Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

An invasive prey provides long-lasting silver spoon effects for an endangered predator

The natal environment can have long-term fitness consequences for individuals, particularly via 'silver spoon' or 'environmental matching' effects. Invasive species could alter natal effects on native species by changing species interactions, but this potential remains unknown. Using 17 years of data on 2588 individuals across the entire US breeding range of the endangered snail kite (Rostrhamus sociabilis), a wetland raptor that feeds entirely on Pomacea snails, we tested for silver spoon and environmental matching effects on survival and movement and whether the invasion of a non-native snail may alter outcomes. We found support for silver spoon effects, not environmental matching, on survival that operated through body condition at fledging, explained by hydrology in the natal wetland. When non-native snails were present at the natal site, kites were in better condition, individual condition was less sensitive to hydrology, and kites fledged across a wider range of hydrologic conditions, leading to higher survival that persisted for at least 10 years. Movement between wetlands was driven by the current (adult) environment, and birds born in both invaded and uninvaded wetlands preferred to occupy invaded wetlands post-fledging. These results illustrate that species invasions may profoundly impact the role of natal environments on native species.

System productivity alters predator sorting of a size-structured mixed prey community

Predator-prey interactions are often size-structured and focused on smaller vulnerable size classes. Predators are also predicted to sort prey communities according to relative vulnerabilities. Increased system productivity and juvenile growth may benefit some species more than others, making relative vulnerability non-static and growth-mediated. We hypothesized that increased system productivity would weaken juvenile-stage predation generally, and potentially shift the community sorting effects of a predator. Using replicated wetland mesocosms we quantified the effects of a generalist size-specific crayfish predator (Procambarus fallax) on juveniles of two species of apple snails (Pomacea spp.) under two levels of system productivity (low vs. high). After 6 weeks of exposure, we quantified predator and productivity effects on snail survival, biomass, and composition of the assemblage. Crayfish depressed the final density and biomass of snails, and sorted the assemblage, selectively favoring survival of the native P. paludosa over the intrinsically more vulnerable invasive P. maculata. Both snails grew faster at higher productivity, but growth differentially increased survival of the invasive snail in the presence of crayfish and weakened the sorting effect. The native P. paludosa hatches at a larger less vulnerable size than the invasive P. maculata, but higher productivity reduced the relative advantage of P. paludosa. Our results are inconsistent with predictions about the sorting effects of predators across productivity gradients because the more resistant prey dominated at low productivity. Our findings highlight that the relative vulnerabilities of prey to a common predator are not always fixed, but can be growth-mediated.

Rapid morphological change of a top predator with the invasion of a novel prey

Invasive exotic species are spreading rapidly throughout the planet. These species can have widespread impacts on biodiversity, yet the ability for native species, particularly long-lived vertebrates, to respond rapidly to invasions remains mostly unknown. Here we provide evidence of rapid morphological change in the endangered snail kite (Rostrhamus sociabilis) across its North American range with the invasion of a novel prey, the island apple snail (Pomacea maculata), a much larger congener of the kite's native prey. In less than one decade since invasion, snail kite bill size and body mass increased substantially. Larger bills should be better suited to extracting meat from the larger snail shells, and we detected strong selection on increased size through juvenile survival. Using pedigree data, we found evidence of both genetic and environmental influences on trait expression and discovered that additive genetic variation in bill size increased with invasion. However, trends in predicted breeding values emphasize that recent morphological changes have been driven primarily by phenotypic plasticity rather than micro-evolutionary change. Our findings suggest that evolutionary change may be imminent and underscore that even long-lived vertebrates can respond quickly to invasive species. Furthermore, these results highlight that phenotypic plasticity may provide a crucial role for predators experiencing rapid environmental change.

Individual Movement Strategies Revealed through Novel Clustering of Emergent Movement Patterns

Understanding movement is critical in several disciplines but analysis methods often neglect key information by adopting each location as sampling unit, rather than each individual. We introduce a novel statistical method that, by focusing on individuals, enables better identification of temporal dynamics of connectivity, traits of individuals that explain emergent movement patterns, and sites that play a critical role in connecting subpopulations. We apply this method to two examples that span movement networks that vary considerably in size and questions: movements of an endangered raptor, the snail kite (Rostrhamus sociabilis plumbeus), and human movement in Florida inferred from Twitter. For snail kites, our method reveals substantial differences in movement strategies for different bird cohorts and temporal changes in connectivity driven by the invasion of an exotic food resource, illustrating the challenge of identifying critical connectivity sites for conservation in the presence of global change. For human movement, our method is able to reliably determine the origin of Florida visitors and identify distinct movement patterns within Florida for visitors from different places, providing near real-time information on the spatial and temporal patterns of tourists. These results emphasize the need to integrate individual variation to generate new insights when modeling movement data.