Trends in mean growth and stability in temperate vertebrate populations

Abstract

Aim: Considerable controversy exists over the nature of the “Biodiversity Crisis.” While some studies suggest declining diversity, others suggest no loss on average. Population declines necessarily precede species loss and may therefore be a more sensitive metric. We examine trends in abundances to test the hypotheses: (1) losses are experienced disproportionately by some taxa, in particular amphibians; (2) positive trends in alien or unexploited species mask declines in native or exploited species, respectively; (3) populations are becoming more unstable, and variances of within-population growth rates are increasing. Location: Freshwater and terrestrial habitats in North America and Europe. Methods: We analysed abundances of 4,343 population time series of 983 species of temperate amphibians, birds, bony fishes, reptiles and mammals from 1970 to 2014. Population mean trends were estimated using a state-space model to account for observation error, and then, linear mixed-effects models were used to test mean differences among groups. As a measure of instability, trends over time in variance of population growth rates were analysed using a Breusch–Pagan test for heteroscedacity, and integrated across populations using meta-analysis. Results: Mean trends for amphibians suggested a systematic decline, whereas birds, reptiles and mammals were increasing, on average, and freshwater fishes showed no net change in general. Origin and exploitation status did not explain the mean patterns. Large-bodied birds and range-restricted mammals exhibited stronger increases compared to their counterparts. For all taxonomic groups, population fluctuations have been increasing overall. Main conclusions: Addressing amphibian declines should be a priority in temperate regions. Comparatively, the lack of broad declines in the other vertebrate classes is promising and not attributable to increases in alien or non-exploited species. Yet, caution remains warranted given that patterns of increasing variance suggested that populations are generally becoming more unstable, even in temperate, developed nations, with arguably the strongest environmental regulations.