(2) the phenological niche defines the pattern of seasonal development; (3) the regeneration niche specifies the requirements for a high probability of success in the replacement of one mature plant by a new mature plant of the next generation; and (4) the life-form niche relates to an expression of size and annual productivity as well as three-dimensional pattern. The niche concept may best be described under genetics because the characteristics of the niche determine the genetic attributes of the species and its populations. According to Stern and Roche (1974), the niche can be defined as those environmental conditions which allow a population to survive and with which this population interacts. It is often possible to estimate a niche width by studying the environmental components, e.g., the length of the growing season, which may be taken to commence when the temperature reaches a certain threshold level in the spring and to finish when a critical day-length is reached in the autumn. The genetic structure of a population is strongly determined by its environment; a few individuals can constitute a good example of that population, depending on its mating system which itself is genetically controlled.
The niche concept has an important bearing on the theory of evolution by natural selection. Zoologists generally accept the idea that in a community at equilibrium, every species must occupy a different niche (see Krebs. 1972). Botanically, however, it is difficult to comprehend how all the species in a species-rich plant community can possibly occupy different niches (Grubb, (1977); the important distinction in the two cases is that whereas the autotrophic plants all require only light, carbon dioxide, water and mineral nutrients, the food requirements of the million or so species of animals are widely different and variable; the food requirements (niches) of most animals can be readily explained in terms of some 3,00,000 species of plants (which vary in respect of secondary metabolites, etc.) and the existence of three or four tiers of carnivores. In the case of plants, although the persistence of numerous species in the more species-rich communities is well known, the basis of such persistence is obscure. According to Grubb (1977), most botanists tend to ignore the important phenomenon of regeneration in plant communities; most non- successional communities are longer lived than their component individual plants. Death of an individual plant creates a gap which is then filled by another individual either of the same species of a different species.
Grubb has emphasized the great importance of this replacement stage not only for understanding the species-richness but also for understanding the basic processes of evolutionary divergence in plants and for the conservation of plant communities. Niche breadth plays an important role in evolution of plant and animal populations. The breadth of a population’s niche is affected by such factors as resources available, competitors, and the physical environment. The common practice among ecologists so far has been to measure niche breadths without consideration of the relative frequencies of the various resources available to the plants or animals. Feinsigner et al. (1981) feel that wherever possible, due notice should be taken of the quantities of the resources available.
According to these workers, niche breadth should be defined as the degree of similarity between the frequency distribution of resources used by individuals of a population and the frequency distribution of resources available to them. Some communities which have only a few species, e.g., those in tundra, are extremely simple. Others, such as those in tropical rainforests, have thousands of species and are very complex. One objective of studying species diversity is to find out why some communities support more species than others. Communities are often highly structured and can sometime also be saturated with species. The component populations within a community specialize into their specific niches within the overall structure of the community.
Such niche specialization enables them to make efficient use of resources in a given habitat. Some other species may actively compete with a given species for such resources as are available in short supply. An efficient predator or competitor can materially affect the role played by a population in a community. Niches can also be broadly classified into ecological, food, time or place niches. The position an individual occupies in the community is called its ecological niche. The principle of competitive exclusion states that no two species can have exactly identical ecologies; if their niches overlap fully then one population will eventually outcompete the other, especially in a situation where resource is scanty. In other words, two sympatric species having identical ecological niches cannot coexist in a stable equilibrium.
Several different food niches may occur within a trophic level: weasels eat mice, shrews eat insects, lions prey on deer, etc. Time niche is illustrated by those animals which tend to be active at different times of the day. For instance, there are diurnal and nocturnal species. The place niche is exemplified by different species foraging in different ways and in different places. The time, place and food niches are intimately interdependent, and it is a common observation that animals occurring together in a given community tend to differ in at least one of these three kinds of niches.
Recently, attempts have been made to investigate microbenthic communities of sea bottom in the light of modern ideas on resource partitioning, niche packing and species richness. The term meiofauna is generally defined to designate the metazoan fauna that passes through a sieve of 0.5 mm mesh size or whose individual wet weight is less than 104 gm. Microfauna comprises the protozoans. Both meio- and microfauna exhibit elegant morphological adaptations to their mode of life and have high species richness.
Many animals can find their preferred habitat niches. For nematode communities it has been demonstrated that species diversity is generally higher for the interstitial fauna of sands than for that of non-capillary sediments, and also that sandy sediments tend to harbour more “specialist” species than do silty sediments. Nematode communities also show distinct annual successional patterns. Earlier it used to be a common belief that the meio- and microfauna of sea bottom feeds on detritus and diatoms.
Recent studies have shown that this is an oversimplified and unwarranted generalization; these studies have revealed distinct, fairly specialized food niches for many species (see Fenchel, 1978). Fenchel has encountered the following different kinds of feeding mechanisms in a study of food and feeding of 260 species of benthic ciliates alone: filter feeding, browsing, hunting, scavenging and attacking injured animals. This diversity corresponds well to the diversity of feeding organelles in the ciliates. Many species eating diatoms are specialized in the sense that they eat prey of only a certain definite size. For instance, Diophrys scutum ingests much smaller-sized diatoms than does Frontonia marina. Nematodes also exhibit a fairly high degree of food specialization which is remarkably correlated with their mouth morphology. Recent researches show that a considerable part of the animal production at the sea bottom is due to the meio- and microbenthos.
To what extent these benthic communities serve as food for the macrofauna is not known though Giere (1975) has suggested that the meiofauna largely represent a final link in the food chains. One frequently asked question is to what extent can niches overlap? It seems that among coexisting congeneric pairs of ecologically similar species (competing?) living in the same place, the feeding (mouth) parts tend to differ in size by a factor of at least 1.3. When resource is plentiful and competition low, the habitat is unsaturated and the niche overlap can be quite high. Sometimes communities become fully saturated with species. In such cases, differences in diversity from one place to another can arise from either of two factors, viz., (1) species may be more tightly packed in at one place than another, and (2) there may be a greater variety of usable resources at one spot as compared to another.
These two situations are analogous to the presence of smaller niches and larger numbers of niches respectively. Some important factors which influence species diversity include geological history and geographical location of the region, climatic and edaphic characteristics, food resource availability and the range of food items available, environmental productivity, complexity or habitat heterogeneity, and competition and predation. Species diversity has been reported in some cases to be increased by predators also. Predators may reduce competition between their preys species by cropping prey population, leading to a greater niche overlap between prey species which in turn produces greater species diversity. More diverse communities tend to support proportionately more species or individuals of predators as compared to those in less diverse communities as, for instance, in marine invertebrates.
The general modes of community diversity have been found to differ in tropical and temperate areas. In the usually variable climate in temperate regions, natural selection favours high- reproductive rates rather than competitive ability, and selection is in favour of faster development, earlier reproduction and increased fecundity. This kind of selection is called “r- selection,” and it maximizes the intrinsic rate of natural increase usually designated by r. On the other hand, in the fairly constant climate in the tropics, population sizes are more constant and there is increased competition; selection in this case tends to favour competitive ability.
This kind of selection is called “A.’-selection,” since it results in a larger number of individual organisms per unit of resource and since it increases the carrying capacity (designated K) of the environment. It shows the relationship between population density and traits selected for r and K. It may be mentioned, however, that the two modes are not necessarily mutually exclusive. In fact they usually occur simultaneously in most populations though the relative importance of each can vary. Ever since the publication of Gilpin and Ayala’s (1973) paper, ecologists have pursued vigourously the study of density dependence of population dynamics in different kinds of animals, with a view to delineating any predictable patterns. Most researches have tended to support the conclusion that in large mammals, most density-dependent change occurs at high population levels close to the carrying capacity of the environment. Many insects and other short-lived species, e.
g., some fishes, on the other hand, show most density-dependence at lower population levels. Species diversity has been reported in some cases to be increased by predators also. Predators may reduce competition between their preys species by cropping prey population, leading to a greater niche overlap between prey species which in turn produces greater species diversity. More diverse communities tend to support proportionately more species or individuals of predators as compared to those in less diverse communities as, for instance in marine invertebrates.
The general modes of community diversity have been found to differ in tropical and temperate areas. In the usually variable climate in temperate regions, natural selection favours high- reproductive rates rather than competitive ability, and selection is in favour of faster development, earlier reproduction and increased fecundity. This kind of selection is called “r- selection.” and it maximizes the intrinsic rate of natural increase usually designated by r. On the other hand, in the fairly constant climate in the tropics, population sizes are more constant and there is increased competition; selection in this ease tends to favour competitive ability. This kind of selection is called selection,” since it results in a larger number of individual organisms per unit of resource and since it increases the carrying capacity (designated K) of the environment.
It shows the relationship between population density and traits selected for r and K. It may be mentioned, however, that the two modes are not necessarily mutually exclusive. In fact they usually occur simultaneously in most populations though the relative importance of each can vary. Ever since die publication of Gilpin and Ayala’s (1973) paper, ecologists have pursued vigourously the study of density dependence of population dynamics in different kinds of animals, with a view to delineating any predictable patterns. Most researches have tended to support the conclusion that in large mammals, most density-dependent change occurs at high population levels close to the carrying capacity of the environment (Fig. 2.
6). Many insects and other short-lived species, e.g., some fishes, on the other hand, show most density-dependence at lower population levels According to many ecologists, species with high reproductive rates, short life-spans and populations maintained below the environmental resource limits generally show most density-dependent change at low population levels (Fowler, 1981). Similar data for species having low reproductive rates, long life-spans and populations that are more limited by resources (e.
g., large mammals) suggest that most density-dependent changes occur at population levels approaching closely to the carrying capacity. Most large mammals are either herbivorous or carnivorous. The populations of herbivorous mammals are mainly regulated by those of the primary producers. In contrast, the carnivorous mammals are regulated by the higher order consumers, and these mammals tend to show the most change in density- dependent mortality at low population levels.
According to Fowler (1981), as a general rule, large mammals show most of their density dependent change at high population levels whereas those whose populations are highly variable and show high maximum rates of increase exhibit most change at low levels. The former group of species broadly correspond to the “^-selected” group whereas the latter correspond to the “r-selected” category. The involvement of density dependence and other factors in regulating the populations of a leaf-eating insect has been examined by Faeth and Simberloff (1981). Contrary to expectation, these workers found that survivorship was not inversely related to population size if intraspecific competition regulated this insect’s populations in a density-dependent manner. They found that intraspecific competition does not play a major role in regulating population size of Cameraria sp. at the densities commonly recorded in nature. Rather, parasitism and predation maintain the insect’s population so low that there are few competitive interactions operating. The contentions of Faeth and Simberloff are that (1) certain natural enemies of Cameraria maintain Cameraria populations much below the levels at which intraspecific competition might operate; and (2) some apparent cases of density compensation in herbivorous insects are a result of escape from parasitism and predation rather than niche expansion as a result of release from competition.
Diverse aspects of the population biology of animals have been studied in several countries during the last several decades (see Bodenheimer, 1938; Andrewartha and Birch, 1954; Ito, 1980). Some of the views expressed by Andrewartha and Birch (1954) have not stood the test of time, and these have been criticized by other ecologists (see Ito, 1980). Ito mainly focusses on the evolution of reproductive strategies and the consequences for population dynamics.
He has assigned reproductive strategies to two chief categories, viz., (1) high fecundity where food is readily acquired by progeny, and (2) low fecundity coupled with parental care where the young ones have to reckon with difficulties in food procurement throughout their life. The chief factor dictating the evolution of reproductive strategies is, according to Ito, the procurability or accessibility of food to the young. The implications of the two contrasting patterns with reference to survivorship are high early mortality coupled with high fecundity, and mortality being mainly concentrated at reproductive age associated with low fecundity coupled with parental care. The population dynamics emanating from these strategies are, respectively irregular and subject to drastic fluctuations as opposed to fairly small population changes. Ito visualizes an inevitable link between fecundity and survivorship.
Murray (1979) has discussed and pointed out some flaws and demerits in our ideas of some popular population models. According to Murray, ecologists have overemphasized the significance of density-dependence in relation to population dynamics. He feels that the sizes of natural populations do not commonly reflect equilibria under density-dependence. Murray’s criticism of density-dependence, however, stems from his own way of defining density-dependence, viz., as a ‘component of the environment whose intensity is correlated with population density and whose action affects survival and reproduction”. This definition is substantially at variance with that generally accepted by most ecologists, viz.
, that density-dependence operates as survival, fecundity, or both, decrease with increasing population density.