Ecology and its Principles, Ecological Succession

Ecology is defined as a scientific study of the interaction of living organisms with each other and with their environment. It deals with the ways in which organism are surrounded by their environment, how they make use of environmental resources including energy flow and mineral cycling.
The study of ecology has presently contributed a lot to the socio-economic and political issues of the world as it plays an important role in agriculture, fishery, pest control, conservation of soil, forests and water resources. All the international issues of environment such as pollution and resource management need a sound knowledge of ecology. Ecology is a human science as well. There are many practical applications of ecology in conservation biology, wetland management, natural resource management (agro-ecology,  agriculture, forestry, agro-forestry, fisheries), city planning (urban ecology),community health, economics,  basic and applied science, and human social interaction (human ecology).

Ecologists study nature on different levels, from a local to a global scale. These levels reveal the complex relationships found in nature.

Organism: An organism is an individual living thing, such as an alligator.
Population: A population is a group of the same species that lives in one area, such as all the alligators that live in a swamp.
Community: A community is a group of different species that live together in one area, such as groups of alligators, turtles, birds, fish, and plants that live together in the Florida Everglades.
Ecosystem: An ecosystem includes all of the organisms as well as the climate, soil, water, rocks, and other nonliving things in a given area. Ecosystems can vary in size. An entire ecosystem may live within a decaying log, which in turn may be part of a larger wetland ecosystem.
Biome: A biome (BY-ohm) is a major regional or global community of organisms. Biomes are usually characterized by the climate conditions and plant communities that thrive there. Ecologists study relationships within each level of organization and also between levels. For example, researchers may study the relationships within a population of alligators, as well as the relationships between alligators and turtles in a community.

Principle 1: Protection of species and species’ subdivisions will conserve genetic diversity.
At the population level, the important processes are ultimately genetic and evolutionary because these maintain the potential for continued existence of species and their adaptation to changing conditions. In most instances managing for genetic diversity directly is impractical and difficult to implement. The most credible surrogate for sustaining genetic variability is maintaining not only species but also the spatial structure of genetic variation within species (such as sub-species and populations). Maintenance of populations distributed across a species’ natural range will assist in conserving genetic variability. This ensures the continuation of locally adapted genetic variants. Retaining a variety of individuals and species permits the adaptability needed to sustain ecosystem productivity in changing environments and can also beget further diversity (future adaptability).

Principle 2: Maintaining habitat is fundamental to conserving species.
A species habitat is the ecosystem conditions that support its life requirements. Habitat can be considered at a range of spatial and temporal scales that include specific microsites (e.g., occupied by certain invertebrates, bryophytes, some lichens), large heterogeneous habitats, or occupancy of habitat during certain time periods (e.g., breeding sites, winter range areas). Therefore conserving habitat requires a multi-scale approach from regions to landscapes to ecosystems to critical habitat elements, features and structures

Principle 3: Large areas usually contain more species than smaller areas with similar habitat.
The theory of island biogeography illustrates a basic principle that large areas usually contain more species than smaller areas with similar habitat because they can support larger and more viable populations. The theory holds that the number of species on an island is determined by two factors: the distance from the mainland and island size. These would affect the rate of extinction on the islands and the level of immigration. Other factors being similar (including distance to the mainland), on smaller islands the chance of extinction is greater than on larger ones. This is one reason why larger islands can hold more species than smaller ones.

Principle 4: All things are connected but the nature and strength of those connections vary
Species play many different roles in communities and ecosystems and are connected by those roles to other species in different ways and with varying degrees of strength. It is important to understand key interactions. Some species (e.g., keystone species) have a more profound effect on ecosystems than others. Particular species and networks of interacting species have key, broad-scale ecosystem-level effects while others do not. The ways in which species interact vary in addition to the strengths of those interactions. Species can be predator and/or prey, mutualist or synergist. Mutualist species provide a mutually beneficial association for each other such as fungi that colonize plant roots and aid in the uptake of soil mineral nutrients. Synergistic species create an effect greater than that predicted by the sum of effects each is able to create independently.

Principle 5: Disturbances shape the characteristics of populations, communities, and ecosystems.
Natural disturbances have played a key role in forming and maintaining natural ecosystems by influencing their structure including the size, shape and distribution of patches. The more regions, landscapes, ecosystems and local habitat elements resemble those that were established from natural disturbances, the greater the probability that native species and ecological processes will be maintained. For example, high frequency, low intensity fires have shaped ponderosa pine ecosystems while low frequency, high intensity fires have shaped lodgepole pine ecosystems. Maintaining these ecosystems means restoring fire and/or designing management practices such as harvesting to reduce the differences between a managed landscape and a landscape pattern created by natural disturbance.

Principle 6: Climate influences terrestrial, freshwater and marine ecosystems.
Climate is usually defined as all of the states of the atmosphere seen at a place over many years. Climate has a dominant effect on biodiversity as it influences meteorological variables like temperature, precipitation and wind with consequences for many ecological and physical processes, such as photosynthesis and fire behaviour. Because of the key role of climate, rapid climate change profoundly changes ecosystems.

a) Habitat: Habitat is the physical environment in which an organism lives. Each organism has particular requirements for its survival and lives where the environment provides for those needs. The features of the habitat can be represented by its structural components namely (1) space (2) food (3) water (4) and cover or shelter.

b) Niche: The term niche means the sum of all the activities and relationships of a species by which it uses the resources in its habitat for its survival and reproduction.

c) Species: A species is defined as; “a group of similar populations of organisms whose members are capable of interbreeding, and to produce fertile offspring (children)”. A tiger, a lion, a lotus and a rose are examples of different species.

d) Speciation: Speciation is the process by which new species are formed and evolution is the mechanism by which speciation is brought about.

Ecological Succession

Ecological succession is a fundamental concept in ecology. It is the process by which a natural community moves, through a sequential change in the structure and composition, from a simpler level of organization to a more complex community. It is a long-term cumulative, directional and largely predictable process of natural development of different communities at the same site, in a definite sequence, over a period of time.
Such changes occur either in response to an environmental change or induced by the intrinsic properties of the community itself. Succession continues till a community develops maximum equilibrium to the environment. It is called climax community.

Reasons for ecological succession
• The original environment may have been optimal for the first species of plant or animal, but the newly altered environment is often optimal for some other species of plant or animal. Under the changed conditions of the environment, the previously dominant species may fail and another species may become ascendant.
• A forest fire, wind storms, and human activities like agriculture all greatly alter the conditions of an environment. These massive forces may also destroy species and thus alter the dynamics of the ecological community triggering a scramble for dominance among the species still present.

Features of Succession
The four characteristics of succession are as follows:
1. Succession is an orderly sequence of changes in the vegetation of a previously bare area. It is directional and is hence predictable.
2. The rate of change, the pattern of change and the limit of development are determined by the alteration brought about in the physical environment by the existing community. Succession is therefore, a fundamentally biological process.
3. It produces a relatively stable or climax community which is in dynamic equilibrium with the climate. It has the maximum heterogeneity, community relationship and biomass while the net productivity is minimum.
4. The final or climax community is controlled by climate and not the habitat. Thus succession beginning water or a rock ends up in similar communities under similar climates. The phenomenon is called convergence.

Types of Succession
Ecological Successions have been described using several criteria. Accordingly, there are several types of succession.

1. Autogenic Succession: It is a series of developmental changes in the structure of vegetation caused by the plants themselve. Plants of a developmental state produce changes in the habitat initially to favour their growth but the changes go on beyond the optimum to that the habitat becomes unsuitable for them. It paves the way for the growth of another type of plant community.

2. Allogenic Succession: Here the habitat is changed by action of outside factors like change in climate, leaching of soil nutrients, increase in salt content of the soil and deposition of salt on sand. The habitat then becomes unsuitable for the colonizers. It favours the growth of a different set of plants.

3. Induced Succession: Man has controlled succession in such a way as to obtain a managed steady state in which good amount of organic matter can be harvested. It is called induced succession. In induced succession, like agriculture, a young state is maintained by various types of inputs and protective measures.

4. Deflected Succession: It is a succession in which the vegetation does not pass through the normal stages of development but either adds or replaces a successional type, e.g., ABB’CDE or AB’CDE instead of the normal ABCDE.

5. Primary Succession: It is the succession that takes on a primary bare area or an area which was not previously inhabited by plants. Such an area is biologically sterile and is, therefore, quite hostile in strating. Succession is also slow.

6. Secondary Succession: It occurs on a site which has become bare secondarily due to destruction of previous vegetation. The area is biologically fertile and hence favourable for reappearance of plant life. Succession is quite rapid.

7. Autotrophic Succession: The succession involves in appearance and continued dominance of green or autotrophic plants. Autotrophic succession begins on a medium having little organic matter but quite rich in inorganic substances. Slowly the organic content of the substratum increases.

8. Heterotrophic Succession: The succession occurs in an area rich in organic matter, e.g., water bodies receiving sewage, litter, etc. and there is a progressive decrease in the energy content. It is dominated by microbes, actinomycetes, fungi, detrivores and other animals. It usually occurs in a microhabitat like dead plant matter, dead animal, dung, etc.

9. Cyclic Succession: A pattern of succession where the climax community is destroyed again and again and a similar pattern of secondary succession repeats itself every time.

The Process of Succession
Succession is a long-term cumulative, directional and largely predictable process of natural development of different communities at the same site in a definite sequence over a period of time. Such changes occur either in response to an environmental change or induced by the intrinsic properties of the community itself. Succession continues till a community develops maximum equilibrium to the environment. It is called Climax Community.
The characteristic sequence of the successional stages includes 8 elementary processes, namely:

Nudation: It is the creation of bare area. Nudation can occur due to physiographic, climatic or biotic agents.

Migration: This is the leaving of the parent area by a gemmule and its arrival in a new area. A gemmule consists of reproductive structure like seed, spore or propagule. Propagule is commonly a vegetative structure. It does not help immigration over an appreciable distance except in case of lichen soredia. The gemmules which take part in migration are also called Migrules or Disseminules. Migration is influenced by four factors–mobility, agent, distance and topography.

Colonization: The nature of topography of the bare area also determines the type of the initial vegetation. For example, on bare rock only the spores of some cyanophytes or the soredia of lichens can stick and germinate while in a saline area only the seeds of some halophytes can grow. The first arrivals in a bare area are called Pioneers or pioneer colonizers. The occupation of a bare area by the pioneers and other invaders is called colonization.

Ecesis: The establishment of plants in a new place is called ecesis. It consists of three processes- germination, growth and reproduction. Germination depends upon dormancy, and viability of seeds or spores. The most critical stage of life cycle is the seedling stage. It may face predation from animals and adverse environment in the form of lack or defieincy of water, light, temperature and soil depth. Maximum mortality occurs in the seedling stage. If the seedling gets established, it grows in size. During its growth the plants may have to pass thorugh adverse conditions and die or remain vegetative. Reproduction is the final stage of ecesis.

Aggregation: It is the increase in number of the colonizing individuals. In the beginning the pioneers are few in number and grow far from one another. They produce a large number of disseminules which spread in the open areas and increase the number of pioneers. If invasion continues and the invaders are also able to multiply, the phenomenon is called Mixed Aggregation.

Competition: It may be intraspecific (among individuals of the same species) or interspecific (among individuas of the different species). Competition occurs when the availability of a necessity becomes inadequate to meet the optimum requirement of all the individuals growing in that area. The competition can be for space, light and heat among the epiterannean parts and for space, water and minerals among the subterranean parts of different individuals. Intraspecific competition is generally more acute because of the similar requirements and similar adaptations of the individuals of the same species.

Reaction: It is the change brought about by colonizers in the habitats. The first reaction is localized. It consists of such changes as bindings of soil particles, assisting in weathering or building soil at the bottom of a water reservoir. Later on, besides the soil the climate of the habitat is also changed. Vegetation has retarding effect on wind. It protects the ground from the direct action of ration fall and insulation. Death of roots produces channels in the soil for quick absorption of rain water. Humus produced by the death of older or weak plants increases water retention, aeration and nutrition of the soil. The reaction of the early colonizers is such as to make the habitat less favourable to themselves and more favourable to invaders. The reaction of the latter invites new invaders and so on.

Stabilization: Continuous competition invasion and reaction give rise to continuous changes in the environment and structure of vegetation. After a long interval some individuals arise which are in complete harmony with the climate of the area. This is termed as stabilization.
The sequence of the above stages is termed as sere. The traditional view, most notably expressed by Frederic Clements, holds that there is an orderly, predictable sequence of seres, or plant communities resulting from a single stage of succession, culminating in a stable climax. The presence of one set of colonizers changes the environment in such a way that the next set of colonizers can join or replace it.



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