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

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 south-eastern Australia, we
collated data from 2,132 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 localised 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 eight 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.