EVOLUTION INTERNATIONAL JOURNAL OF ORGANIC EVOLUTION PUBLISHED BY THE SOCIETY FOR THE STUDY OF EVOLUTION Vol. 36 July, 1982 No.4 Evolution, 36(4), 1982, pp. 637-657 NICHE SHIFTS AND COMPETITION IN DARWIN'S FINCHES: GEOSPIZA CONIROSTRIS AND CONGENERS B. R. GRANT AND P. R. GRANT Division of Biological Sciences, University of Michigan, Ann Arbor, Michigan 48109 Received February 5, 1981. Revised October 20, 1981 The idea that interspecific competition titative ecological data to the hypotheses, is an important process in structuring because quantitative data were not used communities stems largely from David to construct the hypotheses. In fact, Lack Lack's work with Darwin's Finches (Lack, had almost no ecological data (see Abbott 1940, 1945, 1947, 1969). Lack made eco­ et al., 1977). logical inferences about feeding niches In our initial studies we analyzed eco­ from analyses of bill sizes and shapes. He logical and morphological data from six listed several instances where the niches species on eight Galapagos islands (Abbott of coexisting species were different, and et al., 1977; Smith et al., 1978). To follow where the niche of an absent species ap­ this general approach, we selected for more peared to be occupied by one or more detailed study three situations which have species present. These examples he inter­ been heralded as especially clear illustra­ preted as evidence of competitive dis­ tions of competitive effects (e.g., Lack, placement and exclusion. 1969; Williamson, 1972; Arms and Camp, Lack's aim was to offer a coherent the­ 1979), the small size of Geospizafortis on oretical framework for understanding the Isla Daphne Major in the absence of G. adaptive radiation of Darwin's Finches, fuliginosa (Boag, 1981), mutually exclu­ not to test specific hypotheses. In contrast, sive distributions of G. fuliginosa and G. our aim is to test the hypothesis of inter­ difficilis (Schluter and Grant, unpubl.), specific competition. This is needed be­ and differences in bill size between pop­ cause the evidence in general for interspe­ ulations of G. conirostris. Here we report cific competition as an important factor in the results of the third study in which we determining niche relationships and com­ test the hypothesis of competition between munity patterns has been critically debat­ G. conirostris and congeneric species. ed recently (e.g., Connell, 1975, 1978, 1980; Wiens, 1977; Connor and Simber­ The Interspecific Competition loff, 1978, 1979; Simberloff, 1978; Strong Hypothesis Applied to et al., 1979; Abbott, 1980; Grant and Ab­ G. conirostris bott, 1980; Ricklefs and Travis, 1980; Ro­ Geospiza conirostris occurs on Isla Es­ tenberry and Wiens, 1980; Wiens and Ro­ panola (Hood) and its satellite Gardner in tenberry, 1980). We can return to Darwin's the south of the archipelago, and on Isla Finches to perform such tests without cir­ Genovesa (Tower) in the northeast (Fig. cularity by obtaining and applying quan- 1). It has been recorded also on Islas Pinta 637 638 B. R. GRANT AND P. R. GRANT -DARWIN Kilometers , WOLF 1· ~ PINTA o MARCHENA Q GENOVESA O· AN SALVADOR (jBortolom' , .Seymour RABIDA· DoAHNEBALTRA PLAZA SUR SANTA CRUZ LOS G • HERMANOS SANTA FE 1· SAN CRIST6BAL ISABELl. On,low Enderby • OChomPion SANTA MARIA . Coldwell GARDNER 'Gordner ESPANOLA~ 92· 91· 90· FIG. 1. The Galapagos Islands. (Abingdon), Wolf (Wenman) and Darwin nirostris, G. fortis and G. scandens, and (Culpepper), but has not been seen on these that the niche on Genovesa, in the pres­ three islands recently (Harris, 1973; Bow­ ence of magnirostris, is a composite of the man, 1979, and pers. comm.; D. Schluter, niches of absentfortis and scandens (Fig. pers. comm.), so these latter three islands 2). According to this argument, the inter­ will be ignored. island difference in the niche of conirostris Lack (1945, 1947) argued from the beak is attributable to just one variable identi­ shapes of conirostris on Espanola and fied, namely the competitive influence of Genovesa that the niche on Espanola is a magnirostris. A competitive displacement composite of the niches of absent G. mag- between conirostris and magnirostris is DARWIN'S FINCHES 639 8 presented in the form in which it was con­ o structed. In his book Lack (1947) first pre­ J: sented his facts, then followed them with I- Q" an interpretation. But Richards (1948) W pointed out that there was almost as much 0 6 ..... lID interpretation in the presentation of facts ..... as in the section devoted to interpretation: <Q • "It is impossible to collect or arrange sci­ entific data without some theory which 4 • automatically introduces an element of in­ 10 12 14 16 terpretation" (Richards, 1948 p. 83). To BILL LENGTH put Lack's argument into a testable frame­ FIG. 2. Average beak size in millimeters of four work we must rephrase it. His observa­ finch species: Geospizajortis (+) from I. Daphne, G. tions to be explained were the distribution scandens (.) from I. Daphne, G. magnirostris (0) from I. Genovesa and G. conirostris (0) from I. of species and inter-island differences in Genovesa (G) and I. Espanola (E). beak shape; his hypothesis was that dis­ tribution and morphology were causally influenced by interspecific competition for food; and his main assumption was that also implied. Lack (1969) also proposed the feeding niche of a population was re­ that conirostris has competitively exclud­ flected in the average beak characteristics. ed magnirostris, fortis and possibly scan­ The above assumption and the corollary dens from Espanola, and that fortis and that diet differences parallel beak differ­ scandens have been competitively exclud­ ences have been supported by the results ed from Genovesa by conirostris and/or of recent work with several species of Dar­ magnirostris. The three species which ap­ win's Finches (Abbott et al., 1977; Grant parently have been excluded by coniros­ et al., 1976; Smith et al., 1978; Grant and tris are widely distributed throughout the Grant, 1979; Grant and Grant, 1980a; archipelago on almost all the major is­ Grant, 1981). Therefore we might expect lands, including those nearest to Espanola that conirostris on Espanola, with mean and Genovesa. However magnirostris has beak characteristics intermediate between become extinct on Sta. Marla and S. Cris­ those of the absent magnirostris, fortis and tobal (see Fig. 1) since Darwin's visit to scandens (Fig. 2), has an intermediate the archipelago in 1835 (Sulloway, 1982). feeding niche position too. However niche breadth and overlap are functions of niche Tests of the Competition Hypothesis size and shape as well as position (Pianka, Testing the hypothesis is difficult for two 1969; Colwell and Futuyma, 1971; Col­ reasons. The first is that it deals with well, 1979). Therefore an intermediate events in the past. Historical processes feeding niche position does not necessarily cannot be tested directly in the way that mean that the feeding niche of conirostris contemporary processes can, except in the combines the niches of the three missing unlikely circumstances that the reactants species or that the conirostris niche is par­ are brought together under the same con­ ticularly broad. For this reason we regard ditions as prevailed in the past. Instead, Lack's statements about niche similarities, the hypothesis can be tested through its differences and breadths to be testable, i.e., consequences (predictions). The assump­ falsifiable predictions of the competition tions upon which it rests may also be hypothesis. For example, if we found that tested, and if they are shown to be wrong conirostris on Espanola did not feed in a they must be replaced or the hypothesis manner exhibited by magnirostris, fortis must be discarded in favor of another. and scandens elsewhere, we would con­ The second difficulty is that Lack's ar­ sider the competition hypothesis, as it is gument was neither entirely explicit nor applied to the absence of these three 640 B. R. GRANT AND P. R. GRANT species, to be falsified by the failure of the nectar (Grant and Grant, 1980a, 198Ob; prediction. Grant and Grant, 1981). Our random Itis also predicted from the competition sampling of seeds and fruits on the ground hypothesis that missing species would be and on bushes, method of measuring them, less efficient than the species present in and our systematic recording of feeding locating and dealing with the foods on a behavior and diets have been described in particular island if these missing species Abbott et al. (1977)and Smith et al. (1978). were to arrive. Lack (1969) used this ar­ The only difference is that the study grid gument when invoking the competitive on Espanola was changed from the area exclusion hypothesis to argue for the com­ in 1973 to one 500 m south that contained petitive superiority of conirostris on Gen­ Opuntia megasperma trees. We did this in ovesa and Espanola, but inferiority else­ order to have Opuntia in both study areas where. We consider this to be another on Espanola and Genovesa where feeding prediction of the hypothesis that can be observations were made. tested by comparing the feeding efficiency of different species on the same species of RESULTS seeds. There are alternative explanations to the .Prediction of Niche Expansion competition hypothesis for the absence of on I. Espanola fortis and scandens from both islands and To test the prediction of the competition magnirostris from Espanola. One is that hypothesis that the feeding niche of con­ their food niches are not present on these irostris on Espanola combines the feeding islands (Bowman, 1961). We will examine niche of magnirostris,jortis and scandens, this possibility when testing the niche ex­ we first ask if conirostris feeds on the same pansion predictions. Other explanations food items taken by magnirostris, fortis are that they have been unable to reach and scandens on other islands.