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Interspecific Competition, Island Biogeography and Null Hypotheses

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Interspecific Competition, Island Biogeography and Null Hypotheses Evolution, 34(2), 1980, pp. 332-341 INTERSPECIFIC COMPETITION, ISLAND BIOGEOGRAPHY AND NULL HYPOTHESES P. R. GRANT AND I. ABBOTT Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan 48109, and Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands 6009 Received April 6, 1979. Revised October 8, 1979 We conducted a field study of some of Darwin's Finches. Finally, we draw at­ Darwin's Finches (Geospiza species) in or­ tention to some unsolved problems in bio­ der to assess the relative importance of in­ geography, concerning principally the terspecific competition and habitat fea­ separation of potentially conflicting effects tures in determining the observed of different processes such as dispersal and biogeographic, ecological and morpholog­ competition. ical characteristics of these species (Abbott et aI., 1977). Strong et al. (1979) have crit­ Why Their Methods are icized one of our methods and have rean­ Unsatisfactory alyzed a small portion of our data. They Strong et al. (1979) generated expected employed stochastic models to generate ratios of beak sizes among birds on the expected beak size differences between California Channel islands, the Tres Ma­ sympatric species, and then compared ex­ rias islands and the Galapagos. For the pected with observed differences. Finding first two groups of islands they used a a generally close correspondence between computer to draw randomly the observed expected and observed differences, they number of species on each island from concluded that random processes are suf­ within families of birds on the adjacent ficient to account for the observations, and mainland. They repeated the exercise 100 that therefore there is no need to invoke times to obtain an estimate of sampling deterministic processes such as competi­ error. For the Galapagos they chose tion as we had done. Strong et al. (1979) species at random, then a population from obtained the same results and drew the one of the islands at random for each of same conclusion from analyses of beak the species already chosen, until the num­ size differences among birds on the Tres ber of species populations matched the Marias islands of Mexico and the Channel observed ones on all of the major islands. islands of California. Simberloff and his Ratios were calculated separately from associates have also drawn the same con­ observed species combinations and ran­ clusion from a series of other analyses per­ domly paired species, and then compared. formed in like manner (Connor and Sim­ There are several problems with these berloff, 1978; Simberloff, 1978). procedures. In several ways the analyses We take issue with the procedures tend to maximize the risk of making a Strong et al. (1979) have used in their Type II error, that is, favoring acceptance analyses and with the way in which our of the null hypothesis when it is false. statements and interpretations have been 1. The first problem is that the choice represented. We identify five problems in of species within families is unfortunate. their analyses and five sources of confu­ Species of birds in different genera are less sion in the interpretation of results. We likely to be in the same feeding group or find no evidence in their analyses or ar­ guild than are species in the same genus, guments to change our previous conclu­ and hence are less likely in general to be sion that interspecific competition has in potential competition for food (excep­ played a role in the adaptive radiation of tions are easy to conceive, and observe, 332 INTERSPECIFIC COMPETITION 333 Grant, 1966). We know of no systematic ence in results stems from a difference in study undertaken to test this assertion, but the species pool. We consider our species it is a view commonly held on the basis of pool to be appropriate and theirs to be observations and from the reasoning that inappropriate. the more closely related two species are This point is of general importance. the more they will have similar adaptive Strong et al. (1979) could find no reason systems for locomotion, food gathering, to reject the null hypothesis for the total breeding, etc. (e.g., Darwin, 1859; Hair­ Tres Marias and Channel island data. Yet ston, 1964; Lack, 1965). the beak sizes of members of the same A simple illustration can be given with feeding guild suggest that competition has Darwin's Finches. Species in the genus influenced part of the structure of the Tres Geospiza show strong similarities in feed­ Marias avian community (Grant, 1966). ing methods, locations and actual diet The only biological justification for us­ (Snodgrass, 1902; Lack, 1945, 1947; Bow­ ing the family level in treating the data man, 1961, 1963). Our study provided a that we can find is in Simberloff (1978, p. quantitative confirmation of this similari­ 714)-"Still more important than the low ty. Species in the genus Camarhynchus visibility of interspecific competition in also show strong similarities in feeding obscuring its relationship to biogeographic methods, etc., but differ markedly from distributions is the fact that putative com­ all Geospiza species on the one hand and petitors are rarely pairs of taxonomically from Certhidea olivacea on the other (ref­ [congeneric?] and morphologically similar erences above). Pooling all species from species." This claim is unsubstantiated, these three genera, then choosing pairs at which should therefore serve as a stimulus random to examine beak ratios, is a mean­ for further research. An analytical justi­ ingless exercise as a test of competitive fication is that the family level of analysis effects because there is no a priori reason is less sensitive to specific taxa than is the to believe that unrelated species would genus level (Strong et al., 1979). Be that compete, except under the most extreme as it may, resorting to the family level of view that diffuse competition noticeably analysis runs the risk of throwing the baby affects all species in a community. We out with the bath water; or, to be more suggest that it may be worse than mean­ correct, drowning the baby by using too ingless; it may obscure real competitive large and deep a tub. effects within genera by diluting the phe­ 2. The second problem is in the choice nomenon to the point of disappearance. of a mainland area for comparison with Caswell (1976), Colwell (1979) and Inger the island (Grant, 1966). Geographical and Colwell (1977) have made a similar variation in community membership and point. beak sizes of the members within the Grant (1980) has shown with a multi­ mainland region could influence the re­ variate analysis of beak dimensions that sults of a random simulation of the process populations of sympatric pairs of Geospiza of island colonization. species are not a random sample of all In an earlier analysis of the species/ge­ possible paired populations of those nus ratio of island bird communities, Sim­ species; observed pairs tend to differ more berloff (1970) showed that observed ratios than randomly chosen pairs. Character tended to be higher than those expected displacement is a possible, but clearly not from the random sampling of mainland necessary or sufficient, explanation (Grant, species, and certainly not lower as would 1979). In some of their analyses Strong et be expected from an hypothesis of com­ al. (1980) used Geospiza and Camarhyn­ petition (Grant, 1966). With regard to the chus species together, and found that ob­ problem of correctly identifying the source served sympatric assemblages of species pool of species on the mainland, he wrote, (pairs, triplets, etc.) can be considered "A study currently in progress indicates random samples. We suggest the differ- there is little geographical variation in S/G 334 P. R. GRANT AND I. ABBOTT [species/genus] values for land birds, par­ islands is strongly influenced by the equi­ ticularly when localities of similar latitude probability assumption. We suspect that are compared" (Simberloff, 1970, p. 27). other expected biogeographic, ecological The source pool for the California Chan­ and morphological properties of island nel islands and Tres Marias islands prob­ populations are similarly affected. This is ably comes from areas more extensive lon­ not intended to belittle the attempts to gitudinally than latitudinally. One come to grips with a very complex prob­ therefore wonders how errors of choosing lem; only to point out that the problem the correct source pool might influence the has not yet been solved, and the interim results of species/genus and beak differ­ report is not convincing. ence analyses using the stochastic model 4. The fourth problem concerns the approach. Given this uncertainty, and for generation of expected beak size ratios for maximum biological realism, the safest archipelago islands (e. g., Galapagos) procedure is to use the species in an ad­ when there is no identifiable mainland jacent mainland area equal in size to the source area and species pool. The solution islands (Grant, 1966; Abbott, 1975), rath­ adopted by Strong et al. (1979), Connor er than all those in a whole country such and Simberloff (1978) and Simberloff as Mexico (Simberloff, 1970). The chances (1978) in similar analyses, is to treat the of detecting nonrandom colonization are archipelago as a universe from which in­ greatly reduced by using excessively large dividual islands are populated. Beak ra­ mainland species pools when the main­ tios are calculated (Strong et al., 1979) land communities vary geographically in from randomly combined populations; a systematic way. then the results of repeated random pair­ 3. In the original analyses Simberloff ings are compared with the actual data. (1970) assumed equiprobability of dispers­ Our criticism here is that the two sam­ al among all species in the mainland pool, ples tested for similarity (the real data is in order to obtain expected species/genus one sample, and the set of randomly gen­ ratios on islands.
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