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Transcript
UNIT 11 ECOSYSTEM STABILITY Structure 11.1 rntroduction Objectives 11.2 Population Ecology : Introduction 11.2.1 Properties of Population 11.2.2 Regulation of Population 11.2.3 Function of Population 11.2.4 Law of Minimum and Maximum 11.2.5 Law of Tolerance 11.3 Community Ecology 11.3.1 Community Orgauization 11.3.2 Interrelationship 11.3.3 Species Diversity 11.3.4 Concept of Homeostasis and Stability 11.4 Ecological Niche 11.5 Ecological Succession 11.5.1 Process of Succession 11.5.2 Nature of Succession 11.5.3 Influence of Environmental Factors on Succession 11.6 11.7 11.8 11.9 11.10 11.11 Ecotone and Edge Effect Concept of Climax Summary Keywords Further Readings Answers to SAQs 11.1 INTRODUCTION In this Unit, we will study what a population is, what are its properties and how it is regulated. We will d s o study how abiotic environmental factors act as limiting factors and determine survival of a population. In community ecology, we will briefly study the organization of a community and the interrelationship of its constituent populations. We will also discuss the meaning of the term species diversity and relationship between species diversity and stability of the ecosystem. We will elaborate upon the key concept in ecology - the ecological niche and try to follow how niche separation is essential for co-existence of different populations. We will also see that diversity of niches are essential to develop a diverse community. Ecological succession, which means replacement of one communily by another in the course of time, is also the subject of discussion here. We will see what is the general pattern of succession, how each community modifies the habitat so that it becomes suitable for future community. The factors that affect succession will also bc considered along with the climax condition of the community. Ecotone or intergradation zone of two communities and its features will also be the topic that will be considered in this unit. As we shall see this zone often has greater diversity than the bordering communities. On the whole, this unit gives brief idea about populations and communities. Earlier we have discussed what is an ecosystem, how abiotic and biotic components together form an ecosystem as well as functions of ecosystem. We will now go further and discuss some aspects of the biotic component of the ecosystems. Ecology Objectives .. . . After studying this unit, you should be able to explain what a community is and how community is organized, define intra- and inter-specific relations within a community, describe the pattern of ecological succession as well as the concept of niche, and assess concept of climax community and stability. 1.2 POPULATION ECOLOGY : INTRODUCTION Generally, population is +fined as an assemblage of individuals of one species occupying a given area, For taxonomists a population is a local aggregation of individuals that differs slightly, but characteristically,from other local aggregates. Geneticists define a population as a reproductive community sharing a common characteristic gene pool. A community is made up of many populations. Populations exist, in fact, because they are parts of communities. Populations are characterized by place and .function in a community. The place of any population is its habitat. Its function is its ecological niche. The properties ~Lpopulationsinclude things like abundance and density, range of distribution, genetic variability, adaptations, resource needs and other demands on the environment. Factors that influence population growth, birth and death rates, etc. are also properties of a population. Population ecplogy is concerned with interrelation between individuals. of the same population as:,well as those of other species. We call these as intra-specific and inter-specific relationships. The interrelations may be of beneficial nature to the participants, then we can call it as cooperation. If the interrelation is harmful we can call it as disoperation. 11.2.1 Properties of Population Having defined what a population is, we will now see some of the properties that are characteristic of each population. a) ' Size: One of the obvious characteristicsof any population is its size. If possible, we can count all the ixidividuals within a population - which is abundance or absolute number. This is not always possible and we have to count only individuals in a limited unit area (or unit volume in case of aquatic animals). This is called as numerical density. We can also measure biomass per unit area or volume and this is known as biomass density. Some populations maintain a constant density over a period of time while others fluctuate widely. These variations are responses of the population to signals from the surrounding environment. The signals may come from abiotic or biotic environment. I b) Fecundity : Plants and animals differ greatly in the number of generations and broods produced per year. Size of the brood is also highly variable. For example planktonic small animals breed every 2 or 3 days while large vertebrates breed only once a year. For a given population fecundity does not vary greatly. c) Innate capacity : The maximum number of offsprings produced is the innate capacity of the species. It depends on the physiological and morphological characteristics of the species. For example, in mammals the size of the uterus and body cavity are important. d) Life table :Species differ widely in the number of young produced per year, in the average age to which these offsprings live and the average rate of mortality. When / this data are available for a given species population, we can prepare life-table. The ' life table tabulates vital statistics of mortality and life expectancy for each age p u p , in the population. I I I I Ecosystenr Stability u p L'LUW. r x nu.. -6" .urv -.-I -I" VUY- w. ..w I... Y ...I V . Y U C V Y Y"vl I"yI"UU"I..." season. Adaptation : This is the most important characleristic. Each population is fully adapted to its environment and is capable of using the available resources. Adaptation is a consequence of evolution and it depends on the genetic variability in a population. g) 11.2.2 Regulation of Population The population of any species may be said to be stabilized or regulated when population fluctuates in restricted manner about a mean. When population encouilters very favourable environmental conditions, the fluctuation may go well above the average size of the population. Soon, however, there will be a tendency to revert back to average level. If we examine the growth curve of a population (Figure 11.1 (a)), we will find that population grows slowly at first bul soon accelerates its growth. After this also population continues lo grow but at a decelerating rale and becomes stabilized a1 the upper asymptote. Under natural conditions one rarely finds such a clearcut sigmoid curve, because growth of animal population is subject. to many variable factors. Figure 11.1 (a) represents a cyclic and periodic fluctuation when the amplitude of population size remains constant (cyclic) and time inlervals, between increases and decreases, are constant. 'A' represents the positive acceleralion phase, 'B' the logarithmic growth phase, 'C' the negative acceleration phase, and 'D'the upper asymptote. Dotted line is the actual growth curve of a hypothetical populalion. L Time Figure 11.1 r (a) Growth Cuwe of a Populatior~ Figure 11.1 (b) is a cyclic and non-periodic fluctuation - the amplitude is constant, but time intervals of fluctuation vary. Figure 11.1 (c) represents an oscillatory, periodic fluctuation where the amplitude varies, but time intervals of population change do not vary. Figure 11.1 (d) represents an oscillatory and non-periodic fluctuation. The figure also indicates that population size is in a constant change. There will be always some variation in number over a period of time. Though not shown here, there may even be 'explosion' in numbers. Ecology I I T ime Figure 11.1 : (b) P n i t e r ~ ~ofs Population Eluctuations After the Negative Acceleration Plrase The processes or factors that affect population are broadly divided into two groups. One group includcs all biotic factors and the other group includes all abiotic Ficlors. Biotic faclors are also called as density-dependent factors while abiotic factors arc densityindepcndent factors. Density dependent factors vary in the intensity of their actton with the sizc (or densily) or the population. Competition for space or food, for example, will be intense in rl very . dense population. Density even affects rate of reproduction. Predation, disease, etc., are also densily dependent. An infectious disease, for example, will rapidly spread in a very dense population. Dispersal and emigration of individuals, which affect populations, also depend o n the density. I Density-indepcudent l'aclors are abiotic factors. Variation in available space, presellcc or absence of' a suitable cover (or refuge), availability of suilable food; favourablc weather, etc, are all density-independent factors. Sudden change in weather, for cxample, may wash out major proportion of a population irrespective of the size (densily) of U1e population. Drying up of a pond will kill all tadpoles oC the frog irrespcclive of Ulc density. When most other conditions are favourable, food often sets an upper limil to Ule size of population. In general, we find that density-independent factors set the ultimate limit or carrying capacity which cannot be permanently exceeded. The density-dependent factors, on the other hand, generally stabilize populations at or somewhat below lhat ultimate level. 11.2.3 Function of Population The function of each population in a community depends upon its position in a trophic hierarchy. Producer populations produce food using solar energy (photosynthetic autotrophs). Some use chemical energy to do so (chemosynthetic autotrophs). Consumers feed on autotrophs and we call them as herbivores. Carnivorous animals feed on herbivorous (and other) animals. A large and diverse community has many producers and consumers. A'very complex food chain is the result in such case. Aput from providing food, one population may provide shelter to other population, may help propagate other population, may help dispersal of other population and do such 22 1 I other functions. As we shall see in ecological succession, each old population paves the way for new populations by modifying the habitat. Role of each population is, thus, unique and we often call it as ecological niche. Scavenger animals, like vultures, utilise dead animals as energy source. They fill up a very important ecological niche. Without them a large amount of chemical energy will not be efficiently harnessed. Decomposer populations (fungi and bacterh) also break down dead and decaying organisms and recycle the minerals. Thus, different populations carry out different functions in a community. To understand fully the function of any one population, one must study the ecology of the species in detail. Such a study of single species is autecology. The study that involves entire community is called as synecology. 11.2.4 . Law of the Minimum and Maximum An organism and its surrounding environment cannot be considered in isolation. All the abiotic factors are continuously affecting the functioning of the living systems. All living systems consist of certain number of chemical elements that are essential for building the biological molecules. There are 40 such chemical elements found in living organisms. All these have precise and significant role to play in the functioning of the organisms. The Law of the Minimum, as proposed by Justus von Liebig'in 1840,states that the growth of a plant is possible as long as all the essential elements are present in sufficient quantities in the soil. The &wth will be controlled by the essential element that is present in Ule lowest concentration. Growth ceases if an essential element is in insufficient concentration. Just as the particular element can be in low concentration, it can also be very high in concentration. In fact most environmental factors vary through a wide range. There can be a minimum and maximum'ternperature or rainfall and such other factors. If an environmental factor essential for life is absent or depleted below the critical minimum, or if the factor exceeds the maximum tolerable level 'for the species, it becomes a liMting factor. For example, the rate of photosynthesis is dependent upon : (a) amount of C02 presedt, (b) amount of water available, (c) intensity of solar radiation, (d) amount of chlorophyll present, and (e) temperature of the chloroplast. Even if one of the factors is insufficient it may stop photosynthesis+even when other factors are in abundance. In reality thus, biological functions depend on simultaneous action of several factors, though some factors exert more influence than others. 11.2.5 Law of Tolerance Since Liebig's time, many biologists showed that a number of other environmental factors (i.e. other than chemical elements proposed by Liebig) are able to act as limiting factors with respect to organisms. Temperature, humidity, amount of light, etc. have been shown to play an important role. In 1913, V.E. Shelford expanded Liebig's observations to include the maximal limits of an environmental factor. Shelford's law of tolerance (Figure 11.2) states that the value (of environmental factor) below a critical minimum or above a critical maximum would exclude certain organisms from that environmental area. Any value lying between these two critical limits falls within the tol~rancelimits of the organism. In Figure 11.2, it is pointed out that animals are abundant within the range of optimum. This range lies within the upper and lower limit of tolerance. If these limits of tolerance are exceeded for a certain species, the species will not survive there. I Ecology Lower limit of tolerance Upper limit of tolerance RANGE OF OPTIMUM High - C .-0 + (d 4 2 a 0 CL Low = GRADIENT r.High Figure 11.2 : Diagrammatic Repraentation of Shelford's Law of Tolernnce Every environmental factor varies through a wider range of intensity than any single organism can tolerate. Each animal has therefore, an upper and a lower limit for every limiting factor that it can tolerate. Between these two extremes alone the animal can function efficiently. It is not always easy to determine these limits, however. It is also known now that a plant or animal may have a wide range of tolerance for one factor in the environment, but a comparatively very narrow range of tolerance for some other factor. Some freshwater fishes, for example, tolerate wide variation in temperature but d o not tolerate much change in the salinity. Some animals even vary in their limits of tolerance to the same factor at different times. The fish Atlantic salmon lives in sea as an adult but goes to rivers for breeding. Ofcourse, it has to undergo considerable physiological adjustments to do so. In the case of most other marine fishes this is not possible. Similarly, freshwater fish dies instantly in matine waters. t Prefix 'steno' is used to indicate narrow range of tolerance for a particular factor, while prefix eury indicates wide range of tolerance. Thus, stenohaline fish means that the fish has a narrow range of tolerance for salinity. Euryhaliine fish, on the other hand, can tolerate wide range of salinity. Similarly terms stenothermal or eurythermal indicate temperature tolerance. In the end we must also remember that the range of tolerance for a particular factor in any given organism depends on genetic makeup, natural selection and adaptation. SAQ 1 (a) Define the t e r n population and con~mtkit~. (b) Distinguish between cooperation arad disoperalioal. (c) Define innate capacity of a population. (d) Name a few density independelat factom hat can regulate population. 1313 CO ITY ECOLOGY We have already seen that a population is a monospecific assemblage of living organism occupying a defined area. A group of several or many species living together in the same locality is a community. A community (also lmown as biocenose) is a distinct ecological unit. This unit may be defined in t e r n of fauna (animals) or flora (plants) or both. Usually we speak of fauna of say Weslern Ghats or fauna of India. On a larger scale, we can speak of larger units like fauna of Oriental region or fauna of Ethiopian region, etc. But, whenever we consider larger units, the habitats present in a vast area are often significantly different. This difference is immediately apparent in plantlanimal communities. In general, wherever thgre is a great degree of habitat diversity, the community is also very diverse. Uniform habitats like ice-covered polar areas, do not support diverse community. A biotic community along with its habitat is called an ecosystem. Each biotic community is characterized by dynamic interaction among its constituent populations. In fact, populations do not have separate existence; populations exist becayse they are parts of communities. Community ecology is concerned with all kinds of interactions among populations. It studies ranges of various populations, the habitats occupied by populations, the food chains and patterns of energy flow, ecological niches, etc. The subject is complex and vast. We will only broadly consider some aspects of community, such as community structure (or organization), species diversity and community stability, etc. 11.3.1 Community Organization In a large community the different constituent populations occupy different ecological niches - that is they play different roles. Some species, however, may have importan1 control over other species. Such species are called as dominant species. In a thick eucalyptus forest, for example, if you remove some small shrubs completely from an area there may be no significant effect on the community. The role of that shrub will soon be taken over by other shrub. On the other hand, if you remove large number of eucalyptus trees, it will have very significant effect on the ecosystem. Same thing is true in our Sal forests. The eucalyptus tree then becomes a dominant species. A community is often thus named after its dominant species. A community may have one (or sometimes more than one) species that is dominant. A dominant species may not always be pumerically abundant. In a tropical forest insects are abundant but dominant species are usually trees, In fact, in most terrestrial communities, plants are dominant. Dominant species usually have maximum impact on the total community. Dominants usually have greater productivity and greater biomass than others. Dominant species often shoulder the full impact of climate and modify the climate to the benefit of other species. In a forest community trees that are dominant affect light intensily reaching ground, intercept rain, monopolize moisture and nutrients in soil, decrease wind velocity and provide shelter to other plants and animals. Despite all that is said above, it is riot an easy malter to identify dominant species in an ecosystem The roles of many species may be subtle, yet these species may have great effect on ecosystem. Comnunity organization is not fixed. There is a definite degree of periodicity with which the components of a community undergo significant change. In a large deep lake, for example, the planktonic community will be different in early hours lhan in afternoon, due to thermal stratification. Seasonal variation is evident in most communities, but it is especially prominent in temperate regions. In North America, for example, early springnate spring, early summer/late summer, autumnal, and winter comnlunities are recognizably different in the same locality. Even during 24 hours we will notice that day time active community consists of different animals than the night-active community. In subsequent sections, we will study about communities which change with time - one community replacing other. As we will discuss in the following subsection species diversity is also 'one aspect of community organization. This is so because interaction among species (Interspecific) and interaction among individuals of the same species (Intraspecific) are integral part of the Ecosystcrn Stability I Ecology community organization. The complex food webs and resulting patterns of energy flow between various trophic levels are specific for each community. Diverse communities naturally possess much complex organization as compared to simple, less diverse communities. All the major communities, however, possess producers (that produce food) and consumers (that eat the food). Consumers may be directly feeding on plants (herbivores) or other animals (canrivores). Decomposers take care of excreta, wastes and dead organisms and are an integral part of the community. Which producers, consumers or decomposers shall be there depends on the habitat and available niches. The whole community organization is a product of evolution. Various evolutionary forces acting over a prolonged period and various stages of succession ultimately establish a climax community and its organization. In between stages are often made u p of rather transient communities with rather simple organization. As we shall see separately, niche separation and resource partitioning are equally important in deciding h e structure of a community. Lastly, environmental variables or abiotic factors, also affect and decide what kind of community can develop and sustain itself in any given place. In fact, the whole organization of community depends on abiotic and biotic factors operating over time. In any given habitat there are variations of different environmental features, especially topography, climate, and substrate. Often these features change gradually (i.e. along a gradient). Topography is probably one of the most fundamental aspect of enviroiunent which can cause community structure to change. Minor topographic changes can cause local variation in distribution of species but continued topographic variation, as seen on high mountains, can cause complete community changes. As one ascends a mountain, the temperature drops, precipitation changes, wind action changes, etc. This is immediately apparent in communities. At each height regime the community is entirely different (Figure 11.3). Note from the figure that vegetational zones vary in altitudinal extept and location, depending on whether it is located on a north-facing or south-facing slope. , -.. -- __--..- Deciduous Forost Tropical Rain' Forest I Deciduous Forest I Rain Fore& Line Figure 11.3 r Comrndty Change rt Dldtennt Height on r Motmtain (EkvaUor~h A uc in fee&) 11.3.2 Interrelationship This is the most complex, yet interesting, issue of community ecology. As we have said earlier, populations &st because they are part of communities. Each population is linked to some other population in one or the other way. The interrelationship is of two types. Intraspecific relations are the relations between individuals of the same species while interspecific relations are between two different populations (that is between different species). These relations are of a variety of types and we will only briefly summarise some of the important ones. a) Mutualism is an association between two or more species in which all derive benefit in some way. This association may be facultative, in that both the parties are capable of independent exislence or obligatory, in which case the relationship is imperative to one or both the species... Association of algae and fungi to form lichens is an often quoted example of mutualism. I i b) Commensalism is the coaction in which two or more species are mulually associated in activities centering on food and at least one species derives benefit. The other may neither be benefited nor harmed. The commensals may be external or internal. A remora fish attached on the underside of a large fish gets mobility and food; the large fish receives nothing and is not harmed. Many small protozoa attach themselves to aquatic plants or other animals like crabs. Protozoa derive benefit while the plmts/crabs are no1 affecled. c) Parasitism is the type of relation in which a parasite derives benefit at the expense of the host. A parasite generally does not kill its host. It does however cause tolerable harm to its host. The parasites may be external or internal, Tapeworms are internal parasites while ticks, mites and fleas are external parasites. Another kind of parasitism is seen in birds like Indian koel which is a nest parasite of crow in Ulat it lays its eggs in the nests of crow. d) Competition refers to the interaction in which two individuals or two species struggle for limited amount of a resource (food, water, nesting space, etc.). As a rule when such competition occurs, the species thal is more efficient in using the resourc2 will stay, other will be eliminated, It is true that if two populations have identical resource requiremenls then the populations cannot coexist. In real communities therefore there is always certain amount of difference in resource requirements of various populations. Competition is usually keenest between individuals of the same species (intraspecific competition) because they have identical requirements for food, shelter, mate, elc. Besides, individuals of the same species are structurally and functionally equal. Interspecific competition is also severe if the two species !lave similar or overlapping resource requirement. Physical combat, antagonism and sbuggle are kinds of direct competition. Efficient utilization of a resource, so as to monopolize the resource, is indirect competition. A bird defending a nesting site and territory is direct competition while a large plant (with well developed root system) utilizing all the available water is indirect competition. e) Prey-predator relationship is perhaps the most complicated. A single species may have many predators and a single predator may feed upon several prey species. It is a relationship in which one animal (prey) is removed from the ecosystem by the other (predator). A lion feeding on a deer or a frog feeding on an insect are the examples of this relationship. Prey-predator relationship is believed to be useful in population control. f) Allelopathy is a relationship in which one plant produces a toxin that affects growth, health or behaviour of other orgipism, Volatile toxins produced by plant like Salvia prevent other plants from growing nearby, Antibiotics produced by bacteria also come under this category. Apart from the above mentioned types of interactions there are many more obvious or subtle kinds of interactions that are commonly observed in the communities. Lack of space prevents us from dwelling anymore on this topic. Ecosystem Stability , . Ecology 11.3.3 Species Diversity It is cornmon knowledge that communities differ from each other in number of species they contain. If one community contains twenty species and the other contains ten species then, generally, the community with twenty species may be called as more diverse than the other. In ecology, the number of species present in the community is termed as species richness. Sometimes this is also called as species abundance. Species richness can be expressed as the number of species in a sample or habitat. There are also mathematical indices to express species richness. This can be a simple measure of biodiversity of a community. Species diversity, on the other hand, considers both the number of species present and the number of individuals of each species (i.e. their relative abundance). This is a better measure sirice it allows us to compare various communities more meaningfully. The degree to which different species populations approach equality in their relative abundance is called as equitability. In no community are all species equally abundant. In recent times species diversity is calculated using vlarious mathematical formulae. One such formula is tliat given by Pielou (1966). s H; = - pi log pi I where, s = total number of species in a sample pi = the fraction which the number of individuals of one species (i) is to the total number of individuals = relative abundance of ith species ) Suppose we have 100 individuals in a given community and 10 differenl species. Now if all the ten species are represented by ten individuals each then the species diversity (H;) will be 2.30. But if there are 10 species with the following number of individuals per species, say 45,'25, 15, 8, 2, 1, 1, 1,1, 1, then the diversity will be 1.5 Thus, even though the number of species present is the same, the species diversity calculated using their relative abundance is different. The community of more equitable relative abundance has higher diversity. In our example, community with 10 species with 10 individuals each has more even distribution of individuals (greater equitability) hence, it is more diverse. Two equally rich communities will thus have different species diversities. In most communities we often find that few species are represented by large number of individuals and a large number of species are represented by few individuals each. Some species may be very rare. It must be again poihted out here that species diversity is also not constant in a given community. Temporal changes in species diversity do occur. Also the changes in one or more abiotic factors can bring about change in the species diversity. Neilher it is true that higher species, diversity is better than lower species diversity. Some communities have naturally low species diversity. , 11.3.4 Concept of Homeostasis and Stability In general, mature or stable communities are more complex, contain many more species, are resistant to invasion by new species, possess very complex food chains, possess low, productivity to biomass ratio and maintain constant size populations, As we shall see later, community stability increases with each step of ecological succession and it is i maximum in climax community. Such communities are quite resilient against disturbance, so that they are not easily destroyed. 'lhls is homeostasis. The constituent , populations also maintain their' uniformity over time - although this kind of uniformity I and stability is not infinite. I ; To sustain high degree of species richness and species diversity, the environment must possess resources. Niche diversity is essential and the species must have attained greater efficiency in their utilization of resources. Progression towards stability depends not only on the increase in species richness but also on higher equitability of species populations. In general, stable communities possess specialist species which live together without competition. Such specialist species are often more efficient in using their resources. This is essential for stability. Ecosystems with nearly constant environmental conditions (such as those of equatorial rain forests).tend to have communities with high species diversity. The constantness (and hence high predictability) of environment allows high degree of specialization within the community. Populations living in unpredictable environments must be made of generalists than specialists. Overall productivity of the environment is an equally important factor. An environment with stable conditions will not support many species if the productivity is very low, In recent years there is considerable discussion going on regarding the relation between diversity and stability of an ecosystem. Most feel that diverse communities are stable. Our monocultural agriculture is not stable. Such monocultures are more vulnerable to attack by pests. Similarly island communities with low diversity often fluctuate widely than the mainlilnd communities. Diverse communities thus appear to be stable and diversity has ol'tcn been assumed to buffer the community against change. A diverse community is bclieved to be characterized by a very high number of niche interactions, many of which are regulatory in nature. A simple community with few regulatory interactions is less well buffered against change. Most ecologists today feel that Ulis is not always true. Many scientists, in fact, are convinced that the correlation of diversity and stability has been discredited. The main problem is that the word 'stability' is not properly defined. Different people use the word differently. We need not go into details of this controversy but remember that no community that is perturbed beyond its normal limits can survive. l h e specialist species of diverse ecosystem cannot tolerate major perturbation and such systems can collapse. (a) Elaborae~:tiow a ii2onlia1ant tree species can affect bi)ncst i:olull\ur;'ir3ir (b) Enlist iii~corsthat detern~necorrununity stams:turc. (d) Disiiik?{:disffn butweell illtraspccific ;uld intcrspccifil; corllpc:Litic~al. (e) l)isOinag~liislabr:llwceal species richraess and sptcic!; diversity. 11.4 ECOLOGICAL NICHE The key concept in community ecology is the ecological niche. It expresses a population's role in a community and in the ecosystem. It is based on the bonds between the given population and all other populations with which it interacts, directly or indirectly. It also takes into account the bonds between the population and its inorganic (abiotic) environment. Ecology Just as the place of any population is its habitat, the function of a population is its ecological niche. since the function of a population depends on the position of a population in a community and the habitat occupied by that population, the definition of ecological niche includes both these factors. Ecological niche is often defined as the particular position in a community and habitat occupied by a given population as a result of its peculiar structural adaptations, its physiological adaptations and the special behaviour patterns that have evolved to make best use of these potentialities. Each species has its own peculiar niche. No two species can permanently occupy exactly the same niche. There will be obviously severe competition. Many populations can coexist in a community simply because each population has its own niche requirements. . To occupy a particular niche the species must be fully adapted to the ecological conditions around and the other species that surround it. It is the evolutionary process that moulds the individual species so that it is fully adapted to survive in specific environment and to live together in particular relations with the other species. The particular combination of physical (abiotic) factors required by a species is often called as a microhabitat. m e habitat of a species may be broad, perhaps covering thousands of square kilometers or it may be small, restricted place: Microhabitat on the other hand is a subdivision of the.habitat. Microhabitat is the specialized requirement within the total habitat. The specific environmental variables UI the microhabitat are referred to as the microenvironment or the microclimate. For example, if there is a fungus decomposing the decaying vegetation in a tropical forest, its habitat is tropical forest. But the fungus is specialized to feed only on a certain species of vegetation, under certain conditions of temperature, moisture and light. These latter aspects are then the rnicrohabitat and microclimate. . In general if there are diverse kinds of habitats within a given area, there are diverse ecological niches available for colonization. This situation ultimately gives rise to a diverse community. Evolutionary processes like natural selection act in such a manner that the're is usually no niche overlap. During development of a community there may be some degree of niche overlap but in the long run natural selection will tend to redu2e overlap and increase niche separation. This will minimize the competition for resources. As we shall see, this can be achieved in a number of ways. For example, a given stream may have many species of fish. If we look into their microhabitat and their role we will understand that there is no niche overlap. There will . be some fishes that cling to rocks in swift currents and feed on vegetation that grows on such rocks. Some other species may be occupying similar rocks but is carnivorous, some species will be purely pelagic, feeding at the surface, while some other species may prefer aquatic vegetation and stay there to feed. Thus, although there are many species, each occupies its own niche. If two or more species are competing for the same resource, the comptition can be eliminated if one species feeds during day time and the other feeds during night time. Time then becomes a dimension in which species of the same community m y differ in niche. There are many other aspects like behaviour patterns of a species, requirements of food and shelter, etc., that determine what niche it can occupy. In short ecological niche of a species is defined by the features of substratum (rnicrohabitat) and microclimate to which that species is fully adapted, the time of the day ahd the season of the year when the species is active, the type of shelter it requires, the type of food it consumes and the predators that prey on it. Interspecific competition leads to segregation of hiches. Occupancy of different niches reduces interspecific competition, furnishes the species with a microhabitat to which it is adapted, reduces confusion and disturbance, and permits a greater variety of species to occur together without severe competition. I 11.5 ECOLOGICAL SUCCESSION The communities are not static but dynamic entities. The communities and ecosystems change with time. The replacement of one community by another is called as ecological succession. This is a continuous process, at least until a final stage or climax is reached. Even when a climax stage is reached, it is doubtful if the community always keeps its composition unchanged. Emlogical succession is a process. It involves a series of steps or communities - each being called as a sere. The seres are sometimes classified according to the dominant force that is responsible for a particular sere. The forces are biotic, climatic, physiographic md geological. Thus, if dominant force is biotic Ulen we call the sere as biosere. In this section we will consider only biotic succession involving bioseres. 11.5.1 Process of Succession We can recognize two general types of succession. One type is called primary succession because the living organisms establish themselves for the first time in that habitat. Prior to that the habitat or subskatum was life-less or sterile. Recently cooled lava or sand provide such a substratum The community that establishes itself for the first time is to be called as pioneer community. This pioneer community modifies the subtratum and then the secondary communities come in to replace the pioneers. Each community will exist for some time and then it will be replaced. Each such community is a seral stage in succession. The second type of successio~lis known as secondary succession. This differs from the first in the fact that the substratum was occupied previously by a living community. Some catastropllic event like flood causes the community to disappear and then new organisms start colonizing that area. Whether it is primary or secondaj succession, the irmount of water present will always govern ttie entire character-of succession pattern. We can recognize three types of succession depending upon water content : Hydrarch succession - where water is plentiful, a) b) Xerarch succession - where water is very scarce, and c) Mesarch succession - where water availability is intermediatebetween that of hydrarch and xerarch. Modern ecologists recognize two kinds of succession depending on whether the successional event is largely due to community itself or due to some external input. Whcn the community itself modifies the physical environment to bring about succession we call it as autogenic succession, When factors like nutrients coming in from outside changc the community hen we can call it as allogenic succession. Conversion of a bare rocky substratum to a forest is largely autogenic while a pond becoming marsh or wetland is largely allogenic k i d of succession. Thus succession is change from one community to other. This is brougbL about by thc activities of plants and animals themselves. The most important of Uicse activities are the interactions of various organisms that modify the habitat. When Ulc hahilat is modified, new organisms invade the area, and, if they are successful, they replacc previous organisms. This kind of biotic succession is usually rapid and appears to reach a final, permanent stage in a few decades. This is the so called climiyr stage. It is believed that in the climax community, all species are continually able to reproduce successfully and there is no evidencc that new species are invading. Preclimnx or seral community on the other hand is often unstable and hence replaceable community. 11.5.2 Nature of Succession To understand fully what is succession, we must now consider an example of primary succession. It is taking place on a bare, sterile rock. Let us see how different forms of life grow here and modify the environment (habitat). Ecosystem Stability The bare rock will be first occupied by lichens. Among lichens also the pioneers mstose type of lichens which are later replaced by fruiticose type lichens. These begin the long and slow process of mineral decomposition. Carbon dioxide secret the rhizoids forms a weak acid that dissolves the rock. l'he rhizoids also penetrate rock weakening its structure. These plants also trap wind blown dust and detritus. the lichens die, there is further addition of organic matter to the trapped soil. The environment is now sufficiently altered from that of the bare rock. It allows a organisms to settle. The next ones to come are the mosses of different type and fex The lichens are gradually replaced. Animals like mites and spiders also begin to oc this place. Some smaller insects like springtails join and a small community is established. As the mats of moss becomes dense and extensive, more soil accumula This soil usually comes as wind blown dust. More mineral material is added by the action of vegetation and the layer of the soil becomes thick. Once soil is formed, the mosses will be replaced by weeds and grasses. The grass community and shrubs fiuther spread their roots and breakdown the,rock. ?be anim like nematodes, larval insects, ants, spiders become established. Grasses make the sc rich and fertile for seedlings of larger trees. Once these seedlings are established, lar trees that grow totally alter the picture and, if climate is suitable, a small wooded are will develop. This wood will be the climax community. Provided the climatic conditions do not change drastically and no catastrophic event occurs, the community will sustain itself indefinitely. Such climax communities are found in virgin forests. The climax can maintain itself, since the conditions that exist within the area are ideal for continual reproduction, nourishment and normal biological activities of the species involved. A small pond or a lake formed newly will undergo succession in a similar manner - 01 the communities will be different. In the lakes there is continuous input of silt. This eventually makes the lake so shallow that the entire plant - animal communities change. The lake become a marsh, a marsh becomes a marsh thicket and ultimately a climax forest. The anim species associated with each sere will also be different. J Large Lake Low Praire -Climax prcurle Mars Thicket -Climax (If water is plenty) Forest Figure 11.4 : Steps in Succession from a Lake to Foreat Conununity Ecological succession is thus, continuous community change. It is charateristic of all ecosystems., In general the pioneers are generalist species with wide ecological niches and are able to withstand greater fluctuations in their environment. The overall primary production and biomass is low but ratio of production to biomass is high. Diversity of pioneer community is also very low so the food webs are poorly developed. Interspecific interactions are naturally minimal since there are only a few kinds of animals. This pioneer community is not well organized but it can still survive because no other community can withstand the conditions. Figure 11.5 shows progression of community parameters through succession iollowing a catastrophic event, such as, ftte in a terrestrial environment. Biomass increases gradually at the beginning, when the community is composed of herbs and shrubs, then rises rapidly as the trees grow. Species diversity rises then falls slightly as all the pioneer species are eliminated. The ratio of primary production/biomass reaches its maximum during early stages of succession then drops as the large trees with high biomass dominate. Biornps s Production Species Diversity Product ion / 'Biomass Ratio I i 1a1' oil ut Pioneer Early Forest [ti Fire - Time -+ Mature Farest t Climax Figure 11.5 On the other hand, if we look at the climax community, it is very diverse. There are complex food webs where energy is transferred very efficiently. Biomass increases considerably so that productivity to biomass ratio falls. The species in such community are often specialists, occupying rather narrow ecological niches. 11.5.3 Influence of Environmental Factors on Succession All abiotic factors have signiiicant elfect on succession. As remarked earlier, moisture plays a very important role in determining whal kind of succession will follow. Aside from Ule moisture factor, the soil type (clay, sand, silt etc.) will also influence Lhe course of succession and what type o i climax vegetation will develop. These are what we call as edaphic factors. Topogrdphy, that is, general contour of the land area will also affect Ule vegelalioll type. Succession in Creshwaler areas will vary in rate and pattern, depending on Ule extent of the aquatic area and the net rate of water movement. In a lake the succession of communities will also depcnd on the inpul of nutrients (energy) by incoming streams. The excess growth of planktonic forms and algae that takes place in lakes is called as eutrophichtion. This is entirely due to excess nutrients (such as phosphates added by detergents). Eutrophication is a natural process but activities of llum,ul society are accelerating the process. Modem agricultural practices also cause increased erosion of land and corresponding siltation of the lakes. Like energy, the amount of silt coming in is .important in succession. In addition to above, factors like dissolved oxygen, dissolved solids, hydrogen ion concentration and lemperature are also important abiotic factors that can aifect succession in freshwater environments. In riverine environments, where water is flowing, the successional pattern is quite variable. The speed of the current, the depth of the stream, the type of substrate and the general topography of the area - all these factors will cause variation in successional pattern. Overall climate is yet another factor that influences succession. Regional climates are not constant, though the change takes place over a considerable period of time. The fossil record of communities, however, provides us the necessary information about the communities that existed before. Ecosystem Stability So several factors are important as far as ecological succession is concerned. The plmts and animals that occupy a given sera1 stage are also hteracting with each other and are affected by evolutionary forces (like natural selection). These things also influence the course of successiol~although we will not consider them here. 6 ECOTONE AND EDGE EFFECT In terrestrial communities, the vast array of climatic and physical factors become more adverse as we approach the outer limits of that community.Once this limit is reached then many organisms fmd it difficult to inhabit that area. Here then we find a different community. In nature we do not generally find a sharp line indicating beginning of one community and the end of another. Instead there is a Zone bf transition, a zone of tension where conditions for each of the adjacent communities become more adverse. This area is then often occupied by a mixture of species from both ~0n'u'mnitie~. Such a zone or region is called as an ecotone or tension zone. This area may be narrow or large depending upon the physical factors. If the physical factors change abruptly, the ecotone will be narrow. Communities like marsh communities often abruptly change into other terrestrial comunity due to change in water contents and nature of the soil. A general characteristic of the ecotone is that very often there are a greater number of species. 'lhe'&nsity of some of the species may be highest as compared to the neighbguring communities. The outer edges of forests, for example, often show abundance of game species of interest to man. 'Ihis phenomenon is therefore called as edge effect. The reason for the greater number of species found 4 ecotone is that there are tolerant species from both the bordering communities plus the.species that are characteristic of the tension zone itself. This means that transition zone has its own potential to provide habitat for species that do not exist in either communities. According to some authors this potential of the transition zone is the edge effect. Estuary, which is an intermediate zone between riverine community on one side and marine community on the other side, is also considered as a highly productive ecotone. The existence of ecotone depends on both the communities that border it. If one or both communities are modified (either by man or nature) the ecotone may disappear. Alternately, it may undergo considerable modification. 1.7 CONCEPT OF CLIMAX In the section on ecological succession, we learned that the climax community is the last aggregation in the successional series. This community sustains itself if climatic h d other factors do not disrupt it. In recent years ecologists have been studying various ecosystems in detail and have pointed out some important facts in this regard. The fact that climax community represents equilibrium and that the end point of succession is an ecosystem with stable configuration, is not really the case in most ecosystems. In a forest, for example, when a tree species forming canopy dies, its place may be taken by some other tree species. There are certain so called climax communities (red wood forests for example) in which the tree species tends to replace itself because juveniles of these species cannot grow beneath the adults of the same species. It is thus accepted now that it is extremely difficultto find a stable climax stage in nature. It is considered so because during the course of our studies these communities do not show appreciable change, Factors like topography, &iin, soil variation, etc, can lead to significant variations in the clhax stage. The climax is thus not an immutable end point. ; In the words of Connell and Slayter: "we have found no example of a con'nunit~of sexually reproducing individuals in which it has been demonstrated that the average I I I I I , I , I species composition has reached a steady state. Until this is demonstrated, we conclude that, in general, succession never stops" sAQ 3 (a) Fbrcpaul.a= a shod note on ecologicd niche. (b) DistingmisBa betwec~lprimary and secondary successiana (c) Disthaguish between a sead wcorat~laitya11d clirnax c c c s m ~ ~ ~ t y . (d) Elaborate the abiotic factors that will affect dB~epattern of succession in ;a riverme conamunity. (e) Explaiaa why ecotone lamray Baave more diverse comwumity. Population is a monospecific assemblage of individuals in a given area. Populations possess certain properties like density, fecundity, innate capacity, age ratio, adaptation, etc. Populalions are regulated by density dependent as well as density independent factors. Each population has its own function - a role - in a community. Community is an aggregation of many differenl populations. All animals and plants are affected by biotic and abiotic factors. They often grow well under optimum conditions and are absent from areas where the conditions are beyond tolerance. Community or biocenose is an ecological unit, Communities differ from place to place. No two communities are exaclly alike. Community organization depends on abiotic environment as well as the domindantspecies present, species richness,and diversity, habitat diversity (heterogeneity) and slage of succession. Ecological succession is change in cotnmunities over time. It involves replacement of one community with the other. It is a natural process wherein the pioncer species modify the abiotic environment and make il suitable for other species that invade. The final stage of succession is called as climax. Generally climax stage is called as a stable stage. Climax stage is also a more c'omplex and diverse community thari the previous communilies. Ecotone is an area of intergradation of two communities. It may be a large or small area, where some species from both the conlrnunities are found in additional to those Ulnt are native to this zone. Interspecific and intraspecific relationships form an integral part of each cornrnunily. Predation, parasitism, commensalism, etc have profound effect on community structure and function. 11.9 KEY WORDS Productivity : Total mass of organic food thal can be manufactured in a parlicular area for a cerlain period. It is the nel yield of producers and consumers. Respiration : The process in which food matter is 'burned' in the cells to obtain energy slored in tl~echemical bonds. Biomass : It is the total quantity of living matter (often expressed as dry weight) present at a given time for particular area. Say biomass of iish in a given tank. Ecosystem Stability Ecology Species : In simple terms it is equivalent to kinds of organisms. Lion, elephant, neem tree, etc. are kinds of orgapisms. Each is a different species. Resource : It is a useful commodity. Water, minerals, food organisms, etc. are resources, 11.10 FURTHER WADINGS (1) Dash, M.C. (1993), Fundamentals of Ecology, Tata McGraw-Hill Publishing Co., New Delhi. (2) Clapham, W.B., Jt (1983), Natural Ecosystem, 11Edition, Macmillan Publishing Company, New York. (3) Benton, A.H. and Werner, W.E.(1976), Field Biology and Ecology. 111Edition, Tata McGraw-Hill Publishing Company Ltd, New Delhi. (4) Kendeigh, S.C. (1974). Ecology - with Special Reference to Animals and Man, Prentice-Hall of India Private Ltd. (1980 reprint). (5) Knight, C.B. (1965), Basic Concepts of Ecology, The Macmillan Company, New York. 11.11 ANSWERS TO SAQs SAQ 1 (a) (b) (c) (d) A population is an assemblage of individuals of one species in a given area. A community, on the other hand, is an assemblage of many populations in a given area. Beneficial interaction between two species is cooperation, while harnIful interaction is disoperation. Innate capacity of a population is the maximum number of offsprings that can be produced by a given species. ~ o l l o $ i ndensity ~ independent factors can regulate population (i) food, (ii) availability of space, (iii) availability of shelter or cover, and (iv) in general all abiotic factors. SAQ 2 (a) A Dominant species can : (i) have maximum impact on the ecosystem (ii) modify the climatic and other abiotic conditions for other species, especially if dominant species is a tree in a forest monopolize moisture and nutrients in soil (iii) (iv) provide shelter to other plants and animals. (b) Factors that affect community structure are : species richness and species diversity (i) (ii) interrelationshipbetween populations (iii) season or climate and such other abiotic factors (iv) evolutionary processes. ( c ) Parasitism is the term used to define the interaction in which one species, derives benefit at the cost of other species. Allelopathy is production of inhibitoryltoxic substances that affect growth, health or behaviour of Vle other species, (d) Competition between the individuals of one species is intraspecific competition, while that between two different species is interspecific competition. (e) Species richness only denotes total number of species in a give11sample or area. Species diversity takes into consideration, number of individuals of each species along with the number of different species present. SAQ 3 (a) Answer can be prepared from the preceding text. (b) When living organisms establish themselves for the first time on a barren area and undergo succession it is primary succession. When an existing community is deslroyed by some catastrophe the following sera1 communities represent secondary succession. (c) Sera1 community or a sere is 'an iirilermediate community in the process of successio~i.It usually has generalists that modify abiotic Pctors and allow other species to invade. The climax conmluliity is a slable final stage of succession. Generally il does no1 allow invasion by other species. It consists of specialist organisms. Similarly sere is a simple, less diverse community while climax co~nmunityis very complex aid diverse. (d) Following major abiotic factors can affecl succession in aqualic colnnlunity (i) current of water (ii) depth of waler (iii) extent of aquatic habitill (iv) input of nutrielits and sedime~lts (e) (v) whethcr water body is permanent or transient like rain wider pools, elc. Ecolone may havc higher divcrsily because this arca is colonized by members from both thc adjacent communities as well as some specialists adapled to ecolont: itself. Thesc latter species arc no1 found in eiher of the communities. Ecosyste~riStability