geographical synchrony in microtine population cycles: a theoretical evaluation of the role of nomadic avian predators

A simple mathematical model is developed and analyzed to evaluate the hypothesis that regional synchrony in microtine population cycles is caused by nomadic avian predators. We show that predation by nomadic predators, capable of rapidly tracking the local prey densities, may indeed have a synchronizing effect. However, in contrast to the original hypothesis, our model requires no additional mechanism than predation (e.g. behavioural response of prey to predation) to produce synchronous prey cycles. In addition to the synchronizing effect, heavy predation pressure by nomadic predators is expected to shorten the period and dampen the amplitude of the prey cycles. Non-migrating predators having impact on their prey year round have a stronger synchronizing effect than migrating predators absent during the winter. We predict that geographic regions with few non-migrating predators are less likely to have synchronized microtine populations than regions with many such predators and that highly synchronized microtine populations have cycles with smaller amplitudes and shorter periods than microtine populations in geographic regions where local populations are not synchronized.