INVESTIGATING THE DIRECT AND INDIRECT EFFECTS OF INTRODUCED GREENBACK CUTTHROAT TROUT ON BOREAL TOAD RECRUITMENT

Lanier, Wendy

ABSTRACT INVESTIGATING THE DIRECT AND INDIRECT EFFECTS OF INTRODUCED GREENBACK CUTTHROAT TROUT ON BOREAL TOAD RECRUITMENT Worldwide, numerous amphibian species are at great risk of extinction. Currently, a third of all amphibian species are listed by the International Union for Conservation of Nature as threatened and this number is likely underestimated as there are many species whose status is unknown due to insufficient data. Species in the family Bufonidae, the true toads, are one of the most threatened groups of amphibians. One such declining bufonid is the boreal toad, Anaxyrus boreas boreas. While much of the decline in boreal toad populations can be attributed to the chytrid fungus, Batrachochytrium dendrobatidis (Bd), not all populations are impacted by this pathogen. Declining populations of boreal toads in protected areas in the absence of Bd are largely enigmatic. However, there is one hypothesis that might explain these declines. The greenback cutthroat trout, a federally threatened species, have been introduced into alpine lakes in an effort to bolster their population. These trout were introduced into some lakes that supported breeding populations of toads and could be causing the declines observed in the toad populations. Adult boreal toads are mostly terrestrial and beyond the gape limitation of greenback cutthroat trout. As such, trout are not likely to reduce adult toad survival. What is more likely is that introduced trout are reducing or eliminating recruitment of individuals to the adult population. The life stages where trout could interrupt recruitment are: embryos, tadpoles, and postmetamorphic individuals. Among these life stages, trout may reduce recruitment via various iii direct or indirect effects. The direct effect of predation is unlikely as boreal toads contain bufotoxins at all life stages and are, therefore, unpalatable to trout. However, trout may not recognize palatable verses unpalatable prey and must taste a prey individual to determine if it is palatable. For early developmental stages, this ―tasting‖ process could result in physical damage and stress to those individuals and reduce survival. Trout presence may also indirectly affect survival, growth, and development of toads. One indirect effect is a reduction in tadpole activity, thereby reducing time spent foraging. Trout presence could also cause tadpoles to shift habitat use away from energetically favorable habitat and towards habitat that offers protection but is less energetically favorable. Lastly, in the presence of trout, a tadpole might expedite its development in an attempt to emerge out of the water and away from the trout pressure. The impacts of trout on the growth and development of tadpoles could also result in carry-over effects in the postmetamorphic stage and influence subsequent vital rates such as survival and growth. I used field and laboratory studies to investigate various direct and indirect effects of trout on early life stage survival, growth, and development of toads. In the first chapter, I investigated the effect of trout presence on embryo survival in the field. I also analyzed trout microhabitat use to determine the probability of trout presence around the toad egg masses. I found no effect of trout presence on embryo survival but sample sizes were small. Furthermore, I found that trout are unlikely to use the areas around the egg masses. Adult toads select protective habitat to deposit their egg masses, and it is likely this careful placement of the egg masses that confers protection to the embryos.