Environmental Variation is Good for Some Species
Conventional wisdom says that species living in environments with drought, fire, and other variations are more likely to go extinct. But this is not always true: some species do better when their environment is variable, according to a paper in the December issue of Trends in Ecology and Evolution.
“This phenomenon is likely to be common,” say Steven Higgins and his co-authors. Unfortunately, conservation biologists don’t always account for the fact that some species flourish in variable environments.
The difference between species that thrive and species that fail in variable environments is simple: the former can survive adverse conditions long enough to take advantage of favorable ones. In other words, species adapted to variable environments can often “store” the ability to reproduce across generations. A striking example of this “storage effect” is the aspen in Yellowstone National Park, which were observed to recruit from seeds for the first time after the catastrophic fires of 1988.
While the storage effect is hardly a new concept, its application to conservation is new. “Its potential implications have been ignored by conservation biologists,” say Higgins and his co-authors. “This seems peculiar because the fate of rare populations is the central business of conservation biology, and the storage effect…allows populations to increase when rare.”
Accounting for the storage effect is simple in theory. To determine whether a given species would benefit from environmental variability, managers need to know how change affects the species’ ability to reproduce. This in turn depends on two factors: storage (being able to store reproductive potential across generations) and recruitment (an increase in the number of reproductive adults). The problem is that managers usually don’t consider storage and recruitment separately in most population viability analyses (PVA), a common method of deciding how to manage threatened species.
To illustrate how managers can apply the storage effect in PVAs, Higgins and his co-authors present two case studies. The first is a protea, a South African shrub that both dies and recruits after fires. Because the protea cannot resprout after fires and lacks a persistent seed bank, it has no storage and so depends entirely on steady recruitment. If there is a poor seed crop (due, for instance, to insect predation) during a fire year, recruitment will be low. Because a few poor recruitments in a row could lead to local extinction, the best management option in this case is to add new recruits by, for instance, planting seedlings.
The second case is a banksia, a rare shrub in southwestern Australia that can resprout after fire and drought. However, there are few adults left because so much land has been cleared. In other words, while this banksia has storage, recruitment of adults is rare. In this case, the best management option is to protect more adult shrubs by setting aside more of their habitat in reserves or by keeping fire intensity low.
“These case studies suggest that the recognition of the storage effect requires the redefinition of what constitutes a threatened plant population,” say Higgins and his co-authors. However, they caution against taking action based on suspected storage mechanisms. Instead, they urge conservation biologists to do long-term studies on the variability in storage and recruitment in threatened plants. Then biologists can use these data to re-evaluate priorities in lists of threatened and endangered populations, moving those with storage down the list and moving up those without storage to the top of the list.
Higgins, S. I., S.T.A. Pickett, and W. J. Bond. 2000. Predicting extinction risks for plants: environmental stochasticity can save declining populations. Trends in Ecology and Evolution 15(12):516-520.