Predispersal Seed Predation in a Tropical Shrub (Mabea occidentials, Euphorbiaceae)

Abstract

A moth, Laspeyresia sp. (Olethreutidae), was responsible for approximately 80 percent of immature fruit and seed mortality in Panamanian Mabea occidentalis (Euphorbiaceae). Predation intensity (proportion of the fruit crop damaged) was not significantly related to plant fecundity within Mabea stands; however, more fecund plants matured a greater absolute number of viable fruits and seeds than did less fecund plants. Fruits initiated earliest in the season tended to escape predation completely. Relative spatial isolation within Mabea stands did not result in lower predation intensities; the moths can probably easily travel the short interplant distances observed within these populations. Between-stand, rather than within-stand, distances may be important to the dynamics of seed predation in this and other patchily distributed species. PREDATION ON SEEDS by specialized consumers has been recognized for its potential influence upon the distribution, abundance, and reproduction of plants (Janzen 1969, 1970; Harper et al. 1970; Smith 1970; Vandermeer 1975; Connell 1978). Recent discussions (e.g., Janzen 1975, Moore 1978a) emphasize that the role of seed predation as a selective factor in plant populations is related to the extent of differential predation upon conspecific hosts. Identification of those factors that result in consistent variation in predation intensity, within and between populations, is crucial to understanding the impact of seed mortality upon plant population dynamics. In tropical communities, relationships between plants and seed predators are poorly known, although a few studies have made an important beginning in quantifying the effects of seed mortality in nature (e.g., Vandermeer 1974, Janzen 1975, Moore 1978a and b, Silander 1978). Here I report a study of predation on the seeds of a tropical shrub, Mabea occidentalis Bentham (Euphorbiaceae), by a moth, Laspeyresia sp. (Olethreutidae), on Barro Colorado Island, Panama Canal Zone, where Laspeyresia accountted for the majority of Mvabea seed mortality. I describe the interaction and examine predation intensities within two Mabea stands with respect to plant fecundity, degree of spatial isolation, and the phenology of fruit production. These factors do not encompass all possible influences on predation intensity, but as a group they represent some plant characters likely to affect population dynamics. A separate category of factors are those affecting moth numbers and success. NATURAL HISTORY Mabea occidentalis is a shrub or treelet ranging from Central and northern South America to Brazil (Webster and Burch 1967). It is locally abundant in secondary forest (60 years old) on Barro Colorado Island, where it occurs in distinct stands of 15-40 plants. Reproductive individuals under study ranged from 3 to 5 m high with stems ranging from 2 to 9 cm dbh (diameter at breast height) (n 35). Larger plants may have two or three main stems. Mabea is monoecious; the inflorescence is a raceme consisting of three to seven proximal pistillate flowers and numerous distal staminate flowers grouped in clusters of three. Fruits are woody, explosively dehiscent capsules (fig. 1). On Barro Colorado Island, flowering is concentrated in the period from December through February, and fruiting from late December through March. Individual fruits begin development immediately following floral anthesis. Fruits grow to, full size over a three-week period and remain on the plant an additional five weeks or more befo-re dehiscence. During maturation, capsule walls become hard and woody. Fruits are susceptible to oviposition by Laspeyresia (possibly an undescribed species-R. L. Brown, pers. comm.) during their initial three-week growth period when the pericarp tissue is soft. Damaged fruits are easily distinguished from undamaged fruits by a superficial scar of dried latex that indicates the point of entry of the moth larva. Usually only one such scar is found per fruit, but 24 percent of 412 infested fruits examined in the laboratory had multiple scars. Upon hatching, the moth larva burrows into the fruit and eventually consumes all three seeds. The plant aborts fruits soon after infestation; these fruits drop to the ground and larval development continues 146 BIOTROPICA 13(2): 146-15