass while in the detritus food chain the source of energy is dead organic matter or detritus. The two food chains are linked. The initial energy source for detritus food chain is the waste materials and dead organic matter from the grazing kosystem - Structure food chain. and Function - I

17.3.5 Food Web A food chain represents only one part of the food or energy flow through an ecosystem and implies a simple, isolated relationship, which seldom occurs in ecosystems. An ecosystem may consist of several interrelated food chains. More typically, the same food source is part of more than one chain, especially when that is at one of the lower trophic levels. For instance a plant may serve as food source for many hertrivores at a time. For example, grasses can support rabbits or grasshoppers or goats or cows. Similarly a herbivore may be food source for many different carnivorous species. Also food availability and preferences of herbivores as well as carnivores may shift seasonally e.g. we eat watermelon in summer and peaches in the winter. Thus there are interconnected networks of feeding relationships that take the form of food webs (Fig. 17.9). A food web illustrates, all possible transfers of energy and nutrients among the organisms in an ecosystem, whereas a food chain traces only one pathway of the food. (Table 17.1)

Fig.17.9: a) A complex network or web of primary producers, consumers and decomposers illustrated in tvoical terrestrial food web. b) Food web in a Pond. Ecology and Environment Table 17.1: Roles of trophic levels in an ecosystem.

Classification Description Examples Producers Organisms that convert simple Trees, flowers, grasses, ferns, inorganic compounds into complex mosses, algae, cyanobacteria organic compounds by photosynthesis. Consumers Organisms that rely on other organisms as food. Animals that eat plants or other animals. Herbivore Eats plants directly. Deer, goose, cricket, vegetarian human, many snails Carnivore Eats meat. Wolf, pike, dragonfly Omnivore Eats plants and meat. Rat, most humans Eats food left by others. Coyote, skunk, vulture,-i crayfish Parasite Lives in or on another organism, Tick, tapeworm, many insects --using it for food. Decomposers Organisms that return organic conlpounds to inorganic compounds. Important components in recycling.

Human vs. Natural food chains As we all know human civilization is mostly dependent on agriculture. Agriculture means manipulating the environ~iientto favour plant species that we want to eat. In this way we the humans manipulate and allowing only favoured species (crops) to thrive. Like this we are creating a very simple ecosystem. It has only three levels - producers (crops), primary consumers (livestock, humans) and secondary consumers (humans).

Though it may seem good for humans but agricultural ecosystems have problems. By planting only one type of crop (same species) we create monocultures, an ideal situation for disease and insect pests, even the most inept insect can find a new host plant with a jump in any direction. Disease also spread quite easily in one type of crop and if the whole area has same crop it will be completely destroyed. It takes a lot of chemicals (pesticides) to keep a monoculture going.

We also pollute water by putting lots of fertilizers and pesticides in fields, and also damage the ecosystems.

Activity 1

i) In India we are growing high yield varieties of wheat. In a way we are encouraging a monoculture. What will happen if disease strikes the field? Can you suggest some remedies? ii) Draw a food web for a specific biome. Be sure to include as many decomposers, producers and different level consumers as you can. Draw the figures of organisms with the help of reference book in your library.

SAQ 2

i) What is the source of the energy entering a food chain or web? ii) Define producer and consumer. iii) Present each of the following groups of organisms as a food chain. a) Lion, Grass, Zebra b) Cat, field mouse, owl, wheat c) Fish, animal plankton, human, plant plankton d) Fly, oak leaf, ladybird e) Frog, hawk, grass, snake, grasshopper iv) Construct a food chain in which you are the final consumer. Ecosystem - Structure and Function - I v) Compare and contrast food chain and food web. vi) Complete the following using additional arrow. Plant Plankton

Animal plankton Cockle \ Herring \ Mussel Man Cod Walrus

Seal / 17.3.6 and

I11 this subsection, we will examine how pollutarits specially nondegradable ones move through the various trophic levels in an ecosystem (Fig. 17.9). By nondegradabale pollutants we mean those materials, which cannot be metabolised by the living organisms. For example chlorinated hydrocarbons. Movement of these pollutants involve two main processes: i) bioaccu~nulationand ii) biomagnification.

i) Bioaccumulation: refers to how pollutants enter a food chain. In bioaccumulation there is an increase in concentration of a pollutant from the environment to the first organism in a food chain. ii) Biomagnification: biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next. Thus in biomagnification there is an increase in concentration of a pollutant from one link in a food chain to another.

We are concerned about these phenomena because together they enable even small concentrations of chemicals in the environment to find their way into organisms in high enough dosages to cause problems. In order for biomagnification to occur, the pollutant must be 1. long-lived; 2. mobile; 3. soluble in fats; 4. biologically active.

If a pollutant is short-lived, it will be broken down before it can become dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water, it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time. It is traditional or customary to measure the amount of pollutants in fatty tissues of organisms such as fish. In mammals, we often test the milk produced by females, since the milk has a lot of fat in it and because the very young are often more susceptible to damage from toxins (poisons). If a pollutant is not active biologically, it may biomagnify, but we really do not worry about it much, since it probably, will not cause any problems. --

Ecology and

Fig.17.10: An example of biological magnification (a) Single-celled plant species at the bottom of a food chain have picked up a small amount of a stable non-excretable chemical. (b) Cyclops, a small crustacean eats it, incorporates the chemical from the plants into its own tissues. Like the other organisms in the chain, it lacks the biochemical pathways necessary to metabolise or excrete the novel substance. (c) A dragonfly nymph stores all the chemical acquired from the numerous Cyclops it eats. (d) Further magnification occurs when a minnow eats many of the dragonfly nymphs that have stored the , chemical. (e) when a bass, the top predator in this food chain eats many such minnows, the result is a very high concentration of a chemical in its tissues that was much less concentrated in the organisms lower in the chain.

Box 17.1: Effect of DDT on osprey.

In 1950 there were over 200 mating pairs of osprey nesting at the mouth of the Connecticut River. By 1970 only six mating pairs were observed, the decline in the local population being attributed to the detrimental effects of DDT and related hydrocarbons on the calcification of eggshells produced by these birds. The hydrocarbons had been introduced into the runoff of local streams and rivers in insecticides and consumed by the fish that were the osprey's prey. A high percentage of the weakened eggs broke during incubation: approximately 10 eggs, or two or three nestings, were needed €0 a single offspring. Since 1970 the local osprey population has grown considerably, largely because of a ban on the use of the chemicals.

DDT is a persistent pesticide that has become so pervasive in the biosphere that it can now be found in the fatty tissues of nearly every living organism. This chemical has had more severe effects on predatory birds such as the bald eagle and peregrine falcon than on seed-eating birds because of biological magnification. Some investigators have reported that the reproductive rates of eagles and falcons have been drastically reduced because DDT - and its metabolite DDE - interfere with deposition of calcium in the eggshells, with the result that the thin- shelled eggs are easily broken by the incubating parents and few young birds hatch.

50 Ecosystem - Structure 17.4 PYRAMIDS and Function - I

You have studied trophic levels in subsection 17.3.3. These steps of trophic levels can be expressed in a diagrammatic way; and are referred to as ecological pyramids. The food producer forms the base of the pyramid and the top carnivore forms the tip. Other consumer trophic levels are in between. The ecological pyramids are of three categories.

Pyramid of numbers, Pyramid of biomass, and I Pyramid of energy or productivity. I 17.4.1 Pyramid of Numbers This deals with the relationship between the numbers of primary producers and consumers of different levels (Fig. 17. I 1). It is a graphic representation of the total number of individuals of different species, belonging to each trophic level in an ecosystem. For example, we might have the following pyramid for a grass field as depicted in Fig. 17.11 (a) where the base of the pyramid represents the food production base for other higher trophic levels. The pyramid consists of a number of horizontal bars depicting specific trophic levels which are arranged sequentially from primary producer level through herbivore, carnivore onwards. The length of each bar represents the total number of individuals at each trophic level in an ecosystem. The number of individuals drastically decreases with each steps towards higher trophic levels and the diagrammatic representation assumes a pyramid shape and is called pyramid of numbers.

Fig.17.11: Pyramid of numbers shows the number of organisms at each trophic level in the ecosystem (a) An upright pyramid of numbers (b) an inverted pyramid of numbers.

However, it is very difficult to count all the organisms, in a pyramid of numbers and so the pyramid of numbers does not completely define the trophic structure for an ecosystem. A pyramid of numbers does not take into account the fact that the size of organisms being counted in each trophic level can vary. A count in a forest would have a small number of large producers, the big trees, which support a large number of herbivores. As a result the pyramid will assume an inverted shape as you can see in Fig. 17.1 1 (b). This is because the tree is a primary producer and would represent the base of the pyramid and the dependent herbivores and carnivores will represent the second and third trophic levels respectively. Thus, depending upon the size and biomass, the pyramid of numbers may not always be upright, and may even be ---- l-*-l-. :--.---.a i Ecology and Environment 17.4.2 Pyramid of Biomass In order to overcome the shortcomings of pyramid of numbers, the pyramid of biomass is used. (Fig. 17.12). In this approach individuals in each trophic level are weighed instead of being counted. This would give us a pyramid of biomass, i.e., the total dry weight of all organisms at a each trophic level at a particular time. Pyramid of biomass is usually determined by collecting all organisms occupying each trophic level separately and measuring their dry weight. This overcomes the size difference problem because all kinds of organisms at a trophic level are weighed. Biomass is measured in g,lm2 (g=granf, mzmeter). At the time of sampling, the amount of biomass is known as standing crop or standing biomass. For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller trophic level perched on top.

Fig.17.12: The pyramid of biomass depicts total weight of organisms supported at each level.

In contrast, in many aquatic ecosystems, the pyramid of biomass may assume an One caloiie (cal) is the amount of heat needed to inverted form. This is because the producers are tiny phytoplanktons that grow and raise the temperature of one reproduce rapidly. Here, the pyramid of biomass can have a small base, with the cubic centimetre of water consumer biomass at any instant actually exceeding the producer biomass. The through one degree phytoplanktons are consumed about as fast as they reproduce, it is just that the centigrade. One kilo calorie survivors (they may be few) are reproducing at a phen~menalrate. (K cal = 1000 cal) 17.4.3 Pyramid of Energy When we wish to compare the functional roles of the trophic levels in an ecosystem, an energy pyramid is probably the most informative, for the pyramid shape is not distorted by over emphasis on variations in the size and weight of the individuals. An energy pyramid more accurately, reflects the laws of thermodynamics, with conversion of solar energy to chemical energy and heat energy at each trophic level and with loss of energy being depicted at each transfer to another trophic level-(See section 17.5). Hence the pyramid is always right side up (Fig. 17.13), with a large energy base at the bottom. A pyramid of energy must be based on a determination of the actual amount of energy that individuals take in, how much they bum up during metabolism, how much remains in their waste products, and how much they store in body tissue. Fig. 17.13: The pyramid of energy depicts the amounts of energy available at each trophic level.

In energy pyramid, a given trophic level always has a smaller energy content than the trophic level immediately below it. This as you may recall from section 17.3.3 is due to the fact that some energy is always lost as heat while going up from one trophic level to the next. Let us explain this with an example. Suppose an ecosystem receives 1000 calories of light energy in a given day. Most of the energy is not absorbed; some is reflected back to space; of the energy absorbed only a small portion is utilised by green plants, out of which the plant uses up some for respiration and of the 1000 calories, therefore only 100 calories are stored as energy rich materials.

Now suppose an animal, say a deer, eats the plant containing 100 cal of food energy. The deer uses some of it for its own metabolism and stores only 10 cal as food energy. A lion that eats the deer gets an even smaller amount of energy (1 7.14). Thus usable energy decreases from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright (see Figure 17.13). Each bar in the pyramid indicates the amount of energy utilised at each trophic level. The energy inputs an4 outputs are calculated so that energy flow can be expressed per unit area of land or Fig. 17.14: Pyramid of energy showing energy loss volume of water per unit time. The unit of measurement is ~cal/m*/~,where K cal at each higher level. represents energy, m2.represents unit area and y represents years.

17.5 ENERGY IN ECOSYSTEM

As you know, by now energy used for all life processes is derived from solar energy. Sun is the ultimate source 01 The flow of solar energy is unidirectional. Its immediate implication is that an all our energy, which caters to the need of our ecosystem will collapse if the sun stops giving out energy. In the previous svbsecfion ecosystem. It has been you read that solar energy along with nutrients is converted by producers, into food observed that 30% of the materials and is stored within their bodies. All the food materials or nutrients that we total solar radiation entering or other animals consume are obtained directly or indirectly from such producers. As our atmosphere is reflected a result there is continuous flow of energy from the sun through various organisms by the earth -atmosphere system. The remaining 700, and then to outer space: This process maintains the life on the earth. Trapping and of the radiation is absorbed flow of energy involves circulation of nutrients as well, which include the basic by the earth's atmosphere. inorganic elements such as, carbon, hydrogen, oxygen and nitrogen as well as, Of this 19% is absorbed sodium, calcium, and potassium, which occur in small amounts. In addition, directly by the atmosphere and the rest by the earth compounds such as; water, carbonates, phosphates and a few others also form part of surface. living organisms. For an ecosystem to function, it is essential that there is a continuous flow of energy and cycling of nutrients.

17.5.1 Flow of Energy in an Ecosystem With the help of the following flow chart, we can interpret the functional aspect of an ecosystem or the interactions between various components, which involve the flow of energy and cycling of materials (Fig. 17.15). Ecology and Environment

(herbivass-edscanposas)

metabd~heat

Fig. 17.15: Energy flow through an ecosystem.

Implicit in the system, such as (producer) + , (consumer), or producer +,herbivore + carnivore relationship, is the direction of energy movement through the ecosystem. In the process, the flow of solar energy is unidirectional and it is converted into chemical energy through photosynthesis by plants, which also incorporate in their protoplasma number of inorganic elements and compounds. These green plants are grazed subsequently by heterotrophs. This means that chemical energy in the form of carbohydrates, fats, and proteins as well as a host of other nutrients are transferred into herbivores. This process continues upto the level through the carnivores. Another feature of the process is that the energy trapped by green plants when transferred from one food level or trophic level to another also undergoes losses at each transfer along the chain. This is because in an ecosystem, energy is transformed in an orderly sequence (Refer again to Fig. 17.12) and is governed by the two laws of thermodynamics. The first and second law of thermodynamics are given below.

17.5.2 Laws of Thermodynamics

1) The first law of thermodynamics deals with the conservation of matter and energy and states that energy cannot be created or destroyed but can only change from one form to another. For example the energy of visible light is absorbed by green plants through photosynthesis and is changed into chemical energy, which is stored in glucose molecules. So in biological context, this Ecosystem - Structure principle means "Energy may be transferred or transformed, but it is not lost". and Function - I This chemical energy is transformed and used by the cells of the organisms An ecological rule of through the process of the metabolism for their various actlvities. Most of the thumb allows a magnitude of 10 reduction in energy as energy is used to for metabolic activities, movement, and other activities of living it passes from one trophic organisms. level to another. If herbivorea eats 1000 k cal 2) The second law of thermodynamics states that part of some useful energy is of plant energy, about 100 k cal will get converted into degraded into unusable waste as heat energy during every energy herbivore tissue, 10 k cal transformation. The waste heat energy escapes into the surrounding will get into first level environment. This law clearly operates in the trophic levels where at each carnivore production, and 1 succeeding-level some chemical energy of food is transformed into unusable k cal into second level heat energy. This is because cells of organisms continuously need energy, carnivore production. However, data suggests that which is provided to it mainly in fopof ATP while some energy gets a 90% loss of energy fiom transformed into unusable heat (energy). Since heat energy cannot do usehl one trophic level to another works, more energy must be supplied to a biological system from outside to may be too high. Transfer compensate the inevitable energy loss. In order to continue to function - of energy from one trophic level to another tells the real organisms and ecosystems must receive energy supply 'on a continuing basis story, but such data is hard which is provided by the sun. If the energy supply is interrupted, the cell will be to collect. unable to function and becomes disordered. Such disorganization can be either a cause or a consequence of cell death. The following diagram (Fig. 17.16) depicts the energy transfers and energy losses and nutrient movement in an ecosystem.

Human intervention in natural ecosystem is growing significantly. Human impact on the pattern and quantum of energy flow has changed significantly because of the considerable amount of fossil fuel used by urban, industrial and rural communities. The developing countries of the third world like Fig. 17.16: A diagram illustrating the manner in which nutrients cycle through an ecosystem. lndia face perpetual Energy does not cycle because all that is derived from the sun eventually dissipates as energy shortage. In the heat. present day world, energy and prosperity go hand-in hand. The rich countries From the above figure, we can conclude the following: have a high rate of consumption. As energy movement is unidirectional (unlike the nutrients which cycle) in an compared to a citizen in ecosystem, so the initial energy trapped by an autotroph does not revert back to India, a typical person in solar input, the U.S. uses: 50 - times more steel energy that passes from herbivore to carnivore does not pass back to herbivore 56 - times mo.re energy from carnivore. As a consequence of this unidirectional and continuous energy 170 - times more flow, the ecosystem maintains its entity and prevents collapse of the system. synthetic rubber and newsprint nutrients cycle in the ecosystem and transfer of nutrients does not involve loss of 250 - times more motor fuel nutrients such as that of energy which suffers less during transfer. This is because 300 times more plastic the faecal matter, excretory products and dead bodies of all plants and animals are as much grain as five broken down into inorganic materials by decomposers and are eventually returned Kenyans, and as much to the ecosystem for reuse by the autotrophs. (Refer Section1 8.3) energy as 35 or 500 (a whole village!) Pth;n..;onc -

Ecology and Environment Flow of energy through the ecosystem is a fundamental process, which can be easily quantified if the energy input to the ecosystem and its subsequent transformation from one trophic level to another can be expressed in terms of calories.

SAQ 3

i) State two ways in which energy may be lost from a food chain. '

ii) Which of the following is a more effective use of 1000 kg of rice? Give reason a. As food for humans. b. As food for pigs and then the meat from the pig used as food for humans.

iii) Why are the majority of pyramids "bottom heavy" (lots of producers biomass and energy) and "top light" (relatively few consumers biomass and energy).

iv) Explain what the term pyramid of biomass means.

-- 17.6 PRODUCTIVITY

Uptil now you must have understood that ecosystems can function properly if there is constant supply of energy from external source and that is the Sun. The solar energy is captured by the primary producer (plants) and enters the ecosystem through photosynthesis. In this section you will study about productivity of ecosystem.

17.6.1 Primary Production Energy which is accumulated by primary producers (plants) during photosynthesis is production or more especially it is called primary production because it is the first and basic form of energy storage. The rate of accumulation of energy is known as primary productivity. The total energy assimilation by the plants is the total photosynthesis and also gross primary production.

Plants like other organisms spend energy in production, maintenance and reproduction. This energy is produced through respiration. Energy which remains after respiration and in stored as organic matter is net primary production (See Box 17.2). Net primary production can be shown through the following equation.

Net primary production (NPP) = Gross pfimary production (GPP) - respiration by the autotroph (R)

I Production is usually expressed in units of energy per unit area - kilocalories per square meter (~cal/m~).However, the production can also be measured and expressed as the mass of dry organic matter: grams per square meter g/m2. i Over a period of time net primary production is accumulated as plant biomass. Another expression is standing crop biomass, which is accumulated amount of organic matter found in an area at a given time. Biomass is usually expressed as (g./m2)or cal/m2). Biomass differs from productivity; Biomass is the amount (mass) present at any given time whereas the productivity is the rate at which primary producers through photosynthesis create organic matter. You can understand and explain the relationship between GPP and NPP with the following formula.

GPP - R = NPP or GPP = NPP + R

56 In the above equation you can observe that whatever energy is fixed by plants (GPP) Ecosystem - Structure and Function - I some of it is used for their own maintenance (R) and only remaining (NPP) is available for the next trophic level. So net primary production is the only energy available for the next trophic level. ,

Different ecosystems have different productivity. There are several factors such as sunlight, temperature, rainfall and the availability of nutrients that affects the productivity of an ecosystem. For example net primary productivity increases with increasing temperature and rainfall because both these factors the rate of photosynthesis and amount of leaf area that can be supported and duration of growing season. (Fig. 17.17) Some area have low productivity because one essential component is missing. See the figure and draw your inferences about the influence of . rainfall and primary productivity.

1 ~rooicalrain forest

Y 5 I , wlnber dec'duous forest

6 Rairie ~oniferwsforesk Taiga r:3 P: 14 P: 4-20 B: 1-18 B: 25-70 100 I R: 2.4 R: 1.8 P: 0.14 Tundra B: 5 R: 1.3

Mean annual temperature (O C) Fig.17.17: Distribution of primary production, standing biomass, and radiation input relative to rainfall and temperature P = primary production (tnlha), = biomass (tnlha); R = PAR solar radiation (~callm~l~r)from Elements of Ecology - by Robert Leo Smith, Thomas M. Smith.

Now answer the following questions.

in Fig. 17.17 which ecosystem has the highest productivity explain why and tundra have low productivity ! do the critical thinking along with the group of students and various causes that I will affect productivity of an ecosystem.

Production efficiency: Green plant harvest only 5 per cent of energy but on the whole it is only a small fraction of sunlight i.e. 0.02 per cent reaching the atmosphere. The production efficiency, that is the ratio of net primary production to gross primary production (green plants) is on the average rather high. It varies between 40-85 per I cent. Ecology and Environment Box 17.2: Net primary productivity (NPP) for selected ecosystems of the World..

Ecosystem Total area on Range Average Earth (1o6 km2)

Terrestrial Tropical rainforest 17 1000-3500 2200 Cultivated land 14 100-3500 650 Temperate forest 12 600-2500 1240 Grassland 24 200-2000 790 Tundra and alpine 8 10-400 140 Community Desert 42 0-250 40

Aquatic Coral reef 0.6 500-4000 2500 Swamp and marsh 2 800-3500 2000 I Estuary 1.4 200-3500 1500 Lake and stream 2 100-1 500 250 Ocean upwelling 0.4 400-1 000 500 Zone Open ocean 332 2-400 125

17.6.2 Secondary Production Now you know that net primary production is only energy available to consumers (including man) at trophic level. Herbivores graze on plants and utilize primary production. Some of the material is utilized in various activities and remaining part is utilized for producing new tissues for reproduction. Production of animal biomass on account of growth as well as addition of new individuals of animals is referred to as secondary production or the rate of new biomass production by consumers is called secondary productivity. Thus secondary productivity is the :ate of formation of new organic matter by heterotrophs. (Fig. 17.18)

In terms of energy content very little of the plant matter that is consumed is actually converted into animal tissue and conversion is only 10 per cent. For example:

1000 Kcal -> 1 2 Kcal - 10 Kcal 1 Kcal (herbivores)

from Sun GPP NPP , Secondary production

.005 K cal productions by carnivores

You must clearly understand that in contrast to primary production, secondary production is usually not differentiated into 'gross and net' categories because heterotrophs consume only already manufactured food.

New biomass made by plants and other producers is consumed by decomposers such as bacteria and fungi. Sooner or later all organisms die and all biomas's produced by plants herbivores and carnivores is consumed by decomposers. Decomposers are tremendously important to all life. They recycle nutrient, thereby ensuring that nutrients do not remain locked up as dead material once a organism dies. Ecosystem - Structure and Function - I Decomposers are extremely important to any ecosystem

Wbdic heat loss Hetabdk heat lass

Fig.17.18: In ecosystems of all types, more than 50 percept of net primary productivity is used directly by decomposers. This example shows a forest in which 80 percent of the NPP is used directly by decomposers, and the remaining 20 percent is used by other consumers (herbivores and carnivores).

17.7 HOMEOSTASIS

In order to find solutions for environmental problems the understanding of systems and rates of change occurring in the systems including the ecosystem is essential.

17.7.1 System

A system may be broadly defined as any part of the universe that can be isolated for the purposes of observation and study. Some systems may be physically isolated - for example bacteria culture in a petri dish -or may be isolated in our minds or in a computer database. In another way you can visualise a system as a set of co~nponentsor parts that function together to act as a whole. A single organism may t be considered a system, as may be a river, your office, a city or a thermal power plants. On a much larger scale, you already know that biosphere is also a system.

At every level in environmental science we have to deal with a variety of systems that may range from simple to complex and irrespective of how we approach environmental problems it is necessary that we have an understanding of the systems and of how various parts of the systems interact with one another. Systems may be open or closed. A system, which is open with respect to some factor, exchanges that factor with other systems. The ocean is an open system in regard.to water, which it exchanges with the atmosphere. A system that is closed in regard to some factor does not exchange that factor with other systems. Earth is an open system in regard to energy and a closed system (for all practical purposes) in regard to material.

All these systems in order to operate smoothly need to maintain their existing constant condition. This capacity of a system to self regulate or self maintain itself is called homeostatis. What keeps the systeni fairly constant is a feedback mechanism. The feedback mechanism provides environmental information to which a system responds. Ecology and Environment 17.7.2 Feedback Mechanism Systems respond to inputs and have outputs. Our body for instance is a complex system. If by chance you encounter a snake, which you think is poisonous, then the sight of the snake is an input. Our body reacts to the input. The adrenaline level in our blood rises; our heart rate increases and the hair on our body may rise. Our response perhaps standing still or moving away from the snake is an output.

Feedback, a special type of system response occurs when the output of the system also serves as input and leads to changes in the state of the system. A classic example of feedback is temperature regulation in human (Fig. 17.19). The normal temperature for humans is 37°C. We call such r norm r a set point. When the temperature of the environment rises; the sensory mechanisms in the skin detect the change (input) and the body responds physiologically.

Fig. 17.19: Negative and positive feedback mechanisms. In negative feedback the response inhibits or reverses any change from the normal. Positive feedback leads to further change in the same direction. Negative feedback brings the system back to the set point. Positive feedback leads away from the set point and can damage the system.

A message is sent to brain, which automatically relays the message to the receptors which enhances increase in blood flow to skin, induces sweating and stimulates behavioral responses. Water excreted through the skin evaporates, cooling the body. The person may also respond behaviorally: as on feeling hot (input) he or she miiy move into the shade as a result of which the temperature would return to normal., This is an example of negative feedback since the systems response is in the opposit~: direction from the input, and halts or reverses any deviation or movement away :from set point (an increase in temperature leads to a response causing a decrease in temperature).

In the case of positive feedback, an increase in input leads to a further increase in output. For example if the environmental temperature becomes extreme and the body temperature keeps on increasing correspondingly, the homeostatic system of the body breaks down, which is because when it gets too hot, the body is unable to lose heat fast enough to maintain normal temperature, as a result of which the metabolism^ speeds up, raising body temperature until the person dies of heat stroke. Thus a situation in wliich feedback reinforces change, driving the system to higher and higher Ecosystem - Structure and Function - 1 or lower and lower values is called positive feedback.

Negative feedback is generally desirable as it is stabilizing. It usually leads to a system that remains in a constant condition. Positive feedback often called vicious circle is destabilizing.

17.7.3 Ecosystem Homeostasis Let us see how the feedback in an ecosystem helps to maintain homeostasis or balance. The ecosysteln as you must know by now is a dynamic system, where a lot of events occur, like plants eaten by animals, which in turn are eaten by other animals. Water and nutrients flow in and out of the system and the weather changes. However, despite all these events the ecosystem persists and recovers from minor disturbances due to liomeostatis. Consider a grassland which has suffered froin drought due to which plants do not grow well and which have a mice population. The mice that feed on grass beconie malnourished due to lack of food. When this happens, their birth rate decreases. Furthermore, the hungry mice retreat to their burrows and sleep. By doing so, they require less food and are exposed less to predators. As a result their death rate decreases. Their beliaviour protects their own population balance as well as that of the grasses, which are not being eaten, while the mice hibernate (sleep). Thus you can see that the ecosyste~nhas maintained its balance or ecological homeostasis as a result of negative feedback ~nechanism,which is the prime regulatory mechanism for the ecosystem as a whole. You must be fully aware by now that in an ecosystem several kinds of organisms are present. Thus all the organisms in an ecosystem are part of several of different feedback loops. A feedback loop may be defined as a relationship in which a change in some original rate alters the rate of direction of further change.

Now let us consider another important parameter of ecosystem balance, which is . Species diversity, that is, the number of different species and their relative in a given ecosystem accord the stability or persistence the ecosystem under small or moderate environmental stress. High species diversity tends to increase long-term persistence of the ecosystem. This resilience is due to the fact that risk is spread more widely with the presence of many different species and the linkages between them. An ecosystem having several well-adjusted species has more ways available to respond to [nost environmental stresses. For example, in an ecosystem endowed tvith colnplex food web, the loss or drastic reduction of one species does not threaten the existence of others, as most consuniers have alternative food supplies. In contrast, tlie highly specialised ecosystem, planted with only one kind of crop (monoculturing) plant like wheat or rice is highly vullierable to destruction from a single plant pathogen or pest. The essence of the above discussion is that the most balanced ecosystems contain many different types of species and that the presence of many types of or high species diversity imparts stability to the

The ability of an ecosystem to cope with any dislurbance or disruption is however limited and fails in cases of positive feedback like tires (destruction of landscape), overexploitatioli (widespread mining, deforestation), ekcessive simplification (monoculture, plantation, crop fields) or extreme and prolonged stresses (like drought, pollution). In extreme cases the homeostatic mechanism is overshadowed leading to ecosystem degradation.

It is essential that we should check and control our actions, so that we do not overload the ecosystem and disrupt its homeostasis. Ecology and Environment 17.8 COMMUNITY AND ECOLOGICAL SUCCESSION

A community is also called biotic community. It is a group of interacting populations living in a given area. It represents the living part of an ecosystem and functions as a dynamic unit with trophic levels, an energy flow and a cycling of nutrients as described earlier. Some of the species i~~teractionssuch as predator-prey relationships, nlutualism and competition will be discussed in Unit 18, section 18.4.

The organizational components of a biotic community seen1 to us for the most part to be static or suspended in time. However, biotic commu~iitiesdo exhibit progressive change as part of their normal development. Communities for example, change in response to climatic and geological forces as well as in response to the activities of their inhabitants. In sonie cases even within a particular climate, the inhabitants of a location are not the same from one year to the next. Organisms that live in a given location may change the environment by their very presence or activities. An environment that favoured an organism earlier may over time become progressively less favourable arid may become more favourable to other life forms. 'Thus one type of organism may make way for another.

The question arises why does succession occur? The cause cannot be year-to-year changes in weather, because succession will take place even if the climate remains the same. Climate may be a major factor in determining what sorts of species will follow one another, but the succession itself must result from other causes. In particular two factors are thought to important (i) the modification of the physical environment produced by the community itself over the course of the succession and (ii) the rate at which different species involve a new area. The traditional view has been that the most important cause is the modification of environment. In this view each set of species in the succession sows the seeds of its own destruction.

In recent years, however, ecologists have come to realize that the rate at which species disperse and the element of chance also play important role in succession. Some species are more easily dispersed than others, or grow and reproduce more rapidly. Therefore these species become established on recently abandoned farmland than the seedlings of slow growing trees even though the trees may eventually become dominant plants.

The orderly process of change or replacement of some inhabitants or species of the community in an area, through time is known as community development or more traditionally as ecological succession.

Ecological succession, on the basis of the force responsible for the change or succession is of two kinds: (1) Autogenic succession -where ecological succession is the product of the organis~nsthelnselves and (2) Allogenic succession - where succession occurs due to outside forces, particularly physical forces such as fire or flood which regularly affect change. In most cases, succession is a result of both autogenic and allogenic f:nctors although one or the other may have triggered the process.

Allogenic s~~ccessionis less predictable than autogenic succession. For example, the sudden bloom of unexpected opportunistic species such as weeds often interrupts an orderly progression of species during succession. The accidental introduction of congress weed (Parthenlum sp.) along with wheat imported from the USA (during PL 480 scheme) into India is a good example of opportunistic species. Often one population does not give up its place for the next gracefully. On the contrary species are often quite persistent and seemingly resist their own displacement.

Ecological succession includes both (I) primary and (2) secondary succession. Each succession stage or the series of sequential changes in its entirety is known as a Ecosystem - Structure and Function - I sere and each sere is made up of a series of seral communities (seral stages).

17.8.1 Succession in Terrestrial Community 1. Primary succession occurs where no community exists before, such as rocky Seres of particular environments tend to outcropping, newly formed deltas, sand dunes, emerging volcanic islands and lava follow similar flows. An example, which can be used as a model showing development of successions and may primary succession, is the invasion and colonisation of bare rock as on a recently therefore be classified created volcanic island. according to environment for example, a hydrosere develops in an aquatic Trees and shrubs are unable to grow on bare rock due to insufficient soil. Primary environment as a result of succession sere thus begins with lichens. Lichens and mosses are the first to the colonisation of open colonise because they have no roots but schizoids, which fix them on barren rock water; and a halosere and can survive without soil. Lichens can invade and colonise such areas, coming develops in a salt marsh. in, by various methods of dispersal and gaining a foot hold by means of their tenacious, water-seeking fungal component and thus forming the first community, very appropriately often called the pioneer community. (Fig.17.20). Lichens are soil builders, producing weak acids that very gradually erode the rock surface. As organic products and sand particles accumulate in tiny fissures, mosses and larger plants, such as grasses also get an opportunity to establish them and begin a new seral stage. In time, lichens that made the penetration of plant roots possible are no longer able to compete for light, water and minerals and will be succeeded by larger and more nutrient demanding plants such as shrubs and trees.

Ultimately "the final stable and self perpetuating community which is in equilibrium with its environment", is formed and this is called . The climax community is the most productive community that the environment can sustain. The animals of such a community also exhibit succession, which to a large extent is governed by the plant succession, but is also influenced by the types of aninials that are able to migrate from neighbouring communities.

Box 17.3: Comparison of pioneer and climax communities.

1 Characteristic ) Pioneer community I Climax community I 1 Growth rate 1 rauid 1 slow 1 1 Height of vegetation 2 low I high I Short Long Relative number of seeds Many Few , Relative size of seeds Small~arge 1 Relative distance seeds are 1 Long i Short 1 dispersed I Food uroductivitv Low High Biomass Small Large Species diversity Low High 1 Nutrient supply in soil Low High Food chains and webs -Few and simple

A climax community is more complex and is dominated by a few species that came late in the succession. The comniunity becomes self perpetuating and its appearance remains the same though there is constant replacement of individuals. The nature of the climax is determined by environmental conditions such as temperature, humidity, soil characteristics, topographic features and so on. A climax community has much less tendency than earlier successional communities to alter its environment in a manner injurious to itself. Fig.17.20b illustrates the primary ecological succession in a terrestrial habitat. Ecology and Environment The succession on bare rock out croppings is initially an extremely slow process with a sere often lasting hundreds of years or more. But once soil formation has Although succession ends with the establishment of begun, the process usually accelerates. Succession in other types of habitat may be a climax community, this slow. It has been estimated that succession from sand dune to climax forest does not mean that a community on the shores of Lake Michigan took about a thousand years. climax community is static. It does change though slowly, even when the climate is constant. It will change rapidly. Ilowever. if the community is disturbed in some way.

r 1 Intermedbte commun#les Oak Wes (final inhabitants)

pioneer cnmmunky -Grasses and shrub6

...... soil fertility Infertile soil ...... increases

Grasses, shrub, Shadetolerant trees shadeinWrant bees

b

Fig.17.20: (a) Succession an abandoned land (b) In primary succession formation of soil is important step because soil can support a large amount of vegetation. Vegetation also modifies the surrounding environment and increases the organic matter of the area. The plants eliminates the earlier pioneer stages of succession and after long time climax community may develop. Box 17.4: Succession in Glacial Moraine Alaska. Ecosystem - Structure and Function - I Glacial moraines are good place to study primary succession. There we can see a sequence of successional stages. Far away from the glacier where the ice have melted a longer time ago, the successional sequence is quite advanced. Next to the glacier, where the ice melted recently, the successional sequence is in early stage. Primary succession on bare rock after retreat of a glacier is given below in form of a table.

Primary Succession on a Glacial Moraine in Alaska

Time since The plants that Are present Glacier 7 0 years Bare rocks are colonized by lichens and mosses (no roots). As they die, they (Glacier just decompose and form a soil on the rock. melted) -- 3 years after melt Some soil has formed and plants with roots begin to colonize. These are grasses, wildflowers, and bushy willow.

10 years Alder trees colonize. 'These trees contain nitrogen-fixing bacteria, which add nitrogen to the soil. Alders grow taller than the grasses, wildflowers, and bushy willows, which die because they cannot grow in the shade of alders.

50 years Spruce trees (a form of conifer) colonize the nitrogen-rich soil. They grow 1 taller than alder. Young alder cannot grow in the shade of spruce trees, but young spruce can.

170 years Hemlock trees (a form of pine, which is a conifer) colonize and coexist with the spruce trees. The two conifers have slightly different niches: heinlock grows in drier places (higher ground) and spruce grows in wetter places (lower ground).

250 years A climax forest of spruce and hemlock exists and remains until disturbed by , fire or logging. Spruce-hemlock is the climax forest where the climate is cool and dry.

Fig.17.21: Primary succession on a Glacial moraine in Alaska where ice has just melted. Ecology and Environment fir, birch and white spruce

Fig. 17.22: The orderly series of species replacement during succession can be seen in this sequence of plants from a bare rock outcropping to a fir-birch-spruce community. Pioneering lichens and mosses begin the soil-building process, followed by the invasion of Secondary succession in increasingly larger plants until a more stable long-lived, climax forest community grassland communities is emerges. much faster, taking 20 to 40 years while on the other 2. Secondary succession (Fig. 17.23) occurs where a community has been disrupted, hand, fragile disturbed such as previously burned or neglected farms reverting to the wild, or a forest tundra may require many hundreds of years to recover, community that has been subjected to 'forest clearing' or a mining area that has if it ever does. been reclaimed.

200 years (variabk)

Fig.17.23: Secondary succession.

In secondary succesqion, the basic features are similar to those of primary succession, but the seres occur at a more rapid pace. This is possible because the soil is already formed and available. Secondary succession is said to occur when the surface is completely or largely denuded of vegetation but has already been influenced by living organisms and has an organic component. In such areas seeds, spores and plants propagate, such as rhizomes may be present in the ground and thus influence the succession. As secondary succession progresses, the initial invader species is eventually replaced Ecosystem - Structure by plants from surrounding communities. Larger, fast growing trees appear and may and Function - I block the sunlight and so a new generation of shade -tolerant shrubs emerge below the canopy of trees. Finally there is a general blending with the surrounding community. Such a transition may take well over 100 years, depending on the community for eg, secondary succession on a term in North Carolina Box 17.5.

Box 17.5: Secondary Succession on a farm in North Carolina.

Years Plants Factors that Facts that since help them to Cause their I Plowing ( Colonize

0 (fall) Crabgrass Already present; Die in the shade (annual)* can life in dry soil of daisies

Daisies Tiny fruits; can live Young killed by (annuals and in dry soil decaying roots of diennials)* adults; roots too small to compete with grasses and shrubs for water

Grasses and Tiny fruits; large Young killed by shrubs roots to adult shade and (perennials)* monopolize water from pine and withstand needles temperature allelopathic substances iand shrubs to

Pine trees Monopolize light Young killed by (tall) Mycorshizal shade and toxic root substances from adult needles; roots too shallow to compete with oaks and hickories for water

1 50-1 50 1 Transition from pines to oaks and I I

Young can grow in None - this forest remains: leaves and in oaks in drier shade: adults have places than A I deep roots - * An annual -grows during one season and then reproduces and dies. A biennial grows for twoyears and then reproduces and dies. A perennial grows for more than two years and then reproduces every year for many years. Note: The original forest, before it was cut down, consisted of oak and hickory trees.

In both primary and secondary succession the flora and fauna, of surrounding areas are major factors influencing the types of plants and animal entering the succession through chance dispersal and migration. 1 Ecology and Envimment 17.8.2 Succession in Aquatic Habitat Aquatic habitats also undergo community development or succession although such changes may be held in check by shortages of nutrients (Fig. 17.24). Succession in ponds and lakes take place as a result of eutrophication. Eutrophication means changes brought about by increase in nutrients carried by streams and runoff from the land.

The general trend in fresh water bodies is towards increased eutrophication and thus increased community growth, but the deficiency of any of the essential nutrients may reverse the trend.

As the community development in a fresh water body progresses, the sediments increase and the depth decreases, The shores are crowded by littoral zone plants, which extend further and further into the water body, followed by increasing numbers of water tolerant shore plants (Fig.] 7.24). Unless this progress is interrupted, the water body will be transformed into a marsh and will be invaded by terrestrial plants from surrounding community as a result of which the water body would be lost. Lake Baikal in Russia has shown alarming indications of eutrophication, sadly though not through natural means. The nutrient enrichment of Lake Baikal is anthropogenic due to inflow of nutrients added to runoff water from the surrounding agric~ilture landscapes.

Fig. 17.24: Succession in a pond.

General Characteristics of Succession Communities in succession tend to produce more organic material than they use, while in climax communities equilibrium is attained between net production and utilization. In the early stages of succession the rate of exchange between organisms and the environment is slow, but as the climax stage is reached, nutrients cycling is speeded up and often nutrients cycle directly, through exchange pools, between organisms and the decomposing material. The species composition tend to become highly diverse as the community enters the climax state. As a result a greater number of increasingly specialized niches develop. Sin~ultaneously,some ecologists are of the view that feeding relationships transform from a simple chainlike sequence to an intricate food web. Some evidence also exists that climax communities are more stable than their transitional several stages and less susceptible to external influences. Table 17.2: Traits of Successional Plants. Ecosystem - Structure and Function - I Early Successional Late Successional r-Selected traits K-selected traits Many tiny fruits Few large fruits Reproduce once I Reproduce many times Small body Large body Rapid development Slow development Cannot grow in shade Can grow in shade Large niches Small niches Poor competitors Good competitors -No - herbivore poisons Herbivore poisons

17.9 ECOSYSTEN AND HUMAN USE

As the human population increase the natural ecosystems are replaced with agricultural ecosystem. In several pasts of the world the demand for food for human population is so large that it can be fulfilled only if humans occupy the herbivore trophic level rather than the carnivore trophic level. We have discussed this in section 17.6.4. In less developed countries the primary food is grain. Only in developed countries people can afford to eat meat. This is true from both an energy and monetary point of view. Major parts of Africa, Asia and Latin America have diets that ared deficient in both calories and protein. The main source of food in this diet is food from plants. Incidentally these are the parts of the world where the human population growth is very high. As the scarcity of food arise people move from one place to another destroying the natural ecosystem for agricultural practices. Several , specially the drier grasslands, cannot be used for agriculture but they can still be used as grazing land to raise livestock. Raising the domesticated cattle, sheep and goats diplaces the native species of animals from grasslands. The over grazing sometimes alters the plant community also.

Even aquatic ecosystem has significantly altered by human activity. In coastal areas over fishing of many areas of the ocean has resulted in the loss of some important commercial species.

The man-made lakes or farm ponds often have weed problem because they are shallow and provide ideal conditions for the normal successional processes that lead to fill them.

Thus humans have altered certain ecosystems substantially to increase the productivity of food crop. In doing so, they have destroyed the original ecosystem and replaced it with an agricultural ecosystem.

17.10 ECOSYSTEM AND HUMAN INTERVENTION

As you are aware, humans can and do change natural communities. We are often guilty of accidentally or deliberately altering the complex and myriad factors that maintain the delicate equilibrium of ecosystems. Prime examples are highly artificial communities created by modern agriculture. which has tended to emphasise monoculture -the planting of a single crop species in enormous field or area. These areas of cultivation can be maintained only at the price of constant vigilance and the investment of much energy to reduce insect infestations, diseases, in maintaining soil fertility and taking out or killing the weeds. As you all know that multiple-species crops have greater to pests and disease but this type of farming is difficult and expensive and impractical for large-scale farming.

Efforts to eliminate undesirable species from a community often several hidden linkage to other organisms atid inay dramatically demonstrate the complex Ecology and Environment interactions on which community stability rests. In Borneo State of Malaysia, WHO began a campaign to eradicate malaria carrying mosquitoes where 90 percent of population suffer from this malaria.

The inside of the huts of villagers was sprayed with DDT and Dieldrin, two powerful insecticides. The incidence of malaria dropped precipitously. But soon the villagers began to notice that thatch roofs of their huts were rotting and beginning to collapse. Investigation showed that the deterioration, which occurred roofs of only in huts sprayed with DDT was due to the larvae of a moth that normally lives in small numbers in the thatch roofs. Whereas the thatch-eating moth larvae avoided food sprayed with DDT, the moth's natural enemy, a parasitic wasp, was adversely affected by it. The net result was a substantial increase in the population of the thatch-eating larvae because of the near eradication of the wasp.

That would have been an interesting story in itself, but there was yet another potentially serious side effect. Cockroaches and a small house lizard, the gecko, are two normal inhabitants of the village huts. DDT-contaminated cockroaches were eaten by the geckos, which were in turn eaten by house cats (as were solne cockroaches). The cats, poisoned by the accumulation of the insecticide, died. What ensued was a population explosion of rats, which are potential carriers of such diseases as typhus and plague. In an attempt to restore the population, WHO and the Royal Air Force undertook a remarkable venture, Operation Cat Drop, in which they parachuted cats into the villages. With the cat population restored the rates and the consequent threat of serious disease subsided.

Fortunately, not every pest-control effort entails such complications, Insecticides rapidly broken down in the environment and more selective in their toxicity have been developed and these are less disruptive of biological communities much more preferred approach, however, is biological control e.g. the use of the natural predators, parasites, or pathogen ofthe pest rather than chemical treatments.

Often in order to correct the wrongs of the past intervention we tend to undertake well-intended but uninformed measures - however, our efforts falter or fail because of lack of basic information. All this shows that we have still not learnt to live in harmony with the ecosystems of which we are a part. Our technology has far outpaced our basic knowledge and understanding of the environment. As we turn to the scientific community for answers and solutions, ecologists will play an increasingly important role in changing the ways in which we interact with the natural world.

SAQ 4

i) Why are lichens particularly suited for the pioneer stage of succession on bare rock surface? ii) What is ecological succession, is the species characteristic of the earliest stage of succession (the pioneer community) more likely to become climax community? Prescribe the two important causes of succession. iii) Give the difference between primary and secondary succession. What kind of usually proceed faster and why? iv) Can a climax community changes? Why or why not?

17.1 1 SUMMARY

An ecosystem is a unit of nature, consisting of a community of interacting population as well as aspects of the physical environment. All living (biotic) factors and nonliving (abiotic) factors together interact in an ecosystem. It has an array of producers, consumers, detrivores and decomposer and their environment. The pliysical environment provides heat, light and inorganic materials. Sunlight is the initial energy source for nearly all ecosystems. The primary Ecosystem -Structure and Function - I produces are photoautotrophs and they convert the sun's energy into chemical bond energy of ATP. The heterotrophs include consumers. Herbivores feed upon algae and plants. Carnivores eat animals. Parasite withdraws nutrients from tissues of living host. Decomposers and are heterotrophs too. The major decomposers are fungi and bacteria that have energy and nutrient from organic remains and wastes. Detrivores such as crabs and earthworm ingests bits of dead or decomposing materials. Feeding relationship in an ecosystem are known as trophic levels which is "who eats whom" (a hiearchy of energy transfer in ecosystem). Food chain is the sequence of organism including the producers (autotrophic organisms), primary consumers (herbivores), secondary consumers (Carnivores who eats herbivores), and decomposers through which energy may move in a community. In most of the communities food chains are completely interwined to form a food web. The successive levels of nourishment in the food chains are called trophic levels. A food web consists of interconnected food chains. Modem agriculture and industry have been releasing new chemicals into environment (such as PCB's and DDT). They are persistant chemicals and show bioaccumulation and biomagnification; because they tend to concentrate as each trophic level as they are passed up the food chain upto predators. The flow of energy through the ecosystem is one way. At each successive trophic level there is loss of energy from the system; about 10 percent of the energy at one trophic level is available for the next. Because of this is an ecosystem there are limited trophic levels. Law of thermodynamics governs the flow of energy in an ecosystem. Trophic relationships of an ecosystem can be represented graphically in the form of ecological pyramids. They are of types (i) Pyramid of Members represents the numbers of organisms in each trophic level (ii) Pyramid of biomass represents the total weight of organisms in each trophic level. (iii) Pyramid of energy - depicting the amount of energy utilized at successive trophic levels, Ecosystem is maintained by self-regulatory mechanism, which is called homeostatsis. It is a feed mechanism, which can be negative feedback or positive feedback. In Ecosystem succession occurs when a series of communities replace one another. Each community changes the environment to make conditions favourable for a subsequent community and unfavourable for itself till the climax community is established. The first plants to colonize an area are called pioneer community. The final stage of succession, which is quite stable is called climax community. Primary succession is development of an ecosystem where one did not previously exist. While secondary succession is development of an ecosystem where a previous one has been disturbed.

17.12 TERMINAL QUESTIONS

1. Make a pyramid of numbers using following organism, water plea alga, sticklebacks, and pike (pike are big fishes than stickleback). 2. Keep a vigilant eye and record the human intervention in ecosystem around you. 3. Discuss the trophic level structure of ecosystems and using the term, photoautotroph, chemoautotroph heterotroph, herbivore, carnivore and omnivore make good chains and food web. 4. Describe and discuss the major pathway of energy flow and distinguish between grazing and detrital food webs. Use the terms primary productivity (net and gross), biomass, and . 5. Define succession and describe and distinguish between primary and secondary succession. 6. Make a diagram to shows a number of stages in an ecological succession in a lake.