Overall competitor density, not species identity, affects facultative paedomorphosis in the mole salamander, Ambystoma talpoideum

Background/Question/Methods:

Factors that promote phenotypic plasticity frequently include environmentally-based mechanisms centered on both abiotic and biotic processes.  Phenotype production based on biotic components often has been linked to factors such as density-dependent competition.  Specifically, phenotypic plasticity is thought to develop in environments with low interspecific competition, which increases the number of exploitable niches that would then promote alternative phenotypes to reduce intraspecific competition intensity.  Direct tests of the effects of interspecific competition on discrete phenotype production have been rarely performed, however. Mole salamanders (Ambystoma talpoideum) have the ability to undergo metamorphosis, or remain aquatic as sexually mature paedomorphic adults or immature larvae that mature the following year.  The role of interspecific competition in producing each phenotype has not been explored, but could be important given the high incidence of sympatry with competitors. We examined this question by experimentally manipulating larval densities of A. talpoideum and A. maculatum (spotted salamander). Larvae were reared in outdoor experimental mesocosms within a response surface design, which manipulated both overall larval density and frequency of each species. The fate of each individual (i.e., phenotype) was recorded, as well as body size/condition.  Conspecific and congeneric densities were analyzed as potential predictors of phenotype production.

Results/Conclusions:

Overall competitor density was a significant factor that influenced phenotype production for A. talpoideum, but interspecific competitor frequency had no clear effect; survival was nearly equal across all treatments.  Paedomorphs occurred significantly more often in the low and medium overall densities compared to the high overall density.  Metamorph production for A. talpoideum was approximately equal across the three overall densities. There were significantly more overwintered larvae produced in the high density treatment compared to the low and medium density treatments.  These results indicate that competition is important to the production of the different phenotypes and delaying of maturity in A. talpoideum, and that overall competitor density, not species identity, was most critical.  These findings also provide some support for previous hypotheses of paedomorphosis, but augment our understanding of this process in A. talpoideum by incorporating delayed maturity as a potential outcome.  As survival was not affected by competition, overall density of larval salamanders likely has important consequences that affect the size and age structure of aquatic A. talpoideum, and can influence both population and community dynamics.