Changes in the Spatial Configuration and Strength of Trophic Control Across a Productivity Gradient During a Massive Rodent Outbreak

Abstract

Understanding the determinants of spatial and temporal differences in the relative strength of consumer–resource interactions is an important endeavour in ecology. Here, we explore the necessary conditions for temporal shifts in the relative strength of rodent–plant interactions in an area characterised by profound spatial differences in trophic control, with predator–prey interactions prevailing in productive habitats and rodent–plant interactions dominating unproductive habitats of the forest–tundra ecotone. We report data obtained during the exceptionally massive rodent outbreak of 2010–2012 in northernmost Fennoscandia, including an experimental manipulation of herbivore access to vegetation plots across a large-scale productivity gradient, multiple observational measures of plant–rodent interactions linked to rodent abundance data and a large-scale survey of breeding avian predators and mammalian predator activity. Unexpectedly, rodent grazing impacts documented during the rodent outbreak were uniformly strong across the landscape, regardless of habitat productivity. The runaway response in rodent populations was facilitated by a high population growth rate in the early phase of the outbreak due to the extended absence of predators in productive habitats, concomitant with an exceptionally long-lasting lemming outbreak in unproductive habitats. Our results showed that spatio-temporal variation in trophic control also occurs in ecosystems structured according to the exploitation ecosystems hypothesis and emphasises the importance of long-term studies to capture nonlinear and stochastic features that shape ecosystem functioning. In this context, the temporary release from top–down regulation in productive habitats caused strong grazing impacts that may be crucial for the resilience of tundra ecosystems under the threat of climate change-driven shrub encroachment.