Tilman (1990) shows, for a number of models, what we have already seen demonstrated empirically in Section 8.2.6, that when two species compete exploitatively for a single limiting resource the outcome is determined by which species is able, in its exploitation, to reduce the resource to the lower equilibrium concentration, R*. (Satisfyingly, for apparent competition the reverse is true: the prey or host able to support the greatest abundance of predators or parasites is the winner (see, for example, Begon & Bowers, 1995) - a prediction we have seen borne out in Figure 8.17.)
Different models, based on varying details in the mechanism of exploitation, give rise to different formulae for R*, but even the simplest model is revealing, giving:
Here mi is the mortality or loss rate of consumer species i; Ci is the resource concentration at which species i attains a rate of growth and reproduction per unit biomass (relative rate of increase, RRI) equal to half its maximal RRI (Ci is thus highest in consumers that require the most resource in order to grow rapidly); and gi is the maximum RRI achievable by species i. This suggests that successful exploitative competitors (low R*) are those that combine resource-utilization efficiency (low Ci), low rates of loss (low mi) and high rates of increase (high gi). On the other hand, it may not be possible for an organism to combine, say, low Ci and high gi. A plant's growth will be most enhanced by putting its matter and energy into leaves and photosynthesis - but to enhance its nutrient-utilization efficiency it would have to put these into roots. A lioness will be best able to subsist at low densities of prey by being fleet-footed and maneuverable - but this may be difficult if she is often heavily pregnant. Understanding successful exploitative competitiveness, therefore, may require us ultimately to understand how organisms trade off features giving rise to low values of R* against features that enhance other aspects of fitness.
A rare test of these ideas is provided by Tilman's own work on terrestrial ... tested on grasses plants competing for nitrogen (Tilman
& Wedin, 1991a, 1991b). Five grass species were grown alone in a range of experimental conditions that gave rise in turn to a range of nitrogen concentrations. Two species, Schizachyrium scoparium and Andropogon gerardi, consistently reduced the nitrate and ammonium concentrations in soil solutions to lower values than those achieved by the other three species (in all soils but those with the very highest nitrogen levels). Of these three other species, one, Agrostis scabra, left behind higher concentrations than the other two, Agropyron repens and Poa pratensis. Then, when A. scabra was grown with A. repens, S. scoparium and A. gerardi, the results, especially at low nitrogen concentrations where nitrogen was most likely to be limiting, were very much in line with the exploitative competition theory (Figure 8.32). The species that could reduce nitrogen to the lowest concentration always won - A. scabra was always competitively displaced. A similar result has been obtained for the nocturnal, insectivorous gecko Hemidactylus frenatus, an invader of urban habitats across the Pacific basin, where it is responsible for population declines of the native gecko Lepidodactylus lugubris. Petren and Case (1996) established that insects are a limiting resource for both. The invader is capable of depleting insect resources in experimental enclosures to lower levels than the native gecko, and the latter suffers reductions in body condition, fecundity and survivorship as a result.
Returning to Tilman's grasses, the five species were chosen from various points in a typical old-field successional sequence in Minnesota (Figure 8.33a), and it is clear that the better competitors for nitrogen are found later in the sequence. These species, and S. scoparium and A. gerardi in particular, had higher root allocations, but lower above-ground vegetative growth rates and reproductive allocations (e.g. Figure 8.33b). In other words, they achieved their low values of R* by the high resource-utilization efficiency given to them by their roots (low Ci, Equation 8.19), even though they appeared to have paid for this through a reduction in growth and reproductive rates (lower gi). In fact, over all the species, a full 73% of the variance in the eventual soil nitrate concentration was explained by variations in root mass (Tilman & Wedin, 1991a). This successional sequence (see Section 16.4 for a much fuller discussion of succession) therefore appears to be one in which fast growers and reproducers are replaced by efficient and powerful exploiters and competitors.
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