SPM Journal Club: Shrinking body size as an early warning signal of population collapse
Body size is often linked intricately with survival and reproductive rates, and therefore affects population dynamics. It is not unlikely for population collapses to be preceded by a change in body size distributions. If those changes happen long enough in advance, they may serve as early warning signals to predict population collapses.
We discuss a recent paper illustrating how declining body size indeed was an early warning signal for the global collapse of whale populations during the last century of commercial whaling:
"Body size shifts and early warning signals precede the historic collapse of whale stocks" (Clements et al. 2017, Nature Ecology & Evolution)
Predicting population declines is a key challenge in the face of global environmental change. Abundance-based early warning signals have been shown to precede population collapses; however, such signals are sensitive to the low reliability of abundance estimates. Here, using historical data on whales harvested during the 20th century, we demonstrate that early warning signals can be present not only in the abundance data, but also in the more reliable body size data of wild populations. We show that during the period of commercial whaling, the mean body size of caught whales declined dramatically (by up to 4 m over a 70-year period), leading to early warning signals being detectable up to 40 years before the global collapse of whale stocks. Combining abundance and body size data can reduce the length of the time series required to predict collapse, and decrease the chances of false positive early warning signals.