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ENVIRONMENTAL SCIENCE

ECOSYSTEM – Unit of Stability

An organism cannot live in isolation. It needs other organisms, nutrients from its environment, and so on, to survive. So, nature has provided functional units in which different organisms of a given area can live and interact among themselves and with their surroundings. All ecosystems are stable systems. This means that they maintain a natural balance. An ecosystem involves the flows of nutrients and energy (in the form of food). If the organisms having in an ecosystem use up nutrients, like nitrogen, from their environment, without replenishing them, soon the system will collapse. However, a balance is maintained between the availability and use of nutrients by recycling them through natural processes. You already know how things like nitrogen and carbon are recycled in nature. A balance is also required to provide different amounts of energy (from food) needed by different organisms. As we shall see, the numbers of different organisms in an ecosystem are balanced in such a way that each organism gets the required amount of food. For example, in a forest ecosystem, the numbers of the prey (like rabbits) are always more than the numbers of the predator (like foxes), to ensure adequate food for the predator.

ECOSYSTEM – How They Work

An ecosystem consists of two components-the abiotic component the biotic component (living component). Abiotic component: The abiotic, or nonliving, component consists of the physical environment, nutrients and climatic factors. The physical environment consists of soil, water and air. Inorganic substances such as carbon dioxide, oxygen, nitrogen, water, phosphorus, sulphur, sodium, potassium and calcium constitute nutrients. Things like sunlight, rainfall, temperature, humidity and atmospheric pressure constitute the climatic factors. Biotic component: The biotic, or living, component of an ecosystem can be classified on the basis of how the organisms get their food, i.e., whether they are producers, consumers or decomposers. Producers Organisms which make their own food are called producers: They are also called autotrophs. (In Greek, autos = self, trophe = nutrition.) All green plants and certain blue-green algae act as food producers in ecosystems. Consumers Organisms that depend on other organisms for food are called consumers or Heterotrophs: (In Greek, heteros = other.) All animals which eat plants or other animals are consumers. Bacteria and fungi that depend on dead plants and animals for food are also in a way consumers. Consumers can be classified as herbivores, carnivores and omnivores. Herbivores eat only plants and plant products. Cows, deer and rabbits are herbivores. Carnivores eat only the flesh of other animals.

Tigers, snakes and hawks are carnivores. Omnivores eat plants as well as the flesh of other animals. Man and crow are examples of omnivores. Sometimes it is useful to classify the consumers in an ecosystem on the basis of ‘who eats whom’. Primary consumers are those who feed directly on the producers (plants). In other words, herbivores are primary consumers. Carnivores who feed on plant-eating animals (herbivores) are secondary consumers. For example, a grasshopper that feeds on plants is a primary consumer, and the frog that eats the grasshopper is a secondary consumer. The frog could be eaten by a larger carnivore like a snake. A carnivore that feeds on smaller carnivores is called a tertiary consumer. This consumer may be eaten by the largest carnivore, or the top carnivore, of the ecosystem. The top carnivore is not killed and eaten by other animals of the ecosystem. The top carnivore belongs to a higher order of consumers. For example, a hawk could be the top carnivore of an ecosystem. Other examples of top carnivores are tigers and lions. (Primary, secondary and tertiary consumers are also called consumers of the first, second and third order respectively.) Decomposers Organisms which feed on dead plants and animals are called decomposers: Decomposers are also called saprotrophs or saprophytes (in Greek, sapros = rotten). They include bacteria, fungi and worms. Decomposers break down (decompose) the compounds present in dead plants and animals into simpler substances and obtain nutrition from them. The substances formed in decomposition are released into the soil and the atmosphere. Thus, decomposers play an important role in the recycling of materials, replenishment of the soil’s nutrients, etc. They also clean up our surroundings by decomposing dead organisms and wastes from animals and plants. Take a large glass bowl or jar and put some soil and aquatic plants in it. Fill three-fourths of the bowl with water and place it near a window through which sunlight comes in. Put some fish in the bowl. You will need to put some fish food in the bowl from time to time. The oxygen needed by the fish will be liberated by the aquatic plants through photosynthesis. After a few days the water in your aquarium will become dirty. This is because of the waste generated by the fish and the plants. We do not need to clean natural aquatic ecosystems like ponds and lakes. In these, wastes are consumed by decomposers.

ECOSYSTEM – Population and Succession Population

The different components of an ecosystem can be thought of as existing at different scales. A Population is a group of individuals from the same species, and populations from various species make a community. The area that these communities live in is called a habitat. Overall, an ecosystem is a community living in a fairly uniform habitat (rain forest, pond) that consists of interactions between organisms and their physical environmental (abiotic factors). The size of a population in an ecosystem tends to stay fairly constant, albeit with fluctuations. There, however, are certain factors that will affect the sizes of populations, FACTOR Abiotic -

EFFECT Things like temperature, pH and mineral nutrients are vital for growth. If these were to change, it would make a certain niche less favorable.

Predation -

The populations of predators and prey are very closely linked. Other factors (other than prey) may reduce the population of the predator, and this in turn influences the prey.

Competition - As previously discussed, species have a certain niche, whilst they cannot occupy exactly the same, they very often have overlapping niches which results in competition. Where two species occupy the same niche, only one or the other can survive. For example: the red and grey squirrel in Britain. The grey (Sciurus carolinensis) was introduced in the 19th century, and has replaced the native red squirrel (Sciurus vulgaris). In some areas (near Norwich for example) they do coexist, but together their numbers are lower than if one or the other wholly populated the area. This is inter-specific competition.

Succession Many millions of years ago, the earth was devoid of any visible life but over time organisms began to colonize. The sequence of events is called succession and ends with what is known as the climax community This is the most stable community possible in that particular area. So succession could be thought of as the process of getting to the climax community. There is generally a certain pattern to this process - where small plants initially grow for example, which changes the environment and makes it suitable for other species. Take the example a sand dune where a sequence of succession can be seen moving away from the sea, so it is on a geographic gradient.

Next to the shore, are pioneer plants such as grasses. These make the dune more stable for more complex organisms. And this process continues and the dune is made more stable until the most stable community is reached: the climax. Other examples of where succession can occur are on recently formed volcanic islands or in wetland formations. In each instance, the climax community is again woodland. Secondary succession happens in a habitat that has been previously colonized, but has been disturbed; or example, an area of forest that has been cleared. Secondary succession usually occurs more quickly than primary succession because it is made more favourable as a result of previous colonization by existing factors (seeds, roots, soil fertility).