1.5 Evolution And Development Of Ecosystem

DEVELOPMENT AND EVOLUTION OF ECOSYSTEM  Introduction  Development of Ecosystem (or) Ecological Succession  Evolution of Ecosystem

INTRODUCTION An ecosystem is not constant and as per law of nature, it experiences changes. It experiences changes in its constituent organisms and thereby undergoes changes in its structure and function. The study of these changes is important to predict the future of the ecosystem or understand the past of ecosystem and also to understand the principles and conditions under which the ecosystem functions. While development (also referred to as succession) is a study of changes in ecosystem over a small time scale, evolution is a large scale study of the changes that perhaps starts from the origin of the life to the proposed future. DEVELOPMENT OF ECOSYTEMJ (OR) ECOLOGICAL SUCCESSION

When stripped of its original vegetation by fire, flood, or glaciation, an area of bare ground does not remain devoid of plants and animals. Beginning with plants, area is rapidly colonized by a variety of both plant and animal species that subsequently modify one or more environmental factors in the ecosystem. This modification of the environment may in turn allow additional species to become established. This starting stage is called the pioneer stage. The transitional series of communities which develop in a given area are called sere or seral stages, while the final stable and mature community is called the climax.

The development of the community by the action of vegetation on the environment leading to the establishment of new species is termed succession or development. Succession is the universal process of directional change in vegetation during ecological time. It can be recognized by the progressive change in the species composition of the community. Retrogression in

community development does not occur unless succession is disturbed or halted by fire, grazing, scraping or erosion.

CAUSES OF SUCCESSION : Since succession involves a series of complex processes, so there exist many causes of its occurrence. Ecologists have recognized the following three primary causes of succession: 1. Initial or Initiating causes. These are climatic as well as biotic in nature. The climatic causes include factors such as erosion and deposits, wind, fire, etc., which arc caused by lightening or volcanic: activity. The biotic causes include various activities of organisms. All these causes produce the bare areas or destroy the existing populations in an area. 2. Ecesis or Continuing causes. These are processes as migration, ccesis, aggregation, competition, reaction, etc., which cause successive waves of populations as aresult of changes, chiefly in the edaphic (soil) features of the area. 3. Stabilising causes. These include factors such as climate of the area which result in the stabilisation of the community.

TRENDS OF SUCCESSION: The following trends may be noted in ecological development or succession. 1. A continuous change occurs in the kinds of plants and animals. 2. An increase in the diversity of species takes place. The general appearance of the community or the physiognomy keeps on becoming more and more complex as the succession proceeds. 3. There is a progressive increase in the amount of living biomass and dead organic matter. Such an increase occurs in gross as well as net primary production in the initial and seral stages. Thus, there is more biomass accumulation, gradually reaching a huge biomass structure in the climax. 4. Green pigment (Chlorophyll) go on increasing during the early phase of primary succession. The ratio of yellow/green pigments remains around 2 in the early stages and increases to 3 to 5 in the climax stage. Pigment diversity also increases.

5. The community respiration increases but the P/R (i.e., Production/Respiration) ratio remains more than 1 in the sera stages. The huge living biomass respires a lot in the climax stage and the P/R ratio equals 1 (i.e.. P/R = 1). Thus, in the early stages P>R and in the climax stage. P = R. 6. The food chain relationships become more complex as succession proceeds. 7. Nutrients in the young stage are allocated mostly in the soil, but as the seral stages advance, nutrients get allocated more in the vegetation and less in soil. Further the nutrient cycling becomes more closed or intrabiolic with an efficient cycling mechanism whereas in the young stage the nutrients easily leak out from the system, i.e.. the cycling is more of an open type. 8. The role of detritus becomes progressively more and more important. 9. The quality of the habitat gets progressively modified to a more mesic condition from either too dry or too wet condition, in the early seral stage. 10. The niche specialization increases, i.e., different functions arc more effectively performed by specialist species in mature serai stage, whereas in early stage many functions arc performed but less efficiently by a few species. 11. The life cycle of mature community species are longer and more complex. 12. Dispersal of seeds and propagates is by wind in young stage, while by animals in mature stage.

BASIC TYPES OF SUCCESSION : Based on different criteria, there are the following kinds of succession: 1. Primary succession. If an area in any of the basic environments (such as terrestrial, fresh-water or marine) is colonized by organisms for the first time, the succession is called primary succession. Thus, primary succession begins on a sterile area (an area not occupied previously by a community), such as newly exposed rock or sand dune where the conditions of existence may not be favourable initially. 2. Secondary succession. If the area under colonization has been cleared by whatsoever agency (such as burning, grazing, clearing, felling of trees, sudden change in climatic factors, etc.) of the previous plants, it is called secondary succession. Usually the rate of secondary' succession is faster than

that of primary succession because of better nutrient and other conditions in area previously under plant cover. 3. Autogenic succession. After the succession has begun, in most of the cases, it is the community itself which, as a result of its reactions with the environment, modifies its own environment and, thus, causing its own replacement by new communities. This course of succession is known as autogenic succession. 4. Allogenic succession. In some cases replacement of one community by another is largely due to forces other than the effects of communities on the environment. This is called allogenic succession and it may occur in a highly disturbed or eroded area or in ponds where nutrients and pollutants enter from outside and modify the environment and in turn the communities. 5. Autotrophic succession. It is characterized by early and continued dominance of autotrophic organisms such as green plants. It begins in a predominantly inorganic environments and the energy flow is maintained indefinitely. There is gradual increase in the organic matter content supported by energy flow. 6. Heterotrophic succession. It is characterized by early dominance of heterotrophic organ-isms such as bacteria, actinomycctcs, fungi and animals. it begins in a medium which is rich in organic.

STAGES OF SUCCESSION: The ecologists have studied how the process of succession and the entire process of succession can be described in these five sequential steps. 1. Nudation – This is the development of a bare area without any form of life. The exposure of a new surface may may occur due to several causes such as landslides, erosion, deposition, etc and other topographic, climatic and biotic causes. 2. Invasion – Invasion is the successful establishment of a species in a bare area. Invasion involves migration, establishment and aggression. 3. Competition and Coaction - Due to aggregation of a large number of individuals of the species at the limited place, there develops competition (i.e., interspecific and intraspecific competition) for space and nutrition. Individuals of a species affect each other's life in various ways and this is

called coaction. The species which fail to compete with other species are ultimately discarded. 4.

Reaction - Reaction in-cludes mechanism of the modification of the environment through the influence of living organismsonit. Due to this very significant stage, changes take place in soil, water, light conditions, temperature, etc., of the en-vironment. As a result of reaction, the environment is modified and become unsuitable for the existing com-munity which sooner or later is replaced by another community

5. Climax - Finally, there occurs a stage in the process, when the final terminal community becomes more or less established for a longer period of time. This final community is not replaced nnd is known as climax community and the stage as climax stage.

EXAMPLES OF SUCCESSION: In this section, a couple of examples related to ecological succession is provided.

(1) POSSIBLE SUCCESSION IN THE AQUATIC ECOSYSTEM Climax community ↑ Open scrub land = Deciduous forest ↑ Terrestrial communities ↑ Mesic communities ↑ Reeds and sedges ↑ Free floating and rooted plants ↑ Rooted and aquatic plants ↑ Phytoplankton

(2) POSSIBLE SUCCESSION IN THE FOREST ECOSYSTEM

COMMUNITY EVOLUTION Like the responses of communities to changing abiotic conditions, community evolution involves progressive changes in climax communities. Because the evolution is exceedingly slow, it cannot be observed in operation, and few instances from the fossil record are sufficiently complete to show the process in action. The example mat best demonstrates evolution of the basic structure of the community is that of the development of a terrestrial community of a modern type by early reptiles some 250 million years ago. Between the time when vertebrates (amphibians) first became able to lead a predominant terrestrial xistence some 350 million years ago and the establishment of an essentially modern type food web some 100 million years later, the structure of terrestrial community was decidedly different from what it is now. Development of the modern type of community structure required not only a complete rearrangement of the niche structure of the community but also the evolution of new species • that could fill the new niches (Olson, 1961, 1966). Attainment of the adaptations needed for terrestrial life by the first amphibians did not in itself establish a land-based vertebrate community. These early amphibians were carnivores, and the only animals inhabiting the land

environment were insects. It is unbelievable that the clumsy locomotor system of early amphibians would have allowed them to prey effectively on animals such as insects. Thus, the first communities inhabited by terrestrial vertebrates are best regarded as extensions of aquatic communities, with the land habit as an adaptation to improve the capabilities of organisms whose prime food supply was aquatic invertebrates and fish. By some 300 million years ago, reptiles had evolved that could feed effectively on terrestrial invertebrates. An entirely land-based community was theoretically possible in which all herbivore niches were assumed by invertebrates and some of the carnivore niches uy vertebrates. However, the palaeoccological evidences suggest that most contemporary carnivorous vertebrates were unable as yet to realize an entirely terrestrial carnivore niche, so that 'he great majority of the energy flow through the community continued to pass through the aquatic route. The typical food chain to the highest terrestrial vertebrate carnivore was plant → aquatic invertebrate → aquatic invertebratefeeding vertebrate → semi-aquatic predator → terrestrial predator. By 250 million years ago terrestrial herbivorous vertebrates had evolved and a fully terrestrial vertebrates community could come into being. From this time onward the basic structure of the terrestrial community was of an essentially modern sort, with all consumer trophic levels occupied by a wide range of animals, both vertebrates and invertebrates. Such evolutionary changes in the structure of communities are caused by a large number of factors. One factor is changes in the regional climate. It became progressively drier during the period under consideration, and the development of a land-based community reasonably responded in this sort of change. Indeed many evolutionary changes in community structure can be explained on the basis of responses of major changes in the regional abiotic factors of the environment (Axelrod, 1950, 1958). But other chief factors of evolutionary change in community include reorganization of the community's structure in response to the realization of niches that had not previously existed in the community.