Habitat destruction can change the degree of isolation and shape of remaining habitats, and thus can have dramatic consequences for biodiversity and the functioning of ecosystems. Although less appreciated, disproportionate destruction of one type of habitat will also change the ratio of distinct habitat types in a landscape. For example, wetland draining will not only decrease the amount of aquatic habitat in a landscape, but will also decrease the ratio of aquatic to terrestrial habitats. For organisms that require the use of both aquatic and terrestrial habitat types, such as amphibians, changes in the ratio of aquatic to terrestrial habitats can alter their abundance in either habitat, as well as their overall population dynamics. Preliminary surveys of a long lived amphibian, Ambystoma maculatum (the spotted salamander), indicate that the abundance of larval A. maculatum in aquatic habitats depends critically upon the ratio of aquatic to terrestrial habitat available. Because larval A. maculatum are top predators in small fishless ponds, they can have dramatic effects on community composition and diversity of their prey; alterations to the ratio of aquatic to terrestrial habitats can alter the strengths of this effect. This research seeks to elucidate the relationship between the ratio of aquatic to terrestrial habitats in a landscape and the impacts of larval A. maculatum on the aquatic community. It will specifically focus on the influence, of A. maculatum predation on the population dynamics of two frog species that utilize terrestrial and aquatic habitats to different extents, using mark-recapture and demographic modeling approaches. Studies of declining populations of amphibians typically focus on aquatic habitat and factors that affect larval survivorship and growth therein. Yet, there is increasing evidence of the importance of terrestrial habitat for many amphibian populations. Understanding how amphibians will respond to different types of habitat destruction (e.g., deforestation, draining of wetlands) requires consideration of the entire life-cycle of amphibians, and the ratio of aquatic to terrestrial habitats within the landscape. This research will be a timely contribution to the conservation literature and to management efforts aimed at preventing future extinctions of amphibians.
The spotted salamander, Ambystoma maculatum is a long lived amphibian that spends the majority of its life on land, only returning to the aquatic habitat for a very short period once a year to breed. Although amphibians are often associated with aquatic habitats, the abundance of A. maculatum is more dependent on the amount of forested habitat in an area rather than the amount of aquatic habitat. As a result, A. maculatum can reach very high numbers in isolated ponds and wetlands as long as there is abundant forested habitat surrounding the pond. A. maculatum larvae are very strong predators in the aquatic habitat. They often prey on tadpoles of other amphibians. This project focused on how the ratio of aquatic habitat to forested habitat can change the abundance of larval A. maculatum in ponds and the consequences of that for two frog species, the grey treefrog (Hyla versicolor) and the cricket frog (Acris crepitans) which are common prey of A. maculatum larvae. Using 12 experimental ponds at Washington University’s Tyson Research Center, we studied the effects of presence or absence of A. macualtum larvae on the population dynamics of H. versicolor and A. crepitans. To do this, we marked adult frogs to determine the survivorship of individuals from one year to the next. We constructed a matrix population model and projected population growth rate. We found that predation by A. maculatum larvae was predicted to decrease the rate of growth of the populations of both frog species. However, A. crepitans was predicted to experience a dramatic decline in the population, which could drive the population to extinction in the experimental ponds, while H. versicolor was predicted to still grow in population size from one year to the next, but just at a slower rate. Hyla versicolor lives for up to four or five years once it metamorphoses from an aquatic habitat. Acris crepitans on the other hand lives for only one year after it metamorphoses from the aquatic habitat. The shorter lifespan of A. crepitans means that it spends a larger percentage of its life in the aquatic habitat in comparison to the longer lived H. versicolor. Based on theories of population ecology, we would predict that a perturbation that decreases the survivorship of the aquatic life stage (e.g. predation), would have a bigger impact on the overall population size of the shorter lived species, since a greater proportion of its life occurs in that habitat. This project showed a similar decrease in the population sizes and population growth rates of both species, even though we would predict that A. crepitans would be impacted more. Follow up experiments revealed that A. maculatum preferred to eat H. versicolor tadpoles over A. crepitans tadpoles. Therefore, the survivorship rate of H. versicolor tadpoles was much lower than that of A. crepitans. Even though changes in larval survivorship would lead to larger decreases in the population size of A. crepitans in comparison to H. versicolor, mortality was much higher for H. versicolor which led to similar effects at the population level. This project highlights the importance of understanding the habitat use of amphibian species in order to determine how factors such as predation may translate into population level responses. Unfortunately, amphibians are easily seen and measured in aquatic habitats and their dependence on terrestrial habitats is often ignored. However, for many species, the terrestrial habitat can be more important in terms of population growth and persistence than the aquatic habitat. This project has implications for understanding the global decline of amphibian species because it calls attention to the complex life cycle of amphibians which limits the ability to predict how a perturbation such as contamination or disease will affect the overall population. This project was used to help educate St. Louis area youth about amphibian species and potential effects of agricultural runoff, climate change, and disease on amphibians locally and globally. As part of the St. Louis Science Center’s Youth Exploring Science (YES) program, A. Burgett worked with area teenagers to develop a protocol for frog call surveys in St. Louis’s Forest Park, which included night hikes, frog call identification sessions, and collection of water chemistry data. Additionally, this project has supported several area teenagers through and NSF funded Informal Science Education grant, in which students are partnered with researchers to have realistic field experiences. Students who participated in the Tyson Environmental Research Fellowship program presented data from this project at their annual symposium. Finally, several undergraduate researchers participated in this research and were trained in aquatic ecology and experimental design.
