Many ground-breeding birds suffer high juvenile or nest predation in years when rodents are scarce, as predator diets and densities are shifting with rodent numbers. The phenomenon is widespread in many systems with resource pulses, and is called an apparent ecological interaction, i.e. an interaction between two prey species through their shared predator. Often we observe apparent competition, i.e. prey 1 and 2 compete indirectly with each other. But positive interactions are also possible.
The case of a vole-raptor-grouse system in UK moors, where the primary prey is temporarily superabundant, has served as a springboard to investigate how primary prey dynamics (summarized through mean, variance, cycle periodicity) can affect the level of predation experienced by the alternative prey [joint work with J. Matthiopoulos, L. New, S. Redpath].
We found that while increases in mean primary prey density are usually good for alternative prey, increases in variability (coefficient of variation) are usually detrimental. Unless predators numbers react fast and very strongly to rodent numbers, in which case the results can be completely reversed. This possibility for reversal highlights the importance of measuring top predator demography to understand indirect interactions between herbivorous species. I am also puzzled by the conditions for the emergence of apparent mutualism between prey species. True mutualism might be rare, but (+/0) interactions could be many (such as in this case, where increased mean rodent abundance benefits ground breeding birds).
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