Spatially explicit power analysis reveals challenges for a long-term threatened species monitoring program in Australia

Abstract

Long-term monitoring programs are crucial to assess trends in biodiversity and so make informed decisions for conservation and resource management. However, disregarding the statistical power of a monitoring program can lead to incorrect conclusions about species population trends, potentially resulting in ineffective management and misdirected resource allocation. In Australia, predation by introduced red foxes (Vulpes vulpes) and feral cats (Felis catus) remains a major cause of native faunal decline and extinction. Australia spends more than $16 million yearly in controlling foxes for biodiversity conservation, primarily through landscape-scale poison baiting. Using a long-term fox baiting and threatened species monitoring program in southeastern Australia, we collated data from 2132 camera-trap deployments to: (1) explore drivers of the distribution of threatened native mammals and introduced predators, (2) conduct a spatially explicit power analysis to assess the program's ability to detect trends in native and introduced species occupancy for the next 10 years, and (3) provide recommendations for improving monitoring efforts through alternative scenarios. We found that threatened native mammals were more likely to occupy areas with high densities of fox baits, whereas foxes were less likely to occupy these areas; however, these areas were quite localized within baited regions. The power of the existing monitoring design was sensitive to the magnitude of change in occupancy, but robust to approximately 15% changes in the number of survey sites. The monitoring program showed adequate power (>0.8) to detect its original aims: increases in threatened native mammal occupancy and decreases in fox occupancy in baited areas. Hence, the lack of a strong signal of increasing native mammal occupancy in the last 8 years likely indicates that the system has reached a stable state under current management, rather than poor statistical power. This may potentially be the case in many long-term predator management programs. If removing some sites from an existing monitoring design does not considerably vary power, managers could consider diverting these resources to, for example, improving understanding of species–habitat relationships or intensifying predator management efforts.