POPULATION CYCLES IN SMALL MAMMALS: THE PROBLEM OF EXPLAINING THE LOW PHASE

Abstract

Cycles characterize the demography of many populations of microtine rodents and snowshoe hares. A phase of low numbers often follows the decline and introduces a lag that lengthens the cycle. This low can last 1–3 yr in microtines and 2–4 yr in hares. Understanding the low phase is critical in explaining population cycles. Two major classes of hypotheses try to account for the low phase. The first proposes that something may be “wrong” with the extrinsic environment. The most promising of these extrinsic explanations is that predation, acting either directly or indirectly, has delayed density-dependent effects on prey populations during the low phase. The second class of hypotheses proposes that something may be “wrong” with the animals themselves. The most likely intrinsic factors are maternal effects or age effects on fitness during the low phase. Experimental tests for each of these sets of hypotheses are needed, and we suggest replicated experiments on focal species in two continents to resolve these unknowns.