Abstract
Resource concentration effects occur when high resource density patches attract and support more foragers than low density patches. In contrast, resource dilution effects can occur if high density patches support fewer consumers. In this study, we examined the foraging rates of pollinators and seed predators on two perennial plant species (Rudbeckia triloba and Verbena stricta) as functions of resource density. Specifically, we examined whether resource-dense patches (densities of flower and seeds on individual plants) resulted in greater visitation and seed removal rates, respectively. We also examined whether foraging rates were context-dependent by conducting the study in two sites that varied in resource densities. For pollinators, we found negative relationships between the density of flowers per plant and visitation rates, suggesting dilution effects. For seed predators, we found positive relationships consistent with concentration effects. Saturation effects and differences in foraging behaviors might explain the opposite relationships; most of the seed predators were ants (recruitment-based foragers), and pollinators were mostly solitary foragers. We also found that foraging rates were site-dependent, possibly due to site-level differences in resource abundance and consumer densities. These results suggest that these two plant species may benefit from producing as many flowers as possible, given high levels of pollination and low seed predation.
Highlights
Theory suggests that consumers should forage in ways that will maximize the benefits received from resources, while minimizing the energetic costs of obtaining those resources [1]
Because pollinators and seed predators have been known to respond to resource variation at both small and large scales [12,13,14,25], we examined whether foraging rate and resource density relationships were site-dependent
These findings indicate that pollinators and seed predators may respond differently to variation in differently to variation infuture resource density andconsider that future should consider life history when traits resource density and that studies should life studies history traits and foraging behavior and foraging behavior when investigating resource concentration effects
Summary
Theory suggests that consumers should forage in ways that will maximize the benefits received from resources, while minimizing the energetic costs of obtaining those resources [1]. Because of the greater density of resources, foragers can minimize the energetic costs of moving between patches to find food [1]. Many studies have looked for the existence of RCE in a variety of different consumer-resource systems with predators [4], herbivores [5,6,7], pollinators [8,9] and seed predators [10]. While theory suggests that foragers should distribute their foraging efforts in ways to maximize fitness [11], results from empirical studies are mixed. Otway et al [6] found a negative relationship between resource density and insect load (a resource dilution effect) for nine specialist herbivores, while Tubbesing et al [9] found resource concentration effects for bumble bees
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