Biotic factors refer to the living organisms that affect plant growth and development in various ways. These organisms, both macro- and micro-organisms, are the living components of the environment which influence the manifestation of the genetic factor
on phenotypic expression. Macroorganisms refer to the animals
such as humans and other mammals, birds, insects, arachnids,
molluscs, and plants while microorganisms include fungi, bacteria,
virus and nematodes.
The effects of these biotic factors on plant expression may be advantageous or disadvantageous, depending on how they interact with the plant. These interactions include mutualism, herbivory, parasitism, and allelopathy.
Mutualism is a species-to-species interaction in which both the biotic factor and the plant are benefited by the relationship. Examples of beneficial influence on plant growth and development through mutualism are:
(1) The symbiotic relationship of the Rhizobium bacteria and leguminous plants. The rhizobia live in the roots of the legume and obtain their supply of energy from the host plant. In exchange, the rhizobia fix atmospheric nitrogen and supplies it to the plant in absorbable form. Nitrogen is an essential macronutrient for plant growth and development. It is a component of enzymes, DNA, and the chlorophyll molecule.
(2) Forest and fruit trees and other plants including annual crops associate with micorrhizal fungi which aid in the absorption of water and nutrients, such as phosphorus and zinc, from the soil.
(3) Birds, insects and bats serve as vectors of pollination, the transfer of pollen from an anther to a stigma, which is a precondition for the development of fruits and seeds from flowers in the angiosperms. As a reward, the pollinators feed on the nectar that the flower secretes or obtain some other benefit from the plant.
In herbivory, plant-eating organisms called herbivore, such as ruminant animals, rodents, insects, and molluscs feed on plant parts. Herbivores with significant deleterious effects on crop growth and yield are called pest. Damage caused by these biotic factors are varied such as death of the entire plant or organs, reduced root, stem, leaf or inflorescence mass, total defoliation, bores and holes on plant parts, and other marks of feeding.
It has been observed that continuous grazing by ruminant animals results to miniaturization of leaves and dwarf stature of molave (Vitex parviflora) and other trees growing in pasture lands. Some insects are associated with plant diseases that may reduce crop yield or kill the entire plant. The aphids, mealy bugs, and scale insects are associated with the sooty mold.
To prevent damage from herbivores, plants have evolved anti-herbivory defenses including chemical and physical strategies. Various plants produce toxic compounds such as nicotine, morphine, caffeine, cyanogenic substances like atropine; mustard oils, terpenoids and phenylpropanes. Some plants produce high amounts of tannins and resins in leaves that prevent the digestion of food (Stiling 1999). Others have spines, as in cacti, and stinging hairs.
Parasitism is an interaction between two organisms in which one organism, called parasite, is benefited but causes harm to another, called host. The parasite steals its food from the host. Microorganisms such as fungi, bacteria and virus injure crops by causing diseases and are called pathogens.
Examples of parasitic plants are the dodder, mistletoe, Rafflesia, and some orchids. (click to read Plant Classification Terminology Based on Natural Adaptation)
According to Papadakis (1978), there is a universal observation that when two plants grow in proximity to one another, both are injured, either one dies, or both grow stunted. He cited experiments started in the early 1930s showing that between very wide limits, plant density per unit area may be increased many fold without any appreciable difference on total yield although yield per plant is lower at high density.
Went (1973) likewise observed that in established vegetation the germination and growth of plants (with a few exception) is inhibited; plant growth is suppressed with increased plant density but the suppression equally applies to all plants. In parts of tropical rain forest where light is limiting and little photosynthesis is possible, only a few herbs grow and most tree seedlings have few leaves. He noted that the growth and development of these seedlings were suppressed by shade and not by plant competition.
He concluded that plant competition exists, but not in the same way as in the animal kingdom where active competition leads to the elimination of competitors. In the following cases, he explained that the term competition may apply: (1) strife against another plant which may lead to its ouster through the excretion or exudation of allelopathic substances; and (2) scramble for a limited commodity like water, light and nutrients which causes mutual inhibition;
Through a comprehensive review including his own experiments which confirmed the Pickering root toxin theory of 1903 and 1917 the results of which were published in 1940, Papadakis (1978) concluded that plants exude toxic substances from their roots; these substances are exuded by all plant species and are noxious to all plants including the root that exuded them; but their exudation and extent of sensitivity to them may vary from plant to plant. He argues that this plant interaction, now called allelopathy, is responsible for the injurious or inhibitory effects of one plant against another.
Plants that cause deleterious effects to another plant that is intentionally grown, or crop, are called weeds. Tolerance to this biotic factor (weed) varies from crop to crop. Plants with large canopies produce shade which suppress the growth of certain plants and yet the same shade favors many sciophytes. Likewise, the resistance of plants to the other biotic factors, herbivores and pathogens, are varied.
(Ben G. Bareja, January 2011)