When animals get smaller, population decline may be imminent

Average whale sizes dropped drastically before their populations collapsed. Now researchers say it’s time to start measuring other threatened species.

Conservationists may have a new early warning mechanism for the collapse of threatened animal populations. Researchers analyzing the sizes of whales caught between 1900 and 1985 found that the whales became noticeably smaller—up to 25 percent—prior to each species collapsing. The findings suggest that monitoring the size of other threatened species could give researchers and conservationists early warning of a population collapse, increasing the likelihood it can be prevented. We spoke with study author Christopher Clements to learn more.

ResearchGate: Why is it important to know when a population is likely to collapse?

Christopher Clements: Humans as a species have had an almost unbelievable impact on the world around us. One of the consequences of an ever-increasing human population is the worldwide decline in the biodiversity we rely on for food, freshwater, and many other services. The loss of species or collapse of populations can significantly impact these services our societies are based upon, and so predicting if a population is at risk of collapse would mean we could potentially intervene to save it. The other reason for saving a species is of course its intrinsic value as a living organism—something we have shared our planet with for perhaps hundreds of thousands of years. It would be a tragedy if we had the opportunity to save a species but let it go extinct.

RG: How did you determine that size was a good indicator of impending population collapse for whales?

Clements: The size of an individual is a key determinant of many different factors, including how well that individual lives, how likely it is to successfully have offspring, and how affected it is by environmental change. The loss of the largest individuals affects the speed at which the population as a whole reproduces, and how susceptible it is to environmental change. Previous work has shown that when you take the largest individuals out of a population—say by hunting or fishing—then the population becomes less stable. Thus, size is a critical factor in determining the health of a population, and rapid shifts in size can indicate increased instability and potential collapse.

RG: What species did you look at, and how did their sizes change over this period?

Clements: We looked at the four species from 1900 until 1985: blue, sei, fin, and sperm whales. In general, the body size of all the species decline through time, but sperm whales were the best example of this: The average body size of individuals caught declined by four meters between 1900 and 1985 about 25 percent.

RG: How can today’s scientists and conservationists apply this information?

Clements: Our research suggests that shifts in body size might indicate an approaching collapse, but the ultimate goal is to try and help conservation managers make quantitative decisions more easily. We see this research as being an easy to implement, first step to identifying populations that might be at risk of collapse, which would allow time and resources to be invested in understanding what the processes driving this might be, and exactly how at risk the populations are.

RG: Which modern-day species do you foresee this approach being applied to?

Clements: Sharks could be a great target species for this research. They are incredibly important determinants of how well our oceans function, but are endangered all over the world. In some areas, the practice of shark finning for food is of particular concern, as it is very hard to keep track of how many individuals are being caught. However, shifts in body size might be calculable from the fins put up for sale, and thus could act as an indicator of any change in the size distribution of the populations.

RG: Could it work for non-marine species as well?

Clements: Body size is a key trait in terrestrial as well as marine species, and in both animals and plants. So yes, I would expect this to be applicable to non-marine species.

RG: Once an impending population collapse is identified, is there really time to prevent it?

Clements: That is still in many respects an open question. We don’t know how fast we need to act to save a species once we think it will collapse. I think this will depend a lot on the generation time of the species. For slow reproducing species it might already be too late, but I get some hope from the whale populations worldwide, which are slowly increasing again after the 1985 whaling moratorium.


Featured image courtesy of Harald MM.