Date Published: August 22, 2019
Publisher: Public Library of Science
Author(s): S. Rinehart, J. D. Long, Robert B. Srygley.
Pollen can decrease (via reduced consumption) or increase (via numerical response) an omnivores consumption of animal prey. Although pollen can increase predation pressure through numerical responses of omnivores, pollen may also suppress predation by increasing omnivore interactions with conspecifics. Despite this potential, studies of the impacts of pollen on predation by omnivores often overlook the effect of these tissues on intraspecific interactions between omnivores. We designed three studies to examine how Spartina foliosa pollen and conspecific density impact scale insect prey consumption by ladybeetle (Naemia seriata) omnivores. First, we assessed how pollen impacts scale insect consumption by isolated ladybeetles. Second, we measured how pollen influences ladybeetle prey suppression when numerical responses were possible. Third, because initial experiments suggested the consumption rates of individual ladybeetles depended on conspecific density, we compared per capita consumption rates of ladybeetles across ladybeetle density. Pollen did not influence prey consumption by isolated ladybeetles. When numerical responses were possible, pollen did not influence total predation on prey despite increasing ladybeetle density, suggesting that pollen decreased per capita prey consumption by ladybeetles. The discrepancy between these studies is likely a consequence of differences in ladybeetle density—the presence of only two other conspecifics decreased per capita prey consumption by 76%. Our findings suggest that pollen may not alter the population level effects of omnivores on prey when omnivore numerical responses are offset by reductions in per capita predation rate.
Omnivory (i.e. consuming resources from multiple trophic levels)  is ubiquitous within several taxa (e.g. birds, mammals, reptiles, insects, and fishes) and influences the structure and function of communities [2–3]. Interactions between omnivores and their plant and animal prey can account for up to 78% of species’ links in food webs . Despite their prevalence, we lack a basic understanding of how pollen and other plant resources (e.g., seeds) affect interactions between omnivores and their prey in natural systems. Some studies suggest that plant resources decrease prey consumption by omnivores [5–6], whereas others suggest the opposite . This discrepancy may be exacerbated by methodological approaches and the spatial scale of the study [8–9]. In fact, many omnivory studies focus on isolated omnivores feeding on a sub-set of possible resources, which only allows omnivore consumption to depend on resource density and the availability of plant resources [8,10]. Such approaches fail to allow important intraspecific interactions (e.g., mating, cannibalism, and competition) and interspecific interactions (e.g., predation and competition), whose occurrence may be altered by plant resources [9, 11–13]. For example, the availability of pollen reduced the magnitude of predation and intraguild predation on western flower thrips (Frankiniella occidentalis) by altering the distribution of predators and intraguild predators on plants . Understanding how plant resources and conspecific density affect prey consumption by omnivores may help predict when and where omnivores exert top-down control on prey populations.
Pollen can increase or decrease prey consumption by altering omnivore foraging behavior. In laboratory mesocosms, isolated adult ladybeetle prey consumption was unaffected by cordgrass flowers (Fig 1). In the field, habitat patches containing access to cordgrass flowers attracted 4x as many adult ladybeetles as habitats lacking flower access (Fig 2a). However, elevated ladybeetle densities in habitats with cordgrass flower access did not result in greater loss of scale insect prey (Fig 3), suggesting that pollen resources reduced ladybeetle per capita consumption of scale insects. This discrepancy (pollen had no effect in the lab but reduced per capita prey consumption in the field), may be related to intraspecific interactions (e.g., interference competition) between ladybeetles which were absent in the lab study with isolated ladybeetles. This hypothesis is supported by our finding that increasing conspecific density reduced per capita consumption of scale insects by ladybeetles. Overall, these observations suggest that pollen may not impact the population level effects of omnivores on prey when numerical responses of omnivores are offset by reductions in their per capita predation rates.