Intraspecific Variation in Life History Traits among Two Forms of Ambystoma Barbouri Larvae
Venesky, Matthew D., Parris, Matthew J., The American Midland Naturalist
Variation in available resources (e.g., environment or food) can influence life history variation. Populations of the streamside salamander (Ambystoma barbouri) breed in both streams and ponds and larvae exhibit diverse life history patterns related to the environment within which they develop. Previous experiments demonstrated that environmental factors are largely responsible for differences in larval period, mass and size at metamorphosis between the two forms of A. barbouri; however, it is unclear which environmental factors influence the variation. Under controlled laboratory conditions, we tested if hydrological differences between lotic and lentic habitats contribute to life history divergence in A. barbouri. We reared laboratory born larvae from stream- and pond-collected eggs in laboratory environments with hydrological patterns similar to their natal environments. Ambystoma barbouri stream larvae metamorphosed faster and were smaller in mass and size compared to pond larvae. Although we did not test whether genetic or environmental factors influence larval life history traits, our data provide support that hydrological variation alone may contribute to the life history variation in the two forms of larval A. barbouri.
Many amphibians have complex life cycles consisting of an aquatic larval and terrestrial metamorphic stage (Duellman and Trueb, 1986). In the aquatic larval stage, amphibians express intra- and interspecific differences in growth rates, which are likely evolutionary responses to the variety of environmental conditions in which they develop (Wilbur and Collins, 1973). For example, larval period and growth rate can vary with differences in hydroperiod and food availability (Wilbur, 1980). In ephemeral environments, a shorter larval period can reduce the risk of desiccation (Wilbur and Collins, 1973). However, a reduced larval period may carry costs, including an inadequate body mass to survive the first winter and decreased reproductive success (Semlitsch et al., 1988; Morin, 1983). An understanding of the mechanisms driving life history variation is a fundamental goal in evolutionary ecology; however, distinguishing which, if any, environmental factors influence life history traits may be equally important.
Ambystomatid salamanders have a broad geographic range and express variation in their life history traits (Petranka, 1998), specifically inter- and intraspecific variation in larval growdi and developmental rates (Bruce, 1982; Petranka, 1984; Whiteman, 1994). Accordingly, they provide an excellent model for understanding variation in life history traits. Unlike most pond-breeding ambystomatid salamanders, individuals of the streamside salamander (Ambystoma barbouri) breed in bodi first-order streams and ponds (Petranka, 1984). Larvae in these two habitats differ in their behavior, morphology and life history traits, such as larval period and size at metamorphosis (Petranka, 1998). It appears that life history variation between larvae in these two habitats is due largely to environmental factors. In nature, pond A. barbouri grew faster and were larger at metamorphosis compared to stream A. barbouri; however, when larvae from bodi habitats were reared in the laboratory in still water, no differences were observed in their size at metamorphosis or duration of their larval period (Petranka, 1984). In ponds and streams, many biotic and abiotic factors (e.g., dissolved oxygen, food availability) may contribute to these life history differences. It is unclear if water flow alone may contribute to this life history variation.
Under controlled laboratory conditions, we mimicked the hydrological differences between streams and ponds and tested if this specific environmental difference contributes to life history variation in Ambystoma barbouri. By placing stream and pond larvae in laboratory environments with hydrological patterns similar to their natal environments, we tested wheuier differences in hydrological conditions alone influenced developmental rate, size and lengdi of larval period. …