Inflorescence spiders: A cost/benefit analysis for the host plant, Haplopappus venetus Blake (Asteraceae).

Predators on flower visitors, such as spiders, could influence plant reproduction by determining the balance between pollination and seed predation by insects. This study examines the net effect of predation by the inflorescence spider, Peucetia viridans (Hentz), for seed production by a native plant species on which it hunts. Both pollination and seed set of Haplopappus venetus (Asteraceae) were reduced on branches with spiders; however, the release of viable, undamaged seed was higher on inflorescence branches with spiders than on those without. Occurrence of P. viridans was associated with the flat-topped inflorescence branch structure characteristic of H. venetus rather than with the vertical structure of its congener, H. squarrosus. Thus, the interaction should be a reinforcing selective pressure on inflorescence branch morphology of H. venetus over time. Two factors providing constraints on the degree and rate of coevolution of the plant-spider interaction are suggested by the results: (1) the critical role of phenological synchrony and (2) the opposing requirements of interacting species and of subsequent life history stages within a species.