Capture-recapture dataset of Svalbard voles (1990-2007) with trap locations and rain-on-snow measurements

Ecologists are still puzzled by the diverse population dynamics of
herbivorous small mammals that range from high-amplitude, multi-annual
cycles to stable dynamics. Theory predicts that this diversity results
from combinations of climatic seasonality, weather stochasticity and
density-dependent food web interactions. The almost ubiquitous 3-5-yr
cycles in boreal and arctic climates may theoretically result from
bottom-up (plant-herbivore) and top-down (predator-prey) interactions.
Assessing empirically the roles of such interactions, and how they are
influenced by environmental stochasticity, has been hampered by food web
complexity. Here, we take advantage of a uniquely simple High-Arctic food
web, which allowed us to analyze dynamics of a graminivorous vole
population not subjected to top-down regulation. This population exhibited
high-amplitude, non-cyclic fluctuations - partly driven by weather
stochasticity.  However, the predominant driver of the dynamics
was overcompensatory density dependence in winter that caused the
population to frequently crash. Model simulations showed that the seasonal
pattern of density dependence would yield regular 2-year cycles in absence
of stochasticity. While such short cycles have not yet been observed in
mammals, they are theoretically plausible if graminivorous vole
populations are deterministically bottom-up regulated. When incorporating
weather stochasticity in the model simulations, cyclicity became disrupted
and the amplitude was increased - akin to the observed
dynamics. Our findings contrast with the 3-5-yr 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.