American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“When do shifts in trait dynamics precede population declines?”

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Gaurav Baruah, Christopher F. Clements, Frédéric Guillaume, and Arpat Ozgul (May 2019)

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Shifts in fitness related traits could precede population decline, and is influenced by a host of eco-evo factors

How monitoring traits may help predict population declines

Predicting population declines in response to environmental change is an ongoing challenge in ecology. Recent studies suggest that predicting such rapid declines might be possible by monitoring statistical signals (called early warning signals, EWSs) embedded within abundance time series. Both theoretical and empirical studies have shown that before a population declines, EWSs, such as temporal variance and autocorrelation in abundance, increase over time. There are, however, potential pitfalls associated with such signals, including, but not limited to, the requirement of high-quality abundance time-series data. Alternatively, it is possible to strengthen these signals by incorporating information from the dynamics of physical traits such as body size. These trait-inclusive EWSs have been shown to improve our ability to predict population declines over abundance-based EWSs. However, under what environmental, ecological, and evolutionary circumstances it is useful to use traits as an indicator of population decline, is unknown.

In a new study appearing in The American Naturalist, Gaurav Baruah, Christopher Clements, Frédéric Guillaume, and Arpat Ozgul develop a theoretical model to investigate the circumstances under which trait shifts can precede population declines, and thus can act as an early warning signal. They show, using model simulations and empirical data from an experimental microcosm, that under slow to medium environmental change scenarios, shifts in traits are more likely to precede population declines. They further demonstrate that reliable environmental cues, high net reproductive rate, high levels of plasticity and genetic variation lead to shifts in traits preceding population declines, even during fast changes in the environment. Their findings provide crucial information on when biodiversity monitoring programs can target trait dynamics as potential early warning signals of impending population declines.


Predicting population responses to environmental change is an ongoing challenge in ecology. Studies investigating the links between fitness-related phenotypic traits and demography have shown that trait dynamic responses to environmental change can sometimes precede population dynamic responses, and thus, can be used as an early warning signal. However, it is still unknown under which ecological and evolutionary circumstances, shifts in fitness-related traits can precede population responses to environmental perturbation. Here, we take a trait-based demographic approach and investigate both trait and population dynamics in a density-regulated population in response to a gradual change in the environment. We explore the ecological and evolutionary constraints under which shifts in fitness-related trait precedes a decline in population size. We show both analytically and with experimental data that under medium-to-slow rate of environmental change, shifts in a trait value can precede population decline. We further show the positive influence of environmental predictability, net reproductive rate, plasticity, and genetic variation on shifts in trait dynamics preceding potential population declines. These results still hold under non-constant genetic variation and environmental stochasticity. Our study highlights ecological and evolutionary circumstances under which a fitness-related trait can be used as an early warning signal of an impending population decline.