BREEDING SYSTEMS AND DISPERSAL-RELATED MATERNAL REPRODUCTIVE EFFORT OF SOUTHERN SPANISH BIRD-DISPERSED PLANTS.

Abstract

Dioecious plants, characterized by the occurrence of separate male and female individuals, represent a relatively unimportant fraction of the total world's flora (Bawa, 1980). Nevertheless, owing to the nonrandom distribution of the dioecious habit among life forms, successional stages and habitat types, dioecy may become a prominent feature of some regional or taxonomic plant assemblages (Ashton, 1969; Bawa, 1974; Bawa and Opler, 1975; Bawa, 1980; Conn et al., 1980; Freeman et al., 1980; Givnish, 1980). The dioecious condition has been found to be relatively overrepresented in later successional stages of tropical vegetation (Opler et al., 1980) and among trees and shrubs (Bawa, 1980; Conn et al., 1980; Freeman et al., 1980), vertebrate-dispersed plants (Bawa, 1980; Givnish, 1980), small-flowered tropical species (Bawa and Opler, 1975; Opler et al., 1975), wind-pollinated temperate plants (Freeman et al., 1980), and tropical species pollinated by small opportunistic insects (Opler et al., 1975; Bawa, 1980). Although promoting outcrossing has been traditionally suggested as the principal selective pressure underlying the evolution of dioecy (see recent reviews by Willson, 1979; Bawa, 1980; Givnish, 1980), some of the ecological correlates of the dioecious habit have recently served to suggest alternative and/or complementary explanations to its evolution (Freeman et al., 1976; Willson, 1979; Bawa, 1980; Freeman et al., 1980; Givnish, 1980; Cox, 1981). In particular, the strong association found between dioecy and seed dispersal by vertebrates has lent support to the notion of this breeding system having evolved among these plants in response to disperser-mediated, intrasexual competition pressures favoring a disequilibrium in the allocation of resources to male and female functions (Bawa, 1980; Givnish, 1980; but see Thomson and Barrett, 1981). It was assumed by the lastmentioned authors that vertebrate dispersers will select for higher dispersal-related maternal expenditure, thus ultimately favoring the spread of dioecy. In this context, it is of interest to compare female investment in disperser-attracting structures shown by dioecious and nondioecious vertebrate-dispersed plants. Increased returns in fitness with increasing maternal absolute reproductive effort were considered by Bawa (1980) and Givnish (1980) in relation to the size of fruit crops only, assuming that plants with larger fruit crops have greater attractiveness to dispersers (Howe and Estabrook, 1977; Howe and Vande Kerckhove, 1979; Howe, 1980). Nevertheless, crop size is just a single component of the total female reproductive investment associated with vertebrate-dispersal. The amount of flesh (the food reward which attracts dispersers to fruits; Snow, 1971; McKey, 1975) per fruit, and the energy and nutrients per weight unit of flesh, are two further important aspects of a plant's fruiting pattern which influence fruit consumption by frugivores and hence, presumably, the fitness of the plant. At an intraspecific level, frugivores have been shown to favor plant phenotypes with a higher flesh content per fruit (Howe and Vande Kerckhove, 1980) and, among species, dispersers seem to prefer those with the most nutritious fruits and/or those with higher flesh/seed ratios (Herrera, 1981a, 1981b; Jordano and Her-