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

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. 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. 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. 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. 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