LibriumArcana.Games

Invasive organisms can have many and varied impacts ecosystems, and it is important to know what these impacts are before we decide what actions to take (or indeed whether or not we should or can do anything) to control the introduced species or mitigate their ecological effects. Central to this decision making is the possibility that the system itself is self regulating and natural selection will eventually integrate the new organism into the biotic community.

A large part of answering this question is to determine what effects predation will have on the introduced organism once it becomes established. As a result, there are three questions I would like to answer, both relating to the American Bullfrog.

1.Are potential predators of the bullfrog within their introduced range capitalizing on the new source of food, and does this to the extent to which it exists in a given taxa consist of a pre-adaptation (such as learning, or simply being a generalist which will eat anything that moves) or did it evolve recently as a response to selection?

2.Have bullfrogs to the extent that predation exists on them within their introduced range developed strategies to mitigate predation risk either by adapting pre-existing strategies to new predators, or developing new strategies? And if the former, did this adaptation come as the result of selection within their new habitats or does it consist of a pre-adaptation?

3.What effects do these interactions have on native amphibians?

So, what my hypotheses and how do I test them? As the predator in question, I will probably use snakes in the genus Thamnophis (garter snakes)

A. My hypothesis for the first question is that potential predators are in fact actual predators, and that they capitalized on bullfrogs as the result of selection. Snakes are not particularly visual predators and tend to use scent to find prey. If they do not have the proper chemoreceptors they might not recognize a bullfrog tadpole as a potential source of food. Those that do will be at a marked advantage over other snakes because they will be able to obtain more energy and thus produce more, healthier offspring

Testing will be accomplished by the use of three species. The first is Thamnophis sirtalis, which is sympathetic with bullfrogs through the eastern portion of its range, and with introduced bullfrogs on the west coast. Three treatments groups will be prepared. The first will consist of snakes from a population sympatric with native bullfrogs (treatment A), the second from a population sympatric with non-native bullfrogs (treatment B) and the third from a population not sympatric with bullfrogs at all (treatment C). It is expected that snakes from treatments A and B would respond to bullfrogs or bullfrog cues while treatment C would not. This experiment would need to be confirmed with lab-raised snakes to control for learning.

This exact procedure could be used with other species of garter snake. The checkered garter A similar procedure (excluding treatment A) could be used for garter snakes species such as the western terrestrial garter snake, and the pacific aquatic garter snake which are not sympatric with bullfrogs within the bullfrog's native range. It is worth noting of course that these will be measured in terms of a statistically significant difference in a response measure such as tongue flicking, or time spent searching. Depending on what is found in the literature or seen in exploratory trials if nothing is present therein.

B. The hypothesis for the second question depends on the answer to the first and is that in the event that bullfrogs are preyed upon by a garter snake within their introduced range, they will have developed counter measures to mitigate predation risk. These counter measures are predicted to be applications of behaviors they used in their native environment to a new predator as a response to selective pressures.

Testing will be accomplished by using frogs from 2 or three populations depending on the snake predator in question. Treatment Alpha will consist of frogs from a population sympatric with the tested snake in their native range (this treatment will be excluded when it does not apply). Treatment Beta will consist of frogs from a population sympatric with the snakes in their introduced range. Treatment Gamma will consist of frogs not sympatric to the snake in question. They will be tested (not using the same frogs in different tests) using different predator cues such as scent and visual cues. The prediction being that frogs from all three populations will respond the same way if the frog and the snake species co-exist within the frogs native range (A possible confound is that frogs in a population without the snake may have lost the ability to respond to them). The corollary being that when snake and frog do not coexist in the frog's native range, the non-sympatric population will show no response or a statistically significant reduced response compared to the sympatric population. All tadpoles will of course be predator naive.

C. The hypothesis for the third question depends so heavily on the first and second that it is impossible to predict right now. There are several possibilities however. The first is that the addition of a new prey item might increase the number and density of garter snakes which will put further pressure on native amphibian populations. The second is that this happens, but the snakes might preferentially feed on the bullfrogs, thus relieving native species from competition and predation by bullfrogs. A third possibility is that both effects will occur and there will be some interaction, or that there will be no effect because snakes either do not feed on bullfrogs at all, or do not do so sufficiently to impact their numbers or relieve native species.