Phenology and density influence intraguild predation between two larval salamanders
Phenology shifts are occurring for many species, which may have strong consequences for species interactions and community structure. These altered interactions can be further compounded by other processes, such as density-dependent competition within or across species. For size-dependent species interactions, fluctuations in either mechanism (phenology and density-dependent competition) can lead to variable growth rates that subsequently determine the type and strength of interactions that occur. We tested whether the outcome of intraguild predation between a pair of larval salamanders (Ambystoma annulatum and A. maculatum) varied in response to altered phenology and density. We reared salamanders in outdoor cattle tank mesocosms from fall 2013 to summer 2014. We added each species at one of two time points (early or late) in their normal breeding phenology, in four phenology treatments. We added the intraguild predator (A. annulatum) at two different densities, totaling eight treatments. Additionally, we isolated the predominant mechanism by which A. annulatum affected A. maculatum (resource depletion or predation) with additional treatments that manipulated whether A. annulatum was present or absent over the previous winter. We tested whether body size, larval period length and survival varied among phenology and density treatments for each species.
We found that body size, length of larval period and survival of A. annulatum varied through an interaction of each species’ phenology. Body size of A. annulatum was larger when species had synchronous (both early or both late) phenologies, but larval period length was shorter when A. annulatum bred later, regardless of when A. maculatum bred. Body size and larval period length were negatively and positively correlated with the number that survived, respectively. Survival was higher at lower densities, and was lower under synchronous compared to asynchronous phenologies. For A. maculatum, body size was greater when they bred earlier. Larval period length was affected by a three-way interaction of each species phenology and the number of surviving A. annulatum, with asynchronous phenology treatments resulting in stronger covariance of larval period length and A. annulatum survival. Survival of A. maculatum was only affected A. annulatum survival and not phenology. We also determined that the predominant mechanism by which A. annulatum limits A. maculatum is predation. Overall, we found that normal variation in phenology and density has strong effects on intraguild predation, suggesting that continued shifts in either factor via climate change will further alter species interactions.