Recent shifts in the occurrence, cause, and magnitude of animal mass mortality events

Abstract

Mass mortality events (MMEs) are rapidly occurring catastrophic demographic events that punctuate background mortality levels. Individual MMEs are staggering in their observed magnitude: removing more than 90% of a population, resulting in the death of more than a billion individuals, or producing 700 million tons of dead biomass in a single event. Despite extensive documentation of individual MMEs, we have no understanding of the major features characterizing the occurrence and magnitude of MMEs, their causes, or trends through time. Thus, no framework exists for contextualizing MMEs in the wake of ongoing global and regional perturbations to natural systems. Here we present an analysis of 727 published MMEs from across the globe, affecting 2,407 animal populations. We show that the magnitude of MMEs has been intensifying for birds, fishes, and marine invertebrates; invariant for mammals; and decreasing for reptiles and amphibians. These shifts in magnitude proved robust when we accounted for an increase in the occurrence of MMEs since 1940. However, it remains unclear whether the increase in the occurrence of MMEs represents a true pattern or simply a perceived increase. Regardless, the increase in MMEs appears to be associated with a rise in disease emergence, biotoxicity, and events produced by multiple interacting stressors, yet temporal trends in MME causes varied among taxa and may be associated with increased detectability. In addition, MMEs with the largest magnitudes were those that resulted from multiple stressors, starvation, and disease. These results advance our understanding of rare demographic processes and their relationship to global and regional perturbations to natural systems.

Keywords: catastrophes; death; defaunation; rare demographic events.

Fig. 1.

Occurrences of animal MMEs and taxon-specific publication trends through time. Colored bars indicate the number of events during a 5-y interval (e.g., 1940 stands for the 1940–1944 period), and dashed lines show trends in the total number of papers published each year for each taxon. For all taxa, the increase in the number of MMEs is coincident with an increase in the number of publications (SI Appendix, Fig. S1).

Fig. 2.

Magnitude of animal MMEs through time. Each point is a single MME (n = 727 total events). Dashed lines represent statistically significant slopes, shading demarcates slope 95% confidence intervals, and hatched shading indicates extrapolation of regressions before the first reported MME.

Fig. 3.

Causes of MMEs and associated variation in the magnitude of events for different causes. (Left) Bars quantify the total number of mortality events for a given cause, and lines within the bars indicate the relative change in the occurrence of each cause from 1940 to 2009. *Significant temporal trends. (Right) Variation in the magnitude of mass mortality across causal categories after taking into account taxon-specific temporal trends. Shown are the residuals (mean ± 1 SE) from an ANCOVA model, including taxa and year; that is, a model that describes the expected mortality magnitude without taking into account the causes. Causes with larger magnitudes of death than the average will have positive residuals (e.g., starvation), whereas causes with smaller magnitude than the average will have negative residuals (e.g., O₂ stress).

Fig. 4.

Relative frequency of each cause of mortality for each taxon through time. Bars indicate the relative frequency of an MME cause for particular taxa for each decade. Decades are defined by their start year (e.g., 1940 represents the period 1940–1949).