In a test of these hypotheses Pimm and Pimm (1982) recorded the feeding choices of three nectar-feeding bird species (Himatione, Loxops, Vestiaria) on the island of Hawaii. There were two main tree species, Metrosideros and Sophora, which came into flower at different times of the year. The evidence for the distinct preference case is seen in Fig. 9.15. When the number of flowers is high, all three species feed on both trees. When flowers per tree are low (and assuming that this indicated limiting resource) only Loxops feed on Sophora, and only Himatione feed on Metrosideros. Thus, both species reduce their niche width and specialize. There was also evidence of shared preference. Vestaria feed on both tree species but only at high flower numbers, and physically exclude the other species by visual and vocal displays. In contrast, both Himatione and Loxops spend much of their feeding time on trees with few flowers. Thus, these two species are confined to poorer feeding areas during times when resources are low, as predicted by the theory.
Rosenzweig's theory predicts that niches contract when resources are limiting and there is interspecific competition. We have seen that the Hawaiian honeycreepers may conform to the predictions, but what about other species? Information from wildlife both agrees and disagrees with the predictions. The overlap in diet of sympatric mountain goats (Oreamnos americanus) and bighorn sheep (Ovis canadensis) is high in summer but reduced in winter (Dailey et al. 1984), as predicted by the theory. In ducks we have already seen that during winter there is a decrease in overlap (Fig. 9.9). Burning grasslands increases the nutrient content of regenerating plants and may produce locally abundant food. Under these conditions mountain goats and mule deer (Odocoileus hemionus) actually increase dietary overlap (Spowart and Thompson Hobbs 1985). In contrast, elk and deer in natural forests increased dietary overlap in winter when resources were assumed to be least available, contrary to expectation (Leslie et al. 1984).
We should note that we do not have actual measures of the food supply in these examples, so we cannot be sure that we are seeing competition. In Serengeti, Tanzania, wildebeest are regulated by lack of food in the dry season (Mduma et al. 1999), so that overlap with this species at this time should result in competition. However, overlap in both diet and habitat between wildebeest and several other ungulate species increases or does not change between wet and dry seasons (Hansen et al. 1985; Sinclair 1985). One interpretation could be that interspecific competition is asymmetrical, with the impact of the rarer ungulates on the numerous wildebeest being real but very slight, while the reverse does not occur because these other ungulates are kept at low density by predation (Sinclair et al. 2003).
9.7 Competition So far we have discussed the patterns of occupancy and utilization of habitats as if in variable they were constants for a species, or that they changed only seasonally. However, environments longer-term studies are now showing that species densities vary in the same habitat and they also change over a longer time scale measured in years. Thus populations may go through periods when there are abundant resources and, although there is overlap with other species, even at the supposedly difficult time of year there is no competition (Fig. 9.16). Occasionally there are periods of resource restriction and it is only at these times that one sees competition and niche separation (Wiens 1977).
Some of the predicted outcomes from interspecific competition include the reduction of populations, the contraction of niches, and exclusion of species from communities. However, these predictions are also to be expected when species have non-overlapping resource requirements but share predators, especially when predators can increase their numbers fast.
Let us suppose there is a predator that is food limited, and which feeds on two prey species. The prey are both limited by predation and not by their own food supplies. If species 1 increases in number then this should lead to more predators, which in turn will depress species 2 numbers. This result is called apparent competition
Fig. 9.16 Changes over time in the mean (thick line) and variance (shaded area) of a selective constraint such as resource availability. At times A and B there are "bottlenecks" when competition is more likely. (After Wiens 1977.)
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Individual generation because it produces the same changes in prey populations as would be predicted from interspecific competition (Holt 1977, 1984). Examples of apparent competition are given in Section 9.10.2 and Chapter 21, where predators are causing the demise of secondary prey, the rare roan antelope in Kruger National Park, South Africa (Harrington et al. 1999; McLoughlin and Owen-Smith 2003), and the wildebeest in Manyara National Park, Tanzania, as a result of a high abundance of buffalo, the primary prey.
If two prey species live in the same habitat, as in the wildebeest and buffalo example in Manyara, then at high intensities of predation coexistence is unlikely. On the other hand, coexistence is promoted if the two species select different habitats, that is, niche partitioning occurs.
Another version of apparent competition can occur through shared parasites. One species can be a superior competitor if it supports a parasite which it transmits to a more vulnerable species. For example, when gray squirrels (Sciurus carolinensis) were introduced to Britain, they brought a parapox virus that reduced the competitive ability of the indigenous red squirrel (S. vulgaris) (Hudson and Greenman 1998). The latter has largely been displaced, occurring now in only a few small locations of its former range. Gray squirrels are displacing red squirrels through competition in Italy, and could be spreading through Europe (Wauters and Gurnell 1999).
9.8.2 Implications Since the observed responses of prey populations to changes in predator numbers are similar to those from interspecific competition, we cannot infer such competition simply from observations or even experiments that show either changes in species population size or niche shifts. We need to know (i) whether resources are limiting; and (ii) the predation rates and predator numbers.
9.9 Facilitation Facilitation is the process whereby one species benefits from the activities of another.
In some cases the relationship is obligatory as in the classic example of the nereid 9.9.1 Examples of worm (Nereis fucata), which lives only in the shell of hermit crabs (Eupagurus bern-facilitation hardus). The crabs are messy feeders and scraps of food float away from the carcass that is being fed upon; these scraps are filtered out of the water by the worm. While the worm benefits, the crab appears not to suffer any disadvantage (Brightwell 1951). In other cases the relationship may be facultative, by which we mean that the dependent species does not have to associate with the other in order to survive, but does so if the opportunity arises. Thus, cattle egrets (Ardea ibis) often follow grazing cattle in order to catch insects disturbed by these large herbivores. Although the birds increase their prey capture rate by feeding with cattle, as they probably do by following water buffalo (Bubalus bubalus) in Asia and elephants and other large ungulates in Africa, they are quite capable of surviving without large mammals (McKilligan 1984). The European starling (Sturnus vulgaris) also follows cattle on occasions. In contrast, its relative in Africa, the wattled starling (Creatophora cinerea), seems always to follow large mammals and in Serengeti they migrate with the wildebeest like camp followers.
Vesey-Fitzgerald (1960) suggested that there was grazing facilitation amongst African large mammals. Lake Rukwa in Tanzania is shallow and has extensive reedbeds around the edges. The grasses, sedges, and rushes can grow to several meters in height, and in this state only elephants can feed upon the vegetation. As the elephants feed and trample the tall grass they create openings where there is lush
Fig. 9.17 The proportion of the population of different ungulate species using short grass areas on ridge tops (upper catena) in Serengeti. The larger species leave before the smaller at the start of the dry season. (After Bell 1970.)
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