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Several hypotheses have been proposed to explain cyclic fluctuationsin abundance of some small mammal populations. These hypotheses havebeen controversial, however, and there is no consensus among biologists as to why population cycles occur. In a demographically based model, we tested the potential influence of phase-specific changes in life history traits (age at maturity, fertility, juvenile survival and adult survival) on population cycles. Our demographic model considers, and is logically consistent with, the empirical pattern of population characteristics during a cycle. The essence of the model is that phase-specific changes in age at maturity, abetted secondarily by changes in juvenile survival, result in cyclic fluctuations in populationsize. Changes in adult survival and fertility may play a minor role,but they are neither necessary nor sufficient by themselves to generate population cycles. Phase-specific changes in age at maturity might be related to primary changes in the quality of the ecological and social environment that permit particularly high densities. [ABSTRACT FROM AUTHOR]

Environmental factors influence the dynamics and regulation of biological populations through their influences on demographic variables, but demographic mechanisms of population regulation have received little attention. We investigated the demographic basis of regulation of Columbian ground squirrel (Spermophilus columbianus) populations under natural and experimentally food-supplemented conditions. Food supplementation caused substantial increases in population density, and population densities returned to pretreatment levels when the supplementation ended. Control (untreated) populations remained relatively stable throughout the study period (1981-1986). Because food resources regulated the size of the ground squirrel populations, we used life-table response experiment (LTRE) analyses to examine the demographic basis of changes in population growth rate and thus also demographic influences on population regulation. LTRE analyses of two food-manipulated populations revealed that changes in age at maturity and fertility rate of females generally made the largest contributions to observed changes in population growth rate. Thus, our results suggested that abundance of food resources regulated the size of our study populations through the effects of food resources on age at maturity and fertility rates. Our results also indicated that different demographic mechanisms can underlie population regulation under different environmental conditions, because lower juvenile survival substantially contributed to population decline, but in only one of the populations. Demographic analyses of experimental data, such as those presented here, offer a rigorous and unambiguous means to elucidate the demographic basis of population regulation and to help identify environmental factors that underlie dynamics and regulation of biological populations.

Columbian ground squirrels (Spermophilus columbianus) exhibit an elevational cline in several life-history traits and in body mass. Body mass is a trait that might interact with the elevational changes in life histories, but it could reflect either or both of two underlying factors: structural size and physiological condition. I studied these factors in several populations of ground squirrels. Structural size was estimated from first principal component scores in an analysis of postcranial bone lengths. Physiological condition was inferred from differences in body mass that were independent of structural size. Field body mass and structural size of yearlings decreased with increasing elevation, which indicates slower growth at higher elevations. For adults, physiological condition was not measured directly, but evidence suggested that it was reflected by changes in body mass: body mass declined with increasing elevation but structural size did not, residuals of body mass on structural size showed the same patterns of change as body mass, most of the variation in body mass was orthogonal to a "general size factor" in principal components analyses, and significant changes in body mass of individuals could be stimulated by experimental supplementation of food. Life-history traits and physiological condition covaried along the elevational cline and were phenotypically plastic, but the structural size of adult ground squirrels was not an extremely plastic trait.