First author

Ecological theory predicts that predator–prey interactions cause large fluctuations in population size. In reality, however, ecologists often find natural populations to be more stable. On page 1240, Edward McCauley, an ecologist at the University of Calgary in Alberta, Canada, and his colleagues show that the availability of resources affects both maturation and mortality in juveniles of a tiny crustacean called Daphnia. This creates a developmental delay that enables small and large fluctuations in population to coexist. McCauley tells Nature that this work opens up new avenues of research.

Had this inconsistency between theory and observation been a long-standing conundrum?

Yes. Less variation than expected has been documented in a number of parasite–host, plant–herbivore and predator–prey systems. This has led ecologists to suggest various mechanisms — such as the diverse evolutionary strategies seen in different organisms — to account for the stability of many natural systems.

How did you realize that something was missing from predator–prey models?

Twenty-five years ago, we showed that Daphnia — freshwater herbivores — and their algal prey have an incredible range of population dynamics, exhibiting different types of population cycle that weren't predicted by theory. We joined forces with a group of theoreticians and explored how time delays caused by food availability and energy requirements might affect population dynamics. When we stripped this complex biological system down to its essential ingredients — by removing competing algae and other predators that might obscure large-scale fluctuations — we found that predator–prey cycles can exhibit both small and large oscillations. In our new work, we wanted to determine how these cycles could coexist. The key was testing whether food availability affected the length of Daphnia's juvenile stage, as predicted by our models. When food is abundant, juveniles mature too quickly for a stable population to be maintained, whereas scarce food leads to longer maturation rates and smaller fluctuations in population.

What next?

Now we need to explain the prevalence of cycle types and how different patterns of population fluctuations coexist in the same environment. We think that combining models with field and laboratory studies will allow us to show how future changes in temperature and climate might affect the dynamics of communities.