Dispersal of Plants: Nature and Factors | Biosphere | Geography

The study of dispersal of plants on the globe involves the consideration of nature and factors of dispersal, dispersal of flowering plants, role of man in plant dispersal etc.

Nature and Factors of Dispersal:

The present-day distribution of plants over the globe is the result of migration of plants over time into ecologically suitable areas.

The spread or migration of plants takes place mainly in two ways e.g.:

(i) Through vegetation propagation, and

(ii) Through seed dispersal.

Since plants do not have mobility like animals, and hence plants spread or dispersal through vegetative propagation is exceedingly slow process and the new shoots which come out from the parent stocks do not acquire the genetic variability of seeds. Thus, the plants coming out from the shoots of the parent stocks are not able to adapt to the variations in habitat where they reach through the gradual process of spreading.

Thus, the dispersal of plants becomes effective through external factors mainly through the dispersal of seeds.

Several features determine the nature of seed dispersal and therefore dispersal and migration of plants e.g.:

(1) Properties of seeds which help in the process of seed dispersal include:

(i) the properties of seeds to stay up in the air while carried away from one place to another,

(ii) the property of seeds of sticking to the bodies of animals,

(iii) the property of sticking to the feathers and feet of birds,

(iv) property of floating in the waters,

(v) the ability of seeds to germinate and establish themselves as plants successfully in new habitats,

(vi) the number and size of seeds and the frequency of their reproduction,

(2) means of transportation,

(3) speed and distance of dispersion,

(4) geographical barriers etc.

The main agents of seed transportation are wind, ocean currents and animals including man who trans­port seeds of various sorts in varying amounts in different ways. Wind transports seeds mainly in sus­pension from one place to another place. Smaller and lighter seeds are more efficiently transported by wind but such seeds are susceptible to high rate of mortality. Birds carry such seeds which stick to different parts of their bodies. Birds are very effective carriers of seeds because they can transport seeds to great distances.

Besides, seeds which are kept in the stomach of birds are also dispersed when the birds release them as wastes. There are certain limitations of seed dispersion by birds i.e., only smaller seeds are carried away by birds; there is high rate of mortality of seeds carried by birds because if seeds are transported to such areas after covering long distances where environmental conditions are not favourable for the germination of newly brought seeds they may die.

Other animals transport seeds through their bodies when seeds are stuck to different parts of their bodies. The nature of seed dispersion by animals depends on the nature of their mobility, slow or rapid, daily, seasonal or annual etc.

Water transports seeds and disperses them through its various types of movements e.g., sea waves, tidal currents and oceanic currents. Man has now emerged as the most potent and effective carrier of seeds be­cause he is capable of increasing the speed and range of seed dispersal to the greatest extent.

There are certain limitations of seed dispersal viz.:

(i) Smaller and lighter (in weight) seeds are more efficiently transported to greater distances but they are susceptible to high rate of mortality;

(ii) Larger seeds are difficult to be dispersed but these have low rate of mortality because they have greater chance of survival for long time due to greater food reserves contained by them;

(iii) The size and number of seeds produced by different plants also affect and control seed dispersal. For example, annual plants produce large quantity of small seeds which can be easily transported to greater distances while many perennial plants produce few seeds but of large size. Such seeds are transported and dispersed as elaborated above;

(iv) Oceans and seas are the most formidable limiting factors of seed dispersal because these are so extensive in size and become so enormous barriers that these are not likely to be crossed over by any means of seed dispersal.

The most signifi­cant carriers of seeds through the oceans are the ocean currents which take very long time to transport them from one end of the land to the other end. On the other hand, it is very likely that seeds will perish during their long journey of water because there are very few seeds of land or fresh water plants which can withstand the long period of their immersion in the oceanic saline waters. Similarly, extensive high mountains and ex­tensive deserts are effective formidable barriers for the dispersal of seeds.

The tolerance factor plays more important role in the dispersal and migration of plants than the rate of seed production and the efficiency of transporting agents of seeds because ultimate success of seed dis­persion depends not only on the ability of seeds to germinate in new environment of the habitat to which they have been transported but also on their ability to establish in new habitat and to adapt to new environ­ment.

The more the tolerance limit of plants to adapt and adjust to new environmental conditions of new habitat, the more is the chance of their survival and greater opportunities and possibilities of their distribu­tion and expansion in extensive areas.