Simultaneous invasion decouples zebra mussels and water clarity.

Species invasions are a leading threat to ecosystems globally, but our understanding of interactions among multiple invasive species and their outcomes on ecosystem properties is undeveloped despite their significance to conservation and management. Here we studied a large lake in Minnesota, USA, that experienced a simultaneous surge in invasive zebra mussel and spiny water flea populations. A long-term (2000-2018) dataset offered a rare opportunity to assess whole-ecosystem shifts following the co-invasion. Within two years, the native crustacean zooplankton community declined abruptly in density and productivity (-93% and -91%, respectively). Summer phytoplankton abundance and water clarity remained stable across the time series, an unexpected outcome given the high density of zebra mussels in the lake. Observational data and modeling indicate that removal of native herbivorous zooplankton by the predatory spiny water flea reduced zooplankton grazing pressure enough to compensate new grazing losses due to zebra mussels, resulting in a zero net effect on phytoplankton abundance and water clarity despite a wholesale shift in secondary production from the pelagic to the benthic food web. This study reveals the extent of direct and indirect effects of two aquatic invaders on food-web processes that cancel shifts in water clarity, a highly valued ecosystem service.

Resource competition and shared natural enemies in experimental insect communities.

Much theory has been developed to explore how competition for shared resources (exploitation competition) or the presence of shared natural enemies (apparent competition) might structure insect and other communities. It is harder to predict what happens when both processes operate simultaneously. We describe an experiment that attempted to explore how shared natural enemies and resource competition structured a simple experimental insect community. Replicated communities were assembled in population cages consisting of the aphid species Acyrthosiphon pisum and Megoura viciae either alone or competing for a resource, their shared host plant Vicia faba. Each combination was set up with and without the parasitoid Praon dorsale which attacked both species of aphid. Population dynamic data show that interspecific resource competition was the dominant process structuring the community. Though juvenile parasitoids could develop successfully inside hosts of both species, they failed to suppress either aphid below their carrying capacities and were unable to persist on one species. We suggest that intense resource competition may reduce the value of individual aphids as hosts for parasitoids such that their population growth rate is less than zero and discuss whether this phenomenon occurs in natural and agricultural communities.

Go big or don't? A field-based diet evaluation of freshwater piscivore and prey fish size relationships.

Body size governs predator-prey interactions, which in turn structure populations, communities, and food webs. Understanding predator-prey size relationships is valuable from a theoretical perspective, in basic research, and for management applications. However, predator-prey size data are limited and costly to acquire. We quantified predator-prey total length and mass relationships for several freshwater piscivorous taxa: crappie (Pomoxis spp.), largemouth bass (Micropterus salmoides), muskellunge (Esox masquinongy), northern pike (Esox lucius), rock bass (Ambloplites rupestris), smallmouth bass (Micropterus dolomieu), and walleye (Sander vitreus). The range of prey total lengths increased with predator total length. The median and maximum ingested prey total length varied with predator taxon and length, but generally ranged from 10-20% and 32-46% of predator total length, respectively. Predators tended to consume larger fusiform prey than laterally compressed prey. With the exception of large muskellunge, predators most commonly consumed prey between 16 and 73 mm. A sensitivity analysis indicated estimates can be very accurate at sample sizes greater than 1,000 diet items and fairly accurate at sample sizes greater than 100. However, sample sizes less than 50 should be evaluated with caution. Furthermore, median log10 predator-prey body mass ratios ranged from 1.9-2.5, nearly 50% lower than values previously reported for freshwater fishes. Managers, researchers, and modelers could use our findings as a tool for numerous predator-prey evaluations from stocking size optimization to individual-based bioenergetics analyses identifying prey size structure. To this end, we have developed a web-based user interface to maximize the utility of our models that can be found at www.LakeEcologyLab.org/pred_prey.

Host switching in a generalist parasitoid: contrasting transient and transgenerational costs associated with novel and original host species.

Parasitoids face challenges by switching between host species that influence survival and fitness, determine their role in structuring communities, influence species invasions, and affect their importance as biocontrol agents. In the generalist parasitoid, Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae), we investigated the costs in encapsulation, survival, and body size on juveniles when adult parasitoids switched from their original host, Plodia interpunctella (Hübner) (Lepidotera, Pyralidae) to a novel host, Ephestia kuehniella (Zeller) (Lepidoptera, Pyralidae), over multiple generations. Switching had an initial survival cost for juvenile parasitoids in the novel host, but increased survival occurred within two generations. Conversely, mortality in the original host increased. Body size, a proxy for fecundity, also increased with the number of generations in the novel host species, reflecting adaptation or maternal effects due to the larger size of the novel host, and therefore greater resources available to the developing parasitoid. Switching to a novel host appears to have initial costs for a parasitoid, even when the novel host may be better quality, but the costs rapidly diminish. We predict that the net cost of switching to a novel host for parasitoids will be complex and will depend on the initial reduction in fitness from parasitizing a novel host versus local adaptations against parasitoids in the original host.