Early warning signs of population irruptions in Eastern Grey Kangaroo (Macropus giganteus)

Abstract

SummaryLeft unchecked, macropods (kangaroos and wallabies) can exhibit irruptive population dynamics, rising rapidly to a peak, then crashing when overwhelmed by inadequate resources. This predictable population trajectory frequently leads to overabundance issues, particularly in peri‐urban parks and nature reserves. Management decisions are usually guided by estimates of population density, which can be difficult to obtain, sometimes inaccurate and often inadequate because long‐term data are needed to estimate population growth. Alternatively, density‐dependent vital rates could be used to predict the growth trajectory of a population before management issues become evident. We applied a framework of sequential changes in vital rates to examine potential indicators of population growth trajectory. We sampled 16 populations of Eastern Grey Kangaroo (Macropus giganteus) in south‐eastern Australia. Using a range of methods, we measured one vital rate (female reproductive rate) and one surrogate rate (adult sex ratio). As population density increased before irruptive peaks, female reproductive rate (90% breeding) was higher than during post‐peak declines (66% breeding). Similarly, sex ratios in increasing populations were at parity and then became more female‐biased (65% females) after peaks. These variables are readily measured in small parks and reserves, and therefore offer promising indicators of population growth trajectory, which can be used to forecast management issues and initiate timely management actions.