This occurs whenever species overlap in their utilization of resources so that any increase in the density of one species produces an adverse effect on the others. The observed persistence of many species in nature is really due to environmental heterogeneity; laboratory studies of competition in a uniform environment with a single resource lead to the elimination of all except one species.
Competitive interactions have been shown to profoundly influence the distribution patterns of birds (Diamond, 1972).
In some cases competition between species is difficult to demonstrate as several mechanisms may operate to decrease competition. Several cases are known where even similar species can have different food or feed at different times or occupy different habitats (Schoener, 1974).
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Several workers have attempted to model these differences in terms of competition but these models have not been very convincing. Others have adopted an alternative approach, emphasizing the species’ utilization of a spectrum of resources, and attempting to determine the extent of overlap in resources that can be permitted if species are to coexist (Turrelli, 1978).
Yet another approach has been adopted by Rosenzweig (1974, 1981) and Pimm and Rosenzweig (1981). The objective of these and some other workers was to find out how a given number of species would partition a given number of resources to maximize their fitness.
Pimm and Rosenzweig (1981) have studied the effects of a species’ density (and also of its competitor’s density) on the choice of habitats exploited by the species, and have proposed a 2- species, 2-resource optimal partitioning model. Using the model, they have examined the relationships between various measures of how species compete, whether they choose one or both resources, and whether they overlap in resource utilization.
Several conditions can collectively contribute to complete niche specialization without there being any overlap between the two species. There is a region of state space within which both species optimize their behaviour if they confine themselves to separate niches.
The state space of the two species densities is partitioned into regions where the species take either their own preferred habitats or both habitats. The functions partitioning the regions are called isolegs, and other functions, called isoclines, are superimposed upon the state space. Both these functions (isoclines and isolegs) are functionally dependent.
The intersection of the isoclines on the state space determines the species equilibrium densities. In those cases where the two species coexist using mutually exclusive habitats, the isoclines will intersect perpendicularly. Such conditions are often met with in nature when each species inhibits its competitor strongly in its perferred habitat (Pimm and Rosenzweig, 1981).
One important relationship that permits species to coexist competitively is differential habitat selection. Rosenzweig (1981) has proposed a 3-step geographical theory of habitat selection that applies to two species in an environment with two usable patch types in a matrix of unusable space. The first step assumes that habitat selection is density independent and free of search costs.
The second step assumes density independence. No assumption is made in the third step but it involves the use of a new analytical device, the isoleg, which is a line in a two-dimensional state space of the densities of the two species. An isoleg is a set of points in such a densities space, such that on one side of this set, individuals of a given species optimize their foraging by being strict habitat selectors, and on the other side of the set, they do this by utilizing at least a fraction of a poorer patch (Rosenzweig, 1981).
These isolegs can be used to analyze the population dynamics of two competing species. According to Rosenzweig, the isolegs suggest that the zero isoclines of the species are warped into nonlinear forms that can generate competitive coexistence. At the equilibrium point of this coexistence, no overt competition remains.
In several angiosperms, pollination is limited by interspecific competition for insect pollinators. This has prompted some biologists to think that such competition promotes temporal segregation of flowering times among co-occurring species (Mosquin, 1971; Waser, 1978). If the overlap between flowering times is too high, one of the competing species may become excluded.
Two Swedish biologists, Agren and Fagerstrom, have done much work on this problem. Fagerstrom and Agren (1980) proposed another mechanism to explain the paenological spread. Phenological spread at flowering tends to weaken the competition for the colonization of space between newly produced seeds.
Thus, even if the competing species disperse their seeds at the same time, segregation of their flowering times will be expected to increase the fitness of the competitors because of the increased probability for one species to have a higher seed production when the other species does not do so, thereby reducing interspecific competition for space.
In the causal pathway proposed by these workers, an increase in phonological spread causes a decrease in competition for pollination and space. The increased competition for pollination decreases seed production. Increased seed production in turn increases fitness.
Fitness is decreased by increased competition for space. The total effect of the various factors may be estimated by multiplying the signs. Thus, the routes ABCE and ADE, lead to increased fitness from an increase in phenological spread, whereas ABCDE decreases the fitness (Agren and Fagerstrom, 1980).
There is much evidence available to the effect that potential competitors partition resources by different mechanisms, but it is also clear that the decree of overlap can change with the availability of the resources. Work done on birds suggests that physical or biological fluctuations generate niches that can temporarily be exploited by more than one species, since full exploitation of a niche always lags behind the appearance of the niche.
Extensive niche overlap between species is sometimes reduced by interspecific aggression and utilization of different feeding levels. Availability of suitable and plentiful resources is known to generate both intra and interspecific interactions which will limit their utilization.
The two major categories of competitive strategy commonly recognized by ecologists are the exploiters and the interferers. The interfering animals have a large body, long generation time, and are fairly independent of the environmental factors. They may not evolve efficient ways of resource exploitation. They achieve success by physically excluding or pushing away their competitors from the resource area or territory.
Some plants which become dominant in stable ecosystems can also become interferers. They succeed either by occupying more space by their growth or by producing chemical inhibitors which curtail the growth of other plants. Species of Pinus constitute a good example of plant interferers.
A hierarchy of models has been developed (Reynolds, 1992) to study the physiological and morphological effects of CO2 on intra- and interspecific competition in plants. These models are based on whole-plant growth and utilize: 1) the behaviour of isolated plants to model the dynamics of monocultures, and 2) the behaviour of monocultures to model the dynamics of mixed species communities.
Competition is realized through the dynamics of “sustainability”, i.e., the number of individuals that can be sustained in either a monoculture or mixed-species community considering canopy light interception, photosynthesis, partitioning of biomass between root and shoot, and competition for nitrogen.
The models reproduce behaviours that are common in plants and should prove useful in understanding the complex interactions among species in response to elevated C02 and the link between physiological ecology and population biology (Reynolds, 1992).