We distinguish factors influencing seed dispersal that are potentially under the control of parent plants from those that are not in a Panamanian population of the neotropical nutmeg, Virola surinamensis (Myristicaceae).In the first category, we find that individual variation in crop size and nutritional components of the aril failed to explain any variation in the proportion of fruits taken from fruiting trees. The ratio of edible aril to indigestible seed explained a significant but small (13%) portion of variation in the fraction of fruits taken by tropical birds. These results suggest that the potential for ongoing natural selection on fruit and crop characteristics by dispersal agents exists, but is periodic or weak. On the other hand, the number of fruits available to birds within 50 m of any given Virola tree profoundly influenced the extent to which it secured dispersal. More than three times as much variation (42%) in relative dispersal could be explained by the number of "competing" Virola fruits during the peak of an individual tree's fruiting season. When isolated plants are eliminated from the analysis, the depressive effect of competition with neighbors explains 72% of the variation in individual dispersal. These results suggest that birds seek out clumps of fruiting trees, but that intraspecific competition for a limited disperser assemblage occurs within the clumps.Our results are a first step towards separating attributes of the plant that are potentially under the influence of natural selection by dispersal agents from emergent population effects that can override individual advantage in crop or fruit characteristics.

The spread of advantageous transgenic genes from crop plants into wild and weedy relatives is a potential ecological problem. The available theory indicates that the spread of a gene, over space and time, will depend in part on the relative selective advantage of that gene, and in part on gene flow, or the probability distribution of gene movement from source to target plants within a single generation. Risk assessment will require knowledge of both. This paper describes an empirical investigation of the effect of spatial dispersion on gene flow in Brassica campestris, utilizing a system of wild genotypes for target plants, and agronomic genotypes with a marker gene for source plants. I have found that gene flow can be reliably modeled with an exponential probability density function, and that the mean of that distribution can vary with the spatial dispersion of plants. Two-dimensional arrays of plants had lower mean gene dispersal distance than did one-dimensional arrays. In contrast, the degree of clumping and the size of patches of plants in my experiments had no effect on gene flow. However, I found an increase in mean gene dispersal distance with increased distance between clumps of plants, or between individual plants in one year. These results imply that large isolation distances accompanied by a rare long-distance pollination event will ultimately increase the risk of gene spread.

Like many angiosperms, Crinum erubescens is partially self-compatible, producing fewer seeds upon selfing than after outcrossing. In this paper we test the relative magnitude of the prefertilization and postfertilization effects of self-incompatibility, inbreeding depression, or both in a natural population of this hermaphroditic tropical herb. We characterize prefertilization effects by examining pollen tube growth, while postfertilization effects are characterized by examination of embryo abortion and seed maturation. Statistical methods are developed to test the magnitude of these effects from one life-cycle stage to the next. We find that although pollen performance in selfed flowers is lower than that in outcrossed flowers, pollen performance is low overall. Postfertilization effects attributable to inbreeding depression account for a larger proportion of the reduction in fecundity in selfed compared to outcrossed flowers. Among naturally pollinated plants, despite ample pollen deposition, the numbers of fruits and seeds set are intermediate to selfed and outcrossed treatments.

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