Biologists sized up an unlikely natural phenomenon: when parasitism actually causes the number of hosts to increase, an effect known as a hydra effect. A study of common water fleas and their fungal parasites includes laboratory components and an analysis of 13 fungal epidemics in nature. The scientists use consumer-resource theory to explain why — and in what types of systems — the hydra effect can occur.

This counterintuitive result — when an action taken to reduce a problem actually multiplies it — is known as a hydra effect. Scientists propose that ecological systems exhibit something akin to a hydra effect when a higher death rate in a species ultimately increases the size of its population.

In other words, what does kill you makes you … more abundant.

New research from Rachel Penczykowski, assistant professor of biology in Arts & Sciences at Washington University in St. Louis, investigates why and in what types of systems this hydra effect can occur. The study includes laboratory components and an analysis of 13 fungal epidemics in nature.

«Disease epidemics can drive declines in host populations, trigger conservation crises for wildlife and sometimes even drive hosts extinct,» Penczykowski said. «Typically we predict that parasites that increase host mortality rate should decrease host density. But this is not always the case.»

Penczykowski’s study in the journal American Naturalist starts with the common water flea. These tiny crustaceans with antler-like antennae and rounded bellies twitch their way through freshwater lakes across North America.

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Materials provided by Washington University in St. Louis. Original written by Talia Ogliore. Note: Content may be edited for style and length.