Unit 2 : Ecosystem

CONCEPT OF AN ECOSYSTEM

    An ecosystem is an area whose environment is unique and recognizable. Natural ecosystems include forests, grasslands, deserts, wetlands such as ponds, rivers, lakes, and the sea. Man-modified ecosystems include agricultural patterns, and patterns of urban or industrial land use. The ecosystem's existence is based on its geographical features, such as hills, mountains, plains, rivers, coastal areas or islands. Climatic conditions such as the amount of sunshine, temperature, and rainfall also influence it. The living portion of the ecosystem is called its biotic component and abiotic component is its non-living portion. All the living organisms in an area live in communities of plants and animals. They interact with their abiotic environment and with each other. Living organisms cannot survive without their non-living environment as this provides food and energy for the former's survival, Thus, the biotic population and its environment work to create a natural self-sufficient unit known as an ecosystem. Ecosystems are the very base of life itself.

Definition of an ecosystem: A natural functional ecological unit comprising of living organisms (biotic community) and their non-living (abiotic or physio chemical) environment that interact to form a stable self-supporting system. All of the earth's habitats are connected to each other. For example, an ecosystem of the river is linked to the ocean ecosystem and a small ecosystem of dead logs is part of a large forest ecosystem. A complete self-sufficient ecosystem is rarely found in nature but situations can occur that approach self-sufficiency. Ecosystems are divided into terrestrial or land-based ecosystems and aquatic ecosystems. These form the two major habitat conditions for the Earth’s living organisms.

Stability of ecosystems

    Many ecosystems are relatively stable and less influenced by some degree of human perturbation. Some are weak and quickly destroyed by human activity. Eg: Mountain ecosystems are extremely fragile, because degradation of forest cover contributes to significant soil erosion and changes in river courses. Island ecosystems are also easily affected by human activity which can contribute to the rapid extinction of many of their unique plant and animal species. Some species may have a significant impact on the environment if eliminated. These are called ‘keystone species’. Extinction is caused by land-use changes and other geographical changes. Forests are deforested for timber, wetlands are drained to create more agricultural land and semi-arid grasslands are turned into irrigated fields. The pollution from industries and the waste from urban settings can also lead to poisoning and extinction of several species.

STRUCTURE AND FUNCTIONS OF AN ECOSYSTEM

    The concept of ecosystem was first put forth by A.G. Tansely in 1935. Ecosystem is an essential unit of ecology. It has both structure and function. The structure is related to species diversity. The more complex the structure, the greater the species diversity within the ecosystem. The functions of an ecosystem are related to energy flow and materials cycling through structural components of the ecosystem. Every ecosystem has two key components from the structural perspective: Abiotic and Biotic.

Abiotic components

    The non-living factors or the physical environment prevailing in an ecosystem form the abiotic components. They have a significant impact on the structure, distribution, behavior and inter-relationship of organisms. Abiotic components are primarily of two types:

(a) Climate Factors which include rain, temperature, light, wind, humidity etc.

(b)Edaphic Factors which include soil, pH, topography minerals etc.

The functions of important factors in abiotic components are given below:

    Soils are much more complex than simple sediments. They contain a mixture of weathered rock fragments, highly altered soil mineral particles, organic matter, and living organisms. Soils provide nutrients, water, a home, and a structural growing medium for organisms. The vegetation found growing on topsoil is closely linked to this component of an ecosystem through nutrient cycling. The atmosphere provides carbon dioxide for photosynthesis and oxygen for respiration for the organisms found within ecosystems. The processes of evaporation, transpiration and precipitation cycle water between the atmosphere and the Earth’s surface. Solar radiation is used in ecosystems to heat the atmosphere and to evaporate and transpire water into the atmosphere. Sunlight is also necessary for photosynthesis. Photosynthesis provides the energy for plant growth and metabolism, and the organic food for other forms of life.

    Most living tissues are composed of a very high percentage of water, up to and even ex-ceeding 90%. The protoplasm of a very few cells can survive if their water content drops below 10% of their saturation level and most are killed if it is less than 30-50% below the saturation level. Water is the medium by which mineral nutrients enter and are trans-located in plants. It is also necessary for the maintenance of leaf turgidity and is required for photosynthetic chemical reactions. Plants and animals receive their water from the Earth’s surface and soil. The original source of this water is precipitation from the atmosphere.

Biotic Components

The living organisms including plants, animals and micro-organisms (Bacteria and Fungi) that are present in an ecosystem form the biotic components. From nutrition point of view, the biotic components can be grouped into two basic components:

(i)Autotrophic components and (ii) Heterotrophic components

The autotrophic components include all green plants which fix the radiant energy of sun and manufacture food from inorganic substances. The heterotrophic components include non-green plants and all animals which take food from autotrophs. On the basis of their role in the ecosystem, the biotic components can be classified into three main groups:

(A)Producers (B) Consumers (C) Decomposers or Reducers

(A)Producers:

    Green plants have chlorophyll with the help of which they trap energy and change it into chemical energy of carbohydrates using simple inorganic compounds, namely, water and carbon dioxide. This process is known as photosynthesis. As the green plants manufacture their own food they are known as Autotrophs (i.e. auto=self, trophos= feeder). The chemical energy stored by the producers is utilized partly by the producers for their own growth and survival and the remaining is stored in the plant parts for their future use.

(B) Consumers:

    Animals lack chlorophyll and are unable to synthesize their own food. Therefore, they depend on the producers for their food. They are known as heterotrophs (i.e. heteros= other, trophos= feeder). The consumers are of four types, namely:

(a) Primary Consumers or First Order Consumers or Herbivores:

These are the animals which feed on plants or the producers. They are called herbivores. Eg: rabbit, deer, goat, cattle etc.

(b) Secondary Consumers or Second Order Consumers or Primary Carnivores:

The animals which feed on the herbivores are called the primary carvivores. Eg: cat, fox, snake etc.

(c) Tertiary Consumers or Third Order Consumers:

These are the large carnivores which feed on the secondary consumers. E.g. wolf.

(d) Quaternary Consumers or Fourth Order Consumers or Omnivores:

These are the largest carnivores which feed on the tertiary consumers and are not eaten up by any other animal: Eg: lion and tiger.

(C) Decomposers or Reducers:

Bacteria and fungi belong to this category. They breakdown the dead organic materials of producers (plants) and consumers (animals) for their food and release to the environment the simple inorganic and organic substances produced as by- products of their metabolisms. These simple substances are reused by the producers resulting in a cyclic exchange of ma-terials between the biotic community and the abiotic environment of the ecosystem. The decomposers are known as Saprotrophs (i.e., sapros=rotten, trophos=feeder)

Fig. 2.1 Schematic representation of the structure of an ecosystem 

Fig. 2.2 Relationship within an ecosystem 

FUNCTION OF AN ECOSYSTEM

In any ecosystem we have the following functional components:

(i)Inorganic constituents (air, water and mineral salts)

(ii)Organisms (plants, animals and microbes) and

(iii)Energy input which enters from outside (the sun).

    These three interact and form an environmental system. Inorganic constituents are synthesized into organic structures by the green plants (primary producers) through photo-synthesis and solar energy is utilized in the process. Green plants become the source of energy for renewals (herbivores) which in turn become the source of energy for the flesh eating animals (carnivores). Animals of all types grow and add organic matter to their body weight and their source of energy is a complex organic compound taken as food. They are known as secondary producers.

    All the living organisms, whether plants or animals, in an ecosystem have a definite life span after which they die. The dead remains of plants and animals provide food for saprophytic microbes, such as bacteria, fungi and many other animals. The saprobes ultimately decompose the organic structure and break the complex molecules and liberate the inorganic components into their environment. These organisms are known as decomposers. During the process of decomposition of organic molecules, the energy which kept the organic components bound together in the form of organic molecules gets liberated and dissipated into the environment as heat energy. Thus, in an ecosystem, energy from the sun is fixed by plants and transferred to animal components.

    Nutrients are withdrawn from the substrate, deposited in the tissues of the plants and animals, cycled from one feeding group to another, released by decomposition to the soil, water and air, and then recycled. The ecosystems operating in different habitats, such as deserts, forests, grasslands and seas are interdependent on one another. The energy and nutrients of one ecosystem may find their way into another so that ultimately, all parts of the earth are interrelated, each comprising a part of the total system that keeps the biosphere functioning.

Thus the principal steps in the operation of ecosystem are as follows:

(1)Reception of radiant energy of sun

(2)Manufacture of organic materials from inorganic ones by producers

(3)Consumption of producers by consumers and further elaboration of consumed materials

(4) After the death of producers and consumers, complex organic compounds are degraded and finally converted by decomposers and converters into such forms that are suitable for reutilization by producers.

    The principal steps in the operation of ecosystem not only involve the production, growth and death of living components but also influence the abiotic aspects of habitat. It is now clear that there is transfer of both energy and nutrients from producers to consumers and finally to decomposers and transformers levels. In this transfer, there is a progressive decrease of energy but nutrient component is not diminished and it shows cycling from abiotic to biotic and vice versa. The flow of energy is unidirectional. The two ecological processes, energy flow and mineral cycling, which involve interaction between biotic and abiotic components lie at the heart of ecosystem dynamics. The principal steps and components of ecosystem are  illustrated in figure 2.3. 

Fig. 2.3 Different components of ecosystem 

PRODUCERS, CONSUMERS AND DECOMPOSERS

    The biotic community of any ecosystem can be divided simply into producers, consumers and decomposers. Producers or autotrophs are organisms that make their own organic material from simple inorganic substances. For most of the biospheres, the main producers are photosynthetic plants and algae that synthesize glucose from carbon dioxide and water. The glucose produced is an energy source and combines with other molecules from the soil to build biomass. It is this biomass that provides the total theoretical energy available to all non photosynthesizing organisms in the ecosystem.

    Consumers or heterotrophs are organisms that obtain molecules by eating or digesting other organisms. By eating other organisms, they gain both food as an energy supply and nutrient molecules from within the biomass ingested. For instance, to build new protein, consumers have to eat protein containing amino acids. Consumers are of three types:

Herbivores – animals that eat only plants-primary consumers. Eg: hare, deer, elephant and fish that live on algae

Carnivores – animals that eat only animals-secondary consumers. Eg: tigers, leopards, jackals, foxes, carnivorous fish.

Omnivores – animals that eat both animals and plants. Eg: humans

    Decomposers are the waste managers of any ecosystem. They are small, like worms, in-sets, bacteria and fungi. They are the final link in a foodweb and break down dead organic material into smaller particles and finally into simpler substance that are used by plants as nutrition. Thus, decomposition is a vital function in nature. Without this, all the nutrients would be tied up in dead matter and no new life would be produced. Decomposers can be divided into two groups based on their mode of nutrition:

1.Detrivores are organisms that ingest non-living organic matter. These can include earth-worms, beetles and many other invertebrates.

2.Saprotrophs are organisms that live on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion. These include fungi and bacteria. 

Fig. 2.4 Relationship between producers, consumers and decomposers

ENERGY FLOW IN ECOSYSTEM

    Every ecosystem has several interrelated mechanisms that affect human life. All the functions of the ecosystem are in some way related to the growth and regeneration of its plant and animal species. These interlinked processes can be depicted as various cycles. All these processes depend on energy from sunlight. During photosynthesis, carbon dioxide is absorbed by plants and oxygen is released into the atmosphere. Animals depend on this oxygen for their respiration. The water cycle depends on the rainfall, which is necessary for plants and animals to live. The energy cycle recycles nutrients into the soil on which plant life grows. Our own lives are closely linked to the proper functioning of these cycles of life. If human activities alter them, humanity cannot survive on earth.

Energy Cycle

    The energy cycle is based on the flow of energy through the ecosystem. The energy from sunlight is converted by plants into growing new plant material like leaves, flowers, fruits, branches, trunks and roots of plants. Since plants can grow by converting the solar energy directly into their tissues, they are known as producers in the ecosystem. The plants are consumed by herbivores as food, which gives them energy. A large part of this energy is used up for the metabolic functions of these animals such as breathing, digesting food, supporting growth of tissues, maintaining blood flow and body temperature. Energy is also used for activities like looking for food, finding shelter, breeding and rearing the young ones. The carnivores, in turn, depend on the herbivores on which they feed. Thus, the different plant and animal species are linked to one another through food chains. Each food chain has three or four links. However, as each plant or animal can be linked to several other plants or animals through many different linkages, these interlinked chains can be depicted as a complex food web. This is called the ‘web of life’ that shows that there are thousands of interrelationships in nature.

    The producers and consumers in ecosystem can be arranged into several feeding groups, each known as trophic level (feeding level). In any ecosystem, producers represent the first trophic level, herbivores represent the second trophic level, primary carnivores represent the third trophic level and top carnivores represent the last level. The energy in the ecosystem can be depicted in the form of a food pyramid or energy pyramid. The food pyramid has a large base of plants called producers. The pyramid has a narrower middle section that depicts the number and biomass of herbivorous animals, which are called first order consumers. The apex depicts the small biomass of carnivorous animals called second order consumers. Man is one of the animals at the apex of the pyramid. Thus, to support mankind, there must be a large base of herbivorous animals and an even greater quantity of plant material. When plants and animals die, this material is returned to the soil after being broken down into simpler substances by decomposers such as insects, worms, bacteria and fungi; so that plants can absorb the nutrients through their roots. Animals excrete waste products after digesting food, which goes back to the soil. This links the energy cycle to the nitrogen cycle.

        On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transferred between trophic levels include respiration, growth and reproduction, defecation, and nonpredatory death (organisms that die but are not eaten by consumers). The nutritional quality of material that is consumed also influences how efficiently energy is transferred, because consumers can convert high-quality food sources into new living tissue more efficiently than low-quality food sources.

    The low rate of energy transfer between trophic levels makes decomposers generally more important than producers in terms of energy flow. Decomposers process large amounts of organic material and return nutrients to the ecosystem in inorganic form, which is then taken up again by primary producers. Energy is not recycled during decomposition, but rather is released, mostly as heat. The Figure 2.5 shows the flow  of energy (dark arrows) and nutrients (light arrows) through ecosystems. 

Fig. 2.5 Energy and nutrient transfer through an ecosystem 

Fig. 2.6 Energy pyramid 

ECOLOGICAL SUCCESSION

    Ecological Succession is the process by which the ecosystem tend to change over a period of time. There are several developmental stages in the ecosystem. Developmental stages in the ecosystem consist of a pioneer stage, a series of changes known as serial stages and finally a climax stage. The successive stages are related to the way in which energy flows through the biological system. Succession usually produces a stable state at the end. For example, an open area will gradually be converted into grassland, a shrub land and finally, a woodland and a forest.

    There are two different types of succession- primary and secondary.

Primary succession occurs in regions in which the soil is incapable of sustaining life. This may occur due to factors like lava flows, newly formed sand dunes, or rocks left from a retreating glacier.

Secondary succession occurs in areas where a community that previously existed has been removed. It is characterized by smaller-scale disturbances that do not eliminate all life and nutrients from the environment.

    The most frequent example of successional changes occur in a pond ecosystem. The different stages may be: dry terrestrial habitat, an early colonization stage by small aquatic species after the monsoon, a mature aquatic ecosystem. It may go back to its dry stage in summer when its aquatic life remains dormant. Succession can be related to seasonal environmental changes, which create changes in the community of plants and animals living in the ecosystem. 

        Other successional events may take much longer periods of time, extending to several decades. If a forest is cleared, initially only a small number of species from surrounding habitats are capable of thriving in this disturbed habitat. As new plant species take hold, they modify the habitat by altering things like the amount of shade on the ground or the mineral composition of the soil. These changes allow other species that are better suited to this modified habitat to succeed the old species. These newer species are superseded, in turn by still newer species. A similar succession of animal species occurs, and interactions between plants, animals, and environment influence the pattern and rate of successional change. 

Fig. 2.7 Ecological Succession

FOOD CHAINS, FOOD WEBS AND ECOLOGICAL PYRAMIDS

    In the ecosystem, green plants alone are able to trap solar energy and convert it into chemical energy. The chemical energy is locked up in the various organic compounds, such as carbohydrates, fats and proteins, that are present in the green plants. Since virtually all other living organisms depend upon green plants for their energy, the efficiency of plants in any given area in capturing solar energy sets the upper limit to long-term energy flow and biological activity in the community. The food manufactured by the green plants is utilized by themselves and also by herbivores. Herbivores fall prey to some carnivorous animals. In this way, one form of life supports the other form. Thus, food from one trophic level reaches the other trophic level and in this way a chain is established. This is known as the food chain.

Definition of food chain: A food chain may be defined as the transfer of energy and nutrients through a succession of organisms through repeated process of eating and being eaten. In a food chain, the initial link is a green plant or producer which produces chemical energy available to consumers. For example, marsh grass is consumed by a grasshopper, the grasshopper is consumed by a bird and that bird is consumed by hawk.

Food chains are of three types:

Grazing food chain, Parasitic food chain, Saprophytic or detritus food chain

1.\ Grazing food chain

    The grazing food chain starts from green plants (autotrophs) and from them, it goes to herbivores (primary consumers) to primary carnivores (secondary consumers) and then to secondary carnivores (tertiary consumers) and so on. The gross production of a green plant in an ecosystem may be utilized in three ways – it may be oxidized in respiration, it may be eaten by herbivorous animals and after the death and decay of producers it may be utilized by decomposers and finally released into the environment. In herbivores, the assimilated food can be stored as carbohydrates, proteins and fats, and transformed into much more complex organic molecules.

    As in autotrophs, the energy in herbivores also meets three routes-respiration, decay of organic matter by microbes and consumption by the carnivores Likewise, when the secondary carnivores or tertiary consumers eat primary carnivores, the total energy assimilated by primary carnivores or gross tertiary production follows the same course and its disposition into respiration, decay and further consumption by other carnivores is entirely similar to that of herbivores.

2.\ Parasitic food chain

It goes from large organisms to smaller ones without outright killing as in the case of predator.

3.\ Detritus food chain

The dead organic remains including metabolic wastes and exudates derived from grazing food chain are generally termed detritus. The energy contained in detritus is not lost in ecosystem as a whole; rather it serves as a source of energy for a group of organisms called detritivores that are separate from the grazing food chain. The food chain so formed is called detritus food chain.

FOOD WEB

    Many food chains exist in an ecosystem, and they are not independent. In an ecosystem, one organism does not depend wholly on another. The resources are shared specially at the beginning of the chain. The marsh plants are eaten by variety of insects, birds, mammals and fishes and some of the animals are eaten by several predators. Similarly, in the food chain eg: grass→mouse → snakes→owls. Sometimes mice are not eaten by snakes but directly by owls. This type of interrelationship interlinks the individuals of the whole community. In this way, food chains become interlinked. A complex of interrelated food chains makes up a food web. Food web maintains the stability of the ecosystem. The greater the number of alternative pathways, the more stable is the community of living things.

Fig 2.8 Food web in an ecosystem 

Ecological Pyramids

    The idea of ecological pyramids was advanced by C.E. Eltron (1927). The trophic structure of an ecosystem can be indicated by means of ecological pyramid. At each step in the food chain, a considerable fraction of the potential energy is lost as heat. As a result, organisms in each trophic level pass on lesser energy to the next trophic level than they actually receive. This limits the number of steps in any food chain to 4 or 5. The longer the food chain, the lesser is the energy available for the final members on the chain. Because of this tapering off of available energy in the food chain, a pyramid is formed and this is known as the ecological pyramid. The higher the steps in the ecological pyramid, the lower will be the number of individuals and the larger their size.

    There are different types of ecological pyramids. In each ecological pyramid, the producer level forms the base and successive levels make up the apex. Three types of pyramidal re-lations may be found among the organisms at different levels in the ecosystem. They are:

1. Pyramid of numbers, 2. Pyramid of biomass (biomass is the weight of living organisms), and 3. Pyramid of energy.

Pyramid of Numbers

    It depicts the numbers of individuals in producers and in different orders of consumers in an ecosystem. The base of pyramid is represented by producers which are the most abundant. In the successive levels of consumers, the number of organisms goes on decreasing rapidly until there are a few carnivores. The pyramid of numbers of an ecosystem indicates that the producers are ingested in large numbers by smaller numbers of primary consumers. These primary consumers are eaten by relatively smaller number of secondary consumers and these secondary consumers, in turn, are consumed by only a few tertiary consumers. In a parasitic food chain starting from tree, the pyramid of numbers will be inverted.

Fig 2.9 Pyramid of numbers of a lake ecosystem

Pyramid of Biomass

    The living weights of the members of the food chain present at any one time form the pyramid of biomass of organisms. This indicates the total bulk of organisms or fixed energy present at one time. Pyramid of biomass indicates the decrease of biomass in each trophic level from base to apex, e.g., total biomass of producers is more than the total biomass of the herbivores. Likewise, the total biomass of secondary consumers will be lesser than that of herbivores and so on. Since some energy and material are lost in each successive link, the total mass stored at each level is limited by the rate at which the energy is being stored below. This usually gives a sloping pyramid for most of the communities in terrestrial and shallow water ecosystems. The pyramid of biomass in a pond ecosystem will be inverted.

Fig 2.10 A pyramid of biomass 

Pyramid of Energy

    This depicts not only the amount of total energy utilized by the organisms at each trophic level of food chain but more importantly, the actual role of various organisms in transfer of energy. At the producer level, the total energy will be much greater than the energy at the successive higher trophic level. Some producer organisms may have small biomass but the total energy they assimilate and pass on to consumers may be greater than that of organisms with much larger biomass. Higher trophic levels are more efficient in energy utilization, but much heat is lost in energy transfer. Energy loss by respiration also progressively increases from lower to higher trophic states.

Fig 2.11 Pyramid of energy 

    In the energy flow process, two things become obvious. Firstly there is only one way along which energy moves i.e., unidirectional flow of energy. Energy comes in the ecosystem from an outside source i.e. sun. The energy captured by autotrophs does not go back to the sun; the energy that passes from autotrophs to herbivores also does not revert back. As the energy moves progressively through the various trophic levels, it is no longer available to the previous levels. Thus, due to unidirectional flow of energy, the system would collapse if the supply from primary source, the sun, is cut off. Secondly, there occurs a progressive decrease in energy level at each trophic level which is accounted largely by the energy dissipated as heat in metabolic activities.

TYPES OF ECOSYSTEMS

Forest Ecosystem

    A forest ecosystem is a population of organisms residing within a tree. In general, a forest is defined as a large group of trees. Other important aspects of a forest, however, are the shrubs, the floor-leaf mulch and the plants that live in conjunction with the trees. A forest ecosystem, however, isn't just about the forest environment. It also deals with the animals that live in the forest.

Characteristic Features of Forest Ecosystems

Forest ecosystems are rich and diverse, and they have many exciting and fascinating features.

1.\ Seasonality: In countries that have seasonal climate, forest ecosystems will change with the seasons.

2.\ Deciduous or evergreen: A forest may be deciduous (i.e. it sheds its leaves in winter) or evergreen (i.e. its leaves stay green and intact all the time), or it may be a mix of both deciduous and evergreen trees.

3.\ Different levels: Some forest ecosystems such as rain forests, features several levels– such as the forest floor, the lower canopy, the upper canopy and the tree tops.

4.\ Attractive to birds: Many bird species nest in tree tops and this makes forest ecosystems attractive to birds.

5.\ Attractive to insects: Many insects live in tree barks, leaf mulch or flowers and as such they find forest ecosystems very attractive places to make their homes.

Importance of Forest Ecosystems

    Forest ecosystems are important not just for the community close to the forest, but for the whole world. The following are the reasons for this.

1.\ The Amazon rainforest is described as a biotic pump-like a giant green lung that releases oxygen into the atmosphere and locks away carbon.

2.\ Some of our forests are truly ancient, and much older than many human civilizations.

3.\ All our forest ecosystems are important for biodiversity. In fact, biologists very often claim that they are still discovering new species in the Amazon rain forest on a regular basis.

4.\ Forest ecosystems are not just habitats for animals. Many human communities including indigenous communities live in forests all over the world.

5.\ Forests keep the earth rich in minerals, protect it from desertification by providing a shield against winds, and so on.

Types of Forest Ecosystems

Taiga: This thin, sparse forest exists at the extreme north of the world, in countries such as Canada and Finland and in the Arctic Circle. It is characterized by chilly conditions and the fact that the animals and birds and other organisms that live there have adapted to the cold. The taiga is a very ancient forest.

Rain forests: Rain forests are huge, humid, highly bio-diverse swathes of forest that are usually found within the global South. Due to the thick canopy created by their leaves, rain forests usually create their own mini ecosystem that seals off heat and humidity.

Boreal forests: Boreal forests exist in the sub Arctic zones of the world (i.e. less far north than the Taiga). Here, you can find a mix of deciduous and evergreen trees and plenty of different animals, insects, birds and so on.

Forests of the temperate zone: Located between the freezing cold of the polar zone and the scorching heat of the equator, the temperate zone is somewhere where forests can truly flourish. Some very ancient forests, such as the New Forest in Britain are examples of how the temperate zone conditions are just right for huge amounts of biodiversity to occur. Again, in this zone, forests can be made of a mix of deciduous and evergreen trees or of mainly one or mainly the other type of tree.

Functions of Forest ecosystems

    Different organisms exist within the forest layers. These organisms interact with each other and their surrounds. Each organism has a role or niche in sustaining the ecosystem. Some provide food for other organisms, other provide shelter or control populations trough predation.

Grass Land Ecosystem

    Grasslands are areas where the vegetation is dominated by grassland Ecosystems and other herbaceous (non-woody) plants. Grasslands occupy about 24% of the earth’s surface. They occur in regions too dry for forests and too moist for deserts. The annual rainfall ranges between 25-75cm, usually seasonal. The principal grasslands includes Prairies (Canada, USA), Pampas (South America), Steppes (Europe and Asia), and Veldts (Africa). The highest abundance and greatest diversity of large mammals are found in these ecosystems. The dominant animal species include wild horses, asses and antelope of Eurasia, herds of Bison of America, and the antelope and other large herbivores of Africa. Grasslands are found primarily on plains or rolling topography in the interiors of great land masses, and from sea level to elevations of nearly 16,400 ft in the Andes. Because of their continental location, they experience large differences in seasonal climate and wide ranges in diurnal conditions.

Desert Ecosystem

    A desert ecosystem is a community of organisms that live together in an environment that seems to be deserted wasteland. A desert ecosystem generally witnesses little rainfall, resulting in less vegetation than in more humid areas of the globe. Look closely at any seemingly deserted piece of land and you will usually be able to see numerous insets living in communities, an abundance of plant life, mammals and birds. In addition, micro organ-isms such as bacteria will also be present in this ecosystem, though they are not visible to the naked human eye.

There are many different types of desert ecosystems. They are:

1. Hot deserts: Hot deserts can be found close to the equator. The Sahara is a good exam-ple. Hot deserts tend to feature scorching hot ground which many plants may struggle to grow on, little shade, and a shortage of water. The plants and animals that live here have evolved in order to adapt to these very hot conditions. For example, cacti have grown a tough outer skin and interiors which can store any fluid that they absorb so that they can stay hydrated during droughts.

2. Cold deserts: Desertification can exist at high altitudes too, and when this happens, the desert will be cold. A good example is the deserted rocky peaks of a mountain. A cold desert may be sandy or rocky, but it will be a harsh environment where organisms have adapted in strange and wonderful ways so that they can survive. Eg: Gobi desert.

3. Ice deserts: Ice deserts are another type of cold desert. Here, instead of a sandy or rocky wasteland, we have a seemingly uninhabited region that is composed of ice. Ice deserts can be found towards the north and south poles of the planet, though they may also be located high up on mountain peaks.

Aquatic Ecosystems

    In the broadest sense, there are two major types of ecosystems-aquatic and terrestrial. Among which aquatic ecosystems are further classified into freshwater, marine and estuarine ecosystems based on their salt content. Composing more than 70% of the Earth’s surface, aquatic ecosystems are not only the dominant feature of earth but are also very diverse in species and complexity of interaction among their physical, chemical and bio-logical components.

Fresh water ecosystems

    Fresh water ecosystems cover about 2% of the earth’s surface, an area of about 2.5 million Km2. These ecosystems are characterized by running water or still water. The running water ecosystems are also known as lotic ecosystems and still water as lentic ecosystems.

Lotic ecosystems

    Fresh water streams (Springs, rivulets, creeks, brooks etc.) and rivers can change over their course from being narrow, shallow, and relatively rapid to become increasingly broad, deep and slow moving. A river is a lotic ecosystem that is formed because of gravity and acts as a catchments delivery/removal system. They are Mother Nature’s pipelines. The water in a lotic ecosystem, from source to mouth, will have atmospheric gases, turbidity, longitudinal temperature gradation and material dissolved in it. 

    Lotic ecosystems have two main zones:- rapids and pools. Rapids are the areas where the water is fast enough to keep the bottom clear of materials, while pools are deeper areas of water where the currents are slower and silt builds up. Temperature is a major abiotic factor for life in these systems. Water found in these systems will freeze much quicker, and thaw much faster than the deep waters of lentic systems.

    Lotic ecosystems depend on precipitation, snow melt, and springs to keep the water flowing. In time of drought these shallow systems will dry up and many organisms will die.

Lentic ecosystems

    Lentic water systems consist of still bodies of water, such as lakes, ponds and seas. During periods of drought, these systems will often last longer than their smaller counterparts and organisms can continue to live despite the shortened supplies. These bodies often experience many things that lotic water systems do not. Lentic water systems are made up of multiple zones: littoral, limnetic, vertical, and benthic.

    The epilimnion, or surface water, is the area in which most life can be found. This zone is high in oxygen content during the summers; the warmer waters that receive the most sun-light will hold the most oxygen, thus allowing the greatest amount of life to flourish here.

    `The metalimnion, or middle mass of water is where the temperature of the water begins to decrease; life is not as plentiful as in the surface waters but there are still many organisms that can be found here. Very often, this is where the thermocline will begin. A thermocline is a point at which the water temperature decreases approximately one degree Celsius per meter.

    The hypolimnion is the warmest zone during the winter time and the coldest during the summer. It is the bottom most part of the body, where sometimes light does not reach. This zone is where the least amount of life will be found throughout most of the year. Overturns occur when water is stratified in bodies; the water from the bottom of the mass is mixed with the water close to the surface. This occurs during spring and fall.

Marine ecosystem

    Marine ecosystems are among the largest of Earth’s aquatic ecosystems. Examples include salt marshes, intertidal zones, estuaries, lagoons, mangroves, coral reefs, the deep sea, and the sea floor. They can be contrasted with fresh water ecosystems, which have a lower salt content. Marine waters cover two- thirds of the surface of the Earth. Such places are considered ecosystems because the plant life supports the animal life and vice versa.

    Marine ecosystems are essential for the overall health of both marine and terrestrial environments. According to the World Resource Center, coastal habitats account for about one-third of marine biological productivity. Estuarine ecosystems, such as salt marshes, sea grass meadows and mangrove forests, are among the most productive ecosystems on the planet. Coral reefs provide food and shelter to the highest levels of marine diversity in the world. Marine ecosystems usually have a large biodiversity and are therefore thought to have a good resistance against invasive species.

        Marine habitats can be divided into coastal and open ocean habitats. Coastal habitats are found in the area that extends from as far as the tide come in on the shoreline out to the edge of the continental shelf. Most marine life is found in coastal habitats, even though shelf area occupies only seven percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf.

        Alternatively, marine habitats can be divided into pelagic and demersal zones. Pelagic habitats are found near the surface or in the open water column, away from the bottom of the ocean. Demersal habitats are near or on the bottom of the ocean. An organism living in a pelagic habitat is said to be a pelagic organism, as in pelagic fish. Similarly, an organism living in a demersal habitat is said to be a demersal organism, as in demersal fish. Pelagic habitats are intrinsically shifting and ephemeral, depending on what ocean currents are doing. Marine habitats can be modified by their inhabitants. Some marine organisms, like corals, kelp, mangroves and sea grasses, are ecosystem engineers which reshape the marine environment to the point where they create further habitat for other organisms.

Estuaries

        An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments. They are subject both to marine influences-such as tides, waves, and the influx of saline water-and to reverie influences-such as flows of fresh water and sediment. The inflows of both sea water and fresh water provide high levels of nutrients both in the water column and in sediment, making estuaries one among the most productive natural habitats in the world.

        Most existing estuaries formed during the Holocene epoch with the flooding of river eroded or glacially scoured valleys when the sea level began to rise about 10,000-12,000 years ago. Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and may be fully saline.

        The banks of many estuaries are amongst the most heavily populated areas of the world, with about 60% of the world’s population living along estuaries and the coast. As a result, many estuaries suffer degradation by many factors, including sedimentation from soil erosion, deforestation, overgrazing and other poor farming practices; overfishing; and filling of wetland; eutrophication due to excessive nutrients from sewage and animal wastes; pollutants including heavy metals, polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs; and damming for flood control or water diversion.

QUESTIONS

Part A (5 marks)

1. What is an ecosystem?

2. Explain the structure of an ecosystem.

3. What are the functions of an ecosystem?

4. Which are the functional components of an ecosystem?

5. Write briefly on the biotic components of an ecosystem.

6. Which are the abiotic components in an ecosystem?

7. Explain with examples producers, consumers and decomposers.

8. What do you mean by ‘web of life’?

9. Explain the energy cycle in an ecosystem.

10. What do you mean by ecological succession. Explain.

11. Which are the different types of food chain ?

12. What do you mean by an ecological pyramid. Name the different types of ecological pyramids.

13. What is meant by pyramid of numbers?

14. Which are the different types of forest ecosystems. Give examples for each.

15. List the characteristic features of forest ecosystems.

16. Why is a forest ecosystem important to the world?

17. What is a desert ecosystem ?

18. Name the different types of a desert ecosystem. Explain each.

19. Explain lotic and lentic ecosystems.

20. Write a short note on freshwater ecosystem.

21. What are pelagic and demersal zones ?

22.\ What is an estuary?

 Part B (15 marks)

23. Explain its structure and functions.

24. Write a note on the biotic and abiotic components of an ecosystem explaining the functions of each with examples.

25. Explain the energy flow in an ecosystem. What is an energy cycle?

26. Explain the terms food chain and food web.

27. Describe ecological pyramid.

28. Explain a forest ecosystem focusing on the different types, its importance and char-acteristics.

29. Which are the different types of ecosystems? Explain the features of each.

30. Describe aquatic ecosystems. Which are the different types ? Explain each.

Part C (10 marks)

1. Define an ecosystem. Explain its structure and functions.

2. Write a note on the biotic and abiotic components of an ecosystem explaining the functions of each with examples.

3. Explain the energy flow in an ecosystem. What is an energy cycle?

4. Explain the terms food chain and food web.

5. Describe ecological pyramid.

6. Explain a forest ecosystem focusing on the different types, its importance and char-acteristics.

7. Which are the different types of ecosystems? Explain the features of each.

8. Describe aquatic ecosystems. Which are the different types ? Explain each.