A good example is furnished by grassland bird communities; in each community, the species separate depending on variations in vertical habitat, horizontal habitat, and food type. Those species which overlap in habitat commonly tend to eat different foods.
The same end can be achieved by temporarily separating the feeding times, e.g., two similarly sized terns often take their food at different times of the day. Other modes of achieving complementarity between species of the community include such combinations as habitat and time, habitat and habitat, food- type and food-type, etc. However, in spite of the many known cases of complementarity, many other cases are also known where species similarity along one dimension implies similarity along another.
On the basis of extensive studies on the ecology of lizards and other animals, Schoener (1974) has proposed a working hypothesis to interpret the feasibility of resource partitioning as it relates to particular dimensions. The hypothesis postulates that: (a) habitat dimensions are important more often than food-type dimensions and the latter in turn are important more often than temporal dimensions; (b) predators separate more often by being active at different times of the day than do other groups; (c) terrestrial poikilotherms (i.e., poorly buffered against external temperature change) partition food by being active at different times of the day; (d) vertebrates segregate less by seasonal activity than do lower animals; (e) segregation by food-type is more important for animals feeding on food that is large in relation to their own size than it is for those animals which feed on relatively small food items.
Those animals which mature in size quickly relative to their life span tend to partition food by size more often than other animals, but one notable exception to this general rule is mammals.
Researchers have also indicated that habitat is less often the most important dimension in aquatic animals than in terrestrial animals; an exception to this is terrestrial arthropods.
However, although niche over-dispersion fails to support the hypothesis of random niches, it does not rule out such alternatives to competition as predation and reproductive isolation—it is well known that these alternatives can also bring about species differences.
It is also known that unlike competition which encourages divergence in time of activity or in time of reproduction, predation often leads to a synchronization of these times so as to saturate the predators.
The extent of competition can be assessed by studying changes in properties of single species across communities. Any correlation of such changes with presence or absence of similar species would imply a causal relationship.
However, such comparisons sometime fail to discriminate between effects of competition and those of other mechanisms, e.g., isolation.
Based on his studies of the effects of food resource limitation on inter-specific competition in salamander populations, Jaeger (1972) has proposed a new hypothesis of competitive exclusion: when a critical resource that is shared by two competing species becomes limited or insufficient, one species will ultimately eliminate the other from that habitat where their distributions overlap.
In this proposal lesser emphasis is placed on the niche and greater importance is attached to such resources as food, space, light, etc. The competition becomes fiercer as the resource supply or availability declines. The crucial factor is not the abundance of the resource but its actual availability to the species and such availability is often determined by biotic and physical limitations on species, e.g., foraging capacity.
Two main methods of competition are: (a) through interference, and (b) by differential resource exploitation. In the former, one species may block or suppress its competitors’ access to the resource by such processes as aggression and territoriality. This method seems widespread in some birds.
A good example of competition through differential resource exploitation has been reported by Haven (1973) in marine gastropods. Along the shore in the intertidal belt, two species (gastropod) share the same food resource consisting of microalgae.
When one of the two competing species was removed from fenced enclosures, there was a significant increase in the numbers of the other species and this was correlated with marked increases in the algal populations. Haven concluded that gastropod grazing exerts a strong limiting effect on the algal population and that interspecific competition by exploitation limits growth rates and maximum population sizes of the less successful species.
One aspect of competition that has aroused considerable research interest is the phenomenon whereby two species may remain sympatric but the stronger species forces the weaker species to abandon the limited resource and start subsisting on a substitute kind of resource, e.g., a different kind of food.
Thus, Cameron (1971) has noted that when two species of desert rat thrive sympatrically, the weaker (defeated) species alters its food habits and starts utilizing a different kind of primary food. However, the same two species can also occur allopathically, i.e., one species pushing the other from a part of its food range with the result that the displaced species becomes confined to a refuge from competition. When allopatric, both the species use the same resource and no alteration in resource utilization is involved.
Many ecologists feel that niches of competitors are shaped by mutual co-evolution and that this allows many species to coexist. Species which depend on each other, e.g., predators on prey, coexist together and also co-evolve.
But competing species, which do not depend on each other, need not co-evolve or co-occur (Connell, 1980). Increased diversity, by decreasing the consistency of co-occurrence also reduces co-evolution. Some species compete for a resource other than space; in these the divergence takes place along the resource axes. Each species has a resource utilization curve along a resource axis as, for instance, seed size in grainivores.
What factors promote co-evolution of competing species? The probability of co-evolution is greater in pairs of species occurring on different trophic levels than in those at the same trophic level. The same holds for communities with low species diversity in which there are low rates of changes in species composition. In fact, quite a number of ecologists believe that co-evolution between competitors is unlikely (Connell, 1980). According to Connell, the best evidence for the effect of various factors on the rate of co-evolution comes not from competitors but from the evolution of resistance by plants to parasites and pathogens.
According to Connell, ‘if the mechanism of niche differentiation contributes at all to coexistence in many-species guilds, it seems unlikely to have commonly arisen by species having diverged by co-evolution. Instead, it is more likely that they diverged as they evolved separately so that, when they later came together, they coexisted because they had already become adapted to different resources or parts of the habitat. Thereafter competition may keep them apart.”
As compared to the effects of competition on species diversity, those of predation are much poorly understood. Nevertheless, it is now known that:
(a) in certain cases, predation can reverse the outcome of competition among prey, and in other cases, predation does not have any effect on the consequences of competition; and
(b) when prey population is small or rare, the individuals may develop differences in appearance so as to escape predators whose prey-specific consumption rates vary with that prey’s abundance.
Indeed, the interaction of predation and competition effects at present constitutes one of the major challenges of ecology. Individual size and trophic position are significantly important factors in this context; thus, herbivores seem to be mostly regulated by predation whereas larger animals and carnivores are more affected by competition.
Recent studies of bird communities have indicated that weak or diffuse interspecific competition can result in a reduction or restriction in niche volume (see Herbert et al., 1974). Bird species have been found to expand niche dimensions in those areas which have very few species. The intensity of competition between species is generally inversely related to their phenotypic isolation, i.e., phenotypically similar species have the severest competition and vice versa.
The results obtained by Herbert et al., on the analyses of abundance patterns in lepidopteron in relation to resource availability have indicated that the generally accepted view that insect populations are chiefly regulated by density-independent factors, may not be tenable. The work of Herbert et al., has suggested that the common species in a community may often be phenotypically isolated from competitors.
That competition is a vital factor in determining community organization, has been convincingly discussed by Cody who found that variation in foliage height diversity can account for some 80% of the variation in species diversity of birds.
However, Cody has also highlighted some of the flaws and demerits of the current competition theory e.g., supposing that the extent of competition for food could be measured separately from that for space or habitat, how to combine the two values meaningfully? Many ecologists would multiply the two figures, but Cody believes that their addition gives better predictions, of community structure.
Two other observations emphasized by Cody are that:
(a) species which occupy a broad habitat range often eat a broad range of foods and that this behaviour is optimal; and
(b) character convergence is a consequence of interspecific competition.
However, Cody has not tested this second postulation in terms of the earlier model of MacArthur and co-workers (see MacArthur, 1965, 1969) according to which there would be character convergence when a species invades a community in which resource overlap amongst species is quite high.
Cody has observed geographical coexistence of several species of sea- birds even in areas characterized by a high food overlap. He attributes this ability to a partitioning of the feeding niche leading to foraging of the different species at different distances from the sea-shore. In those cases where niche partitioning is for some reason impossible, competitive exclusion often occurs.
Watt (1973) is of the opinion that stability should be enhanced when there is a large number of species at the same trophic level competing for a single host-species, at the next lower trophic level. In contrast, a single species feeding on a large number of host species at a lower trophic level may be expected to lead to instability.
In fact the concept of diversity- stability may involve both inter-specific relationships at any particular trophic level, and relationship between the various kinds of trophic levels of a community. Watt has also suggested that unduly high inter-specific competition within the trophic level of a parasitoid complex attacking a common host may stabilize that complex thereby buffering it against markedly quick responses to fluctuations in host numbers.
Recent studies of the ecology of certain insect host-parasitoid communities (especially species of the genus (Rhopalomyia) support this conclusion (Force, 1974). Force states that in cases such as Rhopalomyia, one or two of the most effective parasitoids could by themselves bring about greater control of the host plant (Baccharis pilularis), including perhaps, a more stable system, than the whole complex of parasitoids is able to accomplish. Force found that the insect populations of the Rhopalomyia community fluctuated greatly and irregularly in both percentages and numbers and sometimes these populations also became locally extinct.
Extensive studies of several important parasitoid species have indicated that their interactions apparently fail to confer any great community stability. Each species seems to have evolved merely to snatch or grab whatever is possible from some members of the community and it achieves this goal by perfecting its competitive mechanisms, often at the cost of higher reproductive capacities.
No empty niches that might be filled by organisms of correct specifications seem to exist; this is because new niches tend to be created out of parts of older and broader niches that were previously occupied by reproductively efficient organisms. Force (1974) believes that we might have read too much into community organization: “Perhaps the filling of niches is essentially nothing more than the haphazard result of competitive jostling among species; and that as communities develop, they are not necessarily programmed for such things as greater stability or better energy utilization—the species merely become more closely packed” (Force, 1974).