Availability of food may play a number of different dynamical roles in rodent-vegetation systems. Consideration of a suite of rodent-vegetation models, ranging from very simple ones to a model of medium complexity tailored to a specific system (brown lemmings at Point Barrow, Alaska, USA), suggested several general principles. If vegetation grows logistically following an herbivory event (a standard assumption of previously advanced models for herbivore-plant interactions), then almost any biologically reasonable combinations of parameters characterizing rodent-vegetation systems would result in population cycles. We argue, however, that the assumption of logistic growth of the food supply may be appropriate for only a few species, such as moss-eating lemmings. The dynamics of food supply for many arvicoline (microtine) rodents may be better described by a ''linear initial regrowth'' model, which exhibits globally stable dynamics. If this is so, quantitative interactions with food supply are unlikely to explain multiannual population cycles for most boreal or temperate voles. The role of food in population dynamics, however, is not limited to its potential to generate cycles. A tritrophic model including vegetation, rodents, and their specialist predators suggests that food limitation may provide direct density dependence needed for sustained oscillations in this system (which is usually modeled by a phenomenological logistic term in the prey equation).We relate the general theory that we developed to one specific system where we have enough data to arrive at reasonable estimates for most of the parameters-brown lemmings at Point Barrow. The Barrow model exhibits oscillations of the approximately correct period and amplitude, thus giving some theoretical support to the food hypothesis. Nevertheless, we suggest that this result should be treated cautiously because key events explaining the population cycle in the model occur during winter, but winter biology of lemmings is still poorly understood.

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Nutrition, spacing behavior, and predation have been proposed as major factors limiting the population density of microtine rodents (lemmings and voles). However, there is no general agreement as to the relative importance of these factors, nor have their interactions been carefully examined. This paper reports on a factorial experiment designed to test the general hypothesis that predation and the availability of high—quality food act simultaneously and additively to limit densities of microtine rodent populations. Our results showed that supplemental high—quality food significantly increased body growth rates, proportion of adults in the population, adult male body size, reproductive activity, recruitment, and density of prairie vole populations. Food availability had no effect on survival nor on sex ratio in any of the 3 yr of this study. Protection from predation significantly increased adult survival (somewhat sporadically), recruitment, and density, but it did not directly affect growth, population structure, or reproduction. Improved survival of young animals could not be detected, but increased recruitment suggested that it may have occurred, perhaps before the young reached trappable size. Food supplementation and protection from predation generally had additive and equal effects on density. The only significant food—predator interactions were for body growth rates and adult body size. The presence of predators appeared to inhibit the growth response to supplemental food, possibly because of more restricted movement and, therefore, less access to food.

An understanding of population dynamics requires explanations for both the patterns of fluctuations and the densities at which they occur. To identify the causal mechanisms involved, we need to examine the environmental factors that impinge upon the population, the integrative responses of individuals that link changes in the environment to individual performance, and the population changes that occur as a result of shifting individual performances. A review of the literature on small mammals, mostly rodents, suggests that average population levels are largely determined by a balance of the positive effects of resources and 'the negative effects of enemies and that the strengths of these effects vary from habitat to habitat. Seasonal patterns of population change occur each year largely because of seasonal breeding; differences among years occur because of shifts in weather, resources, and enemies. Multiannual patterns (cycles) may occur because of time lags in responses to environmental change or because of combinations of nonlinear responses to density. The volatility of population density appears to vary directly with reproductive potential and inversely with the strength of density-dependent recruitment. A general approach for determining which factors are the mO,st important for any particular population requires a mechanistic, multiple-factor view of population dynamics. Multiple-factor experiments can and should be used to test our understanding of the complex web of cause-and-effect relationships that influence the density of small mammals. 831 D. R. McCullough et al. (eds.), Wildlife 2001: Populations Environmental Factors ~ Physiological-Behavioral ~ Individual Performance Responses t Population Change / Fig. 1. Model of the relationships among environmental factors, individual responses, individual performance, and popUlation change.

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We compared the effects of habitat quality on dispersal, demography, dynamics, and fitness of prairie vole (Microtus ochrogaster) and meadow vole (M. pennsylvanicus) populations by manipulating habitat patches in experimental landscapes. Four habitat patches in each of four replicate landscapes differed in availability of high-quality food and amount of vegetative cover in a 2 ϫ 2 factorial design. High cover had a strong positive effect on basic habitat quality, as reflected by the performance of founders early in the season, but supplemental food had only a small effect.Population growth ceased for prairie voles after week 18 (mid-October) when densities had reached much higher levels in habitats with high cover (260 Ϯ 27 voles/ha in high cover with either high or low food; mean Ϯ 1 SE) than in habitats with low cover (115 Ϯ 38 voles/ha in high food, low cover and 60 Ϯ 15 voles/ha in low food, low cover). Population growth had ceased in three habitat types for meadow voles by week 20 at much higher densities than for prairie voles in high cover (636 Ϯ 101 voles/ha with high food, high cover; 556 Ϯ 117 voles/ha with low food, high cover; 110 Ϯ 74 voles/ha with high food, low cover; and 51 Ϯ 16 voles/ ha with low food, low cover).Correlates of fitness for prairie voles, particularly the number of young surviving to adulthood per female, indicated greater individual fitness in high cover than in low cover, which suggests ideal despotic habitat selection. The proportion of the total population found in habitat patches with high cover continued to increase until prairie vole populations stabilized late in the season. This result did not agree with expectations of either the ideal free model or the ideal despotic model of habitat selection. Fitness differed little for meadow voles in different habitat types, and the proportions of the populations in different habitats remained constant after the founders had settled and population growth began. Both of these patterns supported the ideal free model of habitat selection for meadow voles.Because positive net recruitment occurred in all habitats for both species of voles, source-sink dynamics could not occur in our experimental system. Two subpopulations of meadow voles in low-food, low-cover habitats did go extinct temporarily, but this habitat type did not appear to be a population sink because the losses occurred primarily from emigration rather than mortality. Emigration rates for both species of voles were inversely related to carrying capacity of the habitat (estimated as the density at which populations stabilized). This relationship and ideal free habitat selection are required by the balanced dispersal model, which produces equal numbers of dispersers between a pair of habitat types. Dispersal of meadow voles was balanced throughout the growing season, but dispersal of prairie voles was unbalanced, with net movement of individuals from lowquality to high-quality habitats until late in the season when populations stabilized. Unbalanced dispersal early in the seas...

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