After reading this article you will learn about:- 1. Concept of Ecological Succession 2. Process of Ecological Succession 3. Strategies.
Concept of Ecological Succession:
Ecological succession is the gradual change that occurs in an ecosystem of a given area of the Earth’s surface on which populations succeed each other. The most familiar ecological successions are those on abandoned farmland, like the succession from weeds to forest.
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A series of plant communities occupies the fields in roughly predictable order while the habitat progressively changes. Identically, newly formed marshlands or sand-dunes are colonized by regular successions of communities even as the habitats change, most strikingly when moist soil develops on dry dunes.
The forcing functions of ecosystem changes often seem to be provided by vegetation, as the biomass of developing plant communities changes the physical state of the ecosystem and provides the habitat for animal or decomposers. But animal communities also change in succession.
They may modify the habitat for plants and replace other animal communities in a colonizing dynamics of their own. These facts of ecological succession have attracted ecologists principally because they suggest ordered development.
Process of Ecological Succession:
Succession has the appearance of a cooperative enterprise. Plants and animals come and go in sequence, each apparently making its contributions to ecosystem change and then departing. The final result is a perfected complexity that persists indefinitely.
Ecologists use several approaches to study processes of community building. One is to model the changing rates of colonisation and extinction when communities are built on isolated raw habitats, the approach is known as island biogeography.
A particular aim of this approach is to understand the conditions setting the equilibrium eventually reached by the number of species. Other approaches are evolutionary, asking how accommodations between species in complex communities come about.
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This approach addresses matters of mutualism, mimicry and the relationship between predators and their prey that allow the coexistence of many species. But the process of building plant communities in relatively short span of time through ecological succession is yet a separate phenomenon requiring its own explanations.
Communities are built not in one quick fitting together of coevolved species but from pools of local species through the more or less regular replacements of succession. The end point is the complex community dominated by a few characteristic species that typically arrive late in the succession.
Succession traditionally is divided into primary and secondary: autogenic and allogeneic. Primary successions colonize bare sites and lead to the first occupation of the habitat by the climax community. Examples are successions on sand-dunes, volcanic mud-flows, glacial till, filled in lakes and marshes.
On the contrary, secondary successions replace a climax community following a disturbance. Old field successions are secondary successions, as are the successions that replace forest in gaps after hurricanes, or fires.
Autogenic succession is the succession directed from within the ecosystem itself. The term particularly refers to habitat changes brought about by the biota, as when soil is built and nutrients collected. If these enrichments of soil promote the next community replacement then the succession is autogenic.
On the other hand, allogeneic succession is succession driven by forces outside the ecosystem, as when a progressive fall in water-table due to stream down-cutting leads to a succession of plant communities suited to progressively drier habitats.
The communities themselves may have had no influence on the critical habitat changes. The conceptual differences between autogenic and allogenic successions are significant.
A particular recognised succession is called a sere. A hydrosere is a successional sequence on wetland. Wetland successions in general are called hydrarch successions. In a like manner, successions on dry sites like sand-dunes are called xerarch successions.
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The communities of a sere are called seral communities. The earliest seral community on a site is the pioneer community. The succession is said to end with the climax, which is the community with which a succession ends and in which species perpetuate themselves through reproduction.
The composition of the climax community depends on the physical properties of the habitat and of the local climate. The climax for each local habitat type can be recognized by its characteristic dominant plants, like the maples of beech- maple forest of the American Midwest or the oaks of English woodlands or the rainforests of India.
A typical primary succession on a terrestrial site is shown in Fig. 7.1. Many of the population replacements of a typical sere can be understood as a necessary consequence of opportunist and equilibrium species living together in the same area. This is the ‘r’ and ‘k’ hypothesis of succession.
The essence of this hypothesis is that both opportunist and equilibrium species are able to coexist within an area of country that might be called reasonable dispersal distance. No evolution of fresh taxa is required for ecological succession because this is a comparatively short-term event.
It happens in ecological time rather than in evolutionary time. We can discount from a hypothesis of causes both invasions from remote places and the production of fresh species by speciation. Both are possible, of course, but their appearance cannot affect our general explanation.
Thus this hypothesis predicts that the plants of the pioneer seral community will be extreme r-strategists, adapted for dispersal and rapid growth but not for sustained existence at a species equilibrium.
They should be highly fecund, short-lived and without elaborate storage organs and the more costly predator defenses. If these plants exist in a neighbourhood, it can be predicted that they will first occupy vacant habitat.
This prediction is upheld by the plants of pioneer communities which are the typical annual weeds of an old field. But the hypothesis predicts that the pioneers will certainly be displaced as k- strategists invade the site from surrounding complex communities.
This is what we observe, most notably in seres that lead to a forest climax. The progressive invasions of perennial herbs, shrubs and trees are by species whose life-history phenomena are of progressively longer life, with all the diversion of resources from reproduction to storage and defence that these life histories-demand.
The final number of species at climax will depend on the rate of continued invasion and the rate of local extinction, which eventually should be very slow.
The hypothesis that succession is the predictable replacement of r-strategists by k-strategists is readily convincing for all seral stages up to forest. But when the whole habitat is covered by large trees, all following a k-strategy of sorts, the continued successional change might not be so self-evident.
And in classic old field successions to forest a long succession of tree populations continues between the first establishment of forest and the final slow change of climax (Fig. 7.2).
Grime (1979) had suggested that a series of three base strategies should be recognised for plants instead of just the opportunist and equilibrium strategies of r and k.
These three strategies would be that of coloniser, competitor and stress competitor—called ruderal, competitor and stress tolerant strategies or R, C and S- strategies by Grime. An R-C-S continuum fits the facts of succession more closely than the simpler r and k continuum (Fig. 7.3).
Successful computer simulations of some temperate forest successions have been made using data for invasion, growth, and death of individual trees. One such programme is called ‘JABOWA’ (Fig. 7.4).
Alongside the population changes of many successions there are changes in the physical system that can be called ‘ecosystem development’. These changes may be minor in successions filling forest gaps but they can be large in old field succession and become striking in many primary successions.
The possibilities of ecosystem development—whether allogenic or autogenic—are best illustrated with classic examples of primary successions; development of forest on fresh glacial till (as shown below); the xerarch succession on sand-dune; and the hydrarch succession of wetlands.
On the whole, Odum (1969) summarised the overall consequences of succession, which are given in Table 7.1:
Strategies of Ecological Succession:
Consciousness of the importance of different strategies to the study of populations grew out of the debate over the importance of density dependent and density independent control of populations.
The debate made clear that there were two extreme types of life history, common in nature, the one suited to marginal habitats with fluctuating weather and the other for more equable places where a population equilibrium seemed more likely. The two kinds of species were at first known as opportunist and equilibrium species, but later named as r- and k-selection species.
In marginal habitats many life histories are clearly adapted to rapid population growth during short favourable seasons, with other adaptations letting individuals survive the hostile times that inevitably come. These are the life histories of opportunist species. Necessary to the opportunist lifestyle is the ability to disperse rapidly so that remote habitats can be colonised or recolonized quickly from the places of refuge.
Another way of looking at dwellers in marginal habitats is to say that they are fugitives from more desirable places, for one who journeys to the desert must have been from a more fertile place. Thus they can be treated as fugitive species.
On the contrary, an equilibrium species represents the alternative strategy to that of opportunists or fugitive species. This is the strong competitor that sends the opportunist away to the desert, or to ephemeral habitats, as a fugitive. Essential to life at a species equilibrium are adaptations that allow persistence.
Each individual of an equilibrium species should compete strongly with its own kind and with others, and be programmed for life in a crowded habitat. Dispersal is less important than persistence. The result is a life history distinctly different from that of opportunist or fugitive species.
The adaptations that result in an equilibrium species are essentially opposite to those of an opportunist. This is most clearly seen by considering the calorie budget of the animals.
If an animal, throughout its life history, is made to invest calories in mechanisms for defense or endurance, then these calories are not available for extra reproduction. An animal can use the strategy of putting calories into structures that lead to persistence or the strategy of putting all possible calories into reproduction so that genes persist even though individuals perish. Different strategies represent direct trade-offs.
i. r- and k- selection:
There are a number of instances of the formation of young volcanic islands, raised from the sea. This island is devoid of terrestrial plants and animals initially, but then the island will be colonised rapidly by migrants arriving from across the sea.
There will be plants coming as wind-blown seeds, birds and insects coming on their own wings, seeds and small animals clinging to the birds, and many things from small mammals and lizards to living plants drifting out on floating flotsam.
But all these immigrants will have in common the ability to disperse. The overwhelming number of arrivals, perhaps all, will be opportunist or fugitive species well-provided with mechanisms for dispersal.
Once settled on the island, the opportunist immigrants, adapted as they are to high fecundity, should achieve rapid population growth. Soon the island will be crowded with their descendants and there ought to establish population equilibrium. But these animals and plants are not adapted to crowded lives at equilibrium; they are usually fugitives from other species that do better in crowds.
On their island, weather may not strike them down and the better competing equilibrium species cannot reach them. Natural selection should then work to find the most competitive varieties from the island’s opportunist. This is the process now known as ‘k-selection’.
The term r-selection can be equivalent to opportunists and fugitives and k-selection to equilibrium species. Pianka (1970) gave a summary statement of how r- and k-selection could be used to organise so many ideas in contemporary ecology (Table 7.2)
However there are three distinct pathways for plant colonisation in any habitat. The details are given in Fig. 7.5.
ii. Population Ecology:
Population ecology is concerned with interrelationships of co-actions between individuals within and between species. Co-actions may either be beneficial to the participants, cooperation, or harmful to them.
Interspecific cooperation includes mutualism, commensalism, and many of the interactions that establish the community as a dynamic unit. Co-actions that are harmful, to at least one of the participants, include parasitism, predation and competition.
An early manifestation of cooperation in the evolution of animals and plants is the grouping of free- living protozoans to form colonies, and the further development of such colonies into multi-cellular metazoans thereafter behave and respond as unit organism.
It is impossible to say whether the first gathering of protozoan cells to form colonies developed for better protection from some predator or environmental condition, for improved utilisation of food supplies or more efficient reproduction.
Colonisation quickly led to division of labour between somatic and reproductive cells and, later, to division of somatic cells themselves, so that different cells or organs became specialised to serve the particular functions of digestion, respiration, circulation and so on.
Cooperation among cells, tissues, and organs gave greater metabolic efficiency to the whole individual and resulted in evolution of the highest types of organisms. Similarly, the aggregation of individuals must have survival value, because it persists.
Cooperation between species that is intimate and beneficial to both participants is called mutualism; where only one participant benefits, it is called commensalism.
These relations may be either facultative or obligatory. Where one or more of the participants is harmed there is no cooperation, of which parasitism, parasitoidism, competition and predation are the examples. Distinction is made between true parasites, social parasites, and parasitoids. True parasites and their hosts have evolved adaptive interrelations so that co-existence occurs for varying lengths of time.
However, the host is generally weakened and virulent strains of the parasite may cause high mortalities. Causes of mortality or disease among organisms are predators, parasitoids, worm parasites, protozoan parasites, bacteria, viruses, fungi, external parasites, nutritional deficiencies, toxication, physiological stress and accidents.
Competition may be exerted directly through interference in the activities of one organism by another, or indirectly in the form of excessive exploitation of natural resources It may be either intraspecific or interspecific. Allelochemic secretions from one organism that affect the growth, behaviour, or health of other organisms are a subtle means of competition, cooperation or intercommunication.
Competition may result in establishment of social hierarchies, establishment of territories, regulation of population size, segregation of species into different niches or speciation. The overall effect of competition is to relegate the individual and species to an orderly place in the structure and organisation of the community with the result that there is decrease in tension and disturbance.
iii. Re-productivity and Population Structure:
The rate at which a species reproduces, and the frequency of its population turnover, can affect the speed with which it occupies new areas, becomes adapted to new niches, or evolves into new races. In order to analyse the population dynamics of a species, it is necessary to know its life history.
This involves the stages in its life cycle, mortality rates of each stage, longevity, sex and age ratios, age at which individuals become sexually mature, fecundity, factors causing mortality, and so forth. The proportion of different ages and sexes gives the population a definite structure. All these essential data may be conveniently summarised in the form of life tables.
In the stabilized population of any species, whatever the number of eggs or young produced per pair of adults, the average number of offspring reaching reproductive status can never be greater than two sexual forms—which is the number required to replace the parents on their death. Therefore, with each new generation there is a population turnover, with newly born individuals replacing the adults that die.
In a stabilized population the rate of increase of a population through the course of several reproductive cycles must equal the death rate, so that the value of one factor is also a measure of the other. Either factor is indicative both of the rate of population turnover and of the intensity of environmental resistance.
Ratios of young animals/plants to adults often indicate whether a population is expanding, contracting or is stabilized.
In stabilized populations the number of offspring reaching reproductive maturity can never be greater or less than the number of adults themselves. The number of young ones that must be produced to permit such a population turnover gives a measure of the rigour of the environment, and how well-adapted a species is to its niche.
iv. Population Growth:
The characteristic sigmoid shape of the cumulative growth curve of organisms was demonstrated by Vehulst in 1839 and the most widely used description of the curve is the logistic equation.
This curve has been used to describe the population growth of such diverse organisms as Yeast, Paramoecium, Drosophila and man and even the growth of communities in particular habitats as represented by the increase in number of species.
Under natural conditions, however, growth of a population is subject to many variable factors, including the change from one morphological stage to another in the life cycle of many species, and the change in the physical environment both daily and seasonally. The Curve is often not fully expressed, even though its trend is present inherently.
The sigmoid curve shows that a finite population grows slowly at first, then at an accelerating rate which is at maximum as the point f inflection, after which the population continues to increase but at a decelerating rate, finally becoming stabilized at the upper asymptote.
Logistic growth curves are symmetrical, and the point of inflection is one-half the value of the asymptote. The lower concave part of the curve is called the accelerating phase of growth.
If the number of new individuals added during each unit of time—absolute growth rate—is plotted against time midway in each period, a bell-shaped curve is obtained, the peak of this curve coinciding with the point of inflection on the sigmoid curve.
In contrast the instantaneous growth rate (r) is the rate of growth at a point on a time scale and is usually expressed in terms of increase per individual or unit biomass per unit of time (Fig. 7.6).
v. Population Regulation:
No single mechanism is responsible for the control of natural population. Some populations sometimes can exist close to equilibrium, with birth rates balanced by death rates, but more often numbers fluctuate between wide limits of changing or repetitive environments, persistently alter the balance between births and deaths. But the possible causes of death vary from species to species and from time to time.
Failures to be born or to find a mate are as important as actual death, being equally effective at removing a unit of reproduction from the next generation. Birth rates vary in some species, actually being kept low by patterns of behaviour that work to regulate births according to supplies of resources.
This birth regulation always gives advantage to individuals by increasing the eventual number of surviving offspring over what would result from more initial births. Natural birth regulation, therefore, always works to increase the rate of population growth. Selection for group birth control is highly unlikely.
Although a plausible mechanism has been put forward for the selection of groups of predators that are below maximum efficiency as hunters, the consensus of ecologists is that all regulation of births works to achieve optimum reproductive success and that all control of populations is by some form of environmental resistance.
For minority species that attain lasting population equilibrium, competition for vacant niche spaces is the decisive mechanism; short-lived opportunistic species, or those suffering heavy predation, may persist with constantly fluctuating local populations.
Some of the longest-lived animals and plants, particularly those of conservative breeding strategies, live in populations the, are always changing very may reach neither Librium no, extinct, on before the onset of climatic or environmental change.
A struggle for existence inevitably follows from the high rate at which all organic bangs tend to increase.
Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year; otherwise, on the principle of geometrical its crease, its numbers would quickly become so inordinately great that no country could support the produce.
Hence, as more individuals are produced that can possibly survive; there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.
On the whole a population can thus be controlled by invoking any or all the seven ways of death:
(i) Starvation (outright energy deficiency: food, calories or light);
(ii) Malnutrition (lack of nutrients or water or essential components);
(iii) Predation;
(iv) Parasitic diseases;
(v) Accident (probably weather-induced);
(vi) Failure to be born; and
(vii) Failure to find a mate.
The importance of these various ways of death vanes widely with different life-history strategies, producing quite different patterns of population control.
The overall fitness stakes of each species are shown in Fig. 7.7:
