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, tem-

perature, etc., of the en-vironment. As a result of reaction, the envi-

ronment 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 invertebrate-

feeding 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.