Joint estimation of habitat dynamics and species interactions: disturbance reduces co-occurrence of non-native predators with an endangered toad.

Abstract/Summary

1. Ecologists have long been interested in the processes that determine patterns of species occurrence and co-occurrence. Short-comings of many existing empirical approaches for these questions include a reliance on patterns of occurrence at a single time point, failure to properly account for imperfect detection, and treating the environment as a static variable. <br />
2. We fit detection and non-detection data collected from repeat visits using a dynamic site occupancy model that simultaneously accounts for the temporal dynamics of a focal prey species, its predators, and its habitat. Our objective was to determine how disturbance and species interactions affect the co-occurrence probabilities of an endangered toad and recently introduced non-native predators in stream breeding habitats. To do this we determined support for alternative processes that could affect co-occurrence frequency in the system. <br />
3. Co-occurrence probabilities of toads with non-native predators in high-disturbance ephemeral streams and low-disturbance perennial streams were directly related to the differences disturbance regimes in each of the stream types. If predators were established at a site, they were rarely lost from the site except in cases when the site dried out. Once a dry site became suitable again, toads colonized them much more rapidly than predators, creating a period of predator-free space.<br />
4. We attribute the dynamics to a ‘storage effect’ where toads persisting outside of the stream environment during periods of drought rapidly colonized sites when they become suitable again. Our results demonstrate that, even in a highly connected stream network, temporal disturbance can structure frequencies with which breeding amphibians encounter non-native predators.<br />
5. Dynamic site occupancy models are a powerful tool for quantifying inter-species and species-habitat interactions. In contrast to previous methods that infer dynamic processes based on static patterns in occupancy, the approach we take allows the dynamic processes that determine species-species and species-habitat interactions to be directly estimated.