Fruit and Consumer Differences in Three Species of Trees Shared by Trinidad and Tobago

Abstract

Three species of trees in the premontane forests of both Trinidad and Tobago have an extremely different coterie of fruit and seed consumers on each island. On Trinidad, legitimate dispersers comprised of principally frugivorous passerines were most abundant, whereas on Tobago seed-predacious parrots and chachalacas predominated. Significant morphological differences between Trinidad and Tobago in the fruits of the three plant species reflected the difference in consumers. Such characters as relatively thickened fruit walls, a higher frequency of multiple seeds and arils, and enlarged seeds were present in the Tobago populations, which were heavily preyed upon. The Tobago populations also had certain attractive features to legitimate dispersers, such as quantity of fruit pulp, red color of fruit linings, and concentrations of sugar reduced or in lower frequency. The continued survival and reproduction of the Tobago tree populations, despite different dispersal than on Trinidad, is attributed to their protective adaptations and the different habitats on the two islands. This study suggests that under strong isolation, adaptation to a new consumer situation may be selected for rapidly in certain tropical fruiting trees. THE INTERRELATION BETWEEN FRUITING PLANTS and seed consumers (dispersers and predators) has received much attention in the past 20 years. However, little or no evidence has been provided concerning a fruiting species' ability to ecologically adapt to a different set of consumers (Smith 1970, 1975; Janzen 1983). Although some fruiting species have been shown to be dependent upon a relatively small group of dispersers (Snow 1970; Crome 1975; McKey 1975; Howe 1977, 1983; Greenberg 1981; Wheelwright 1983), growing evidence suggests that specialized seed dispersal systems are relatively rare (Howe & Vande Kerckhove 1979, Howe 1980, Wheelwright & Orians 1982, Schemske 1983, Fleming et al. 1985). Given the likelihood of geographic and temporal variation in types and densities of dispersers available to a fruiting species (e.g., Howe 1977, 1980; Howe & Vande Kerckhove 1979), it is probable that many fruits have maintained a generalized attractiveness useful under many situations of disperser availability. Just as fruits have developed general traits for attracting dispersers, they also have developed various general traits for protection from predation (Janzen 1971, 1983). Each endozoochorous fruit has had to develop means of both attracting dispersers and preventing predation either by the dispersal agents or by true seed predators. In many cases in the tropics (e.g., Janzen 1970, 1971, 1972; Janzen et al. 1976; Howe et al. 1985), seed predators account for the destruction of a large proportion of the seeds produced. In some cases (e.g., Howe et al. 1985) seed predators are thought to be so influential that, without effective dispersal, the plant may not reproduce successfully. Thus, the balance between seed dispersal and predation in fruits often appears to be a delicate one. Nevertheless, in various types of isolation, fruit species may face situations where the coterie of dispersers and predators is vastly altered. The long-term survival of a fruiting species in such a setting rests upon its ability to adapt to the new situation. Is it possible for a generalized endozoochorous fruit to adapt to a very different selective regime? How do such traits as seed shadow, demography, spatial distribution, and abundance change when a species' coterie of consumers changes? Recently isolated continental islands can offer some answers to these questions. The small island of Tobago was connected to Trinidad and the South American mainland in the Pleistocene (see Discussion for evidence). It contains a tract of largely undisturbed forest that shares many species with similar extensive forests on the large nearby island of Trinidad. Despite the floristic and vegetational similarities, the avifaunas of the two islands' forests are markedly different, with reduced overall densities of birds as well as lower numbers of species on Tobago (Keeler-Wolf 1982, pers. obs.). Certain ecological guilds of birds are also substantially different between the two islands. This is particularly true in the case of seed predators and fruit eaters (Keeler-Wolf 1982, pers. obs.). My long-term census data indicate that the guild of principally frugivorous passerines comprises 35-43 percent of the total number of individuals in the I Received 3 March 1986, revision accepted 6 October 1986. 2 Present address: 943 Aquarius Way, Oakland, California 9461 1, U.S.A. 38 BIOTROPICA 20(1): 38-48 1988 This content downloaded from 207.46.13.153 on Fri, 05 Aug 2016 06:21:09 UTC All use subject to http://about.jstor.org/terms Trinidad premontane forest avifauna, whereas in Tobago premontane forest it is only 10-11 percent of the avifauna. Conversely, individuals of seed predators are relatively more abundant on Tobago where they represent 11-15 percent of the avifaunal densities; on Trinidad they comprise only 2-4 percent. Three species of trees-Sloanea laurifolia (Benth.) Bentham (Elaeocarpaceae), Richeria olivieri Philcox (Euphorbiaceae), and Myrcia fallax (A. Rich) DC. (Myrtaceae)-occur on both islands and are heavily used by frugivorous and seed-predatory birds. Different sets of consumers eat their fruits and seeds on each island. On Trinidad, the majority of species feeding on the fruits are principally frugivorous passerines. These typically ingest the ripe fruits of each species and either defecate or regurgitate the seeds unharmed. On Tobago, the most frequent consumers of the fruits of these three species are parrots and chachalacas. The former chip or split open unripe fruit and chip up the seed; the latter ingest whole fruit, grinding and partially digesting the seeds in the