Scaling the extinction vortex: Body size as a predictor of population dynamics close to extinction events

<ol> <li class="Body">Mutual reinforcement between abiotic and biotic factors can drive small populations into a catastrophic downward spiral to extinction – a process known as the 'extinction vortex.' However, empirical studies investigating extinction dynamics in relation to species' traits have been lacking.</li> <li class="Body">We assembled a database of 35 vertebrate populations monitored to extirpation over a period of at least ten years, represented by 32 different species, including 25 birds, five mammals and two reptiles. We supplemented these population time series with species-specific mean adult body size to investigate whether this key intrinsic trait affects the dynamics of populations declining towards extinction.</li> <li class="Body">We performed three analyses to quantify the effects of adult body size on three characteristics of population dynamics: time to extinction, population growth rate, and residual variability in population growth rate.</li> <li class="Body">Our results provide support for the existence of extinction vortex dynamics in extirpated populations. We show that populations typically decline non-linearly to extinction, while both the rate of population decline, and variability in population growth rate increase as extinction is approached. Our results also suggest that smaller-bodied species are particularly prone to the extinction vortex, with larger increases in rates of population decline and population growth rate variability when compared to larger-bodied species.</li> <li class="Body">Our results reaffirm and extend our understanding of extinction dynamics in real-life extirpated populations. In particular, we suggest that smaller-bodied species may be at greater risk of rapid collapse to extinction than larger-bodied species, and thus management of smaller-bodied species should focus on maintaining higher population abundances as a priority. </li> </ol>