SPECIFICITY OF SEED-ATTACKING BEETLES IN A COSTA RICAN DECIDUOUS FOREST

Abstract

(1) The deciduous forests and the riparian evergreen vegetation that they include in the lowlands of Guanacaste Province, Costa Rica, contain at least 975 species of dicotyledonous plants. At least 110 species of beetles (Coleoptera) whose larvae are seed-predators were reared from more than 3700 samples of fruits and seeds of these plants. (2) At least 100 species. of these plants usually had beetle seed-predators (Bruch idae, Curculionidae, Cerambycidae) in their mature or nearly mature fruits or seeds. (3) Most (75%) of the species of beetles were specific to a particular plant species; 14 preyed on two plant species, 9 on three, and 2 on four. The bruchid Stator pruininus preyed on six species and S. limbatus on eight species. (4) Of the 100 species of plants whose seeds were preyed on regularly by beetles, 63 were in the Leguminosae, 11 in the Convolvulaceae, and the remainder were spread among sixteen other plant families. (5) Of these 100 prey species, 59 were fed on by a single species of beetle, 25 by two species of beetles, II by three, 4 by four and one, Cassia ieptocarpa, by five species of beetles. (6) In at least 90% of seed or fruit samples, all species of beetles attacking that species in the study area were present. Of the 100 species of beetles, eighty were found in the first sample of the appropriate fruit or seeds. The prey of ten additional species of Bruchidae in the study area is unknown, but will be other than the prey species listed here. (7) With some striking exceptions, the prey species of those beetle species which preyed on more than one plant species were closely related. In contrast, in those cases where there were two or more congeneric plant species in the study area, the species of beetle which attacked one or more of them left unattacked an average of 5·8 of the congeneric plant species. (8) There were five unambiguous cases of a prey species that occurred throughout the study area and that had two or more species of beetle seed-predator whose distributions did not overlap at all. (9) Hymenopterous parasitoids were uncommon in most of the samples and were absent from large samples of a number of common tree species whose seeds were heavily preyed on by beetles. Of the 157 predator-prey pairs reported here, 57% of the beetle species were unattacked by hymenopterous parasitoids. (10) The distribution of beetle predator species among the plant prey-species was conspicuously neither random nor uniform. Unexpectedly large numbers of species were either unattacked, or preyed on by two to five species of beetles, while unexpectedly small numbers were attacked by a single species of beetle. (11) The prey-specificity of most of these seed-predators in a species-rich flora is of great importance in understanding the potential impact of animals on plant speciesrichness, but is not proof that seed predation by animals causes extreme plant species-richness. Furthermore, these beetles are only a small fraction of the animals that kill or weaken plants in a manner that may influence their abundance and spatial distribution.