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Description
Predators and omnivores shape community structure and function by consuming (i.e. consumptive effects; CEs) and ‘scaring’ (i.e. nonconsumptive effects; NCEs) prey. Thus, predicting the consequences of predator-prey interactions has been a major focus of ecological research for several decades. For instance, understanding the mechanism(s) by which predators induce trophic cascades (i.e. CEs vs. NCEs) is important because the nature of this indirect interaction can critically influence ecosystem-level processes such as energy flow and nutrient cycling. Despite the vast literature on predator-prey interactions, few studies tested the role of predator and prey traits on the outcomes of these interactions. Recognizing this, I tested how predator traits [e.g. hunting mode (Chapter 1) and facultative omnivory (Chapter 2 & 3)] and prey traits [e.g. habitat domain range (Chapter 1)] impact the outcome of predator-prey interactions in natural systems. In Chapter 1, I examined the trait-mediated indirect interaction (TMII) and total indirect interaction (TII) produced during interactions between an active, broad habitat domain range (BHDR) ladybeetle predator (Naemia seriata) and its narrow habitat domain range (NHDR) prey (scale insects; Haliaspsis spartinae). I exposed scale insects to nonlethal and lethal ladybeetle predators in laboratory mesocosms for 15 weeks. I measured how these interactions indirectly impacted the growth of the scale insect’s host plant (cordgrass; Spartina foliosa) and the population density of scale insects. Contrary to theoretical predictions based on these predator and prey traits, nonlethal ladybeetles did not induce TMIIs. However, lethal ladybeetles increased cordgrass total and root dry biomass by 36% and 44% (respectively), suggesting the presence of strong density-mediated indirect interactions (DMIIs). Additionally, both lethal and nonlethal ladybeetles reduced scale insect population density. My findings suggest that DMIIs, rather than TMIIs, can result from interactions between active, BHDR predators and NHDR prey. In Chapter 2, I used three primary experiments to assess the relationship between habitat use (based on the availability of animal and/or plant prey resources) and performance for an important insect omnivore (ladybeetles). First, I used field manipulations of resource availability (i.e. scale insects and cordgrass pollen) to examine the habitat use of ladybeetle predators. Second, I conducted a series of no-choice laboratory assays to compare the performance (fecundity and longevity) of ladybeetles on these different resources. Third, I quantified adult ladybeetle preference for olfactory cues from cordgrass with and without scale insects using a y-tube olfactometer. In the field, adult ladybeetles selectively used plots containing scale insects. In the lab, diets containing scale insects maximized both adult and larval ladybeetle longevity, and adult fecundity. Adult ladybeetles were attracted to chemical cues associated with scale insects over distances of 10s of centimeters. Overall, my findings suggest that the habitat use and performance of ladybeetles are strongly linked, with ladybeetles preferentially using habitats that maximize their individual performance. In Chapter 3, I designed three studies to examine how alternative resources and conspecific density impact the consumption of scale insects by an omnivorous ladybeetle, as previous work suggests that alternative resources can both increase and decrease omnivore consumption of prey species. First, I used laboratory mesocosms to assess how alternative resources (cordgrass flowers) impact scale insect consumption by isolated adult ladybeetles. Second, I conducted a field study to understand how alternative resources influence ladybeetle prey suppression when numerical responses (i.e. aggregation and reproduction) are possible. Third, because the above experiments suggested that the consumption rates of individual ladybeetles depended upon conspecific density, I used no-choice assays to specifically compare per capita consumption rates of ladybeetles across ladybeetle density. Isolated adult ladybeetles in the lab had a similar rate of scale insect consumption, regardless of the presence of alternative resources. In contrast, ladybeetles decreased their per capita scale insect consumption when alternative resources were available in the field (where numerical responses were present). Laboratory no-choice feeding assays suggested that the discrepancies between the effects of alternative resources in our laboratory and field studies may have been mediated by ladybeetle density, as ladybeetle consumption of scale insects declined with increasing ladybeetle density. My findings suggest that elevated conspecific density, mediated by omnivore numerical responses to alternative resources, reduces omnivore per capita consumption of scale insect pests in a southern California salt marsh. Collectively, my dissertation suggests that the functional traits of predators and prey can provide useful insights into when, where, and how predators may exert top-down effects on ecological communities.
