Ecological Niche

The term niche was first described by Grinnell to represent the ultimate distributional or spatial unit occupied by just one species or subspecies , to which that species is held by structural and instinctive limitations, such as climatic factors , kind and amount of food ,suitable nesting sites and cover.
According to Charles Elton niche is an animal’s place in the biotic environment or community with relation to its food and enemies. Thus Elton’s concept of niche was of trophic niche.

Ecological Niche and its Aspects:-
The three aspects of the ecological niche can be conveniently designated as the spatial or habitat niche , the trophic niche and the multinational or hypervolume niche.
Spatial or habitat niche:-
This concept represents the ultimate distributional or spatial unit occupied by a species. Thus, in a particular habitat shared by several species, each of the species may be confined to its own microhabitat (or spatial niche) because no two species in the same general territory can occupy for long identically the same ecological niche,.
Trophic niche :
Sometimes two species may live in the same habitat but they occupy different trophic niches because of differences in the food habits. Two aquatic bugs, Notonecta and Corixa live in the same pond but occupy different trophic niches. The former is active predator that swims about grasping and eating other animals. But the latter feeds largely on decaying vegetation . Similarly herring and tuna fishes live in the same habitat but occupy different trophic niches as they have different food habits. The tuns feeds on herring and herring on zooplanktons.
Multidimensional or hypervolume niche :-
According to Hutchinson , a niche could be visualized as a multidimensional space of hypervolume. He also made distinction between the ‘Fundamental Niche’ –the maximum abstractly inhabited hypervolume when the species does not face competition with others and the ‘realized niche’ –a smaller hypervolume occupied under biotic constraints.
Gause’s principle of competitive exclusion’ :-
A Russian biologist G.F. Gause has postulated that no two forms can share exactly the same ecological niche for an indefinite period of time , eventually one form will replace the other. Thus according to so-called Gause principle , each mode of life is filled by just one species , niches may overlap , but a single niche is inhabited by only one form.

Gause principle can be very well explained by the following example: In Africa the greater flamingo ( Phoenicopterus antiguorum) feeds in the same shallow lakes as a similar appearing species , the so –called lesser flamingo (Phoeniconaias minor) .

Fine platelets in the mouth of the lesser flamingo restrict its diet largely to microscopic blue-green algae, whereas the coarser filter structure in the bill of the greater flamingo allows it to ingest larger food particles , including such small animals as tiny crustaceans and mud-dwelling insect larvae. Although both species of flamingo eat form the same ponds, they exploit different niches for their food, and thus avoid competition.
Niche Overlapping-
When two species inhabiting very similar , spatially overlapping niches compete , there are two possible consequences . First , one of the forms may be eliminated, or may become extinct-at least in the locality where the two are competing. If a laboratory population cage containing Drosophila pseudoobscura is infested by even a single gravid female of D. melanogaster , the pseudoobscura population is greatly affected. The rate of reproduction of D. melanogaster is so high and the food requirements of the two species are so similar that the pseudoobscura population decreases away rapidly.
Complexity of niches :
Some niches are more complicated than others. For instance , the niches at the bottom of the oceanic trenches or in certain caves are relatively less complex than those in a forest or shallow lake , for in the former many of the conditions bearing on the niche temperature, light . amount of moisture etc. are far more constant than the latter.
Differences in the complexity of niches also depend on the organisms themselves. Those organisms with complex life cycles usually occupy more than one niche. The mosquito larva , for example , feeds on microscopic aquatic forms found in shallow water , but as an adult it is air borne and feeds on vertebrate blood . Malarial parasite also has a different niche for each stage of its life cycle, for different phases in the life cycle are completed in different parts of human and mosquito body.
Ecological equivalents:-
The animal that occupy similar ecological niches, although geographically separated , are called ecological equivalents. For example , we may say that the kangaroos of Australia are ecologically equivalent to the bison of North America.
Character displacement :-
The phenomenon of character displacement has been explained by Brown and Wilson as follows:-
Two closely related species have overlapping ranges. In the parts of the ranges where one species occurs alone, the populations of that species are similar to the other species and may even be difficult to distinguish from it . In the area of overlap . where two species occur together , the populations are more divergent and easily distinguished. i.e. they ‘displace’ one another in one or more characters.

Sympatric and Allopatric Species:-
The species show displacement when they are sympatric (joint fatherland, or species occurring in the same area but not necessarily in the same niche) and active convergence when they are allopatric ( different fatherland , species found in different geographical regions or separated by a spatial barrier.)
There are two adaptive values of character displacement. Firstly, it enhances niche displacement , thus reducing competition , and secondly it enhances genetic segregation by maintaining species distinctiveness (preventing hybridization ) and thereby maintains a greater species diversity in the community.
e.g. When two species of nuthatches (Sitta ) are allopatric , each is so similar to the other that they can be distinguished only by an expert in bird taxonomy. But when the two species are sympatric , there is a striking difference in morphology so that they can be distinguished at a glance. In one species the bill and the black facial strip become enlarged , while in the other these characters are reduced in size. In this way the accentuated difference in the bill size reduces trophic (food) niche overlap, and the marked difference in the facial strip enhances species recognition and prevents interbreeding.

Advantage of niche segregation :-
The major advantage that animals gain by occupying different niches is escape from continuous intense competition . It is also true that the niche occupied is favourable to the species physically in furnishing suitable substratum and microclimate. Automatic segregation of a species into its niche through inherited behaviour patterns avoids the great expenditure of energy and loss of time that would be required if this segregation had to be worked out a new each year or each generation .
Segregation into niches avoids confusion of activities between organisms in the community and permits a more orderly and efficient life cycle on the part of each species. Furthermore, the segregation of each species into different niches permits the occupancy of the area by a larger number of species, since they will better divide the available resources between them. Similarly , the more distinct the niche of a species is, the more it can avoid conflict with its neighbours and lead a life that is orderly , productive and efficient.

Author:-Dr. Anupama Goyal

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