Evolutionary Biology Topics allow us to trace the history of life on Earth.
What is Forest Ecosystem and Role of Decomposers in forest
An ecosystem is a self-sufficient unit of living things and non-living environment needing only the input of sunlight energy for its functioning. The forest is an ecosystem consisting of living things such as plants, animals and decomposers, and non-living things such as soil, water and air. In the forest ecosystem, there is an interaction between the living things and non-living environment. This point will become more clear from the following discussion.
(i) The living organisms like plants interact with soil to obtain nutrients (like mineral salts) for their growth. The plants interact with water (in soil and water bodies) to get water for making food by photosynthesis. And plants also interact with air to obtain carbon dioxide for making food by photosynthesis.
(ii) The living organisms like plants and animals interact with one another through their food chains in which one organism eats another organism. Now, grass is a kind of plant. In a forest, grass (or plant) is eaten by insects (such as grasshopper), the insects are eaten by frogs, the frog is eaten up by snake, and finally, the snake may be eaten up by an eagle. So, a common food chain occurring in a forest habitat is :
Grass(Plant) → Insect(Grasshopper) → Frog → Snake → Eagle
This food chain can be shown with the help of pictures as follows (see Figure).
This is just one example of the food chain taking place in a forest. In fact, many different food chains can be found in a forest. All the food chains in a forest are inter-linked to form a food web. If any food chain of a food web is disturbed, it affects many other food chains. For example, if we remove all the snakes from a forest, then all the food chains in which snakes take part will be broken or affected. In this way, every part of the forest is dependent on its other parts (through various inter-connected food chains occurring in it). Please note that every food chain in the forest starts with plants (grass or other plants). This is because the plants are producers of food.
They are autotrophs. So, the first step in every food chain is the plant. The plants are eaten up by the animals called herbivores. So, the second step in a food chain is always a herbivore (or herbivorous animals). The herbivores (animals which eat plants) are eaten up by the animals called carnivores (which eat meat). So, the third step in a food chain is always a carnivore (or carnivorous animal). But it is a small carnivore. The small carnivore is eaten up by a large carnivore. And the large carnivore is eaten up by a still larger carnivore called top carnivore. The green plants (autotrophs) take nutrients and water from the soil, carbon dioxide from air and energy from sunlight, and make food by photosynthesis. The plants are eaten as food by herbivores. Many herbivores are eaten as food by various carnivores. In this way, food is transferred from plants to various types of animals through the food chains occurring in a forest.
(iii) When plants (autotrophs) and animals (heterotrophs) die, then the decomposer organisms decompose their dead bodies into nutrients, water and carbon dioxide. Nutrients and water are returned to the soil, and carbon dioxide is returned to the air. These nutrients, water and carbon dioxide are then re-used for the growth of new plants in the forest. And this process goes on and on like an unending chain. From this discussion we conclude that the various components of a forest are interdependent on one another.
Crowns of Trees
The branchy part of a tree above the stem is known as the crown of the tree (see Figure). The crown includes the top part (or upper part) of a tree which has all the branches and leaves of the tree. Different types of trees have crowns having different shapes and sizes. There are a large number of crown shapes of trees in a forest. Some of the common shapes of crowns of trees are shown in Figure.
The major functions of the crown of a tree are to absorb sun’s light energy, carry out photosynthesis, release oxygen and carry out the processes of respiration and transpiration. All these functions are performed by the leaves of the crown. The branches of crown only hold the leaves. The crowns (or tree tops) of tall trees form a green cover over the forest land. And the trees and plants of different heights create different horizontal layers in the forest. The crowns of trees shade the forest floor.
The Structure of a Forest
A forest consists of different horizontal layers of vegetation (trees and plants). Scientists usually divide a-forest into five layers (or five zones) of vegetation based on the living environment. The five layers (or zones) of a forest vegetation starting from top and going down are : Canopy, Understorey, Shrub layer, Herb layer and Forest floor (see Figure). The amount of sunlight available in each layer of the forest decides which type of CANOPY
plants grow and survive there. We will now describe all the layers of vegetation in a forest very briefly. Let us start with canopy.
(i) CANOPY. In a forest, the branches and leaves of tall trees form a kind of roof or umbrella over the smaller trees and other plants in the forest. The uppermost branches and leaves of tall trees which act like a roof over the forest ground is called canopy (see Figure). The canopy is more or less a continuous layer of leaves. It is the upper part of the crowns of tall trees in the forest which make up the canopy. Thus, in a forest, the tall trees form the uppermost layer of vegetation called ‘canopy’. So, canopy is the highest layer of vegetation in a forest.
The primary vegetation in a canopy is of mature trees. The canopy is filled by leaves from large, mature trees. Canopy leaves intercept the sunlight available to the forest. Less than 50 per cent of the total sunlight falling on the top of forest can pass through canopy and go down to the plants in forest layers below the canopy. The canopy of forest is full of various type of animals and includes insects, birds, reptiles, mammals (monkeys, etc.), and many more.
(ii) UNDERSTOREY. The layer of vegetation in a forest which is just below the canopy is called understorey (see Figure) (Understorey is above the shrub layer). The understorey layer consists of smaller trees and tree saplings. The understorey layer of forest has comparatively much less sunlight.
The smaller trees found in the understorey grow to become bigger trees only when tall trees die and open gaps in the canopy (which can provide more sunlight to the smaller trees needed for their further growth).
(iii) SHRUB LAYER. The layer of vegetation in a forest which is just below the understorey, is called shrub layer (see Figure). The shrub layer consists mainly of mature shrubs and bushes. The shrub layer has a smaller vegetation which is between 1 metre and 2 metres in height from the forest floor.
The animals such as deer and bears can obtain a lot of food from the shrub layer of vegetation. Many of the shrubs in the forest depend on animals for the dispersal of their seeds. The animals eat the shrubs fleshy fruits and pass out the seeds at different places in the forest through their faeces. There is not much sunlight in the shrub layer.
(iv) HERB LAYER. The layer of vegetation in the forest which is just below the shrub layer is called the herb layer (see Figure). The herb layer consists mainly of herbs, ferns and grasses. The herb layer is the lowest layer of vegetation in the forest (having leafy plants). The herb layer ranges from the forest floor to about 1 metre in height. Thus, herbs form the lowest layer (of leafy plants) in the forest.
Very little sunlight remains for the plants in the herb layer. In the herb layer, the plants grow and flower early in the season so as to get sufficient sunlight before the canopy leaves open and obstruct sunlight. Most of the plants in the herb layer have short life cycles.
(v) FOREST FLOOR. The ground surface of forest is called forest floor (see Figure 6). The forest floor is also considered a layer of vegetation in the forest structure. The forest floor has many kinds of small leafless plants such as mosses, liverworts and lichens, etc. The largest animals in the forest (such as tigers, lions, deer, bear, etc.) live on the forest floor. The forest floor has also many kinds of insects and worms. The decomposer organisms such as bacteria, and fungi (like mushrooms, toadstools, etc.) are also present on the forest floor. The forest floor is covered with dead and decaying plant matter (such as dead and decaying leaves and tree trunks), decaying animal matter and animal wastes.
The Role of Decomposers in a Forest
As leaves, trees, other plants and animals die, they all fall on the forest floor. The animal wastes (animal dung or droppings) also fall on the forest floor and collect there. On the forest floor, there are a large number of decomposers such as bacteria and fungi.
The decomposers break down dead leaves, branches and tree trunks (see Figure 7); dead animals ; and animal wastes to form humus which contains the nutrients (mineral salts). Humus mixes up with the soil in the forest and the nutrients present in it are used for the growth of plants. In this way, decomposers help in maintaining the supply of nutrients to the trees and other plants in the forest. We will now discuss the importance of forests.