Plant Spatial Pattern and Herbivore Population Dynamics: Plant Factors Affecting the Movement Patterns of a Tropical Cucurbit Specialist (Acalymma Innubum)

The effects of plant diversity and host plant patch size on the population dynamics of a cucurbit specialist (Acalymma innubum) were studied in a natural forest community in the Virgin Islands. Host plant patches (Cayaponia americana) varied in size and in the plant diversity of the surrounding habitat; host plants were either growing in nearly monospecific stands on the ground (“open” patches) or were growing up shrubs and trees (“forest” patches). The number of beetles per patch was linearly related to patch size, both in the open and in the forest; however, open patches (low plant diversity) had significantly greater numbers of beetles than did forest patches (high plant diversity) for all except the largest sized patches. In fact, 67% of the forest patches had no beetles on any of the sampling days (in comparison to 28% for open patches). Beetle density (number of beetles per unit plant leaf area) was positively correlated with patch size in the forest, but not in the open habitat. Mark—recapture studies revealed that four times as many beetles initially colonized open patches as compared to forest patches when released outside patches. Beetles released within patches stayed for longer periods of time (greater “tenure time”) in open patches than in forest patches, but this difference was most pronounced for very small patches. Four times as many individuals moved from forest to open patches as from open to forest patches. Four possible factors contributing to these differences in movement patterns, tenure time, and resulting abundances in low—diversity open patches compared to high—diversity forest patches were testes: (1) flight behavior, (2) plant quality, (3) light level, and (4) host plant growth form. Experiments and observations supported all but host plant growth form. First, observations on flight patterns revealed that beetles remain for longer periods of time on host plants than on nonhost plants. Secondly, beetles offered a choice of leaves from host plants growing in open patches (sun) and leaves from the same species of plant growing in forest patches (shade) displayed a strong preference for leaves form open patches, both in terms of the number of choices and the amounts eaten. And finally, manipulations of light level showed that beetle numbers are significantly reduced under shaded conditions. Only differences in host plant growth form did not explain greater abundances in the open habitat. Manipulations of host plant growth form in small experimental plots showed that beetle numbers were significantly increased when their host plants were growing vertically (up other plant species or stakes) compared to growing horizontally on the ground. Comparison with studies of a closely related species (Acalymma vittatum) in an agricultural community suggests that plant diversity can similarly affect population densities and herbivore tenure time in agricultural and natural communities, at least for closely related herbivores. This study emphasizes the interrelationship of patch size and plant diversity in influencing herbivore population dynamics and the multiple mechanisms involved: (1) herbivore preference for sunny habitats and for leaves growing in monoculture—type situations, and (2) greater tenure time on host plants and thus in patches with greater concentrations of host plants.