Estimating effects of species interactions on populations of endangered species.

Global change causes community composition to change considerably through time, with ever-new combinations of inter- acting species. To study the consequences of newly established species interactions, one available source of data could be observational sur- veysfrom biodiversity monitoring.However,approaches usingobser- vational data would need to account for niche differences between species and for imperfect detection of individuals. To estimate pop- ulation sizes of interacting species, we extended N-mixture models that were developed to estimate true population sizes in single spe- cies. Simulations revealed that our model is able to disentangle direct effects of dominant on subordinate species from indirect effects of dominant species on detection probability of subordinate species. For illustration,weappliedourmodeltodatafromaSwissamphibianmon- itoring program and showed that sizes of expanding water frog popula- tions were negatively related to population sizes of endangered yellow- belliedtoadsandcommonmidwifetoadsandpartlyofnatterjacktoads. Unlike other studies that analyzed presence and absence of species, our model suggests that the spread of water frogs in Central Europe is one of the reasons for the decline of endangered toad species. Thus, study- ing population impacts of dominant species on population sizes of en- dangered species using data from biodiversity monitoring programs should help to inform conservation policy and to decide whether com- peting species should be subject to population management.