Hence, our study targets a High Arctic population of the graminivorous (grass-eating) East European vole ( Microtus levis) in a food web that lacks significant top-down regulation (i.e., predation). 28– 30), namely to target populations that are found in exceptionally simple biotic settings. Here, we apply an approach that has proved useful for unraveling the effects of density dependence and weather stochasticity in herbivorous large mammals (e.g., refs. The role of climate forcing is now also emphasized by the recent collapses and dampening of population cycles in several ecosystems that appear to be associated with ongoing climate change ( 15, 19, 20). Considering seasonal dynamics is also crucial to assessing the role of climatic change and weather stochasticity because both differ between summer and winter ( 15, 18). 13, 14), several studies have emphasized that annual density dependence ought to be decomposed into its seasonal components ( 15– 17)-both to accurately account for the density-dependent structure that underlies the observed dynamics and to identify the season-specific biotic mechanisms that cause density dependence. As rodent cycles are most prevalent in northern ecosystems with profound climatic seasonality (refs. 4, 10 for reviews), although overcompensatory direct density dependence appears to be an alternative in some settings ( 11). Based on time series analyses, delayed density dependence is considered to be a main determinant of population cycles (see refs. A central topic has been what sort of density dependence yields the high-amplitude, multiannual population cycles for which voles and lemmings have become so renowned ( 6– 9). Studies of small rodents have contributed much to elucidating the different facets of density-dependent and density-independent population dynamics ( 2, 4). Theory suggests that contrasting population dynamics result from details in the pattern of density dependence, including its strength, whether it acts instantly or with a delay, and how it interacts with deterministic (seasonal) and stochastic (weather) components of the prevailing or changing climate ( 1– 5). Our findings contrast with the 3- to 5-y population cycles that are typical of graminivorous small mammals in more complex food webs, suggesting that top-down regulation is normally an important component of such dynamics. When incorporating weather stochasticity in the model simulations, cyclicity became disrupted and the amplitude was increased-akin to the observed dynamics. While such short cycles have not yet been observed in mammals, they are theoretically plausible if graminivorous vole populations are deterministically bottom-up regulated. Model simulations showed that the seasonal pattern of density dependence would yield regular 2-y cycles in the absence of stochasticity. ![]() ![]() However, the predominant driver of the dynamics was overcompensatory density dependence in winter that caused the population to frequently crash. This population exhibited high-amplitude, noncyclic fluctuations-partly driven by weather stochasticity. Here, we take advantage of a uniquely simple High Arctic food web, which allowed us to analyze the dynamics of a graminivorous vole population not subjected to top-down regulation. ![]() Assessing, empirically, the roles of such interactions and how they are influenced by environmental stochasticity has been hampered by food web complexity. The almost ubiquitous 3- to 5-y cycles in boreal and arctic climates may theoretically result from bottom-up (plant–herbivore) and top-down (predator–prey) interactions. Theory predicts that this diversity results from combinations of climatic seasonality, weather stochasticity, and density-dependent food web interactions. Ecologists are still puzzled by the diverse population dynamics of herbivorous small mammals that range from high-amplitude, multiannual cycles to stable dynamics.
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