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

Cycles characterize the demography of many populations of microtine ro dents and snowshoe hares. A phase of low numbers often follows the dec line 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 hypo theses try to account for the low phase. The first proposes that somet hing may be ''wrong'' with the extrinsic environment. The most promisi ng of these extrinsic explanations is that predation, acting either di rectly or indirectly, has delayed density-dependent effects on prey po pulations during the low phase. The second class of hypotheses propose s that something may be ''wrong'' with the animals themselves. The mos t likely intrinsic factors are maternal effects or age effects on fitn ess 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.