Effects of seasonal aridity on the ecology and behaviour of invasive cane toads in the Australian wet-dry tropics

1. Measuring the costs imposed by specific environmental challenges is difficult, because organisms adapt in ways that reduce those costs. Invasive species provide an opportunity to quantify environmental impacts before organisms can make adaptive changes. 2. The native range of cane toads (Rhinella marina) lies within the wet neotropics; although part of this range experiences seasonal drought, many of the places recently invaded by this large anuran species are much more arid. 3. Five years' fieldwork from a seasonally arid site in the Australian wet- dry tropics shows strong seasonal shifts in the toads' (i) population structure, reflecting seasonality in breeding and recruitment; (ii) adult morphology (secondary sexual characteristics in males); (iii) growth rates; (iv) energy balance; (v) spatial ecology (philopatry, dispersal rates) and (vi) adult mortality rates. 4. Some of these patterns accord with intuition: for example, wet- season conditions resulted in higher rates of growth, reproduction and movement, better body condition and more pronounced secondary sexual characteristics. However, seasonal patterns for other traits were nonintuitive: for example, neither hematocrit levels (reflecting hydration state) nor corticosterone levels (reflecting stress) showed significant seasonal variation, and mortality rates were higher in the wet season than the dry season. 5. The toads' ability to flexibly adjust their behaviour and ecology to local hydric conditions has allowed them to thrive even under climatic conditions that preclude activity, feeding and reproduction for most of the year.