Contrasting Foraging Patterns: Testing Resource-Concentration and Dilution Effects with Pollinators and Seed Predators

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.

The relationship between pollinator community and pollination services is mediated by floral abundance in urban landscapes

“…This may indicate a per-plant dilution effect in which pollinator densities at flowers decrease in resourcedense patches. This has been previously observed in a prairie system when flower density was increased (Wenninger et al 2016), possibly due to competition between pollinator individuals in abundant communities of pollinators. The underlying mechanism for why high pollinator abundance without species richness leads to greater competition is likely the same mechanism that promotes co-existence within all communities: intraspecific competition is assumed to be greater than interspecific competition (reviewed in Chesson 2000).…”

Ants Increase Cloverworm Herbivory Via Nonconsumptive Pathways

“…fect was seen in leaf damage data. The transient nature of ant scouts could mean that more ants would have been observed had a greater length of daily observation time been deployed (Wenninger et al 2016). Also, Buckley (1990) has shown some ant species are more apt to protect herbivores during nocturnal predation events, which were not observed in our study.…”

Plant ontogeny determines strength and associated plant fitness consequences of plant‐mediated interactions between herbivores and flower visitors

“…Interestingly, plant exposure to herbivores affected the correlation between the number of inflorescences and pollinator mutualists, but not the correlation between the number of inflorescences and antagonistic pollen beetles. This may indicate variable importance of resource quantity and quality for mutualists and antagonists (Cariveau, Irwin, Brody, Garcia‐Mayeya, & Ohe, ; Wenninger, Kim, Spiesman, & Gratton, ). Floral antagonists may prefer/better assess resource quantity than quality (Althoff, Xiao, Sumoski, & Segraves, ; Ekbom & Borg, ; Rusman et al, ; Wenninger et al, ), whereas both may be important for floral mutualists during foraging (Kuppler et al, ).…”