Biotic factors are influences, reactions and interactions of living beings not only amongst themselves but also over the environment. All living beings are collectively termed as biota. Major components of biota are flora, fauna and microbes. Interactions of biotic components bring about modifications. Biota also brings about changes in the environment.
Interactions that occur amongst members of the same species is termed as intraspecific interactions. Interactions that occur amongst members of the different species is termed as interspecific interactions. Growth, movements and reproduction are dependent upon mutual relationship between individuals of the same or different species. Biotic factors are, therefore, conveniently studied under the following headings:
- Interrelationships between different plants.
- Relationship among plants and animals.
- Interrelationships between microorganisms and plants.
Interrelationship between different plants:
The individual of the same species growing in the same area show a severe competition for foods, light and water. These could also be termed as intraspecific competition. The following are some major examples of mutual relationships between individuals growing in the same area.
These are woody climbers. The roots grow into the soil while the plant itself climbs along the tree trunks up to the top to obtain sunlight. Lianas are commonly found in tropical or dense forest. Some of the common examples are Lianas are Tinospora, Bauhinia etc.
These plants grow on the stems or branches of other plants but do not obtain food from them. Epiphytes commonly occur in tropical rain forests. Some of the common examples are orchids such as Vanda, many mosses and ferns.
Epiphytes absorb water and minerals from the soil present in cracks and crevices of the outer surface of a tree by their absorbing roots. Epiphytic orchids have special hanging roots. These roots have a special tissue termed as Velamen present just outside the exodermis. It absorbs atmospheric moisture.
These are the organisms which live on other organism (host) and obtain their food from host plant. Parasites do not have contact with the soil. They develop sucking roots or haustoria to obtain food from the host plant. Parasites could be total parasites, i.e., obtain their food, water and minerals from the host plant. Other group of parasites called partial parasites, synthesize their own food but obtain water and minerals from the host plant.
Following are the different types of parasites which grow on stems or roots:
1. Total stem parasites, e.g., Cuscuta.
2. Partial stem parasites, e.g., Viscum, Loranthus
3. Total root parasites, e.g., Rafflesia
4. Partial root parasites, e.g., Santalum album (sandalwood), Striga
The best example of symbiosis between two plants is lichen. In lichens, algae and fungi live together in such a way that they appear as one organism. Both are mutually beneficial to one another. Algae provides food for the fungus which in turn, supplies humidity and mineral elements to the algae in addition to protection.
Interrelationships between plants and animals occupying the same space.
Following are some of the interaction occurring between plants and animals occupying the same habitat.
Honeybees, moths, butterflies etc., are essential for pollination. Salvia, Ficus, many members of Compositae, etc., are dependent on insects.
Dispersal of seeds and fruits
Animals play an important role in the dispersal of seeds and fruits. Birds eat fruit and excrete the undigested seed. These are, thus, distributed over a large area. Many types of fruits and seeds attach to the body of animals and are carried to long distances, e.g., grasses, Xanthium, Achyranthes etc.
These plants feed on insects. They are autotrophic and manufacture their own food material. Insectivorous plants grow on the soil deficient in nitrogen. The deficiency is overcome by digesting the insects. Leaves of these plants are generally modified and secrete proteolytic enzymes. These enzymes digest insects and provide nitrogen to the plants, e.g., Drosera, Nepenthes, Utricularia.
This is an association of ants with tree. The ants find a suitable habitat in the tree while trees get protection from the ants. This is common in plants such as, mango, litchi, guava etc.
This is a direct food relationship between two species of animals. One of these termed as predator, captures and feeds on another animal termed as prey. Tiger eating a deer is an example of predation, where tiger is a predator and deer, the prey.
The herbivores feed upon herbs, shrubs or trees. As a result of grazing aerial photosynthetic organs reduce. Thus there is a decrease in the production of food by photosynthesis. The roots starve, plants become weak and ultimately die due to overgrazing. Selective grazing reduces the number of palatable plants. The remaining plants which are not eaten by animals, grow to form the dominant vegetation of the area.
These eat underground plant organs and leaves causing harm to the plants. Rodents such as, rats squirrels, etc., eat seeds in large amounts and thus reduce the multiplication of plants.
Interrelationships between microorganisms and plants
The soil microorganisms such as, bacteria, fungi etc., are helpful to plants. Following are some of the common phenomenon:
Symbiotic nitrogen fixation
This is a good example of symbiosis or mutualism. Bacterium e.g., Rhizobium, lives symbiotically inside the root nodules of Leguminosae. The bacterium fixes atmospheric nitrogen which is very important and essential for plant growth and metabolism. Besides the member of Leguminosae, bacterial root nodules also occur in angiosperms (e.g., Casuarina, Myrica etc) and gymnosperms (e.g., Podocarpus).
This is a symbiotic association between fungi and roots of higher plants. The fungi may be present inside the root or outside the root. If the fungus is present inside the root, specially the cortex (e.g., members of Ericaceae). The association is termed as endotrophic mycorrhiza. If the fungus occurs on the surface of the root (e.g. Pinus oak etc.). The association is termed as ectotrophic mycorrhiza.
This is a complete or partial inhibition of one organism by another either by secreting substance or by modifying its immediate environment. The substances or conditions produced by an organism are generally harmful for the other. This is also termed as biological antagonism. The production of antibiotic substances which inhibits vital metabolic activities of other organism, is a common example of antibiosis.