Thermal performance curve of endurance running at high temperatures in deer mice

The impacts of warming temperatures associated with climate change on performance are poorly understood in most mammals. Thermal performance curves are a valuable means of examining the effects of temperature on performance traits, but they have rarely been used in endotherms. Here, we examined the thermal performance curve of endurance running capacity at high temperatures in the deer mouse (Peromyscus maniculatus). Endurance capacity was measured using an incremental speed test on a treadmill, and subcutaneous temperature in the abdominal region was measured as a proxy for body temperature (Tb). Endurance time at 20°C was repeatable but varied appreciably across individuals, and was unaffected by sex or body mass. Endurance capacity was maintained across a broad range of ambient temperatures (Ta) but was reduced above 35°C. Tb during running varied with Ta, and reductions in endurance were associated with Tb greater than 40°C when Ta was above 35°C. At the high Ta that limited endurance running capacity (but not at lower Ta), Tb tended to rise throughout running trials with increases in running speed. Metabolic and thermoregulatory measurements at rest showed that Tb, evaporative water loss and breathing frequency increased at Ta of 36°C and above. Therefore, the upper threshold temperatures at which endurance capacity is impaired are similar to those inducing heat responses at rest in this species. These findings help discern the mechanisms by which deer mice are impacted by warming temperatures, and provide a general approach for examining thermal breadth of performance in small mammals.

Do animal personalities promote species coexistence? A test with sympatric boreal rodents

The coexistence of competing species requires density feedbacks that have a larger effect on their own species' population growth than they do on others in the assembly. The feedbacks are often associated with behavioral tradeoffs that enable species to differentially exploit underlying axes of heterogeneity. Conjoining theories of species coexistence with foraging behavior and density-dependent habitat selection reveals that such tradeoffs impinge on invasion probabilities and equilibrium dynamics emerging from species' differences in habitat use. The resulting habitat separation promotes coexistence by reducing the overall interaction among species. Differential habitat selection depends on the behavioral abilities of organisms to identify and exploit the most profitable habitats and resource patches. One might thus expect that each species will evolve behavioral types distinct from those of other potential competitors. Accordingly, we exposed four coexisting species in four genera of boreal rodents to open-field tests. We used principal components (PC) to summarize their behaviors along three independent axes corresponding with clines of exploratory, vigilant, and apprehensive personalities. We confirmed that the axes represented repeatable behaviors (personalities) and assessed differences among species with a general linear model (GLM). The GLM revealed highly distinct differences among species, and between pairs of species, on each PC. Even so, it is difficult to infer the adaptive advantages of personality to the habitat segregation that reduces otherwise high interactions among species. Rather, personalities are best interpreted as co-adaptive behaviors reflecting the complex of morphological, physiological and behavioral attributes that dictate tradeoffs and enable coexistence.

Competitive release during fire succession influences ecological turnover in a small mammal community

Ecologists have long debated the relative importance of biotic interactions versus species-specific habitat preferences in shaping patterns of ecological dominance. In western North America, cycles of fire disturbance are marked by transitions between North American deermice (Peromyscus maniculatus), which predominate after wildfires, and southern red-backed voles (Myodes gapperi), which gradually replace deermice 3-4 years postfire and maintain dominance as forests mature. While this shift has been frequently documented, the processes that mediate this turnover are debated. One possibility is competitive release, which predicts a reduction in vole competition may contribute to niche expansion and population growth in deermice. Alternatively, turnover in both species may be shaped by differences in their preferred habitat and resource base, as predicted by optimum foraging theory. We evaluate these hypotheses using stable isotopes and spatial mark-recapture of deermouse and vole populations sampled prior to and following a fire as part of a longitudinal study in the Greater Yellowstone Ecosystem. Fire disturbance was associated with a 94% decrease in vole abundance but a 102% increase in deermice. Even after accounting for microhabitat, vole and deermouse populations were negatively correlated spatially and temporally (R = -0.45), and competitor abundance was more important prefire than postfire. When vole abundance was high (prefire), vole dietary niche space was seven times broader than that of deermice. Postfire, deermouse dietary niche nearly tripled and was enriched in ¹³ C (i.e., more C₄ plants), while voles occupied a slightly reduced dietary niche (79% of prefire breadth). Our results suggest deermice are experiencing ecological release due to a reduction in vole competition but vole shifts are largely driven by habitat preferences.

Small Prey Animal Foraging Behaviors in Landscapes of Fear: Effects of Predator Presence and Human Activity Along an Urban Disturbance Gradient

Urban environments provide the only or best habitats that are left for wildlife in many areas, promoting increased interest in urban conservation and a need to understand how wildlife cope with urban stressors, such as altered predator activity and human disturbance. Here, we used filmed giving-up density experiments to investigate behavioral coping responses of foraging small prey animals at three sites (close, mid, and far) along an urban disturbance gradient. Our study design included “natural” and experimentally added stressor cues of predators and/or human disturbance. We observed small mammal foraging behaviors, particularly: the common brushtail possum (Trichosurus vulpecula), northern brown bandicoot (Isoodon macrourus), brown antechinus (Antechinus stuartii), black rat (Rattus rattus), and brown rat (Rattus norvegicus), and to a lesser degree several species of native birds. We found that at the close urban-edge environment, coping responses to human disturbances were most pronounced, and predator cues from the red fox (Vulpes vulpes) were perceived as least risky. However, at the mid environment, red fox cues were perceived as most risky, especially when combined with human disturbance. At the far environment, domestic cat (Felis catus) cues were perceived as most risky, again when combined with human disturbance. Impacts from the combined stressors of predator and human disturbance cues appeared to be additive, with higher risk being perceived with increasing distance from urban build-up. Behavioral adjustments were observed to be the primary response to stressors by small prey animals in the close environment. In the mid environment, slight temporal shifts in activity across the night were more evident. In the far environment, habitat components were likely being used differently as the primary coping response to stressors. As mostly the same species were observed along the disturbance gradient, our results suggest a level of response plasticity that is calibrated to the level of exposure to a stressor and the stressor type. To maximize conservation outcomes in urban habitats, we therefore propose that management should be sensitive to the level and history of human disturbance, as this affects the coping responses of wildlife that remain.

The necessity of tailored control of irrupting pest populations driven by pulsed resources

Resource pulses are widespread phenomena in diverse ecosystems. Irruptions of generalist consumers and corresponding generalist predators often follow such resource pulses. This can have severe implications on the ecosystem and also on the spread of diseases or on regional famines. Suitable management strategies are necessary to deal with these systems. In this study, we develop a general model to investigate optimal control for such a system and apply this to a case study from New Zealand. In particular, we consider the dynamics of beech masting (episodic synchronous seed production) leading to rodent outbreaks and subsequent stoat (Mustela erminea) irruptions. Here, stoat control happens via secondary poisoning. The results show that the main driver of the optimal control timing (June) is the population density of the control vector. Intermediate control levels are superior to higher levels if the generalist consumer is necessary as a control vector. Finally, we extend the model to a two-patch metapopulation model, which indicates that, as a consequence of the strong vector dependence, a strategy of alternating control patches yields better results than static control. This highlights that besides control level, also the design impacts the control success. The results presented in this study reveal important insights for proper pest management in the New Zealand case study. However, they also generally indicate the necessity of tailored control in such systems.

Foraging movements are density-independent among straw-coloured fruit bats

Intraspecific competition in large aggregations of animals should generate density-dependent effects on foraging patterns. To test how large differences in colony size affect foraging movements, we tracked seasonal movements of the African straw-coloured fruit bat (Eidolon helvum) from four colonies that range from 4000 up to 10 million animals. Contrary to initial predictions, we found that mean distance flown per night (9-99 km), number of nightly foraging sites (2-3) and foraging and commuting times were largely independent of colony size. Bats showed classic central-place foraging and typically returned to the same day roost each night. However, roost switching was evident among individuals in three of the four colonies especially towards the onset of migration. The relatively consistent foraging patterns across seasons and colonies indicate that these bats seek out roosts close to highly productive landscapes. Once foraging effort starts to increase due to local resource depletion they migrate to landscapes with seasonally increasing resources. This minimizes high intraspecific competition and may help to explain why long-distance migration, otherwise rare in bats, evolved in this highly gregarious species.

Pulsed resource availability changes dietary niche breadth and partitioning between generalist rodent consumers

Identifying the mechanisms that structure niche breadth and overlap between species is important for determining how species interact and assessing their functional role in an ecosystem. Without manipulative experiments, assessing the role of foraging ecology and interspecific competition in structuring diet is challenging. Systems with regular pulses of resources act as a natural experiment to investigate the factors that influence the dietary niches of consumers. We used natural pulses of mast-fruiting of American beech (Fagus grandifolia) to test whether optimal foraging or competition structure the dietary niche breadth and overlap between two congener rodent species (Peromyscus leucopus and P. maniculatus), both of which are generalist consumers. We reconstructed diets seasonally over a 2-year period using stable isotope analysis (δ13C, δ15N) of hair and of potential dietary items and measured niche dynamics using standard ellipse area calculated within a Bayesian framework. Changes in niche breadth were generally consistent with predictions of optimal foraging theory, with both species consuming more beechnuts (a high-quality food resource) and having a narrower niche breadth during masting seasons compared to nonmasting seasons when dietary niches expanded and more fungi (a low-quality food source) were consumed. In contrast, changes in dietary niche overlap were consistent with competition theory, with higher diet overlap during masting seasons than during nonmasting seasons. Overall, dietary niche dynamics were closely tied to beech masting, underscoring that food availability influences competition. Diet plasticity and niche partitioning between the two Peromyscus species may reflect differences in foraging strategies, thereby reducing competition when food availability is low. Such dietary shifts may have important implications for changes in ecosystem function, including the dispersal of fungal spores.

Size dependency of patch departure behavior: evidence from granivorous rodents

Individual size is a major determinant of mobile organisms’ ecology and behavior. This study aims to explore whether allometric scaling principles can provide an underlying framework for general patterns of resource patch use. To this end, we used giving‐up densities (GUDs), that is, the amount of resources remaining in a patch after a forager has quit feeding, as a comparative measure of the amount of resources exploited by a forager of any given size. We specifically tested the hypothesis that size‐dependent responses to both internal (energy requirement) and external (risk management) forces may have an effect on GUDs. We addressed this topic by conducting an extensive meta‐analysis of published data on granivorous rodents, including 292 GUD measurements reported in 25 papers. The data set includes data on 22 granivorous rodent species belonging to three taxonomic suborders (Castorimorpha, Myomorpha, and Sciuromorpha) and spans three habitat types (desert, grassland, and forest). The observations refer to both patches subject to predation risk and safe patches. Pooling all data, we observed positive allometric scaling of GUDs with average forager size (scaling exponent = 0.45), which explained 15% of overall variance in individual GUDs. Perceived predation risk during foraging led to an increase in GUDs independently of forager size and taxonomy and of habitat type, which explained an additional 12% of overall GUD variance. The size scaling exponent of GUDs is positive across habitat types and taxonomic suborders of rodents. Some variation was observed, however. The scaling coefficients in grassland and forest habitat types were significantly higher than in the desert habitat type. In addition, Sciuromorpha and Myomorpha exhibited a more pronounced size scaling of GUDs than Castorimorpha. This suggests that different adaptive behaviors may be used in different contexts and/or from different foragers. With body size being a fundamental ecological descriptor, research into size scaling of GUDs may help to place patch‐use observations in a broader allometric framework.

Cascading effects of predation risk determine how marine predators become terrestrial prey on an oceanic island

Apex predators can suppress the foraging activity of mesopredators, which may then result in cascading benefits for the prey of those mesopredators. We studied the interactions between a top predator, the Barn Owl (Tyto alba), and their primary prey, an island endemic deer mouse (Peromyscus maniculatus elusus), which in turn consumes the eggs of seabirds nesting on Santa Barbara Island in California. Scripps's Murrelets (Synthliboramphus scrippsi), a threatened nocturnal seabird, arrive annually to breed on this island, and whose first egg is particularly vulnerable to predation by mice. We took advantage of naturally occurring extreme variations in the density of mice and owls on the island over 3 years and predicted that (1) mouse foraging would decrease with increasing predation risk from owls and moonlight and (2) these decreases in foraging would reduce predation on murrelet eggs. We measured the giving up densities of mice with experimental foraging stations and found that mice were sensitive to predation risk and foraged less when owls were more abundant and less during the full moon compared to the new moon. We also monitored the fates of 151 murrelet eggs, and found that murrelet egg predation declined as owl abundance increased, and was lower during the full moon compared to the new moon. Moreover, high owl abundance suppressed egg predation even when mice were extremely abundant. We conclude that there is a behaviorally mediated cascade such that owls on the island had a positive indirect effect on murrelet egg survival. Our study adds to the wider recognition of the strength of risk effects to structure food webs, as well as highlighting the complex ways that marine and terrestrial food webs can intersect.

Serotonin promotes exploitation in complex environments by accelerating decision-making

Background

Fast responses can provide a competitive advantage when resources are inhomogeneously distributed. The nematode Caenorhabditis elegans was shown to modulate locomotion on a lawn of bacterial food in serotonin (5-HT)-dependent manners. However, potential roles for serotonergic signaling in responding to food discovery are poorly understood.

Results

We found that 5-HT signaling in C. elegans facilitates efficient exploitation in complex environments by mediating a rapid response upon encountering food. Genetic or cellular manipulations leading to deficient serotonergic signaling resulted in gradual responses and defective exploitation of a patchy foraging landscape. Physiological imaging revealed that the NSM serotonergic neurons responded acutely upon encounter with newly discovered food and were key to rapid responses. In contrast, the onset of responses of ADF serotonergic neurons preceded the physical encounter with the food. The serotonin-gated chloride channel MOD-1 and the ortholog of mammalian 5-HT1 metabotropic serotonin receptors SER-4 acted in synergy to accelerate decision-making. The relevance of responding rapidly was demonstrated in patchy environments, where the absence of 5-HT signaling was detrimental to exploitation.

Conclusions

Our results implicate 5-HT in a novel form of decision-making, demonstrate its fitness consequences, suggest that NSM and ADF act in concert to modulate locomotion in complex environments, and identify the synergistic action of a channel and a metabotropic receptor in accelerating C. elegans decision-making.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0232-y) contains supplementary material, which is available to authorized users.

The functional response predicts the effect of resource distribution on the optimal movement rate of consumers

Understanding how often individuals should move when foraging over patchy habitats is a central question in ecology. By combining optimality and functional response theories, we show analytically how the optimal movement rate varies with the average resource level (enrichment) and resource distribution (patch heterogeneity). We find that the type of functional response predicts the effect of enrichment in homogeneous habitats: enrichment should decrease movement for decelerating functional responses, but increase movement for accelerating responses. An intermediate resource level thus maximises movement for type-III responses. Counterintuitively, greater movement costs favour an increase in movement. In heterogeneous habitats predictions further depend on how enrichment alters the variance of resource distribution. Greater patch variance always increases the optimal rate of movement, except for type-IV functional responses. While the functional response is well established as a fundamental determinant of consumer-resource dynamics, our results indicate its importance extends to the understanding of individual movement strategies.

An island-wide predator manipulation reveals immediate and long-lasting matching of risk by prey

Anti-predator behaviour affects prey population dynamics, mediates cascading effects in food webs and influences the likelihood of rapid extinctions. Predator manipulations in natural settings provide a rare opportunity to understand how prey anti-predator behaviour is affected by large-scale changes in predators. Here, we couple a long-term, island-wide manipulation of an important rodent predator, the island fox (Urocyon littoralis), with nearly 6 years of measurements on foraging by deer mice (Peromyscus maniculatus) to provide unequivocal evidence that prey closely match their foraging behaviour to the number of fox predators present on the island. Peromyscus maniculatus foraging among exposed and sheltered microhabitats (a measure of aversion to predation risk) closely tracked fox density, but the nature of this effect depended upon nightly environmental conditions known to affect rodent susceptibility to predators. These effects could not be explained by changes in density of deer mice over time. Our work reveals that prey in natural settings are cognizant of the dynamic nature of their predators over timescales that span many years, and that predator removals spanning many generations of prey do not result in a loss of anti-predator behaviour.

Ecological correlates of stress for a habitat generalist in a biofuels landscape

Understanding the success of habitat generalist species requires knowledge of how individuals respond to stressors that vary across habitats within landscapes. Habitat structure can affect stress by altering predation risk, conspecific densities, and densities of heterospecific competitors. Increased demand for biofuels will alter habitat structure for species in agroecosystems worldwide. We measured stress responses of deer mice (Peromyscus maniculatus (Wagner, 1845)), a widespread habitat generalist, in a biofuels landscape. We quantified fecal corticosterone concentrations for individuals in four biofuel crops: switchgrass (Panicum virgatum L.), miscanthus (Miscanthus × giganteus Greef & Deuter ex Hodkinson & Renvoize), mixed prairie, and corn (Zea mays L.). We also evaluated stress responses of deer mice to the annual harvesting of corn. Deer mice inhabiting switchgrass and mixed prairie had higher fecal corticosterone concentrations compared with mice in corn and miscanthus. Fecal corticosterone concentrations correlated positively with abundances of conspecifics and behaviorally dominant voles (prairie vole, Microtus ochrogaster (Wagner, 1842); meadow vole, Microtus pennsylvanicus (Ord, 1815)) across habitats. Stress levels of deer mice depended on how habitat structure modified the competitive environment. Deer mice did not exhibit increased fecal corticosterone concentrations in response to corn harvest, a rapid and extensive habitat disturbance common to agroecosystems. Our research is the first to investigate how landscape change due to biofuels expansion can affect stress levels of individuals.

Trophic shifts of a generalist consumer in response to resource pulses

Trophic shifts of generalist consumers can have broad food-web and biodiversity consequences through altered trophic flows and vertical diversity. Previous studies have used trophic shifts as indicators of food-web responses to perturbations, such as species invasion, and spatial or temporal subsidies. Resource pulses, as a form of temporal subsidies, have been found to be quite common among various ecosystems, affecting organisms at multiple trophic levels. Although diet switching of generalist consumers in response to resource pulses is well documented, few studies have examined if the switch involves trophic shifts, and if so, the directions and magnitudes of the shifts. In this study, we used stable carbon and nitrogen isotopes with a Bayesian multi-source mixing model to estimate proportional contributions of three trophic groups (i.e. producer, consumer, and fungus-detritivore) to the diets of the White-footed mouse (Peromyscus leucopus) receiving an artificial seed pulse or a naturally-occurring cicadas pulse. Our results demonstrated that resource pulses can drive trophic shifts in the mice. Specifically, the producer contribution to the mouse diets was increased by 32% with the seed pulse at both sites examined. The consumer contribution to the mouse diets was also increased by 29% with the cicadas pulse in one of the two grids examined. However, the pattern was reversed in the second grid, with a 13% decrease in the consumer contribution with the cicadas pulse. These findings suggest that generalist consumers may play different functional roles in food webs under perturbations of resource pulses. This study provides one of the few highly quantitative descriptions on dietary and trophic shifts of a key consumer in forest food webs, which may help future studies to form specific predictions on changes in trophic interactions following resource pulses.

Foraging decisions and behavioural flexibility in trap-building predators: a review

Foraging theory was first developed to predict the behaviour of widely-foraging animals that actively search for prey. Although the behaviour of sit-and-wait predators often follows predictions derived from foraging theory, the similarity between these two distinct groups of predators is not always obvious. In this review, we compare foraging activities of trap-building predators (mainly pit-building antlions and web-building spiders), a specific group of sit-and-wait predators that construct traps as a foraging device, with those of widely-foraging predators. We refer to modifications of the trap characteristics as analogous to changes in foraging intensity. Our review illustrates that the responses of trap-building and widely-foraging predators to different internal and external factors, such as hunger level, conspecific density and predation threat are quite similar, calling for additional studies of foraging theory using trap-building predators. In each chapter of this review, we summarize the response of trap-building predators to a different factor, while contrasting it with the equivalent response characterizing widely-foraging predators. We provide here evidence that the behaviour of trap-building predators is not stereotypic or fixed as was once commonly accepted, rather it can vary greatly, depending on the individual's internal state and its interactions with external environmental factors.

Foraging patterns of voles at heterogeneous avian and uniform mustelid predation risk

Temporal variation of antipredatory behavior and a uniform distribution of predation risk over refuges and foraging sites may create foraging patterns different from those anticipated from risk in heterogenous habitats. We studied the temporal variation in foraging behavior of voles exposed to uniform mustelid predation risk and heterogeneous avian predation risk of different levels induced by vegetation types in eight outdoor enclosures (0.25 ha). We manipulated mustelid predation risk with weasel presence or absence and avian predation risk by reducing or providing local cover at experimental food patches. Foraging at food patches was monitored by collecting giving-up densities at artificial food patches, overall activity was automatically monitored, and mortality of voles was monitored by live-trapping and radiotracking. Voles depleted the food to lower levels in the sheltered patches than in the exposed ones. In enclosures with higher avian predation risk caused by lower vegetation height, trays were depleted to lower levels. Unexpectedly, voles foraged in more trays and depleted trays to lower levels in the presence of weasels than in the absence. Weasels match their prey's body size and locomotive abilities and therefore increase predation risk uniformly over both foraging sites and refuge sites that can both be entered by the predator. This reduces the costs of missing opportunities other than foraging. Voles changed their foraging strategy accordingly by specializing on the experimental food patches with predictable returns and probably reduced their foraging in the matrix of natural food source with unpredictable returns and high risk to encounter the weasel. Moreover, after 1 day of weasel presence, voles shifted their main foraging activities to avoid the diurnal weasel. This behavior facilitated bird predation, probably by nocturnal owls, and more voles were killed by birds than by weasels. Food patch use of voles in weasel enclosures increased with time. Voles had to balance the previously missed feeding opportunities by progressively concentrating on artificial food patches.

Giving-updensity and dietary shifts in the white-footed mouse, Peromyscus leucopus

Dietary shifts are commonly exhibited by omnivorous consumers when foraging from variable food resources. One advantage of dietary shifts for a consumer is the ability to gain complementary resources from different foods. In addition, dietary shifts often affect food-web dynamics. Despite the importance of dietary shifts to organismal, community, and ecosystem ecology, empirical studies of the ecological mechanisms that control dietary shifts are limited in scope. In this study, we tested the effects of complementary resources on dietary shifts of an omnivorous mammal, the white-footed mouse Peromyscus leucopus, in the context of depletable food patches in the natural environment. We used two complementary resources: seeds that provide a higher energy gain per unit handling time and mealworms that provide a higher protein gain per unit handling time. Stable isotopes of carbon and nitrogen (delta13C, delta15N) in mouse plasma were used to quantify dietary shifts, and we determined giving-up density (GUD), the food density at which a forager leaves a food patch (for an optimal forager, it should correspond to the quitting harvest rate that balances net fitness gain with various costs of foraging). The results showed that GUD increased most significantly when a mixture of seeds and mealworms was added, compared to when only seeds or mealworms were added. This suggests that, given a similar level of food availability, a patch with a mixture of complementary foods is of higher quality than a patch with only one type of food. Moreover, GUD measured with seeds (GUDs) correlated positively with seed availability, and GUD measured with mealworms (GUDmw) correlated positively with mealworm availability, indicating that the marginal value of seeds or mealworms decreases with their relatively availability in the environment. As GUDs increased, P. leucopus shifted their diets toward higher trophic levels, and as GUDmw increased, P. leucopus shifted their diets toward lower trophic levels, suggesting dietary shifts driven by food complementarity. This study demonstrated that the combination of giving-up density and stable-isotope analysis holds great potential for testing ecological mechanisms underlying dietary shifts.

CAN WE MEASURE CARRYING CAPACITY WITH FORAGING BEHAVIOR?

Carrying capacity is one of the most important, yet least understood and rarely estimated, parameters in population management and modeling. A simple behavioral metric of carrying capacity would advance theory, conservation, and management of biological populations. Such a metric should be possible because behavior is finely attuned to variation in environment including population density. We connect optimal foraging theory with population dynamics and life history to develop a simple model that predicts this sort of adaptive density-dependent change in food consumption. We then confirm the model's unexpected and manifold predictions with field experiments. The theory predicts reproductive thresholds that alter the marginal value of energy as well as the value of time. Both effects cause a pronounced discontinuity in quitting-harvest rate that we revealed with foraging experiments. Red-backed voles maintained across a range of high densities foraged at a lower density-dependent rate than the same animals exposed to low-density treatments. The change in harvest rate is diagnostic of populations that exceed their carrying capacity. Ecologists, conservation biologists, and wildlife managers may thus be able to use simple and efficient foraging experiments to estimate carrying capacity and habitat quality.

Rodent seed predation and GUDs: effect of burning and topography

We examined the relationships between seed predation and the habitat into which a seed falls, abundance of rodents within that habitat, and foraging pattern of rodents within that habitat. Using seed plots, we assessed seed predation in burned and unburned tallgrass prairies at biweekly intervals between May and September in 2001 and 2002. Significantly more seed was removed from plots in burned than unburned prairies. Rodent abundance did not differ between burned prairie and unburned prairie, although the abundance of omnivorous–granivorous rodents (herbivores excluded) was greater in burned than unburned prairie. Proportion of seed removed in burned and unburned prairies was independent of both total rodent abundance and abundance of omnivorous–granivorous rodents. We also measured giving-up density (GUD) of rodents in burned and unburned prairies during the spring and summer of 2002. GUDs did not differ significantly in burned and unburned prairies. However, rodents had a higher GUD in uplands than in limestone breaks or lowland habitats. Our results suggest that rodent foraging in tallgrass prairie is affected by microhabitat and that rodent abundance is not sufficient to predict seed predation.

Reproduction, foraging and the negative density–area relationship of a generalist rodent

While many species show positive relationships between population density and habitat patch area, some species consistently show higher densities in smaller patches. Few studies have examined mechanisms that may cause species to have negative density-area relationships. We tested the hypothesis that greater reproduction in edge versus interior habitats and small versus large fragments contributes to higher densities of white-footed mice (Peromyscus leucopus) in small versus large forest fragments. We also examined vegetation structure and foraging tray utilization to evaluate if greater reproduction was a result of higher food availability. There were greater number of litters and proportion of females producing litters in the edge versus interior of forest fragments, which may have contributed to greater population growth rates and higher densities in edge versus interior and small versus large fragments. Data on vegetation structure and giving-up densities of seeds in artificial patches suggest that food availability may be higher in edge versus interior habitats and small versus large fragments. These results, in an area with few or no long-tailed weasels, provide a distinct contrast to the findings of Morris and Davidson (Ecology 81:2061, 2000) who observed lower reproduction in forest edge habitat as a result of high weasel predation, suggesting that specialist predators may be important in affecting the quality of edge habitat. While we cannot exclude the potential contributions of immigration, emigration, and mortality, our data suggest that greater reproduction in edge versus interior habitat is an important factor contributing to higher densities of P. leucopus in small fragments.