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It includes an analysis of population dispersion and various types of interactions.
1. Population Dispersal:
Movement of individuals into and out of the population is called population dispersal. It plays important role in the geographical distribution of organisms even to the areas previously unoccupied by the members of population.
Dispersal of organisms occurs for various reasons such as food, protection, prevention from overcrowding, action of wind and water, environmental factors, such as light, temperature, breeding behaviour, physiological reasons or for interchange of genetic materials between the populations.
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Population dispersal occurs in nature in the following three ways:
(i) Emigration
(ii) Immigration
(iii) Migration.
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(i) Emigration:
It is one way movement of individuals out of a population. This movement is permanent and causes spread of a species to new areas. Emigration under natural condition occurs when there is overcrowding in the population and is generally regarded as an adaptive behaviour that regulates the population on a particular site and prevents over-exploitation of the habitat This type of dispersal offers new opportunity to the individuals of a population to interbreed with those of the other populations leading to more genetic heterozygosis and adaptability.
(ii) Immigration:
Immigration is one way movement of individuals into a population. It leads to rise in density of population. It may result in decreased mortality among the immigrants or decreased reproductive capacity of the individuals.
(iii) Migration:
Migration is two way mass movement of the entire population. It involves a periodic departure and return of the individuals of a population and occurs only in mobile organisms during un-favourable periods. It is shown by many birds, fishes and certain mammals. In most cases, migration of population occurs for food, shelter or reproduction.
Through this type of movement the chances of utilization of resources in the habitats not previously occupied by any organism are great. However, during migration of population, mortality of numerous individuals may occur due to various ecological hazards, such as temperature fluctuation, scarcity of food, predation etc. Migration has certain benefits for populations as it enables wider dispersion of populations. It also avoids intraspecific competition for food, shelter, etc.
2. Interactions among Populations:
Plants and animals exhibit a wide range of relationships. Individuals of one species interact with the other individuals of the same species or with those of other species.
Various types of interactions are as follows:
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1. Neutralism:
When the presence of one species appears to have no effect on the second species (i.e., no interaction), it is a state of neutralism.
2. Commensalism:
It is one-sided relationship between two species in which one species is benefited but the other is neither benefited nor harmed. Some epiphytes, as for example orchids, are the best examples. Epiphytes depend upon the other trees for support and nutrients. They manufacture their own food but do not help supporting plant in any way.
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3. Proto-cooperation:
It is less extreme type of interaction in which two species interact favorably with each other, though both of them are able to survive separately.
4. Mutualism:
It is an obligatory interaction that is beneficial to both species. The term symbiosis has also been used for this relationship. Mutualism is best demonstrated in lichens. The lichen is composed of two components—alga and a fungus. The fungus supports the alga while the alga supplies food to fungus. Green Hydra presents another example of mutualism. This animal has green photosynthetic alga in the protective ectoderm. The alga gives off oxygen benefitting the animal which, in turn, supplies CO2 and N2 to the plant. Root nodules of legumes containing Rhizobium leguminasarum bacterium provide another example of mutualism in which there is reciprocal beneficial relationship between the root and bacteria.
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5. Amensalism:
In this type of interaction between the two species, one species is harmed or inhibited and the other is neither benefitted nor harmed by the association. Many algae produce extracellular toxic metabolites which inhibit the growth of other algae species. Chlorella vulgaris (green alga) produces chlorellin which is toxic to other species of algae.
6. Parasitism:
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When two organisms live together in which one derives nourishment at the expense of the other, the condition is called parasitism. In the parasitic association, the species which provides nourishment and support is called the host and the one which gets support and nourishment is called the parasite. Several species of plants and animals form parasitic associations with other organisms. A parasite usually parasitizes a host which is larger in body size than it and ordinarily it does not kill the host, at least until it has completed its reproductive cycle.
7. Cannibalism:
This type of interaction is limited within a species in which the bigger individuals kill and feed on the smaller ones. It is a natural method of population control.
8. Predation:
In this type of association and interaction one species (predator) kills and feeds on second species (prey). Predation is important process in the community dynamics. Predator is always larger than the prey. From population ecology point of view predation is the action and reaction in the transfer of energy from one trophic level to the other. It represents a direct and complex interaction between two or more species of eaters and eaten.
9. Competition:
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When in the association of two or more species each species is adversely affected by the presence of the other species in respect of food, shelter, space, light, etc., this phenomenon is termed competition.
It is of two types:
(i) Intraspecific competition:
When competition occurs between the individuals of the same species and their requirements are common, the process is called intraspecific competition.
(ii) Interspecific competition:
In this type of competition, the individuals of different species compete for common materials and conditions.
