Meeting Abstract
The invasive cane toad, Rhinella marina, originated in South and Central America, but was intentionally introduced into several countries including South Florida around the 1930s for biocontrol purposes (Lever 2001). In comparison to Australia where they travel east to west, the Florida peninsula provides limited longitudinal movement, and they are required to travel along more latitudinal isothermal gradients in order to increase their geographic spread. Invasive tropical poikilothermic species are generally restricted in their northern range expansion due to thermal minimum sensitivity. Our lab has recently revealed established cane toad populations in three novel locations that are located north of the expected geographical range in Florida (Holcombe et al 2007). As part of my research into physiological limitations, tradeoffs, and phenotypic variation allowing for this northern establishment, we measured field critical thermal minimum (CTmin) across an isothermal gradient as an extrapolation of potential thermal sensitivity to colder winter conditions. We hypothesized that colder regions—invasion front—will have lower thermal minimums than warmer regions—origin. CTmin was measured within 12 hours of collection from four separate populations ranging from south to north: Miami, Lake Placid, New Port Richey, and Deland, FL. Toads were placed within a controlled incubator, measured for body temperature every 10 minutes, and failure to elicit a righting response was used for the endpoint. Preliminary results indicate a trend towards lower thermal minimums at higher latitudes. Deland, the northernmost population had individuals that had significantly lower CTmin than individuals from Miami (p=0.039).
