What is ecosystem and what are the components of an ecosystem?

Ecosystem is the minimal entity that has the properties required to sustain life. An ecosystem is a community of organisms and its non-living environment in which matter (chemical elements) cycles and energy flows.

However, the key to the ecosystem is the processes that occur in them, as ecosystems are very complex entities with no clear and obvious boundaries. Both a small puddle of water and a large tract of forest covering several thousand hectares are ecosystems in their own right, as they support life and entertain various interactions.

The concept of the ecosystem is a broad one, the main emphasis being on interrelationships- obligatory, interdependent and causal, i.e. the coupling of components by relationships to form functional units hence as the system is a complex one, the 'parts' are inseparable from the 'whole' and the 'whole' cannot function without its 'parts'.

The two main components of an ecosystem are biotic and abiotic components. Each of these components interacts and both influence each other.

The abiotic components of an ecosystem are the physical and chemical factors. The physical factors include wind, light, rain or more broadly-climate and the chemical factors include the elements and minerals such as carbon, nitrogen, oxygen, sulphur, phosphorus etc. and their derivatives. The biotic components of the ecosystem are the living organisms- plants, animals and microorganisms.

There are various interactions taking place between the biotic and abiotic factors, and within the biotic factors themselves. Even within the biotic factors, there are interactions at the inter- and intra-specific level.

The biological control of the chemical environment

Although it is general knowledge that the abiotic factors exert their influence on biotic components in an ecosystem, it also happens the other way around. The biotic components too 'have their say' on the chemical environment and to a certain extent on the physical environment too.

This so happens because the biotic components not only constantly adapt themselves to their environment but also modify the environment to suit their own needs. The oxygen and the carbon dioxide content of the air and the nitrate content in the soil and oceans are delicately and finely controlled and cycled by the biotic components.

Energy and material flow in ecosystems

The universe consists of two entities: matter and energy. Matter is physical material that forms the building blocks of physical and biological environments. Energy is the ability to do work.

The existence of the living world depends on the flow of energy and the circulation of materials or nutrients through the ecosystem. Both influence the abundance of organisms and the complexity of the community. As energy and materials flow through the ecosystem together, each of them cannot be segregated from the other.

The flow of energy is one way, and once used by the ecosystem, it is lost. Materials, on the other hand, recirculate. An atom of carbon, oxygen, nitrogen etc. may pass through the living and non-living components many times, or it may even be exchanged between ecosystems. This one-way passage of energy and the round trip of materials (called biogeochemical cycle) are the cogs on which the living world turns.

The expenditure and storage of energy can be described by the two laws of thermodynamics. These two laws are important in the study of energy flow through the ecosystem.

The first law of thermodynamics states that energy can neither be created nor destroyed, it can only be transformed from one form to another or act upon matter in various ways, but regardless of the transfers or transformations taking place, no gain or loss in total energy occurs.

When coal is burned, the potential energy present in the coal is equal to the kinetic energy released and heat is transferred to the surroundings. This is an exothermic reaction.

In an endothermic reaction, energy from the surroundings may be paid into the reaction. For example, in photosynthesis, the molecules of the products store more energy than the reactants. This extra energy is acquired from the sunlight, but even then there is no gain or loss in total energy.

The second law of thermodynamics states that whenever energy is converted from one form to another, some of the useful energy is lost, and no total energy is lost, but some form of energy is produced that cannot be used.

For example, when electricity is converted to light, some part of energy is dispersed as heat to the surroundings. The same thing happens to energy in the ecosystem. As energy is transformed from one organism to another in the form of food, a large part of that energy is dissipated as heat. The remainder is stored in living tissues.

Solar Energy

Energy is constantly arriving on the earth from the sun. The solar energy is primarily in the form of light but the light energy is converted into other forms of energy when it is absorbed by the earth.

All living organisms in the universe use energy in several forms, but all can be grouped under two headings: radiant and fixed. Radiant energy is in the form of electromagnetic waves such as light. Fixed energy is potential energy bound up in various organic substances which can be broken down or reacted with something else in order to release the energy content.

Energy Capture

At the base of an ecosystem, primary producers (plants) actively convert solar energy into stored chemical energy. Photosynthesis is the process of converting solar energy, water and carbon dioxide into carbohydrates and oxygen. This process occurs in two steps.

In the first step, light energy is absorbed by chlorophyll to split a molecule of water releasing hydrogen and oxygen. In the second step, the energy is used to convert carbon dioxide to simple sugars.

The sugar can be converted into starch and stored by the plant or converted to specialized carbohydrates such as cellulose. It can be combined with other nutrient substances such as nitrogen, phosphorus and sulphur to build complex molecules like proteins, nucleic acids and hormones. All these types of reactions are necessary for normal growth and maintenance of body tissues and function of the plants.

Dynamics of Energy and Material Flow in Ecosystem

All life requires energy, and ultimately the energy comes from the sun. Approximately 99.9% of the sun's energy reaching the earth is reflected into space, absorbed as heat, or evaporates water from the water bodies. Plants absorb only 0.1% of this incoming light energy, particularly in the blue and red wavelengths of the visible spectrum.

This energy is used by the plants for photosynthesis to create simple sugars from carbon dioxide and water with the release of oxygen. The simple sugars are used to store chemical energy. Some 20% of this stored energy is used by the plants for respiration.

When respiration occurs, the carbon-carbon bonds are broken and the carbon is combined with oxygen to form carbon dioxide. This process releases energy, which may either be used by the organism or may be lost as heat. The energy stored in the plants is consumed by animals (primary consumers).

These primary consumers convert about 10% of the consumed energy into flesh or organic matter. The 90% of the remaining consumed energy is used by the animals during respiration for motion, maintenance of body function, or is excreted. Consequently, an animal such as wolf which consumes a rabbit that eats grass would be a secondary consumer and would receive 1% of the available energy in the plant. When energy is transferred through the food chain, about 90% of the available energy is lost in each transfer while the remaining 10% of energy is used by the consumer.

The sun provides radiant energy for the plants to produce food in the form of sugar. The energy is transferred from producers (plants) to primary consumers (herbivores) and then to the secondary consumers (carnivores). Dead plants and animals provide energy for decomposers. While a part of the food energy consumed is assimilated by organisms, the rest of it is dissipated as heat during respiration. There is a unidirectional flow of energy from the sun until it is dissipated as heat.

The ecological system maintains its stability by continuous input of energy from the sun and the cyclic movement of nutrients through the system.

The hydrological cycle

Energy from the sun constantly causes evaporation from all water surfaces. Oceans, lakes, ponds, rivers, streams, and the surfaces of terrestrial organisms all lose water due to evaporation.

The sun also provides the energy which drives the weather systems which move the water vapour (clouds) from one place to another. Precipitation (rainfall) occurs when water condenses from a gaseous state in the atmosphere and falls to earth.

Evaporation is the reverse process in which liquid water becomes gaseous. Once water condenses, gravity takes over and the water is pulled to the ground. Gravity continues to operate, either pulling the water underground (ground water) or across the surface (surface run-off). In either event, gravity continues to pull water lower and lower until it reaches the oceans.

The oceans are salty because any weathering of minerals that occurs as the water runs to the ocean will add to the mineral content of the water, but water cannot leave the oceans except by evaporation, leaving the minerals behind.

The carbon cycle

Carbon atoms can be found in molecules of carbon dioxide in the atmosphere. Carbon dioxide molecules are used in the process of photosynthesis to form energy-rich organic sugar molecules. Because plants can make organic matter from inorganic matter, they are often called as producers. Approximately 15% of the C02 in the atmosphere is used via photosynthesis in plants. A byproduct of the process is the release of oxygen molecules.

Once the carbon atoms are part of a large organic molecule like sugar, plants can convert the sugar molecule to other kinds of organic molecules, such as proteins or fats. The organic molecules that are part of the body of a plant are incorporated into the bodies of herbivores when the plant parts are consumed. Similarly carnivores obtain carbon-containing molecules by eating animals that have eaten other living beings. Thus the carbon atoms are passed through a series of organisms.

All living organisms (including plants) must break down complex organic molecules (sugars) for energy. This process is known as aerobic cellular respiration. Oxygen is required to break-down complex organic molecules with the release of carbon dioxide, water and useable energy. This can be represented as follows:

The C02 is released back to the atmosphere through decomposition of dead organic matter and wastes also.

The nitrogen cycle

Although nitrogen is the major component of the earth's atmosphere comprising 78% of the air we breathe, it cannot be used directly by animals or most of the plants. Useable forms of nitrogen are nitrate (N03~), nitrite (N02") and ammonium (NH4 +). The natural mechanisms that exist convert atmospheric nitrogen to useable forms. Nitrogen gas can be taken from the atmosphere in two mechanisms.

First, lightning provides enough energy to burn the nitrogen and fix it in the form of nitrate (N03~). This process is duplicated in fertilizer industries to produce nitrogen fertilizers. The other form of nitrogen fixation is by nitrogen- fixing bacteria. These nitrogen-fixing bacteria are of three different types:

(i) Bacteria such as Rhizobium sp. which live symbiotically in the root nodules of leguminous plants.

(ii) Soil bacteria living freely in the soil.

(iii) The photosynthetic cyanobacteria (blue green algae) which are found most commonly in water.

Rarely inorganic nitrogen in the atmosphere in transformed by lightning approximately 90% is converted by bacterial activity. The process of converting molecular nitrogen in the atmosphere to ammonia is called nitrogen fixation. Nitrogen is fixed, either in the form of nitrate or in the form of ammonium ions. Plants and algae then convert these inorganic nitrogen compounds into organic ones, and the nitrogen becomes available, through ecological food chains, as organic compounds.

When plants and animals die, nitrogen is returned to the soil. The nitrogenous compounds are broken down in three different stages:

Nitrogenous compounds are converted into ammonium (NH4 +) compounds by certain kinds of heterotrophic bacteria. This process is known as ammonification. Then the ammonium compounds are acted upon by two different groups of nitrifying bacteria. One group (Nitrosomonas sp.) oxidizes ammonium compounds to nitrites, and the other group (Nitrobacter sp.) oxidizes the nitrites into nitrates, subsequently. The denitrifying bacteria can reduce nitrates, by a series of biochemical reactions, to molecular nitrogen.

The phosphorus cycle

Of the major nutrient cycles in the environment, phosphorus is the simplest. Phosphorus is found either as inorganic or organic phosphate. The source of phosphorus atoms is rock. Phosphorus-containing rock is broken down by chemical reactions and the eroding action of wind and moving water. Some of the phosphorus becomes dissolved in water and other particles become part of the soil

Biotic components

An ecological community is a set of interacting species that are the living part of an ecosystem.

There are many ways of interaction between species in an ecosystem. One way of interaction is by feeding on one another. Energy, chemical elements and some compounds are transferred by this process from creature to creature along 'food chains', in which more complex cases are food webs.

The organisms in a food web can be grouped into trophic levels, simply put, energy levels. A trophic level includes all those organisms that are at the same number of feeding levels away from the source of energy.

Green plants produce sugars through the process of photosynthesis, using the light energy of the sun and C02 of the air and they invariably belong to the first trophic level -'producers', or 'autotrophs'.

Herbivores feed on the producers and thus belong to the second trophic level - 'primary consumers', or the heterotrophs. The consumers are fed upon by the 'secondary consumers' which belong to the third trophic level. The secondary consumers are consumed by the 'tertiary consumers'. Hence in various ecosystems, the number of trophic levels may vary.

All the dead and the decaying organic matter, the organisms produce, are decomposed and recycled into simpler organic and inorganic matter by the decomposers.

As outlined above, the basis of any ecosystem is the interaction of the various components and particularly the autotrophs and heterotrophs. The greatest autotrophic metabolism occurs in the 'green belt' of the plant and the intense heterotrophic metabolism in the soil of the 'brown belt'. Moreover, there exists a considerable delay in the utilization of the products of the autotrophic components in the heterotrophic region.

This separation in space and time leads to two types of food chains. They are the 'grazing food chain' in which the living plants or plant parts are consumed by the consumers and an 'organic or detritus food chain' which involves the decomposition of food materials.

In summary, an ecosystem consists of three main components: (1) the producers, (2) the consumers and (3) the decomposers.

Biological Pyramids

Communities have a definite trophic structure, which varies across time and space, and is a characteristic of a particular type of ecosystem the interaction within the food chain and the energy loss and gain at each transfer (from one trophic level to the other) can be captured in the form of a pyramid. The trophic structure may be described in terms of standing crop per unit area (i) pyramid of numbers and (ii) pyramid of biomass and energy fixed per unit area (iii) pyramid of energy.

In any ecological pyramid, the autotrophs form the base and the heterotrophs, the subsequent levels. The numbers pyramid overrates the importance of small organisms in an ecosystem because many small units are required to equal the mass of one big unit. The biomass pyramid overemphasizes the importance of large organisms. However, the energy flow pyramid provides a more suitable index for comparing any and all components of an ecosystem.

The energy pyramid is more valid because the number and weight of any organisms that can be supported at any trophic level depends not only on the amount of fixed energy but also on the rate at which food is being produced. The energy pyramid is a picture of the rates of passage of food mass through the food chain.

Inter-specific Interactions

Various types of interactions between species too influence ecosystems.

Neutralism: Neither species affects the other.

Competition: Two different species compete for the same resource, thus having a negative effect on each other.

Inhibition: One species completely inhibits the growth of the other. Certain trees release toxic substances which inhibit the growth or even germination of other plants up to a certain circumference.

Parasitism: The parasite does not kill the host immediately but feeds on it, negatively affecting it.

Predation: One species, the predator, directly feeds on the other, the prey, thus having a direct negative effect.

Symbiosis: Both symbionts benefit from the interaction. The nitrogen-fixing bacteria are found in the root nodules of the leguminous plants and both are benefited.

Commensalism: One of the species is positively influenced by the interaction, whereas the other is neither benefited nor harmed.

The Habitat Approach

In the previous part of this chapter we have looked at ecology in the viewpoint of the individual, community and the ecosystem, i.e. the functional aspects of ecology. Herein, we look at the structure of the ecosystem, although we should not examine both these issues disjointedly. During the analysis of any ecosystem or habitat, we are acquainted with the organisms and the physical factors that are associated with them.

Terrestrial ecology

The terrestrial environment is generally considered to be the most variable in terms of both time and geography hence this chapter will be most concerned with the composition of and the geographical variation in terrestrial communities. The following are the most important features of a terrestrial environment.

(1) Water is a limiting factor on land. Terrestrial biota is distributed and spaced in such a way that it is in consonance with the availability of water.

(2) Temperature is another major factor which has an impact on the nature and distribution of both animal and plant communities.

(3) Oxygen and carbon dioxide, two of the most vital gases necessary for sustenance are fairly uniform in distribution except in places like the high altitudes, where oxygen is a limiting factor.

(4) Land is not continuous and barriers such as mountains, oceans restrict the free movement of biota.

(5) Soil is the most important source of nutrient supply in the ecosystem.

Hence, climate and substrate both interact with the population giving every 'biome' a distinct texture. A'biome' is defined as 'a biotic community of geographical extent characterized by distinctiveness of the life forms of the important climax species'. Thus biome, which is certainly a bigger unit than community constitutes the great regions of the world distinguished on an ecological basis, such as tundra biomes, forest biomes, grasslands, deserts, etc.

The tundra

There are two tundra regions, having a circumpolar distribution: one is in the northern regions of the continents of Asia and Europe and the other lies in the northernmost regions of North America.

The temperature which is very low and a short period of growth (about 60 days) are the major limiting factors. The ground remains frozen except for a short period during the growing season. Some areas are permanently frozen named permafrost. The vegetation consists of lichens, grasses, sedges and dwarf woody plants.

Although growing period is short, a long summer photoperiod during this season enables some amount of primary productivity. The many animals that survive in these regions include reindeer, caribou, arctic hare, musk ox, and their predators like arctic fox, arctic wolves, etc.

Tundra-like conditions are found in high altitudes too. They are termed as alpine tundra and occur on high-altitude Mountains in the temperate regions.

Forest ecosystem

Northern coniferous forests:

Below the tundra region, both in North America and Eurasia lay the northern coniferous forests. The most prominent vegetation is the needle-leaved evergreen trees especially the spruces, firs and pines. A dense shade results in poor undergrowth. As a result of the evergreen nature of the forests, productivity is fairly high, although there is low temperature throughout half of the year.

The animals found in these forests are moose, snowshoe hare, grouse, squirrels, crossbills etc. The coniferous forests often experience bark beetle outbreaks, thus paving the way for succession in the ecosystem.

Moist temperate coniferous forests:

Here, in this type of ecosystem, temperatures are a bit higher than the northern coniferous type and here humidity is very high because of the dense fog which often substitutes for reduced precipitation in certain areas. As water is not a general limiting factor, these regions are called the temperature rainforests also. Rainfall ranges from 30-150 inches.

Trees such as western hemlock, Douglas fir, redwoods and Sitka spruces are the dominant species. Wherever there is any penetration of light, the understory is well developed.

Temperate deciduous forests:

Deciduous forest communities occupy areas with abundant, evenly distributed rainfall (30-60 inches) and moderate temperatures. It covers most of Europe, eastern North America, part of Japan, etc. Hence there are more isolated forest regions and species composition differs immensely. As leaves wither from the trees at least during a certain period in every year, the contrast between every season is great.

These forests represent one of the most important biotic regions of the world, because Europeans and North Americans (settlers) have modified these areas and now prime forests have been modified by the human communities.

Beeches, maples, oaks, hickories, chestnuts and other trees are the most common climax vegetation of these forests.

Broad-leaved evergreen subtropical forests:

Where the moisture remains high and temperature differences between winter and summer are narrowed down, the temperature deciduous forest gives way to the broad leaved evergreen forest climax. Trees as varied as northerly oaks, to the more tropical strangler, wild tamarind, etc. are present in these type of forests.

Tropical rainforests: The variety of life reaches its diverse best in tropical rainforests. These forests are present in the tropical regions and low altitudes. Rainfall exceeds 80-90 inches per year and is distributed all the year round. The forests of South America, the Amazon basin, are the largest and the most contiguous of all the rainforests. The biodiversity of the forests is tremendous.

Any tropical rain forest is highly stratified with generally three to four layers or 'storeys. They are: (i) scattered very large trees that project above the canopy; (ii) canopy layer which forms a continuous carpet-like layer about 80-100 feet tall; (ii) an understory, which is present only where there is a break in the canopy leading to the sun's light to reach the bottom.

If rainfall becomes less during the dry season, another subtype of forests called the semi- evergreen type results. Shrub and herb layers often having ferns and palms are present in the understory. Plants like epiphytes grow on the massive tree trunks and vines and lianas vie for space and light.

Most of the species of animals present here are arboreal and much are herpetological forms and birds. Insect life is also very diverse. The animals like geckos, snakes, frogs, parakeets, hornbills, cotingas, monkeys and predators like clouded leopard are present.

Tropical scrub and deciduous forests:

Where moisture conditions are intermediate between desert and savannah on the one hand and rain forest on the other, tropical scrub or thorn forests and tropical deciduous forests are predominant.

The tropical Asian forests are mostly of the tropical deciduous type. Wet and dry spells of equal length alternate and hence the contrasts between the seasons are very conspicuous.

The scrubland habitat supports thorny trees, shrubs and the main climatic factor is the imperfect distribution of fairly good total rainfall.

Grassland

Temperate grassland:

Grasslands cover extremely large areas and are extremely important from the point of view of human beings. Grasslands provide natural grazing for animals and many of man's staple crops like wheat and rice have been bred from grasses.

Generally grasslands occur where rainfall is too low to support forests but higher than that which results in desert life forms. This means around 10-30 inches of annual rainfall. However, grasslands occur even in places where there is high water table which prevents large trees from taking root and in places where fire is a regular feature. Tall, mid and short grasses are more predominant here.

Most of the perennials have very long roots which are also extensive. The most important grasses are Andropogon, Poa, Panicum, Sorgastrum, Stipa etc. The most prominent animals in these grasslands include large mammals, such as bisons, pronghorns, etc. and their predators including wolves, cougars, etc.

The large herbivores move about in large herds, in migratory patterns, thus reducing overgrazing and leading to the rejuvenation of the soil.

Tropical savannah biomes:

Tropical savannah are predominantly grasslands but interspersed with trees. The trees may be scattered or in clumps. Rainfall is about 40-60 inches per year. The largest savannah grasslands occur in Africa. As fire is a limiting factor for these types, the diversity of species is not very high because of high selection pressure. Often a single species of grass or tree may be predominant over large areas.

The important grasses belong to such genera as Panicurn, Pennisetum, Andropogon, Imperata, etc. and trees such as acacias, euphorbias and palms dot the landscape. The prominent animals are antelopes, large herds of elephants, bison, buffaloes and their predators like lions, cheetahs, leopards, etc.

Desert biomes

Deserts often occur in places where rainfall is less than 10 inches per annum and in some places with much more rainfall which is irregular in distribution. Scarcity of rainfall may be due to high subtropical pressure or geographical positions such as rain-shadow regions or high altitudes. Hence moisture is the limiting factor in this type of ecosystem. When irrigated by artificial means and with good soil structure, deserts can literally 'bloom' because of the abundance of sunlight.

There are three main types of plants which grow in the deserts: (i) the annuals, which avoid drought by growing only when there is adequate rainfall; (ii) the succulents, which store water; and (iii) the desert shrubs with numerous ramifying branches with small thick leaves. The root systems of the plants are very well developed with a very high root shoot ratio.

Desert animals and plants are adapted to survive scarcity of water by a variety of means. Long roots, water-proof coating for desert insects, production of metabolic water and behavioural adaptations of the animals are all the water-conserving mechanisms. The major fauna of the desert are rodents, herpetofauna and birds like raptors and some mammals.

Aquatic ecosystems

Freshwater ecosystems

Freshwater occupies only a relatively small portion of the earth's surface, when compared with marine or terrestrial ecosystems but these ecosystems are very important from the point of view of humans, since human beings thrived wherever there was abundant freshwater, like the basins of the rivers. The Indus valley civilization, the Sumerian civilization, the Nile valley civilization all flourished in the river banks. Freshwater habitats can be classified into:

(1) Standing-water or lentic, e.g., ponds, lakes, etc. and

(2) Running water or lotic habitats, e.g., streams, rivers, etc.

Temperature in freshwater habitats does not show much variation, though it is often a limiting factor. The turbidity of water is due to the type and amount of suspended materials such as silt, clay particles, etc. This is also an important limiting factor because it influences the ability of light to penetrate to a certain level in the water body. The current action, especially in streams has a very important role in the distribution of organisms. The dissolved gases such as 02 and C02 are also limiting factors in any aquatic ecosystem.

In a pond or a lake, there are certain zones which have distinct features, mainly based on the depth of the water body and the light penetration. They are: (1) littoral zone, which is shallow; (2) Limnetic zone, which is open water upto the zone of effective light penetration; and (3) profundal zone, the bottom and deep water area, where there is a paucity of light.

Lentic communities: Various organisms are found distributed in the various zones. In the littoral zone, the rooted plants, floating plants, emergent plants which are rooted plants which emerge out of the water level, submerge plants, and phytoplanktons such as algae are present. The consumers are animals in which a vertical rather than horizontal zonation is evident.

In limnetic zones, the producers are mainly phytoplanktonic algae. In temperate lakes, phytoplanktonic population often shows a marked seasonal variation often leading to uncontrolled algal growth or blooms. The consumers of the limnetic zones are the zooplanktons, some insects and fishes.

In the profundal zone, the organisms mainly depend for their food on the littoral and limnetic zone. The region is rich in nutrients that are carried by currents and swimming animals to other zones.

Lotic community:

In this ecosystem water current is a major limiting factor. The velocity of the current varies greatly in different parts of the stream or river. As streams flow for greater distances, the land-water interchange is often perceptible, with land forms extending into streams and the steams extending into land and other lentic habitats. As a result of the 'flowing1 nature of the streams and rivers, oxygen tension is uniform and there is little or no thermal stratification.

Most of the plants are attached firmly to substrates in order to avoid getting carried away. The animals are either powerful swimmers or they get attached to the substrates by means of certain special structures such as hooks and suckers.

Marine ecology

The sea is very big, covering nearly 70% of all earth's surface. All the oceans are continuous. The sea is in continuous circulation because of the wind stress set up by air temperature differences between the pole and equator. The sea water is salty with an average of 35 parts per 1000 parts of water.

Zonation in the sea:

The shallow water zone is termed the neritic zone. This is present in the continental shelf. The zone between the high and low tides is the intertidal zone. The continental shelf extending upto some distance offshore drops off steeply and is termed the continental slope. The region beyond the continental shelf is termed the oceanic region, which has the continental slopes and beyond which is the abyssal plains. Based on the light penetration, the zones are termed euphotic and aphotic regions.

The communities:

The marine organisms are very diverse and coelenterates, sponges, annelids, echinoderms, crustaceans and fish are dominant in marine waters. Algae are the most important phytoplanktons. The zooplanktons feed on the phytoplanktons. They include the protozoans, crustaceans, tiny jellyfish, free-floating polychaete worms, etc. They all remain as planktons in their entire life cycle and are termed as haloplanktons. Meroplanktons comprise those animals, the larval forms of which are associated with the planktons. The benthos includes a variety of organisms in the inshore region. They are a variety of crabs, amphipods, oysters, mussels, corals etc. The nekton and neuston include fishes, whales, seals, turtles, etc.

Estuarine ecology

River mouths, coastal bays, tidal marshes and any other semi-enclosed coastal body of water which is influenced by tidal action and in which sea water is constantly in interaction with freshwater are called estuaries. Hence they are 'transitional ecotones' between freshwater and marine habitats.

Drowned river valleys, ford-type estuaries, bar-built estuaries, estuaries created by tectonic processes and river delta estuaries are some of the types of estuaries. Various estuaries have various amounts of salinity value, some being homogenous, whole others not. Estuaries are highly productive because of their 'being trapped' in between the sea and the freshwater ecosystem.

The shallow water production rate exceeds the rate of community respiration. The sea weeds, algal mats, sea grass beds, etc. which are profuse in growth are the primary producers. The estuaries also export energy and nutrients to deeper parts of the estuaries and to the sea. In the deeper parts, the nutrients are used up by the consumers. These are sedimentary in nature.

The planktons and nekton often move between both the shallow water and the deeper parts of the estuaries. They have certain periodicities which may be daily, tidal or periodic in nature.

Evolution

Understanding biological evolution is the key to understanding biological diversity. Biological evolution is a never-ending process which refers to the change in the inherited characteristics from generation to generation. This process may lead to a new species. Moreover, once a species is evolved, it cannot evolve backward, i.e., into its parents. This process of evolution is 'one-way Street'.

Four processes have been implicated in evolution: (i) mutation; Hi) natural selection; (iii) migration; and (iv) genetic drift.

(i) Mutation

Genes present in the chromosomes within cells contain the 'codes' which are responsible for all the inherited characteristics. The cells are often in the process of division and chromosomal division is vital during the time. Every division is error-prone and some of the errors are left as such, while most of them are rectified. These changes are called mutations. The mutations may be induced by certain chemical, biological or physical factors, like X-rays, gamma rays, viruses, etc.

These mutations may, in the rarest of rare cases, lead to the evolution of a new species. The new species will not be able to reproduce with the parental species. Mutation just yields a new species but it does not mean that the new species is better adapted to the environment than its parent.

(ii) Natural selection

When there is a variation in the characteristics of a species, some individuals may be better suited to the environment which may place them at an overall advantage. These are better represented by their descendants than those of their less-fitting counterparts.

Hence natural selection is a concordance of the factors, i.e., (1) genetic variability, (2) environmental variability, (3) differential reproduction that varies with the environment, and (4) the influence of the environment on survival and reproduction.

(iii) Migration

A third process which can lead to evolution is migration. The migration of a population of a species to another habitat may happen and the processes of mutation, natural selection, etc. may influence the population in such a way that it may become an entirely new species in the long run.

(iv) Genetic drift

Another process leading to evolution is genetic drift. This refers to the changes in the frequency of a gene in a population due to chance and not to any of the said factors. Chance determines which individuals get isolated from a larger population. Once again, the isolated individuals may not be more adapted to the environment. This chance isolation may lead to a change in gene frequency and lead to genetic drift.

Genetic drift can occur in any small population, i.e., those below a certain threshold. These individuals are doomed to extinction because: (1) characteristics that are less adapted to the environmental conditions may dominate; (2) genetic variability of the species is greatly reduced so that its capability to adapt to future changes too is reduced.