Accounting for detectability and abundance in survey design for a declining species

Aim: As populations decline they become harder to detect simply because there are fewer animals to observe. In such cases, survey effort that was once reliable may become inadequate and the species incorrectly labelled locally extinct, causing management efforts to cease. For declining populations, then, we need to determine the survey effort necessary to detect the species, assuming only a single individual is present at a site. Detectability, however, often varies wildly with seasonal, behavioural and environmental variables. Thus, we need statistical analyses and survey design which can deal with highly variable detection rates and allow us to detect even very small populations. This is especially true in Australia, where many mammal species are experiencing serious decline. Here, we examine a threatened Australian rodent, the New Holland Mouse (Pseudomys novaehollandiae): we determine minimum survey requirements for reliable detection and show that historical survey effort is inadequate to assert local extinction. Location: Victoria, Australia. Methods: Using a 48-year live-trapping dataset, we estimated P. novaehollandiae abundance and detectability using N-mixture modelling in a Bayesian framework, testing the effects of seasonal fluctuations and environmental variables to determine ideal conditions for occupancy surveys. We used these findings to assess the adequacy of historical resurvey efforts in confirming local extinctions. Results: We found that detectability followed seasonal fluctuations distinct from changes in abundance, was strongly reduced by brighter moon phases, and slightly increased over the course of a survey and when raining. When abundance was low, standard historical survey efforts were inadequate to assert the species' absence with appropriate statistical confidence, indicating that further surveys would be required to test purported local extinctions. Main conclusions: Our results highlight the potential for considerable intra-species heterogeneity in detectability caused by shifting local abundances and survey conditions, as well as the importance of accounting for detectability when monitoring threatened and declining species.