The Effects of Invasive African Clawed Frogs on Native Amphibians in Southern California
With increased global trade and human movement, invasive species have established populations in new regions at an unnaturally high rate, threatening native species and ecosystems. One invasive amphibian, the African clawed frog (Xenopus laevis), native to sub-Saharan Africa and shipped globally beginning in the mid 20th century, has established populations across the world, including southern California. Little research has been performed to determine how this invasive species may affect native amphibian populations already at risk from other anthropogenic factors.
The first part of this dissertation explores how X. laevis affects the abundance of the amphibian chytrid pathogen, Batrachochytrium dendrobatidis (Bd). As an asymptomatic carrier of Bd, post metamorphic X. laevis were hypothesized to be large reservoirs of the pathogen responsible for spreading it to globally to susceptible native species. However, field surveys from three populations of X. laevis found few infected individuals and those that were infected had low infection levels. Laboratory experiments were performed to evaluate the potential for larval X. laevis to prey upon the motile zoospore infectious stage of the pathogen from the water column. A reduction in the motile zoospores has been associated with reduced transmission rates and larval X. laevis in laboratory experiments was found to consume live Bd zoospores. However, larval X. laevis also exhibited intraguild predation on a zooplankton, Daphnia magna, which itself preys upon the chytrid zoospores. Intraguild predation may complicate the net effect of X. laevis larvae on Bd zoospore abundance in the water column. Together, these findings suggest that X. laevis are likely not a large source of the pathogen and may prey upon Bd, suppressing pathogen transmission to or between native amphibians.
The second part of this dissertation explores X. laevis as a predatory threat to amphibians native to southern California, and native amphibian response to the potential predation threat. Laboratory predation trials were performed using the Pacific treefrog (Pseudacris regilla) as a representative of native California amphibians, and showed that X. laevis will prey upon larval, juvenile, and adult P. regilla. Behavioral trials were also performed to evaluate if P. regilla larvae and adults recognize X. laevis as a potential predator and alter their behavior to avoid it. These experiments show that while P. regilla larvae do not change their activity levels in the presence of X. laevis, they do display spatially avoidance. Field enclosure experiments with adult P. regilla also found that they will spatially avoid X. laevis. This suggests native amphibians may recognize X. laevis as a predator, invoking a spatial avoidance response.
The third part of this dissertation explores the distribution of X. laevis and its co-occurrence with native amphibians to determine if the spatial avoidance observed in the experimental trials with P. regilla translates into the exclusion of amphibian populations from their native habitat. Amphibian surveys were performed using a new molecular technique, environmental DNA, to detect a species’ presence through DNA it sheds into the environment. Environmental DNA successful detected the presence of X. laevis using a Xenopus-specific primer with quantitative PCR, making it a useful tool to survey X. laevis distribution. Another more general primer was used to detect all amphibian species and explore co-occurrence between X. laevis and native amphibians. Several species of native amphibians were present at the same stream sites as X. laevis.
Together this dissertation suggests that while X. laevis is not acting as an ecologically important reservoir for the chytrid pathogen, it is a threat to native amphibians because of direct predation on larval and adult stages. Native amphibians may recognize and avoid X. laevis, reducing predation risk; but potentially reducing native amphibians in areas invaded by X. laevis. Survey for X. laevis revealed its widespread presence in southern California and co-occurrence with multiple native amphibian species.