Effects of phenotype- and condition-dependent factors on juvenile dispersal of the ringed salamander (Ambystoma annulatum)
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Dispersal is the movement of individuals from their natal population to a different breeding population. Long distance dispersal movements are rare, with most individuals staying within their natal population (hereafter residents). The frequency with which individuals disperse to new population or settle near natal sites has strong consequences for individual survival and fitness, as well as ecological and evolutionary processes. Despite the importance of dispersal, there have been limited empirical tests of the factors affect whether an individual disperses or is philopatric. This is particularly true for amphibians of the family Ambystomatidae. These pond-breeding salamanders have complex life cycles, with aquatic larvae and terrestrial juveniles and adults. Despite abundant research on the breeding migrations and larval life stage of these salamanders, factors affecting juvenile dispersal remain poorly resolved. This is particularly true for phenotype-dependent factors, such as body size and body condition, which are generally controlled for rather than manipulated. An understanding of factors driving dispersal in Ambystomatidae is critically needed. One-third of all amphibians are currently threatened with extinction due to habitat loss and fragmentation, and climate change. By understanding the factors that affect dispersal in this family of salamanders, we can manage at the individual, population, and metapopulation level. My dissertation research used laboratory and field experiments to measure effects of phenotype- and condition-dependent factors on movement in a mole salamander (Ambystoma annulatum). The primary objectives of my study were to 1) describe the effect of natal density on juvenile phenotype, and 2) determine the effect of individual variation in phenotype-dependent factors (i.e., behavior, morphology, and body condition) and condition-dependent factors (i.e., natal population density and juvenile habitat quality) on dispersal. To identify the effects of natal population density on juvenile phenotype, I conducted a replicated pond mesocosm experiment. I manipulated the density of ringed salamanders (Ambystoma annulatum) over 10 levels and compared the model fit of four functional forms of body size, body condition, date of metamorphosis survival, and percent lipids. In general, I found support for non-additive density dependence, with the negative impact of each additional individual decreasing in magnitude as density increased. After measuring the effect of natal conditions on juvenile phenotype, I conducted a series of experiments to describe the effect of phenotype- and condition dependent factors. I reared salamanders from larvae under different natal densities, resulting in juveniles that different in the conditions they experienced (i.e., natal density), as well as their body size, condition, and morphology. I also measured the exploration behavior of individuals. Using PIT telemetry, I tracked the initial movement of juvenile salamanders in two different habitat conditions (forest and field habitat). I found that both phenotype- and condition-dependent factors affect juvenile dispersal. I also found an interaction between these two types of factors, suggesting that redundant cues over ontogeny affect dispersal. Collectively, these studies indicate that both aquatic natal habitat and its effects on juvenile phenotype, as well as juvenile terrestrial habitats should be considered when managing amphibian populations and metapopulations.
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