Predatoriska fåglars effekt på populationsdynamiken hos amfibier

Sammanfattning: The global amphibian population is rapidly declining. Although many threats that affect amphibians are known, there are many contributing factors which are not fully understood. For adult amphibians, the largest part of the mortality comes from predation. We know of many predators which prey upon amphibians, but to what extent they affect the populations has not been subject to much investigation. Neither has the effects of specific taxa or species of predators. Birds are reputed amphibian-eaters, and they often occur in the same habitats. Both birds and amphibians often select for wetlands and other water bodies with high biodiversity, that often support a high diversity and/or density of birds as well as amphibians. The fact that a prey might flourish in an area that one of its strongest predators also inhabits might seem controversial. Especially when taking into account that it is often the density of predators that best explains the population growth of prey species. In this review, I investigate what effects large amounts of birds might have on amphibian populations as a result of their predation on adults. I consider how the effects might differ with high vs low densities of both prey and predator, and whether an eventual effect might impact the survival of a metapopulation differently depending on its size and spatial distribution. As an example of a habitat where both bird and amphibian species occur in large numbers, I use Trönninge ängar - a bird conservation area just outside the city of Halmstad, where populations of both amphibians and birds have been increasing over the last years. My results show that high densities of predatory birds in such communities could potentially cause declines in amphibian populations, but that this impact is softened when the population is more widely distributed in the area. There are also possibilities that birds do not only affect amphibians by predation, but also might facilitate their distribution by reducing other predators. Thus, bird predation in itself might not be a sufficient predictor for their effect on amphibian populations, as the relationship might be more complex than simple predator-prey interactions.