How Predation is Consider as an Important Factor in the Regulation of Natural Communi­ties?

Any population of a prey species includes some individuals that are either very young or very old. These individuals find it difficult to obtain their food, and they are more susceptible to predation than the young, active, middle-aged and healthy (established) individuals.

In some cases, a predator can catch only those individuals that are displaced or are diseased and dying, but cannot capture any of the established individuals. In these cases, the act of displacement is the chief cause of mortality and not the predation per se, and in these cases the predator is only playing the role of an agent of mortality. In contrast, if a predator captures and eats an established individ­ual, then the predator is not only an agent of mortality but also the cause of mortality.

If predators are absent, then the outermost zone B tends to increase in size as the diseased and dying individuals are not being caught—in such a case, displaced individuals would eventually die, not by predation, but because of starvation and/or disease. In this case, starvation and/or disease become the agent of mortality (Jones, 1979).

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Predation is an important factor in the regulation of natural communi­ties and, interacting with resource partitioning, it may possibly assume a significant role in community structure wherever predators occur. Recent researches on predation have been carried out not only on terrestrial and freshwater communities but also in marine biota, especially benthic fishes, invertebrates, and shore birds.

Grant (1981) has observed the effects of predation by shore birds on a population of the sand-burrowing amphipod Acanthohaustorius millsi. His calculations suggest that shore birds remove about 1.2 x 10^-2kcal. m².d^-1 of A. millsi and that this species comprises about 10 per cent of their caloric intake in an intertidal rocky sea coast in South Carolina.

He has further observed that factors such as burrowing depth and predator avoidance may decrease mortality rate due to shore birds feeding both visually and factually. According to Grant, those factors which deter­mine prey availability are critical predator-prey systems.

Predation is expected to reduce prey densities, but the experimental manipulation of predators does not always elicit any predictable numerical responses from prey population (Bradley, 1983). Several recent studies of predation in freshwater and intertidal marine communities have revealed that predators sometime have positive indirect effects on some community components (Kneib, 1988).

Relatively little is known about predator-prey interactions in tropical areas. The oscillations inherent in such interactions are generally reduced when the predator has a high threshold below which it cannot exploit the prey. Certain attributes of the predator e g , territoriality, prevent built-up of dense populations and, when the environment is heterogeneous, it becomes quite difficult for the predators to find and exploit their prey.

In nature, an ecological system has always a more or less heteroge­neous spatial structure and many ecologists have been interested in the formation of spatial distribution pattern of interacting species and also in the relation between the spatial structure and the stability of the interacting populations.

Comins and Blatt (1974) studied a prey predator system in spatially heterogeneous environment and concluded that the heterogeneity of the environment had a pronounced stabilizing effect on the system The effect of spatial heterogeneity on competing species has been investigated by Teramoto et al. (1978), who have shown that the environmental hetero­geneity plays an important role to stabilize the populations of competing and similar species.

In fact, on the basis of this work, an assertion can now be made to the effect that coexistence of two mutually competing prey species can be stabilized by joining with a predator species, otherwise one of the prey species is likely to become extinct (Teramoto et al., 1978).

Three main theories have been put forward to explain the natural regu­lation of vertebrate numbers. One view emphasizes the importance of densi­ty-dependent factors in the regulation of populations (Nicholson, 1933, 1957). In contrast, Andrewartha and Birch (1954) proposed that resource- limitation was the chief factor regulating the populations.

Milne (1957, 1961), who worked on insect populations, carefully considered both the above viewpoints and came up with a proposal of his own. He disagreed with Nicholson, pointing out that the regulatory effect of most predators on their prey populations is never fully density-dependent. He was of the opin­ion that the only perfectly density-dependent factor is interspecific competi­tion for resources.

Thus, Milne, like many other ecologists, supported the resource-limitation concept. However, one demerit of this otherwise popular concept is that it fails to explain regulation in those cases where resources are not limiting.

Jones (1979) has emphasized the involvement of both resource limita­tion and predation in the regulation of animal populations. Both food re­source and predator numbers regulate the size of the prey population. Jones has also suggested how regulation may occur in a situation where food is not limiting.

For instance, in the case of herbivores, it is not the total food available that is critical but rather the amount of food that can be exploited with relative safety is the limiting factor involved. Likewise, for carnivores, the chief limiting factor is not the total number of prey but rather the number of displaced (somewhat weaker or sick) prey individuals.

Jones assumes that interspecific competition for a limited resource creates a dis­placement pressure within the population, leading to the displacement of some individuals that are more vulnerable to early death as compared to the non-displaced or established individuals.

The predator species primarily eats the displaced prey individuals. Displacement of a fraction of the prey popu­lation also leaves a somewhat greater share of the food resources available to the established individuals, though both the displaced and the established fractions may sometime share the same food as, e.g., the carnivores. In the case of terrestrial herbivores, however, the displaced and established prey individuals do not generally share the same food.

Predators undoubtedly play an important role in the population process­es of all species, and efficient predators can be a major regulator of prey population density. Through their effect on prey populations, predators in­fluence the competitive interactions of prey populations with other popula­tions on the same trophic level.

Habitat patchiness is also a determining factor in the life of microtine animals which are vagile and migratory. Some ecologists have assumed that the habitats in which microtines might find themselves probably have a variable capacity to support these populations but Rosenzweig and Abramsky (1980) have argued that the reproductive strategy required of a vole or lemming in a poor habitat should be expected to differ from that in a rich place.

Rosenzweig and Abramsky have postulated two possible hypoth­eses to account for microtine cycles. (Microtine cycles relate to year to year variations in the populations of small rodents, called microtines. These cycles have been studied mainly in certain voles belonging to the genus Mcrotus, and also certain lemmings, genus Lemmus.

The populations show enormous changes from year to year, alternately exploding and crashing). The role of immigrants in generating microtine cycles finds an important place in these hypotheses.

The predation hypothesis which assumes that voles are equally proficient predators in habitats of differing productivity, can account for the observation that the populations in richer habitats are cyclic.

According to this hypothesis, habitat heterogeneity pre­vents the voles and their food from precise coadaptation. Hence, their sys­tems range of dynamic stabilities is expanded over a productivity gradient, and oscillation becomes a permanent attribute of their censuses.

The second hypothesis is termed the phenol-logical hypothesis. The as­sumption in this hypothesis is that voles track their resources by responding to stimulants and inhibitors in the plants which signal periods of plant growth. As the habitat is not uniform but patchy, the voles found in a richer patch are different from those in poor patches. Immigration of new voles is a barrier to effective local adaptation, and voles sometimes also breed at inappropriate times. This overloads and decreases food quality and is fol­lowed by decline in vole population and selection against sensitivity to the plant stimulants. This is how microtine cycles result.

The phenol-logy hypothesis satisfactorily accounts for the observed maintenance of breeding activity by the mouse (Microtusj during the usual­ly quiescent season preceding a peak (Gaines et al., 1979). Furthermore, after a Microtus crash, breeding rate is quite low, and this is what would be expected of the super insensitive mice left behind as a result of natural selection after the population crash and the plant quality deterioration.

How­ever, though the hypotheses of Rosenzweig and Abramsky have much ap­peal, one important phenomenon, viz., the synchrony of vole cycles over wide areas of a continent, and its relation, if any, with the observed cyclicity, have not been considered.

Plant-herbivore interactions and parasite-host interactions are the two chief examples of predation and are described in detail elsewhere.

Bernays and Graham (1988) and Jermy (1988) agree on the following points:

1. Trade-off estimations have not established the advantage of special­ists over generalists;

2. Secondary plant metabolites do not furnish the ultimate cause for narrow host range but often provide the proximate or immediate cause;

3. Deterrents do not betoken toxicity of plants;

4. Secondary plant metabolites cannot be strictly regarded as defenses against insects; and

5. Local host abundance is more a prerequisite for specialization than its cause.

Jermy disagrees with the conclusion of Bernays and Graham that predation might be ‘ the most important factor pushing insect herbivores toward narrow host range” To test this view, further experimental studies are needed.