Range Expansion Dynamics and Interspecific Interactions of the Invasive Cuban Treefrog in Florida

Abstract

Biological invasions simultaneously represent a threat to native biota and provide an opportunity to test ecological and evolutionary ideas. Increased interest in the field of invasion ecology has greatly improved our understanding of invasion dynamics; however, many ideas and assumptions remain inadequately tested. This dissertation focuses on two major themes: (1) native predator response to a toxic, non-indigenous species, and (2) the processes that may promote range expansion following establishment. Here, I used the Cuban Treefrog (Osteopilus septentrionalis) invasion of Florida as a model to explore these aspects of invasive ecology.

Cuban Treefrogs are known prey of native snake predators, despite their possession of a defensive toxin assumed to be novel to their invasive Florida range. I investigated the idea that possession of unique chemical defenses may increase the likelihood of formation of evolutionary traps for predators. Additionally, I explored the possibility that populations of snakes in co-occurrence with Cuban Treefrogs had responded to escape potential trap dynamics. To test these hypotheses, I conducted a series of prey preference assays and feeding trials of Common Gartersnakes (Thanmonphis sirtalis) collected from areas of sympathy and allopathy with Cuban Treefrogs. To assess prey preference, we explored the predatory responses of snakes to cues from Cuban Treefrogs, a native treefrog, and positive and negative controls. To evaluate a potential fitness cost of consuming Cuban Treefrogs, I monitored the growth of 61 wild-caught adult snakes fed exclusive diets of either Cuban Treefrogs, or one of two native prey species. Our results suggest that Cuban Treefrogs represent an evolutionary trap for snakes because consumption of these frogs carries fitness costs, and snakes are unable to recognize this prey as being costly. We failed to detect a regional effect of snake growth or prey preference suggesting, snakes are not responding to escape trap dynamics.

The current range of Cuban Treefrogs encompasses the majority of peninsular Florida and expansion of this range likely exacerbates impacts of these frogs on the native biota. Many biological invasions are characterized by rapid range expansion; however, the factors enabling range expansion are poorly understood. Here, I investigated the idea that dispersal ability is promoted at the invasion front by trade-offs facilitating allocation of resources away from immune defense and towards traits that improve dispersal ability. To investigate this idea, I assayed Cuban Treefrog immunity and several characteristics associated with increased dispersal ability in long-colonized and recently-colonized regions of their invasive distribution. Cuban Treefrogs at the invasion front exhibited decreased innate immune responses coupled with greater locomotor endurance, compared with frogs from a long-colonized region. I failed to detect a regional difference in sprint speed or growth rate. These findings provide support for the idea that immune-based trade-offs promote range expansion and invasion success.