The interactions among living organisms such as plants and animals are called biotic factors, which may cause marked effects upon vegetation. The effects may be direct and indirect and modifies the environment. The plants mostly which lives together in a community and influence one another. Similarly, animals in association with plants also affect the plant life in one or several ways. The different interactions among them can be classified into following two types they are positive interaction and negative interaction

Positive interactions

When one or both the participating species are benefited, it is positive interaction. Examples; Mutualism and Commensalism.

a. Mutualism:

It is an interaction between two species of organisms in which both are benefitted from the obligate association. The following are common examples of mutualism. Nitrogen fixation Rhizobium (Bacterium) forms nodules in the roots of leguminous plants and lives symbiotically.

The Rhizobium obtains food from leguminous plant and in turn fixes atmospheric nitrogen into nitrate, making it available to host plants.

Other examples:

• Water (Azolla) fern and Nitrogen fixing Cyanobacterium (Anabaena ).

• Anabaena present in coralloid roots of Cycas. (Gymnosperm)

• Cyanobacterium (Nostoc) found in the thalloid body of Anthoceros.(Bryophytes)

• Wasps present in fruits of fig.

• Lichen is a mutual association of an alga and a fungus.

• Roots of terrestrial plants and fungal hyphae- Mycorrhiza

b. Commensalism:

It is an interaction between two organisms in which one is benefitted and the other is neither benefitted nor harmed. The species that derives benefit is called the commensal, while the other species is called the host. The common examples of commensalism are listed below:

Epiphytes

The plants which are found growing on other plants without harming them are called epiphytes. They are commonly found in tropical rain forest.

The epiphytic higher plant (Orchid) gets its nutrients and water from the atmosphere with the help of the hygroscopic roots which contain special type of spongy tissue called Velamen. It prepares its own food and does not depend on the host. Using the host plant only they support and does not harm it in any way.

• Many orchids, ferns, lianas, hanging mosses, Peperomia, money plant and Usnea (Lichen) are some of the examples of epiphytes.

• Spanish Moss –Tillandsia grows on the bark of Oak and Pine trees.

Negative interactions

When one of the interacting species is benefitted and the other is harmed, it is called negative interaction .

Examples: predation, parasitism, competition and amensalism.

a. Predation:

It is an interaction between two species, one of which captures, kills and eats up the other. The species which kills is called a predator and the species which is killed is called a prey. The predator is benefitted while the prey is harmed.

Examples

A number of plants like Drosera (Sun dew Plant), Nepenthes (Pitcher Plant), Lamina Lid Dionaea Tendril Pitcher Insect (Venus fly trap), Utricularia (Bladder wort) and Sarracenia are predators which consume insects and other small animals for their food as a source of nitrogen. They are also called as insectivorous plants.

° Many herbivores are predators. Cattles, Camels, Goats etc., frequently browse on the tender shoots of herbs, shrubs and trees. Generally annuals suffer more than the perennials. Grazing and browsing may cause remarkable changes in vegetation. Nearly 25 percent of all insects are known as phytophagous(feeds on plant sap and other parts of plant)

• Many defense mechanisms are evolved to avoid their predations by plants. Examples: Calotropis produces highly poisonous cardiac glycosides, Tobacco produces nicotine, coffee plants produce caffeine, Cinchona plant produces quinine. Thorns of Bougainvillea, spines of Opuntia, and latex of cacti also protect them from predators.

b. Parasitism:

It is an interaction between two different species in which the smaller partner (parasite) obtains food from the larger partner (host or plant). So the parasitic species is benefited while the host species is harmed. Based on the host-parasite relationship, parasitism is classified into two types they are holoparasite and hemiparasite.

Holoparasites

The organisms which are dependent upon the host plants for their entire nutrition are called Holoparasites. They are also called total parasites.

Examples:

• Cuscuta is a total stem parasite of the host plant Acacia, Duranta and many other plants. Cuscuta even gets flower inducing hormone from its host plant.

• Balanophora, Orobanche and Rafflesia are the total root parasites found on higher plants.

Hemiparasites

T he organisms which derive only water and minerals from their host plant while synthesizing their own food by photosynthesis are called Hemiparasites. They are also called partial parasites.

Examples:

• Viscum and Loranthus are partial stem parasites.

• Santalum (Sandal Wood) is a partial root parasite.

The parasitic plants produce the haustorial roots inside the host plant to absorb nutrients from the vascular tissues of host plants.

c. Competition:

It is an interaction between two organisms or species in which both the organisms or species are harmed. Competition is the severest in population that has irregular distribution. Competition is classified into intraspecific and interspecific.

1. Intraspecific competition:

It is an interaction between individuals of the same species. This competition is very severe because all the members of species have similar requirements of food, habitat, pollination etc. and they also have similar adaptations to fulfill their needs.

2. Interspecific competition:

It is an interaction between individuals of different species. In grassland, many species of grasses grow well as there is little competition when enough nutrients and water is available. During drought shortage of water occurs . A life and death competition starts among the different species of grass lands. Survival in both these competitions is determined by the quantity of nutrients, availability of water and migration to new areas. Different species of herbivores, larvae and grass hopper competing for fodder or forage plants. Trees, shrubs and herbs in a forest struggle for sunlight, water and nutrients and also for pollination and dispersal of fruits and seeds. The Utricularia (Bladderwort) competes with tiny fishes for small crustaceans and insects.

d. Amensalism:

It is an interspecific interaction in which one species is inhibited while the other species is neither benefitted nor harmed. The inhibition is achieved by the secretion of certain chemicals called allelopathic substances. Amensalism is also called antibiosis.

• Penicillium notatum produces penicillin to inhibit the growth of a variety of bacteria especially Staphylococcus.

• Trichoderma inhibits the growth of fungus Aspergillus.

• Roots and hulls of Black Walnut Juglans nigra secretes an alkaloid Juglone which inhibits the growth of seedlings of Apple, Tomato and Alfalfa around it.