Simultaneous population fluctuations of rodents in montane forests and alpine meadows suggest indirect effects of tree masting
Resource pulses can generate cross habitat dispersal of consumers, and therefore affect organisms even in areas where the resource pulses do not occur. We investigated this phenomenon at the elevational treeline in the Carpathian Mountains, where beech (Fagus sylvatica) masting caused an increase in abundance of forest rodents and intensified their use of alpine meadows. We tested 3 hypotheses concerning the impact of forest rodent spillover on the abundance of meadow dwelling pine voles (Microtus subterraneus) 1) the competition hypothesis: if the spillover affects pine voles mostly through intensified competitive interactions, then pine voles should decline when forest rodents reach their peak abundance, i.e. 1 year after masting; 2) the apparent competition hypothesis: if predators switch to alternative prey when populations of forest rodents collapse, then pine voles should decline 2 years after masting; and 3) the apparent mutualism hypothesis: if the increase of forest rodents temporarily releases pine voles from predatory pressure, pine voles should increase in synchrony with forest rodents 1 year after masting. Our results, while correlative in nature, supported the apparent mutualism hypothesis: 1 year after masting, both forest rodents and pine voles strongly increased their abundance. Two years after masting, when populations of forest rodents crashed, abundance of pine voles returned to pre masting levels rather than collapse. These findings suggest that pulsed spillover, known mostly from negative effects on organisms in recipient habitats, can also create indirect positive interactions. Furthermore, they illustrate how density dependent spillover of animals might increase the spatial scale of masting effects beyond the habitats where seeds are released.
Journal of Mammalogy