In his study of California thrashers and other birds he suggested that the niche be regarded as 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.
Thus, he considered the niche as a subdivision of habitat (Udvardy, 1957) or microhabitat (Odum, 1971), or habitat (Smith, 1977). Charles Elton (1927), in his classic book Animal Ecology, considered the niche as the fundamental role of the organism in the community—what it does its relations to its food and its enemies.
Basically, this idea stresses the occupational status of the species in the community, its “profession”. In 1957 G.E. Hutchinson proposed that the niche be considered the total range of environmental conditions under which an organism lives and reproduces.
The Hutchinsonian concept initially was limited to environmental variables affecting a species, such as light, temperature, moisture, height of vegetation, and time. If biological variables are added to the environmental variables, then this, concept of the niche considers both the organism’s place and its function in community.
In a way the community can be considered an aggregation of many environmental and functional variables, each of which can be thought of as a point in a volume of space of infinite dimensions. This Hutchinson termed the multidimensional space or hyper volume.
He also recognized the following two niches: the fundamental niche which is the maximum abstractly inhabited hyper volume when the species is not constrained by competition with others; and the realized niche which is a smaller hyper volume occupied by a species under biotic constraints.
In other words, each species has a fundamental niche, the full range of variables within a community to which it is adapted through natural selection and evolution. However, because of competition few species occupy their fundamental niche; rather they occupy a smaller realized niche. The following two examples will explain Hutchinson and concept more clearly:
1. Root (1967) studied the exploitation of the niche by the bird blue-gray gnatcatcher (Polioptila carrulea) in California oak woodland. He characterized the niche of bird in part by the size of its food (prey-insect) and the height above the ground at which it was captured.
For the sake of example one could consider the maximum range of its food size, prey between 1 and 3 mm, and its foraging area, from ground level to 11 feet, as the gnatcatcher’s fundamental niche. The gnatcatcher’s niche centre, indicated by frequency of captures and stomach content analysis, consists of insects 3 to 5 mm in length taken 8 to 18 feet above the ground.
Further the height and food dimensions are diverted from this centre, the more the gnatcatcher’s niche overlaps those of other species. Depending on the degree of competition, the boundaries of realized niche could be defined by any one of the contour lines.
Fig. 13.6. Representation of a niche space for blue-gray gnatcatcher based on the combination of two variables: size of prey (food) and feeding height above the ground. The contour lines map the feeding frequency (in terms of total diet for the two niche axes) for adult birds during the incubation period in July and August in California oak woodlands. The maximum response level is at H. This represents optimum conditions. Contour lines spreading out from this optimum (i.e., H) represent decreasing response levels the outer contour line represents the outer boundary of fundamental niche for these two variables (after Smith, 1977).
2. Putwain and Harper (1971) studied the niches of two species of dock, Rumex acetosa and Rumex acetosella, each growing in two different grasslands. The two species must compete withgrass species and other forbs. To determine the relationship of these two species, they treated the flora with certain specific herbicides to selectively remove in different plots (1) Grasses; (2) Forbs, except Rumex species; (3) The Rumex species; and (4) All species except Rumex. R. acetosa spread rapidly after the grasses were removed, but R. acetosella increased only when both grasses and forbs were removed.
In this case, the fundamental niche of R. acetosella (R) overlaps the fundamental niche of both grasses (G) and forbs (D). Only when these competitors are eliminated does Rumex realise its fundamental niche. However, R. acetosa overlaps only with the grass species whose removal permits the expansion of Rumex throughout its fundamental niche. The niches for the seedlings differ from that of mature plants.
This shows that the fundamental and realized niches of an organism can change *’ith the growth and development of the organism. This fact is further proved by following examples: an insect with a complex life cycle can occupy one niche space as larva and an entirely different niche space as an adult (e.g., mosquitoes). In other organisms niche space can change as the organism matures, because food and cover (shelter) requirements change as the organism grows larger.
Odum (1971) has suggested that an ecological niche includes three aspects: the spatial or habitat niche, the trophic niche and hyper volume niche. We have already explained the concept of hyper volume niche. The other two types of niches can be discussed as follows:
1. Spatial or habitat niche:
It represents the ultimate distributional or spatial unit occupied by a species. 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. In section an example of spatial niches of leaf miners has been given.
O’Neill (1967) has provided another interesting example of microhabitat segregation in seven species of millipedes. All these species live in the same general habitat, the forest floor of a forest and all belong to the same basic trophic level, i.e., all are detritus feeders.
But each of the seven species predominates in a different spatial niche: Eury- urus erythropygus (in heartwood at centre of logs); Pseudopolyde- smus serratus (in superficial wood of logs); Narceus americanus (at outer surface of logs beneath bark); Scytonotus granulatus (in under log, but on log surface); Fontaria virginiensis (in under log, but on ground surface); Cleidogonia caesioannularis (within leaves of litter); and Abacion lacterium (beneath litter on ground surface).
2. Trophic niche:
In this case two species live in the same habitat but they occupy different trophic niches, because of differences in food habits. For example, two aquatic bugs, Notonecta and Corixa live in the same pond but occupy different trophic niches. The former is an active predator that swims about grasping and eating other animals, but the latter feeds mainly on decaying vegetation.