Spatial ecology of the wood frog, Rana sylvatica

Date of Completion

January 2003

Keywords

Biology, Ecology|Biology, Limnology

Degree

Ph.D.

Abstract

The understanding of distributions is a foundational challenge for ecologists. The individuals within a population are often patchily distributed. This pattern is as common as it is poorly understood. I undertook a study of wood frogs (Rana sylvatica) with the goal of documenting patterns and evaluating hypotheses concerning their distributions within wetlands. Specifically, I was focused on the role of light and thermal variation in wetlands. The wood frog is an excellent candidate for such a study. It is found within both open and heavily shaded wetlands, and upon hatching, its larvae disperse across wetlands from a single oviposition site shared by the entire population. I found that within three weeks of hatching, wood frog larvae were completely dispersed throughout even relatively large wetlands (>5000 m2). These larvae, however, did not distribute themselves evenly within wetlands. In fact, distributions were often highly clumped, particularly within heavily shaded ponds, and these heterogeneities in distributions could be related to light and thermal gradients. Light and thermal gradients can impact wood frogs even prior to hatching. In a field experiment, I found that the location of the oviposition site can have strong effects on hatching and persistent impacts that carry over to the development and growth of larvae. These experimental results are corroborated by the observation that wood frog metamorphs tend to be larger in lighter areas within wetlands compared with more heavily shaded areas that may be just a few meters away. Finally, I found that wood frog larvae from shaded wetlands are positively thermotaxic while conspecifics from neighboring unshaded wetlands are not. Collectively, my results show that light gradients within wetlands are of critical importance for the distribution of wood frogs. ^

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