Spatial scale and dispersion pattern of ant- and bird-dispersed herbs in two tropical lowland rain forests

The population level correlates of contrasting dispersal syndromes in closely related species are largely unknown. A family of tropical understory herbs, the Marantaceae, provides the opportunity for comparative studies as it contains many species with contrasting dispersal syndromes. As part of a study of the comparative population biology of ant- and bird-dispersed species, we test the hypothesis that spatial scale and dispersion pattern are related to dispersal type, proposing that bird-dispersed species will have a larger spatial scale than ant-dispersed species, and that, among bird-dispersed species, the scale will vary among three distinct dispersal types. We also propose that ant-dispersed species will show a more clumped dispersion pattern than the bird-dispersed species. Two types of spatial scale are examined: the amount of space occupied by individuals (maximum and actual) and the spacing among individuals within populations. The maximum size of plants showed a trend in the predicted direction. However, this trend was only of marginal statistical significance. The actual distribution of plant sizes, as measured by total leaf area and classified logarithmically, differed significantly among the dispersal types in the predicted direction. Spacing among individuals as measured by nearest neighbor distances also varied significantly by dispersal type, but not entirely in the predicted direction. Dispersion pattern analysis indicated that most study species in most populations had significantly clumped spatial patterning. The aggregation index varied 20-fold among study plots, but did not vary significantly by dispersal type. We conclude that while spatial scale was related generally to type of dispersal, dispersion pattern was not.