In “Cryptic” Population Dynamics, Absence of Evidence Is Not Evidence of Absence

1838 When the populations of two species oscillate together, it's a good bet that they are tightly coupled ecologically. Most often, these oscillations occur between predator and prey (or a pathogen and its host), as demonstrated by the famous case of the Canadian lynx and snowshoe hare, documented in the trapping records of the Hudson's Bay Company. This makes sense—as the predator depletes the prey, its own reproduction suffers and its numbers decline; as predation eases, the prey's numbers can grow again. But is the opposite also true? If the prey's population doesn't fl uctuate while the predator's does, may we assume they are not tightly linked in the food web? Among ecologists, this has generally been thought to be the case. But a new study by Takehito Yoshida and colleagues shows that this assumption is not always justifi ed. The authors noticed some odd population dynamics in studying a rotifer and its food source, a unicellular alga. They observed the pair in a chemostat, an aqueous growth chamber whose nutrient supply is kept steady through a continuous in-and outfl ow of medium. Observations of the two species have shown that indeed they are tightly linked—the algae are the only food source for the rotifers—and under many conditions within this system, their populations co-oscillate like the lynx and hare. But the authors found that under other conditions, the rotifer's population waxed and waned, while the alga's population remained virtually constant. It seemed plausible that this unexpected result was due to changes in the relative abundances of at least two algal genotypes: one type more palatable to the rotifer predator and one less so. The authors reasoned that when rotifers became abundant, loss of the more palatable prey genotype might be compensated for by growth of the less palatable type, which no longer had to compete for access to nutrients. The numbers of the unpalatable algal type might grow large and, since it was unpalatable, cause the rotifer population to decline, giving advantage again to the more competitive but also more palatable genotype. As a test, they constructed a mathematical model that included terms for prey abundance, palatability, nutrient competition ability, birth rates, and other variables in the system, and found that it predicted that the two algal genotypes could exhibit exactly the observed population behavior when the fi tness cost to the algae of being unpalatable was low. Furthermore, the …