Organisms in the environment interact among themselves and this may be intra-specific (between populations of same organism) or inter-specific (between populations of different species). Some of the interactions are cooperative while other is competitive. The networks of interactions are collectively referred to as ‘biotic factors’.
Symbiosis (Living together)
In symbiosis, two different species depend metabolically upon each other and thus are mutually benefited. Such species are known as ‘symbionts’. Rhizobium bacteria and leguminous roots are the symbionts. In this case, the bacteria get protective space to live in and on the other hand fix atmospheric X2 to help manufacture protein by the host plant.
In this ease, one species is benefited while the other is either benefited or remains neutral. The concerned members are called the ‘commensals’ and this association is known as commensalism. For example, some algae and fungi join together, to form a different life form known as lichens.
The algal partner manufactures food through photosynthesis, which the fungi utilize. And in turn, the fungi protect the algae from drying up and help colonize tree barks, rocks, etc. and in reproduction. Some also, refer to this as symbiosis
Some plants and animals share the nest constructed by others without causing any damage to it. For example, termite nests provide niche for many animals from ant to snakes.
A parasite is an organism living in or osn the body of another organism and deriving nutrition from it. Parasitism is virtually universal in all plants and animals. Man is beset with many parasitic organisms including intestinal worms (tape worm, flukes, round worm etc.). Several fungi, like rusts and smuts, parasitise, crop plants affecting their productivity.
There are plants like Loranthus that are partial parasites infesting the stems of hundreds of the tree species. There are some other parasites like Cuscuta on stem and Orobanche on roots have no chlorophyll and are complete parasites. The Cuscuta stem twines around a large variety of hosts and at contact points pierce their haustoria into the host tissue to derive its nutrition. In Orobanche the seeds germinate near the host plant and soon get attached to the host root.
The host and parasite appear as separate plants but the contact is there with in the soil and thereby appreciably affecting, the yield of cultivated plants of family Brassicaceae and Solanaceae.
In this case an epiphyte grows on other plants but do not derive food from them. For example, lianas are woody plants with roots in the ground but taking the support of other plants to climb. Lianas arc common in tropical rain forests where light at ground level is scarce due to dense growth of tree species. Utricularia. (Bladder wort) and some carnivorous plants like, Nepenthes (pitcher plant) grow on other plants, but derive food from insects.
In Nepenthes, leaf lamina is modified into a pitcher-like structure with a lid. Zooplankton enters into the structure through the lid, get trapped and the soft parts are digested. These adaptations help meet the nitrogen requirement ofthe plants growing in humid forest vegetation.
In any ecosystem, the species compete for space, light, nutrient and water. These competitions may be intraspccific or interspecific depending on situations. Plants with similar habit compete severely to get their share of resources from overlapping niche dimensions.
But when there are many a species having different habit and growth pattern and sharing a common habitat, there arc very interesting adjustments among competing species to get their share of resources. Some species have the capacity to secrete chemicals that inhibit the germination of seeds of other species.
This is called ‘allelopathy’ or chemical control of distribution of plants. There arc also some lower organisms that secrete ‘antibiosis’ to eliminate competing organisms (for example Penicillium produces ‘Penicillin’)
Depending upon their water requirement or more specifically upon the quantity of water available in their habitats, we find three ecologically distinct groups of plants viz.
(a) Hydrophytes: Plants growing in or near water,
(b) Xerophytes: Plants growing in habitats where there is poor avail- ability of water (dry habitat).
(c) Mesophytes: Plants growing in habitats that are neither very dry nor very wet.
Ecological adaptations in the two important ecological groups- hydrophytes and xerophytes are described below.