Understanding the relative importance of these two factors is key to designing effective conservation strategies. It may be common sense to focus on boosting numbers if demographics matter more, and to put together breeding programmes that expand the remaining gene pool if genetics rule, but accurately pointing the finger at the primary cause of decline is rarely a simple task.
The cheetah is the poster child for this problem, Wootton told attendees of the 2012 Ecological Society of America annual meeting in Portland, Oregon, on 6 August. Africa’s most at-risk cat species has lost much of its habitat and its former genetic diversity. Consequently, no one knows which factor will more strongly influence its fate.
He and Pfister wanted to do experiments in which they could manipulate both the genetic structures and the sizes of populations. Cheetahs being impractical subjects, they opted instead for the sea palm, Postelsia, a type of kelp that occurs along the western coast of North America. Postelsia barely disperses at all. It grows in clumps on wave-swept rocky shores, reproducing by dribbling spores onto rocks directly below it. Waves remove the adult plants each year, leaving room for spores to grow. As a result, each clump is often genetically isolated from those just metres away.
Wootton and Pfister bred sea-palm populations with varying degrees of genetic diversity. They transplanted them onto rocky patches of shoreline in batches of different sizes. After 12 years, smaller populations were less likely to have survived than larger ones, and, among the populations that did disappear, smaller ones did so more quickly. Crucially, genetic diversity did not influence the odds of a population’s survival.
Wootton found a cut-off point, between 10 and 100 individuals, where the risk of becoming locally extinct changed dramatically. “The rule of thumb when I was a student was that 50–100 individuals was a viable population size,” he says. “Our experiments support that.”
How widely applicable are the findings? The sea palm is edible and heavily harvested, so the results have direct implications for its management. But it is also stationary, whereas many vulnerable species — speedy cheetahs especially — are mobile.
“It’s hard to know [how broadly applicable the results are] because it’s the first experiment of its kind,” Wootton points out. Meanwhile, Robert Paine, an ecologist at the University of Washington in Seattle, who has worked with Wootton and Pfister, praises the new study, describing it as dealing with a very real question with broad application.
Some other studies have claimed that genetic diversity is the more important factor, but these typically report only a correlation between low diversity and species endangerment, Wootton says. “I’d say if you have a limited budget, you should probably study the ecology and the demographics first rather than doing molecular analysis.”