Host-Parasite Relationships and Species Diversity in Mammals: An Hypothesis
1969; Wiley; Volume: 1; Issue: 2 Linguagem: Inglês
10.2307/2989758
ISSN1744-7429
Autores Tópico(s)Plant and animal studies
ResumoOne process for increasing regional species diversity is by the joining of congeners (S1 and S2) which have evolved in geographic isolation. Mechanisms to prevent competitive exclusion of S2 by St in at least one habitat must pre-exist if S2 is to survive. One such mechanism may be the presence of parasites which are well adapted to S2 but which are deleterious to Si. Such secondary interactions in specific habitats may override other differences in morphology, physiology, aggressiveness, and feeding efficiency. Disease and other forms of interference between similar species may suppress total density in regions of high species diversity. THE NUMBER OF similar species that can coexist in a region may be increased through a division of habitats (MacArthur 1965). In many cases, this implies, a) that those species which occupy separate habitats can not coexist in any single available habitat (and hence are incompatible), and b) that there must be mechanisms that provide competitive advantages to each species in its own habitat. The sorts of genetically controlled factors that can provide an advantage of one species over another in a specified environment include morphological, physiological, and behavioral adaptations that affect characters such as tolerance to physical conditions, feeding efficiency, fleeing, fighting, growth, and reproduction. Within a habitat, the outcome of competition may be mediated by higher order interactions such as predation (Brooks and Dodson 1965, Paine 1966), parasitoidism (Utida 1953), and parasitism (Park 1948). The primary mechanisms seem inadequate to explain the reciprocal nature of the competitive advantage among those species of small mammals which have been studied and I suggest here that the parasites of mammals can provide an effective mechanism for dividing habitats that is relatively independent of the primary characteristics of the competitors. EXAMPLES OF COMPETITIVE EXCLUSION Our knowledge of competitive processes is meager (Mayr 1963) and is mainly anecdotal, inferential, and theoretical. Miller (1967) has provided a recent review but additional comments on three examples may serve to emphasize the limits of our understanding and point to the likelihood that second-order interactions explain some of the mutually exclusive distributions of similar species. 1. The situation for the four species of pocket gophers (Geomyidae) of Colorado has been examined by Miller (1964). The fundamental niche of the superior competitor in each case is judged to be a proper subset of the poorer competitor's fundamental niche. Thus, the larger (22% in body length), more specialized Geomys bursarius (Shaw) seems capable of excluding the smaller Thomomysf talpoides (Richardson) from areas of deep, fine soil and the latter can persist in shallow, coarse soil which apparently provides a barrier to the distribution of G. bursarius. As noted by Miller, however, the same explanation is more tenuous when applied to the combination of G. bursarius and Cratogeomys castanops (Baird). The latter is slightly larger (4% ) yet lives in soils that are somewhat less optimal than those occupied by G. bursarius. In addition, Vaughan and Hansen (1964) have demonstrated experimentally that T. talpoides and the somewhat larger (11%) T. bottae (Eydoux and Gervais) are capable of living and reproducing within each other's range in mixed associations. 1 Present address: Center for the Biology of Natural Systems, Washington University, St. Louis, Missouri 63130. I thank R. Z. Brown, Lim Boo-Liat, R. H. MacArthur, Illar Muul, R. T. Paine, 0. J. Sexton, and my family for helpful discussion and criticism at various stages in the development of this paper. My research was supported by a grant from the National Science Foundation, G-22230. BIOTROPICA 1(2): 29-35. 1969 29 This content downloaded from 207.46.13.90 on Wed, 14 Sep 2016 06:19:13 UTC All use subject to http://about.jstor.org/terms It is possible that the present ranges of these four species are not static but if one or more species were actively invading the range of another, the picture described in the field might seem less clear. The mechanisms of competition are uncertain (Miller 1967), but seem to be forms of interference rather than exploitation. 2. One of the most familiar examples of competitive exclusion among small mammals has been observed in both Europe and in North America where the introduced roof rat, Rattus rattus (Linnaeus), has been displaced by the later invading Norway rat, R. norvegicus (Berkenhout) ( see Brown 1960 or Miller 1967 for reviews). The phenomenon has been recorded with good precision in some areas but the actual mechanisms that direct the process have not been carefully examined in the field. In some cases at least temporary coexistence has been achieved by the arboreal roof rat living in the upper stories of buildings while the burrowing Norway rat occupied the lower levels. The usual explanation for the ultimate exclusion of roof rats has been that the Norway rat is larger and more aggressive and is also better adapted for temperate climates than is the semitropical roof rat. In view of the latter notion, it is interesting that some local populations of roof rats still exist on isolated farms in Vermont and in the Appalachian Mountains and that the Norway rat is extending its range in the relatively hot climate of Georgia. The reasons for the displacement are not clear in all cases and the two species have coexisted for many years in the same habitats in Hawaii (Kartman and Lonergan 1955) and some other tropical islands. 3. Various species of microtine rodents (Microtus, Clethrionomys) fit the pattern whereby one species (Si) occupies restricted habitats (H1) in the presence of S2 but uses a wider range of habitats (H1, 2) in the absence of S2 and vice versa; i.e., the fundamental niches overlap (Findley 1954, Ota and Jameson 1961, A. W. Cameron 1964). For example, Findley noted that where the range of M. pennsylvanicus (Ord) overlapped that of M. ochrogaster (Wagner), M. pennsylvanicus was usually restricted to habitats of higher moisture and denser vegetation. One might intuitively expect that the prairie meadow vole (M. ochrogaster) would be better adapted than M. pennsylvanicus to more xeric habitats and Getz (1963) has demonstrated this in the laboratory. Somewhat surprisingly, however, Getz (1962) also observed in the laboratory that the somewhat smaller and intraspecifically docile M. ochrogaster tended to dominate M. pennsylvanicus in interspecific contests. Getz thus surmised that M. pennsylvanicus was excluded from the dry uplands by a combination of behavioral and physiological mechanisms favoring M. ochrogaster. This, however, leaves us with no postulated mechanism to explain the survival of M. pennsylvanicus in the moist areas within the western range of M. ochrogaster, since the latter should be able to exclude the former during periods of drought and during periods of cyclic lows for M. pennsylvanicus. For resource-limited species, MacArthur and Levins (1964) calculate that there is a theoretical limit to the similarity of coexisting species. The conceptual aspects of the model are concerned primarily with food and it is suggested that if two species depend on the same food source, the more efficient feeder (Si) can eliminate the less (S2) by reducing the food supply below the threshold required by S2. To survive, S2 must find a habitat (H2) from which S1 is excluded by other factors or in which the feeding efficiencies are reversed. The above exploitation model may have considerable evolutionary significance for some species but would appear to, have little relevance for those herbivorous rodents which are not generally food limited (Hairston, Smith, and Slobodkin 1960). Thus, for some of the better examples of competitive exclusion or habitat displacement among mammals, the observed differences in morphology, physiology, and behavior among the presumed competitors suggest mechanisms that enable Si to oust S2 from H1 but it is less clear how the reciprocal relationship operates. Such apparent anomalies have led me to speculate on the possible role of parasites as mechanisms effecting exclusion. HOST-PARASITE RELATIONSHIPS The classical concept of a parasite covers a wide varie y of possible relationships whereby one organism lives at the expense of another, contributing no advantages and being at least potentially harmful to the host. My use of the term follows that of Sprent (1963) and is restricted to those organisms obliged tol spend some part of their life cycle in intimate contact with the tissues of the host; one usual but not necessary result of the association being the release of antigens by the parasite and the production of corresponding antibodies by the host. These antibodies normally give the host some degree of immunity to reinfection. Certain characteristics of this relationship may be anticipated and have been summarized, in part, by T. Cameron (1964): 1. The survival of the individual parasite and
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