Effects of Biotic Factors on Vegetation

The biotic factors include the influence of living organisms, both plants and animals upon the vegetation.

Any activity of the living organism which may cause marked effects upon vegetation in any way is referred to as biotic effect.

The biotic effect may be both direct and indirect. It may be beneficial to the plants in some respects but detrimental in other respects.

The plants live together in a community and influence one another. In the forest there are many plant communities, such as trees, shrubs, herbs, mosses, lichens. These communities interact with one another and adjust according to environmental conditions. Trees cast their shadow on many shade-loving plants which grow around or beneath them. The micro-organisms, such as bacteria, algae, fungi, and viruses affect the life of plants of a given area in many ways.

Besides these, the decomposition of dead parts of plant bodies causes significant addition of organic compounds and humus to soil. In this way, vegetation modifies the habitat to a considerable extent. Similarly, animals which are in close association with plants also affect the plant life in one or several ways. Many animals use plants as their food and for shelter as well. Besides animals, the man is most significant agent for modifying the vegetation.

The biotic effects modifying the vegetation can be discussed in the following heads:

(1) Interactions between the plants and local animals and man.

(2) Interaction among plants growing in a community.

(3) Interaction between plants and soil micro-organisms.

1. Interaction between Plants and Local Animals and Man:

These can be described under the following heads:

(i) Effects of grazing and browsing by animals.

(ii) Role of animals in the pollination.

(iii) Role of animals in the dispersal of seeds and fruits.

(iv) Insects and carnivorous plants.

(v) Effects of human activities on vegetation.

(v) Myremecophily.

(vii) Miscellaneous effects.

(i) Effects of grazing and browsing:

Grazing means eating away of un-harvested herbs as forage by animals (Fig. 2.13), as for example, eating away of grasses by goats whereas browsing refers to a similar use of shrubs or trees by animals (Fig. 2.14), as for example, eating away of leaves and small twigs of Margosa (Neem) by camels.

The animals destroy a large part of Vegetation by grazing and browsing. Some animals prefer to graze and browse on some particular plant species they show selective grazing and browsing, e.g., sheep normally prefer forbs, horses and cattle prefer grasses and goats and deer prefer woody and leafy Parts of plant. Small annual plants become uprooted and disappear after being grazed. In browsing, taller plants such as trees and shrubs are little affected.

Various other effects of grazing and browsing are summarized briefly in the following points:

(a) The grazing and browsing adversely affect the aeration of soil and make it compact and hard and finally render the soil unfit for the growth of trees and shrubs. Forests open to cattle are changed first into shrubby vegetation’s and finally into grasslands. Excessive grazing and browsing may thus change the pattern of vegetation and finally lead the area to develop into desert.

Murphy (1951) is of the opinion that Sahara desert developed as a result of unchecked and excessive grazing by goats, sheep and by introduction of camels in the area. Experimental works near the periphery of Sahara by Kassas (1956) have shown that many vegetational changes took place when grazing by animals was completely checked. Unrestricted grazing and browsing are the main causes for the eastward spread of desert in the part of Punjab, Delhi and Rajasthan.

(b) The grazing and browsing reduce greatly the frequency of photosynthetic organs (leaves and apical green parts of stem) and thus curtail the assimilation.

(c)The grazing and browsing reduce the vegetation from the surface of earth to a considerable extent and thus expose the soil for erosion.

(d) The most important effect of grazing and browsing is the trampling. In the trampling complete destruction of small and weak annual herbs is caused by the hoofs, paws and feet of animals, but the shrubs and trees are little affected. Usually trampled area becomes inhabited by special type of plants which can withstand the mechanical effect of trampling. These plants propagate vegetatively and are not dependent upon the seed for their propagation.

Some grasses, Heylendia latibrosa for example, survive better in the grazed habitats than in non-grazed areas! Ecological differentiation due to grazing has been reported in certain other grasses and weeds. Dichanthium annulatum develops underground vegetative buds for reproduction in response to grazing of above ground parts. Plants show certain protective measures such as bitter taste of leaves, stems and fruits, prickles and spines on the surfaces of stems and leaves, which keep the herbivores away from them.

(e) In grazed pasture and meadows, dung avoiding (coprophobic) plants disappear giving place for the colonization of non-coprophilous vegetation.

(ii) Role of animals in pollination:

A large number of plants depend on insects, birds and a number of animals for their pollination. These plants develop coloured flowers. The flowers possess scents, nectar, sap, edible pollens and many other characteristic structures for attracting insects towards them. Insects, birds and other pollinators visit the flowers in search of honey and edible pollens. Flowers in the families Rosaceae, Compositae, Leguminosae, Rutaceae, Umbelliferae, Euphorbiaceae, Cruciferae, Ranunculaceae are pollinated by insects.

Some plants are specialized in their pollination by particular type of animals, for example, Rafflesia is pollinated by elephants and birds, bilipped flowers of Salvia are pollinated by bees, entomophilous flowers of orchids, Ficus and Calotropis are pollinated characteristically by insects.

It is observed that different types of flowers and their pollinators generally live together in the same biotic communities and affect each other’s life. R.F. Daubenmire says that “Extreme specialization is as dangerous in connection with pollination as with other functions in the biological world, for not only must the ranges of two symbionts coincide but also the extinction of the one heralds the doom of the other.”

Besides insects, birds, bats and some other animals, man too is taking active part in pollinating artificially one plant with the pollen of some other plant species. The artificial pollination is being used by man for the production of high yielding and disease resistant plant varieties.

(iii) Role of animals in the dispersal of fruits and seeds:

Many animals, such as birds, bats, monkeys, act as important agents for disseminating the seeds, fruits and spores and thus they play important role in the migration of plants. The seeds of many plants are very hard. Such seeds along with fleshy parts of fruits are swallowed by animals. While passing through the elementary canals of animals hard seeds are not affected by digestive juices.

When the animals leave faecal matter, the uninjured seeds present in it germinate. Passing of seeds through the digestive tracts sometimes facilitates their germination in certain cases. The seeds of tomato, tobacco, guava and many other plants are dispersed in this way.

The hairy, spiny, hooked and sticky fruits and seeds of some plants get entangled with the bodies of birds and other animals and with the clothes of man and are brought to distant places. When the animals clean their bodies at some places the seeds are dropped there. Seeds and fruits of Xanthium, Andropogon, Plumbago, Aegle marmelos are dispersed in this way. Ants are good agents for transporting oily seeds and small grains of cereals.

(iv) Insects and Carnivorous plants:

Semi-autotrophic insectivorous plants, as for example, pitcher plant, Drosera (Fig. 2.15A) Aldrovanda (Fig. 2.15B), Dionaea (Fig. 2.15C), bladderwort, etc., grow in the habitats which are deficient in nitrogenous compounds. These plants have some specialized organs and mechanisms for trapping and assimilating the preys.

Pitcher plants have leaf pitchers containing liquid and enzymes inside (Fig. 2.16). When the insects are trapped down in the pitcher they are digested and assimilated by it. In Drosera spathulate leaves are covered with sensitive glandular hairs which shine in the sunlight and attract insects and small flies. When the insects are entangled in the glandular hairs of leaves, digestive enzymes are secreted immediately which kill and digest the bodies of insects. The digested parts of insects are absorbed by the surface cells of the leaves.

Utricularia (Bladderwort) is an aquatic insectivorous plant which is commonly found in ponds of India. Its bladders catch and digest small swimming aquatic animals (Fig. 2.17).

(v) Effects of human activities on vegetation:

Man affects vegetation in the following ways:

(a) By cutting, felling and replanting the forest trees.

(b) Cultivation:

Besides the old methods of cultivation, man has adopted a number of advanced methods for cultivation of plants. Cutting, budding, grafting and other methods used by man are proved beneficial for certain plants. Now at various research stations men are performing cross breeding experiments to evolve new varieties of plants that give high yields and are disease resistant. In cultivation, the destruction of weeds by man eliminates the competition among the plants. Proper spacing of plants during cultivation also checks the competition among them for food.

(c) Fire:

Fire is a biological factor rather than a physical factor because it is mostly caused by man’s activity. Lightning initiated fires have destroyed plants and animals since their early appearance on earth. In some countries, especially America and Africa, a lot of work has been done on the effects of fire on different ecosystems. A large body of information has developed on the effects of fire on grasslands and forests as well as on the use of fire in land management.

The branch of ecology which deals with the effects of fire on ecosystem is called “Fire ecology” or Ecopyrology. Plants having ability to withstand fire with little or no damage are referred to as Pyropliytes. A number of pyrophytes are known to occur in Siwalik hills. Important examples of pyrophytes are Cochlospermum religiosa, Combretum nanum, Grewia sapida etc. These small plants are supposed to have become permanently dwarf by annual jungle fires. Pyrophytes are mostly woody plants with thick bark.

Fires caused by man’s activity are responsible for complete destruction of vegetation at certain places resulting in temporary or permanent alterations in the characters of vegetation’s. In some parts of tropics and subtropics, especially in Africa, the burning of grassland has been a regular practice for the last many centuries.

Generally in fires the aerial parts of plants are destroyed completely but their roots, rhizomes or other underground parts may sometimes remain unaffected which under favourable conditions may grow and produce new plants. Fire generally makes the area suitable for the growth of grasses and thus improves the quality of forage. Post- bum plants are preferred by herbivores. Animals grazing on burnt grasslands are found to gain weight more rapidly than those grazing on un-bunt grasslands. Fire removes harmful plant and animal parasites and pests.

Litter accumulations physically prevent the healthy production and growth of some plant species in grassland. Fire not only removes the choking litter accumulation but also reduces the organic debris to ash. It affects the productivity by stimulating both the above and below ground growth, increases flowering in forbs and seed production in grasses, increases certain species like legupies and improves nutrient contents of the grassland species.

Burning in normal course does not affect the grassland soils adversely and generally improves them. Mineral salts of calcium, magnesium, potassium and phosphorus increase with burning. Excessive burning may reduce the humus content and the fertility of the soil. Soil acidity increases and erosion is accelerated. Annual burning coupled with continuous heavy-grazing will have detrimental effects on the health of grassland.

The grazing-burning rotation is very useful in the management of Pampas or Savannahs. In the ancient times man used fires to clean forest vegetations and also for lightening purposes. Vansteen (1936) is of the opinion that many deserts in the old world are man-made. Recent works in Australia by Spech et al. (1958), Coaldrabe (1951) and in Canada by Schmidt (1960) have thrown light on the significance of fire in determining the type and composition of plant communities.

(d) Man also clears the vegetation for making houses, roads, etc.

(e) In ancient times many human invasions took place in India which caused great destruction of vegetation. Alexander (330 B.C.), Muslim invasion after 850 A.D. Gori and Gaznavi, and Rajput invasions destroyed dense forests and converted them into deserts. Mohenjo-Daro and Harappa are examples which are supposed to have become deserted as a result of human invasions. The excavations of Mohenjo-Daro and Harappa indicate that shrubby plants were abundant in the desert areas of Punjab and Sindh.

(vi) Myremecophily:

Sometimes ants take their abode or shelter on some trees such as Mango, Litchi, Jamun, South American Acacia (Acacia sphaerocephala) (Fig. 2.18) and so on. These ants act as body guards of the plants against any disturbing agent. In lieu of this defence, the plants provide food and shelter to these ants. This phenomenon is known as myremecophily.

(vii) Miscellaneous effects:

The animals also affect the plant life in many other ways. Some animals, as for example, barkeater, rodents may kill a large number of trees. Juice sucking insects, woodpeckers, bud eating birds, sparrow, squirrel and other animals cause great harm to the vegetation. Elephants detach the branches of the trees and sometimes uproot the gigantic trees. The insects, birds, squirrels, mice and rodents eat abundant seeds. Some animals eat and destroy seeds at the sowing time. Fishes, ducks and other aquatic animals depend upon aquatic plants for food and shelter.

Termites are chief agents for destroying seeds, seedlings and standing crop plants in the fields. Termites and their termitaria serve important ecological function in African Tropical wet-dry soils which has been appreciated and utilized by indigenous African agriculturists in northern Malawi and neighbouring south-western Tanzania Howard W. Mielke (Tropical Ecology. Vol. 19, No. 1, 1978) has highlighted the importance of termites and termitaria in Tropical wet dry Africa. Termites and termitaria contribute to the development of soils.

The enhancement of the mineral content in termite mounds as compared to adjacent soils has been observed by many workers (Hesse, 1955; Stoops, 1964- Lee and Wood, 1971; Watson, 1975; Milne, 1974; Waston, 1969; Bouillon, 1970). This enhancement in mineral contents of termite mounds has been attributed to the interaction of several factors.

Such as:

(1) Termite behaviour in aggregating finer soil particles within their mounds;

(2) Termite behaviour in collecting and storing significant amount of plant matter and accompanying base minerals in their mounds, and

(3) Relatively low rate of leaching occurring on the mounds due to their dome shape and concomitant propensity to shed water. Nitrogen fixing bacteria have been found associated with some species of termites and as a result of this association some species of termites facilitate the fixation of significant amounts of nitrogen.

Termites are important food in the human diet of Africans of the Tropical dry-wet climatic regime. Termitaria soils are used as a raw material for the production of cement, mortar, plaster and bricks and are used in the construction of everything from buildings to roads (Lee and Wood, 1971). Termites and shifting cultivation also provide nutrients and habitats for migratory Palaearctic birds.

2. Interaction among Plants Growing in a Community:

Various plants in a community react with one another in several ways for:

(i) Water,

(ii) Essential minerals and organic compounds, and

(iii) Light and air.

The taller plants modify the habitat for the plants growing around and underneath them by casting shadow, protecting them from injuries by strong wind, by increasing the atmospheric humidity, and by determining the humus content of the soil.

The most interesting instances of interactions among plants growing in a community are as follows:

(i) Action of lianas;

(ii) Effects of some epiphytes;

(iii) Effects of parasitic plants.

(i) Action of lianas:

Lianas are woody vascular Discs plants growing on the ground, maintaining, more or less, autotrophic mode of life and growing upward taking support of some trees and other objects. The woody stems of these plants have well developed alternating vertical columns of secondary xylem and parenchymatous tissues which enable them to twist around the supporting objects (Fig. 2.19).

In tropical evergreen forests, lianas grow at the top of the trees and form the top layer of the forest canopy. This habit enables these lianas to get sufficient light. The lianas affect other plants also because they cast their shadow and check the light from reaching to the plants of lower storeys.

(ii) Effects of some epiphytes:

The epiphytes grow on the leaves and stems of other plants. They are autotrophic and are dependent on other plants only for support. Epiphytes differ from parasites in not taking food from the hosts and also differ from lianas in not having any permanent connection with the soil. The examples of epiphytes may be found in the families Orchidaceae, Asclepiadaceae, Bromeliacease. Cactaceae, etc. Dischidia, Tillandsia are most common examples. Epiphytes are found in humid climates.

The two main problems for these plants are:

(a) Maximum absorption of water from the atmosphere and from the bark surface of the supporting plant and

(b) Maximum economy in the water consumption. These plants develop two types of roots, namely the aerial and clinging roots.

The aerial roots are thick and have special thin walled porous absorptive tissue, the ‘velamen’ on their surface. These roots absorb rain water and moisture from the atmosphere. The clinging roots fix the epiphytes on the surface of supporting plants. Because the epiphytes are autotrophic, they do not affect the supporting plants to any considerable extent.

(iii) Effects of parasitic plants. Some plants are heterotrophic and are dependent on other plants for their food requirements. They are called parasites.

These are of the following two types:

(i) Ectoparasites (external parasite); and

(ii) Endoparasites (internal parasites).

The endoparasites are more destructive than the ectoparasites. Because the parasites take their food from host plants, they check the growth and ultimately cause the death of their hosts. Cuscuta, Loranthus, Orobanche, Rafflesia, and sandal wood tree (Santalum album) are important parasitic angiosperms which may grow either on roots or on stems and some time even on the leaves of the higher plants. The parasites may be either obligate or facultative.

Cuscuta (Fig. 2.20 A & B) is an obligate stem parasite on Acacia, Zizyphus and a number of other angiospermic plants. Loranthus is a partial stem parasite on mango. Orobanche grows very commonly on the roots of crucifers and solanaceous plants as obligate roots parasite. Other important parasites are Rafflesia on the roots of Vitis Viscum album on coniferous trees. Striga, one of the smallest angiospermic parasites grows on the grasses. Arceuthobium minutissimum, an interesting smallest parasitic dicot, is an obligate stem parasite of Pinus excelsa.

Besides angiospermic parasites, fungi and bacteria are also known to parasitize plants and cause several destructive diseases in them. Puccinia graminis is a parasitic fungus causing rust diseases in wheat. Mildews, smuts, white rust, damping off, and blight disease are generally caused by parasitic fungi.

Some algae are also known to parasitise several plants and cause diseases in them. Most common example is Cephaleuros which is found on a number of angiosperms.

3. Interaction between Plants and Microorganisms:

Various kinds of bacteria, protozoa, algae, fungi, worms, nematodes and other soil microbes act as important agents which alter the physical and chemical properties of the soils and increase or decrease their fertility. These changes in the soil properties have great impact on the nature and growth of vegetation.

Very often soil microbes, such as nematodes, bacteria and fungi cause many diseases in the underground parts of plants. Viruses too cause several mosaic and other diseases in many plants, as for example, the curling of tomato leaves, mosaic patterns in papaya and lady’s finger (bhindi), bean mosaic, tobacco mosaic, etc. Some microbes secrete growth stimulating substances in the soil which induce the growth of plants.

Besides above effects, the soil micro­organisms show symbiotic activities and many soil fungi form mycorrhizal association with the roots of higher plants. These two phenomena, i.e., symbiosis and mycorrhizal associations are described below.

Symbiotic influence:

Some soil microbes live in close association with plants, both benefiting each other. In this association both the organisms are interdependent and they do not harm each other. This mutual relationship between two organisms is known as symbiosis and the interdependent organisms are called symbionts.

Many cases of symbiosis in plants are known. The nodulated roots of legumes contain nitrifying bacteria (Rhizobium) (Fig. 2.24). These bacteria fix atmospheric nitrogen into nitrogenous compounds and benefit the legumes by supplying nitrogenous compounds in usable form.

The leguminous plants, in return, provide nutrients, water and shelter to bacteria. When the roots of these plants die, decomposition of dead remains takes place in which the nitrogenous substances; nitrite, nitrate and ammonium ions are absorbed by the plants. Nostoc and Anabaena living symbiotically in the coralloid roots of Cycas also fix atmospheric nitrogen.

Lichens also show symbiosis. These are synthetic plants in which algae and fungi live symbiotically. Generally, the algal component belongs to myxophyceae and fungal components are ascomycetous or sometimes basidiomycetous forms. In this association, algal component fixes the atmospheric nitrogen, prepares food and supplies nutrients to its fungal counterpart. Fungal component gives support to the algal component. It also saves algae from desiccation because of its sponginess and high water holding capacity.

Mycorrhizal association:

Sometimes fungi grow on the surface or inside the roots of higher plants. They are called mycorrhizae.

Micorrhizae are of two types:

(i) Ectotrophic mycorrhiza. Fungus (Fig. 2.25).

(ii) Endotrophic mycorrhiza. Fungus penetrates the deeper tissues of the roots and rhizomes of higher plants (Fig. 2.26).

The roots with mycorrhizae are unbranched and without root-caps and root hairs. Fungal hyphae in this association act like root hairs, absorb water and minerals from the soil and supply them to the roots. The roots in return, provide food and shelter to the mycorrhizae. It has been estimated that some species of plants in about 80% families of seed plants have mycorrhizal association.

Blue berries cannot grow without mycorrhiza. Orchid seedlings do not develop properly unless they become infected with mycorrhizae. Recent researches have shown that mycorrhizae play very important role in the root tissues. They regulate acidity and sugar content in the root tissues and enable the roots grow vigorously.