Population Characteristics-VI

6.                 Dispersal

Geographical differentiation does not allow organisms to freely reach their potential range of total land area as various biological and physical factors prevent their distribution to differential areas.

Although when an organism dispersed to a new area and colonize there by adapting to the external environment conditions, create a new species and thus support evolution.

Hence, dispersal is an ecological process of organism distribution in a habitat. It’s a self made distribution of organism which requires several mechanisms to move across the geographically differentiated habitats.

Plants and immobile animals show a low dispersal rate as a means of transportation. Active process of distribution is adopted by movable animals depicting high dispersal rate. Among all living organisms, human has played a prominent role in worldwide dispersal and redistribution of species. Dispersal is influenced and stimulated by many factors such as food supply, food quality, change in temperature, high reproduction and photoperiod and operates when organism leave their native place where they were born.

Thus, by dispersal, organisms change the area of habitat. The first movement of individual animal towards its first breeding attempt from its birth site is denned as Natality Dispersal. When an adult changes its location and settle to a new place it’s called Adult Dispersal.

The modes of dispersal are:

i.       Migration

It’s a two way regular movement of animals during different seasons, usually for feeding and breeding purposes. For example Roebuck Bay is a migratory bird, Western Australia.

Figure 8: Migratory Roebuck Bay, Western Australia.

Some species aimlessly do regular movements and never settle down to one habitat called as Nomadism, while mostly all other migratory animals return back to their original place after this seasonal movement. Nomadism exhibiting species have unpredictable possibility of food availability from years to years and thus, wandering without settling.

Migration and Nomadism are not the actual dispersal mechanisms.

ii.                 Emigration

It’s a unidirectional outward movement of organism from present population to a new population. These outward movements occurred because of scarcity of food/natural resources, high fecundity rate and many more unfavourable conditions. The migrated organisms never return back to the population where it was born.

Figure 9: Emmigration of birds

iii.                   Immigration

A unidirectional inward movement of organism into a new population from its native population for reproduction and feeding purposes is termed as Immigration.

Figure 10: Immigration of Birds

7.                 Dispersion

Dispersion is the distribution of individuals within its geographical area. The pattern of dispersion by individual defines their relative spacing. Different population of same species and different species can have different pattern of dispersion. The social behaviour or other traits of organism’s resource utilization, and other conditions influence the pattern of organism dispersion.

The general patterns followed by organisms are of two types: Temporal and spatial dispersion

a.     Temporal Dispersion

When the dispersion pattern of organism is regulated by time, it is termed as temporal dispersion. For example some insects are dispersed according to the circadian cycle, while some are regulated by lunar and some by tidal cycle.

b.    Spatial Dispersion

Spatial dispersion is majorly classified into three types of  dispersal: a. Clumped (contignous/ aggregated), b. Random and c. Uniform (even spaced).

i.                   Clumped Dispersion

A population disperse in large geographical area as tightly packed “clumps” or clusters exhibit clumped dispersion.

These clusters are apart and between them nearly no organism can be found. Some organisms form clusters to attain protection from predators and some for the resource utilization.

ii.                 Uniform Dispersion

When the organisms are uniformly or evenly distributed in their habitat, the mode of dispersion is called Uniform dispersion. This tendency is among those species which can survive anywhere.

iii.              Random dispersion

When the organism is distributed unevenly and randomly, it’s known as random dispersion. Random dispersion is possible under uniform conditions where each organism has an equal chance to habitat place. It does not involve aggregation.

Figure 11: Dispersion; (a) Uniform dispersion (b) random dispersion and