Temporal and spatial patterns in rodent and mustelid tracking rates across New Zealand conservation land between 2005 and 2022

Variations in invasive rodent and mustelid populations in time and space have important implications for conservation management in New Zealand. We use models fitted to 24 years of records of small mammal tracks to describe long-term temporal and large-scale spatial patterns in the activity of ship rats, house mice, and mustelids (mainly stoats) across conservation land while controlling for effects of aerial 1080 operations. Ship rats are expected to increase in abundance and range as the climate warms, but our models did not indicate an overall increase in their tracking rates over 17 years, and longer time series will be needed to detect emerging trends. Classification of ship rat and mouse tracking rates together showed differences in the character of rodent dynamics between eastern and western beech forest zones of the South Island. In eastern beech forests, rodent irruptions were mouse-dominated and tracking rates of both rodent species fell to low levels between irruptions, while in western beech forests, inter-mast ship rat tracking rates were higher, and ship rats and mice irrupted together. Geographic patterns of mustelid activity differed from those of rodents. Mustelid tracking rates were highest in the wettest western mid-elevation forests where ship rats and mice were irruptive but usually scarce. The fewest mustelid tracks were recorded in non-forest environments at all elevations and in warmer podocarp-broadleaved forests, where the relationship between mustelid and rodent activity was weakest. These patterns suggest that stoat populations may be food-limited in cool environments and suppressed by larger carnivores (feral cats and ferrets) in places where prey populations (such as ship rats, mice, and rabbits) are usually large. Different monitoring approaches may help to resolve how biotic interactions among rodents, mustelids, and feral cats influence stoat abundance and the outcomes for indigenous prey populations.