Role of Man in Plant Dispersal | Biosphere | Geography

In this article we will discuss about the role of main in plant dispersal.

After the Great Ice Age of Pleistocene period man has emerged as the greatest and most powerful modifier of the environmental conditions of the bio­sphere because of his skill and technological advance­ment. In fact, man has introduced a new period of ‘organic upheaval’ in the organic would (including both plants and animals) known as ‘NOOSPHERE’.

Man whether directly or indirectly, deliberately or inadvertently/unintentionally or accidentally, has af­fected and modified each organism (whether plant or animal) and its habitat in the biosphere. Man has brought changes in the plant and animal equilibrium and has increased the number of other species. He has destroyed a few species and has become responsible for the extinction of some species.

He has played the most important role in the dispersal, distribution and redistribution of plants and animals in the different parts of the world. Man has changed and diverted the direction of evolution of plants and animals, has accel­erated the rate of reproduction, breeding and evolution and development of different populations of organic communities.

He has intentionally created and evolved new species of plants and animals through the process of artificial selection. He has introduced new plant species through ‘cross-pollination’ among different types of plants and has encouraged ‘vegetative propa­gation’. Man has developed new species by creating habitats of new environments.

Man has affected the dispersal, distribution, and redistribution of plants and animals and vegetative propagation through the following methods:

(i) Through his destructive activities (e.g., defor­estation for domestic, agricultural and industrial pur­poses, forest fires etc.),

(ii) Through the discovery and development of new types of vegetation and their establishment in suitable habitats,

(iii) Through the removal of plants from their native places and their transplantation in other areas,

(iv) Through agricultural practices, and

(v) Through changes in the habitats.

Man has created new habitats for his own ben­efits by changing and modifying the local physical environment called as hybrid habitats which have two types of locations e.g.:

(i) Near the camp-sites (this was in practice during pre-historic period), kitchen gar­dens, city parks and national parks etc., and

(ii) Newly created areas for the construction of roads, rails, facto­ries and for agriculture. Such areas are created through land use changes mainly through mass felling of trees or extensive deliberate forest fire. There is very fast rate of growth of plants desired by man in such dis­turbed or hybrid habitats because there is no competi­tion among the desired plants and other plants as unwanted plants have been eliminated.

The plants developed in the habitats created by man (hybrid or disturbed habitats) are classified in two categories e.g.:

(i) Camp Follower Plants and Seeds:

Though these plants are unwanted for man but they are always associated with agricultural activities of man.

Man is himself responsible for extensive dispersion and dis­tribution of such unwanted plants because these plants have inherent qualities of:

(a) Their ability for adaptation and adjustment in different environmental conditions, and

(b) High rate of production of their seeds.

The most unwanted weeds associated with agriculture like dendilion, shephard’s purse plantain, chic weed etc. are in fact the creation of man himself and now these have become problematic and enemy to mankind be­cause they adversely affect the agricultural productiv­ity.

(ii) Cultivated Crops:

Cultivated crops are called ‘cultigens’ or ‘cultivars’. When man began his sedentary life he domesticated a few plants through his choice accord­ing to his needs. Man’s choice of selection of a few species of plants was entirely purposive because he selected only those plants in the beginning which could have immediately fulfilled his requirements.

Such plants include sugarcane, paddy, wheat, cotton, pulses, flowers etc. It may be pointed out that the dispersal and distribution of natural plants depend on a variety of factors mainly natural factors and human factors but the dispersal and dispersion and propagation of plants developed by man depend exclusively on man because these cannot survive in different environmental condi­tions rather these grow more successfully in selected environments only.

It is believed that the first domestication of plants of food crops was started in the north-western part of South America and S.E. Asia. That is why C.O. Sauer has called the aforesaid areas as the planting hearth.

Four areas of the origin of cereal plants have been identified e.g.:

(i) Guatemala-Mexico region-where corn was domesticated for the first time and beans and squashes were domesticated and developed at later date and these were. Inter on, dispersed and distributed by man into North America and South America;

(ii) N.W. India-Eastern Mediterranean region represents the first domestication and cultivation of wheat, bar­ley, rhye, oat etc. which were later on dispersed and distributed by man in other parts of the world;

(iii) North China region where millets and soyabeans were developed as domesticated crops and

(iv) Abyssinian highlands region (eastern Africa) – where sorghums were domesticated.

Man has widely dispersed and distributed cereal plants from these aforesaid four principal areas of their domestication in those areas where he has migrated either permanently or temporarily.

The pace of migration of man to different parts of the world during the past 4-5 hundred years has been greatly accelerated because of:

(i) Exploration of new areas,

(ii) Development of fastest means of transporta­tion and therefore reduction in distances in terms of time, and

(iii) Growth in international trade and commerce etc.

All these have resulted in large scale migration of human beings in the different parts of the world and this migration has caused widespread dispersal and distribution of plants and animals from their places of origin and domestication to other parts of the world. It may be pointed out that in the beginning the dispersal and distribution of plants and animals were limited because of the presence of physical (geographical) barriers (e.g., high mountains, seas and oceans, exten­sive deserts and ice covered areas etc.) but man now has conquered these obstacles because of the develop­ment of aeroplanes and ships which can carry man within short time from one end of the globe to the other end.

Previously plants and animals mainly cereal plants were confined to the limits of hemisphere or-continents but now these limits have been removed and the dispersal and distribution of cereal plants have become world-wide for example:

(i) The introduction of potato, tobacco and corn in Europe,

(ii) Introduction of rubber and cincona in south-east Asia from South America,

(iii) Introduction of wheat, oat, rhye, barley, flax, sugarcane, paddy, banana from Asia and sorghum from Africa in South America etc. are the testimony of man-induced dispersion and distribution of cultivated plants.

It may be pointed out that the dispersal and distribution of plants by man is carried out in two ways e.g.:

(i) Intentional dispersion – when man intentionally brings with him the seeds of desired plants or even plants to those areas where he migrates to settle down or a country officially imports the seeds developed in other countries.

(ii) Unintentional dispersion – some seeds are unintentionally transported by man during his migration from one place to another place but he does not know that he is transporting seeds with him.

Such type of unintentional transport of seeds by man may be possible in a variety of ways e.g., the seeds may be attached to the packing, to the means of transport (vehicles, ships, aeroplanes etc.), to the human bodies and his cloths etc. and may be carried to the desired destinations where a few of them may be able to germinate (if environmental conditions of new places are favourable at least to minimum level) and develop while others may perish. There has been phenomenal increase in the number of plant and animal species due to colonization of New Zealand and Hawaiiland by men because the native flora and fauna before the migration of men to these areas were limited in number.

A few plants introduced in new areas by man through intentional transport and dispersion have now become problems for human society. For example, since the time of first introduction of prickly pear cactus as fodder for animals in Australia from South America it has so enormously multiplied due to rapid rate of increase of its pastures that it has adversely affected and damaged the natural source of animal fodder.

Similarly, kudzu (a type of vine creepers) was introduced in the U.S.A. from Japan to check soil and gully erosion but its growth has been so phenomenal that it has covered many areas of the U.S.A. and now it has become very difficult to eradicate this problem­atic plant from the U.S.A. The growth of Ipomoea Cornea, brought in India from North America, has been so rapid and enormous that it has spread in almost all parts of the country.

This plant has become so problematic that there is need of movement from the government side as well as from public side to eradi­cate this problem-plant otherwise it will cover many of the agricultural fields. The dispersal of animals by man from one part of the world to the other part also poses great danger to native vegetation.

For example, Euro­pean rabbits and red deers brought in New Zealand have done extensive damage to natural vegetation which has a chain effects on the local environment e.g., phenomenal increase in the population of rabbits and red deers has resulted in the loss of natural vegetation (due to mass grazing by these foreign animals) which has caused extensive soil and gully erosion resulting into tremendous loss of good fertile soils and decrease in agricultural production and in the number of native animals.