Stuck in the weeds: Invasive grasses reduce tiger snake movement

Wetlands are particularly vulnerable to degradation in urban environments, partially due to the introduction of non-native plants. Invasive weeds in wetlands can replace native plants leading to alterations in habitat composition and vegetation, in turn, animal movements and ultimately population dynamics might be affected. Here we investigate how home range size and movements of western tiger snakes (Notechis scutatus occidentalis) differ in wetlands dominated by invasive kikuyu grass (Cenchrus clandestinus) compared to wetlands dominated by native vegetation to understand if and how the movement ecology of this top-order predator is altered by vegetation homogenization. To do so, we used Autocorrelated Kernel Density Estimators (AKDE) to estimate home range size, dynamic Brownian Bridge Movement Models to document movement trajectory confidence areas, and compared movement distances using a Bayesian regression model. Home range sizes by tiger snakes were 14.59 {+/-} 9.35 ha smaller in areas dominated by invasive versus native vegetation. Moreover, within-day movement distances tended to be smaller in areas dominated by invasive versus native vegetation (mean {+/-} SD: 9 {+/-} 3 m versus 18 {+/-} 6 m), but there was considerable overlap between the 95% credible intervals between these two groups. Smaller home ranges by tiger snakes in areas dominated by invasive kikuyu grass were likely driven by thermoregulation, with snakes moving vertically between basking locations on top of kikuyu and shelter sites at the base, rather than travelling horizontally along the ground to open basking areas in sites dominated by native vegetation. Additionally, fragmentation of sites dominated by invasive vegetation might have contributed to the comparatively smaller home ranges of snakes there. These findings add to our understanding how changes in habitat composition driven by invasive vegetation can affect animal space use and emphasise the need for further studies to understand how these changes affect population dynamics.