Abstract
Amphibians are currently the most threatened group of vertebrates, with an estimated 40 % of amphibian species currently in danger of extinction. The global spread of introduced fauna has been accepted as a major factor in the decline of amphibians, as native amphibians often have little or no evolutionary history with introduced predators. In particular, introduced small mammals such as rodents pose a major threat, as they are generalist predators that together have an almost global distribution. Wildlife management decisions regarding the control of introduced predators to protect endangered species are often based on predation rates derived from diet studies of the predators concerned. However, many studies have reported difficulties in identifying amphibians as prey in diet analysis, due to the degraded nature of any diagnostic prey components surviving the mastication and digestion processes of the predator. The aim of the research presented in this thesis was to aid global amphibian conservation by assessing the available tools, and developing further tools, to detect frogs as prey of introduced mammals.
I investigated the feeding behaviour of introduced mammals (ship rats Rattus rattus, Norway rats Rattus norvegicus, house mice Mus musculus and hedgehogs Erinaceus europaeus) when presented with frogs (southern bell frogs Litoria raniformis) as prey and explored the usefulness of two diet analysis methods: traditional morphological analysis and novel DNA-based methods. Using these methods I estimated the impact of ship rats on two endangered New Zealand native frogs (Hochstetter’s frog Leiopelma hochstetteri and Archey’s frog Leiopelma archeyi).
The results show that, using morphological diet analysis, 11 % of stomach and faecal samples collected after small mammals had ingested frogs could be classed as containing frogs as prey, but less than 1 % could be identified to species level. Although very small bones can be used to identify frogs as prey, the odds of successful identification dramatically increase as prey bone length in predator stomach and faecal contents increases.
The use of DNA-based methods increased prey identification (to species level) from less than 1 % to 58 %. Prey detection periods exceeded known gastrointestinal transit times for the small mammal species concerned, indicating that it is primarily the passage of prey material through the gastrointestinal tract that limits the successful detection of DNA, rather than the degradative effects of ingestion or digestion. The DNA-based methods were validated for field-collected samples and were successful where morphological analysis was not. Both Hochstetter’s frogs and Archey’s frogs were identified in the stomach contents of wild ship rats and estimates of predation rates at the study sites ranged from 0.01 – 0.9 frogs/ha/night, but it remains unclear whether ship rats alone threaten them with extinction.
This is the first study to investigate the usefulness of DNA-based diet analysis for detecting amphibians as prey, the first to estimate predation rates on New Zealand’s native frogs, and the first to compare stomach versus faecal prey DNA detection over time in any vertebrate. DNA-based diet analyses are highly adaptable and they offer a reliable and cost-effective approach to conservation managers hoping to make assessments of the impacts of introduced fauna on native amphibians, which are necessary to make informed decisions on the implementation of predator control.