COS 160-5 - The role of an ephemeral prey taxon in shaping top-predator inter-patch movements and body conditio

Background/Question/Methods

Top predators can serve as bioindicators of an ecosystem, as they often reflect and determine major changes in food webs. In the era of global anthropogenic change, perturbations to food webs are widely expected, with direct impacts on some species leading to concomitant changes in other food-web members. In an alpine wetland complex in the Rocky Mountains (Colorado, USA), metamorphic Arizona tiger salamanders (Ambystoma mavortium nebulosum) breed in midsummer in ponds with permanent hydroperiods, and then move to non-permanent ponds where they exploit lower conspecific competition and the availability of a calorically rich, ephemeral prey taxon, fairy shrimp (Branchinecta coloradensis). It is unclear, though, how variation in densities of fairy shrimp among ponds drives movement patterns of salamanders and correlates with salamander body condition. We sampled fairy shrimp densities in nine ponds across eight weeks in 2021 and calculated salamander movement and body condition with data from an ongoing mark-recapture study. In addition, we performed gastric lavage on 56 salamanders to investigate whether fairy shrimp densities were reflective of salamander consumption of the prey taxon.

Results/Conclusions

We tested salamander inter-pond movements in an optimal foraging theory framework, and we confirmed several theoretical hypotheses. Salamanders were more likely to emigrate from a non-permanent pond when the average density of fairy shrimp dropped below the average density of the whole wetland complex. Salamanders were also less likely to emigrate from more isolated ponds due to the presumed time and risk costs of inter-pond travel. Furthermore, salamander body condition was positively correlated with fairy shrimp density, with gastric lavage confirming that fairy shrimp densities were casually related to consumption rate. With increased variation in annual snowpack and wetland hydrology in western US mountain ranges, there is a risk that fairy shrimp densities will decrease as non-permanent ponds experience altered hydroperiods. This may lead to fitness consequences for metamorphic Arizona tiger salamanders, a species in which female body condition and fecundity are positively correlated. Overall, our study highlights the role of a calorically rich prey taxon in influencing top-predator inter-patch movements, and the potential consequences that would accompany a decline in the valauable prey taxon.