Ecological Impact of Introduced Crayfish on Benthic Fishes in a British Lowland River

Pacifastacus leniusculus (Dana), a native crayfish of western North America, was introduced into the U.K. in 1976. Our study examined some interactions between P. leniusculus and benthic fish in a British lowland river, the River Great Ouse. In a river survey an inverse correlation was found between the abundance of crayfish and the two dominant benthic fishes, bullhead (Cottus gobio L.) and stone loach (   Noemacheilus barbatulus [L.]) in six riffles. The benthic fishes were least abundant in the riffle nearest the original site of crayfish introduction and gradually increased in abundance both up and down river as crayfish abundance decreased. The hypotheses that crayfish compete with bullheads and stone loach for shelter and prey on fish were tested by laboratory experiments in an outdoor artificial stream (6 × 2 m) with recirculating water and 12 artificial shelters on the bottom. In competition experiments 12 fish of one species were alternatively kept alone and with 12 crayfish for 3-day cycles lasting a total of 12 days. The results showed that crayfish out-competed both fish species for shelter. Predation was measured by keeping 24 fish of each species alone and with 36 crayfish for 10 days respectively in the artificial stream. The mortalities of both fish species were significantly higher when crayfish were present. The loss of fish could be partly due to predation because crayfish guts contained the remains of some lost fish and they were observed preying on both fishes in a tank. In the river crayfish lived at high densities reaching ≥ to 20 m⁻² in riffles, and they continued to disperse. This may lead to a great reduction in benthic fish abundance if not local extinctions.     

The influence of kinship on foraging competition in Siberian jays

Foraging competition in Siberian jay groups was examined in relation to dominance and kinship to determine whether juvenile offspring, by associating with adults, gained in food acquisition relative to juvenile immigrants. Members of the adult pair were dominant over juvenile cohort members and males were dominant to females, although an inter-sexual hierarchy, with male juveniles occasionally overlapping adult females, was suggested. Few competitive asymmetries were found between adults and retained offspring or adults and immigrant juveniles when they were competing for food together, but in kin and non-kin foraging groups, respectively. Male offspring visited the bait site more frequently than adult males, and female immigrants spent less time at the bait site than adult females. Under these circumstances, hoarding activities may limit the ability of alpha members to control resources. In mixed groups containing both juvenile offspring and juvenile immigrants, no difference was found in the number of visits made to the bait site, although load sizes and foraging rates were lower for immigrant birds. Retained juveniles obtained greater load sizes and foraging rates when associating with adults. The social dominance of parents suggests that they control juvenile foraging. Although offspring benefit in the presence of adults, adults may incur a cost to their restraint by spending more time at the bait site when competing with immigrants. These results extend conclusions from previous work describing the role of selective tolerance by adults which relaxes competition with retained offspring in Siberian jay winter groups. The present findings suggest that offspring benefit in both immediate and future energy gains, which may have a direct influence on survival. Received: 18 September 1996 / Accepted after revision: 26 January 1997     

Effects of small-scale habitat fragmentation on predator–prey interactions in a temperate sea grass system

During the last decades, fragmentation has become an important issue in ecological research. Habitat fragmentation operates on spatial scales ranging over several magnitudes from patches to landscapes. We focus on small-scale fragmentation effects relevant to animal foraging decision making that could ultimately generate distribution patterns. In a controlled experimental environment, we tested small-scale fragmentation effects in artificial sea grass on the feeding behaviour of juvenile cod (Gadus morhua). Moreover, we examined the influence of fragmentation on the distribution of one of the juvenile cod’s main prey resources, the grass shrimp (Palaemon elegans), in association with three levels of risk provided by cod (no cod, cod chemical cues and actively foraging cod). Time spent by cod within sea grass was lower in fragmented landscapes, but total shrimp consumption was not affected. Shrimp utilised vegetation to a greater extent in fragmented treatments in combination with active predation. We suggest that shrimp choose between sand and vegetation habitats to minimize risk of predation according to cod habitat-specific foraging capacities, while cod aim to maximize prey-dependent foraging rates, generating a habitat-choice game between predator and prey. Moreover, aggregating behaviour in grass shrimp was only found in treatments with active predation. Hence, we argue that both aggregation and vegetation use are anti-predator defence strategies applied by shrimp. We therefore stress the importance of considering small-scale behavioural mechanisms when evaluating consequences from habitat fragmentation on trophic processes in coastal environments.     

Disease avoidance influences shelter use and predation in Caribbean spiny lobster

Shelter competition is uncommon among social animals, as is the case among normally gregarious Caribbean spiny lobsters (Panulirus argus). However, healthy lobsters avoid sheltering with conspecifics infected by a lethal pathogenic virus, PaV1. These contradictory behaviors have implications for shelter use and survival, especially in areas where shelter is limited. In laboratory experiments, we tested shelter competition between paired healthy and diseased juvenile lobsters in shelter-limited mesocosms. Neither healthy nor diseased lobsters dominated access to shelters, but lobsters shared shelter less often when diseased lobsters were present relative to controls with two healthy lobsters. We hypothesized that exclusion of juvenile lobsters from shelter results in increased mortality from predation, especially for the more lethargic, infected individuals. Field tethering trials revealed that predation was indeed higher on infected individuals and on all tethered lobsters deprived of shelter. We then tested in mesocosm experiments how the contrasting risks of predation versus infection by a lethal pathogen influence shelter use. Lobsters were offered a choice of an empty shelter or one containing a diseased lobster in the presence of a predator (i.e., caged octopus) whose presence normally elicits shelter-seeking behavior, and these data were compared with a previous study where the predator was absent. Lobsters selected the empty shelter significantly more often despite the threat of predation, foregoing the protection of group defense in favor of reduced infection risk. These results offer striking evidence of how pathogenic diseases shape not only the behavior of social animals but also their use of shelters and risk of predation.     

Behavioral responses to a novel predator and competitor of invasive mosquitofish and their non-invasive relatives (Gambusia sp.)

Attributes of the recipient community may affect the invasion success of arriving non-indigenous organisms. In particular, biotic interactions may enhance the resistance of communities to invasion. Invading organisms typically encounter a novel suite of competitors and predators, and thus their invasiveness may be affected by how they cope with these interactions. Behavioral plasticity may help invaders to respond appropriately to novelty. We examined the behavioral responses of highly invasive mosquitofish to representative novel competitors and predators they might encounter as they spread through North America. We compared the behavior of invasive Gambusia holbrooki and G. affinis to that of two close relatives of lower invasive potential (G. geiseri and G. hispaniolae) in order to elucidate whether responses to novelty related to invasiveness. In short-term assays, female Gambusia were paired with a novel competitor, Pimephales promelas, and a novel predator, Micropterus dolomieu. Behavioral responses were measured in terms of foraging success and efficiency, activity, refuge use, predator inspections, and interspecific aggression. Contrary to a priori predictions, invasive and non-invasive responses to novel interactions did not differ consistently. In response to novel competition, both invasive species increased foraging efficiency, but so did G. geiseri. In response to novel predation, only G. holbrooki decreased consumption and activity and increased refuge use. No antipredator response was observed in G. affinis. We found consistent differences, however, between invasives and non-invasives in foraging behavior. Both in the presence and absence of the competitor and the predator, invasives foraged more efficiently and consumed more prey than non-invasives.Communicated by P. Bednekoff     

Impact of cannibalism on predator-prey dynamics: size-structured interactions and apparent mutualism

Direct and indirect interactions between two prey species can strongly alter the dynamics of predator-prey systems. Most predators are cannibalistic, and as a consequence, even systems with only one predator and one prey include two prey types: conspecifics and heterospecifics. The effects of the complex direct and indirect interactions that emerge in such cannibalistic systems are still poorly understood. This study examined how the indirect interaction between conspecific and heterospecific prey affects cannibalism and predation rates and how the direct interactions between both species indirectly alter the effect of the cannibalistic predator. I tested for these effects using larvae of the stream salamanders Eurycea cirrigera (prey) and Pseudotriton ruber (cannibalistic predator) by manipulating the relative densities of the conspecific and heterospecific prey in the presence and absence of the predator in experimental streams. The rates of cannibalism and heterospecific predation were proportional to the respective densities and negatively correlated, indicating a positive indirect interaction between conspecific and heterospecific prey, similar to "apparent mutualism." Direct interactions between prey species did not alter the effect of the predator. Although both types of prey showed a similar 30% reduction in night activity and switch in microhabitat use in response to the presence of the predator, cannibalism rates were three times higher than heterospecific predation rates irrespective of the relative densities of the two types of prey. Cumulative predation risks differed even more due to the 48% lower growth rate of conspecific prey. Detailed laboratory experiments suggest that the 3:1 difference in cannibalism and predation rate was due to the higher efficiency of heterospecific prey in escaping immediate attacks. However, no difference was observed when the predator was a closely related salamander species, Gyrinophilus porphyriticus, indicating that this difference is species specific. This demonstrates that cannibalism can result in the coupling of predator and prey mortality rates that strongly determines the dynamics of predator-prey systems.     

Predicting functional response and size selectivity of juvenile Notonecta maculata foraging on Daphnia magna

A mechanistic model was developed to assess the impact of predation of juvenile Notonecta maculata on size structured Daphnia magna populations and to provide a framework for quantifying the backswimmers uptake of food. Results of experiments and model predictions clearly demonstrate selective predation of backswimmers when fed with a choice of daphnid size classes, with patterns of selectivity differing across N. maculata instars. The model describes the foraging process empirically on the base of a general predation cycle including four conditional events instead of using classic functional response curves. For model parameterisation components of predation, namely probability of encounter, attack and success as well as time spent on handling prey was directly observed by means of video tracking experiments. Since attack rate, capture success and handling time appeared to be a function of prey size differing between Notonecta instars, the model takes into account ontogenic changes in both predator and prey characteristics. Independent data of functional response and size selectivity experiments were used for model validation and proved the model outcome to be consistent with observations.     

The influence of sexual selection and predation on the mating and postcopulatory guarding behavior of stone crabs (Xanthidae,Menippe)

Summary Postcopulatory mate guarding in crustaceans traditionally has been viewed as a behavioral mechanism that prevents predation on the soft post-molt female. This study tests the effects of sexual selection and predation on the postcopulatory guarding durations of male stone crabs, Menippe mercenaria, M. adina, and their hybrid. Male stone crabs were held with a pre-molt female, and either another adult male stone crab, an intermolt female, or a male blue crab, which corresponded to intermale competition, control, and predation treatments, respectively. The mating behavior of the heterosexual pair was recorded with a time lapse video system and the durations of copulation and postcopulatory guarding were measured. Males guarded longer in the intermale competition treatment than either the control or predation treatments. In the competition treatment, agonistic encounters occurred between the males at the den containing the female and several mate takeovers occurred. Females survived the predation treatment in trials in which the guarding durations were the longest, whereas females were eaten by the blue crab in trials with the shortest guarding durations. Sexual selection appears to be important in maintaining postcopulatory mate guarding in stone crabs.     

Patterns of Nest Predation on Artificial and Natural Nests in Forests

Abstract:  Artificial nest experiments have been used in an attempt to understand patterns of predation affecting natural nests. A growing body of literature suggests that neither relative rates nor patterns of predation are the same for artificial and natural nests. We studied nest predation and daily mortality rates and patterns at real and artificial ground and shrub nests to test the validity of artificial nest experiments. We monitored 1667 artificial and 344 natural nests, over seven trials, in three regions, across 58 sites in Ontario. We controlled for many of the factors thought to be responsible for previously reported differences between predation rates on natural and artificial nests. Although artificial nests in our study resembled natural nests, contained eggs of appropriate size, shape, and color of target bird species, and were placed in similar microhabitats as natural nests, the rates of predation on these nests did not parallel rates on natural nests for any region in terms of absolute rate or pattern. Predation rates on artificial nests did not vary between years, as they tended to for natural nests, and the magnitude of predation pressure on artificial ground nests compared with shrub nests did not show the same pattern as that on natural nests. In general, rates of predation on artificial nests were significantly higher than on natural nests. Our results suggest that conclusions derived from artificial nest studies may be unfounded. Given that many influential ideas in predation theory are based on results of artificial nest experiments, it may be time to redo these experiments with natural nests.     

Meta-analysis of foraging and predation risk trade-offs in terrestrial systems

Although there is ample evidence for the generality of foraging and predation trade-offs in aquatic systems, its application to terrestrial systems is less comprehensive. In this review, meta-analysis was used to analyze experiments on giving-up-densities in terrestrial systems to evaluate the overall magnitude of predation risk on foraging behavior and experimental conditions mediating its effect. Results indicate a large and significant decrease in foraging effort as a consequence of increased predation risk. Whether experiments were conducted under natural or artificial conditions produced no change in the overall effect predation had on foraging. Odor and live predators as a correlate of predation risk had weaker and nonsignificant effects compared to habitat characteristics. The meta-analysis suggests that the effect of predation risk on foraging behavior in terrestrial systems is strongly dependent on the type of predation risk being utilized.     

Behavioral resource depression and decaying perceived risk of predation in two species of coexisting gerbils

Summary Behavioral resource depression occurs when the behavior of prey individuals changes in response to the presence of a predator, resulting in a reduction of the encounter rate of the predator with its prey. Here I present experimental evidence on the response of two species of gerbils (Gerbillus allenbyi and G. pyramidum) to the presence of barn owls. I conducted the experiments in a large aviary. Both gerbils responded to the presence of barn owl predators by foraging in fewer resource patches (seed trays) and by quitting foraged resource patches at a higher resource harvest rate (giving-up density of resource; GUD). This reduced the amount of time gerbils were exposed to owl predation, and hence the encounter rate of owls with gerbils, i.e., behavioral resource depression. Thus, the presence of owls imposes a foraging cost on gerbils due to risk of predation, and also on the owls themselves due to resource depression. I then examined how resource depression relaxed over time following exposure to owls. In the days following an encounter with the predator, the reduction in foraging activity for both gerbil species eased. Increasing numbers of trays were foraged each day, and GUDs in seed trays declined. The two gerbils differed in their rate of recovery, with G. pyramidum returning to prepredator levels of foraging after 1 or 2 nights and G. allenbyi taking 5 nights or longer. Interspecific differences in recovery rates may be based on differences between the species in vulnerability to predation and/or ability to detect the presence of predators. The differences in recovery rates may be due to optimal memory windows or decay rates, where differences between species are based on risk of predation or on how perceived risk changes with time since a predator was last encountered. Finally, differences between or among competitors in recovery from resource depression may provide foraging opportunities in time for the species which recover most quickly and may have implications for species coexistence.     

Linking marine predator diving behavior to local prey fields in contrasting habitats in a subarctic glacial fjord

Foraging theory predicts that animals will adjust their foraging behavior in order to maximize net energy intake and that trade-offs may exist that can influence their behavior. Although substantial advances have been made with respect to the foraging ecology of large marine predators, there is still a limited understanding of how predators respond to temporal and spatial variability in prey resources, primarily due to a lack of empirical studies that quantify foraging and diving behavior concurrently with characteristics of prey fields. Such information is important because changes in prey availability can influence the foraging success and ultimately fitness of marine predators. We assessed the diving behavior of juvenile female harbor seals (Phoca vitulina richardii) and prey fields near glacial ice and terrestrial haulout sites in Glacier Bay (58°40′N, ?136°05′W), Alaska. Harbor seals captured at glacial ice sites dived deeper, had longer dive durations, lower percent bottom time, and generally traveled further to forage. The increased diving effort for seals from the glacial ice site corresponded to lower prey densities and prey at deeper depths at the glacial ice site. In contrast, seals captured at terrestrial sites dived shallower, had shorter dive durations, higher percent bottom time, and traveled shorter distances to access foraging areas with much higher prey densities at shallower depths. The increased diving effort for seals from glacial ice sites suggests that the lower relative availability of prey may be offset by other factors, such as the stability of the glacial ice as a resting platform and as a refuge from predation. We provide evidence of differences in prey accessibility for seals associated with glacial ice and terrestrial habitats and suggest that seals may balance trade-offs between the costs and benefits of using these habitats.     

Within-group spatial position in ring-tailed coatis: balancing predation, feeding competition, and social competition

A variety of factors can influence an individual’s choice of within-group spatial position. For terrestrial social animals, predation, feeding success, and social competition are thought to be three of the most important variables. The relative importance of these three factors was investigated in groups of ring-tailed coatis (Nasua nasua) in Iguazú, Argentina. Different age/sex classes responded differently to these three variables. Coatis were found in close proximity to their own age/sex class more often than random, and three out of four age/sex classes were found to exhibit within-group spatial position preferences which differed from random. Juveniles were located more often at the front edge and were rarely found at the back of the group. Juveniles appeared to choose spatial locations based on feeding success and not predation avoidance. Since juveniles are the most susceptible to predation and presumably have less prior knowledge of food source location, these results have important implications in relation to predator-sensitive foraging and models of democratic group leadership. Subadults were subordinate to adult females, and their relationships were characterized by high levels of aggression. This aggression was especially common during the first half of the coati year (Nov–April), and subadults were more peripheralized during this time period. Subadults likely chose spatial positions to avoid aggression and were actively excluded from the center of the group by adult females. In the Iguazú coati groups, it appeared that food acquisition and social agonism were the major determinants driving spatial choice, while predation played little or no role. This paper demonstrates that within-group spatial structure can be a complex process shaped by differences in body size and nutritional requirements, food patch size and depletion rate, and social dominance status. How and why these factors interact is important to understanding the costs and benefits of sociality and emergent properties of animal group formation.     

Patterns of co-occurrence and interactions between age classes of the common triplefin,Forsterygion lapillum

Many marine organisms have pelagic larval stages that settle into benthic habitats occupied by older individuals; however, a mechanistic understanding of intercohort interactions remains elusive for most species. Patterns of spatial covariation in the densities of juvenile and adult age classes of a small temperate reef fish, the common triplefin (Forsterygion lapillum), were evaluated during the recruitment season (Feb–Mar, 2011) in Wellington, New Zealand (41°17′S, 174°46′E). The relationship between juvenile and adult density among sites was best approximated by a dome-shaped curve, with a negative correlation between densities of juveniles and adults at higher adult densities. The curve shape was temporally variable, but was unaffected by settlement habitat type (algal species). A laboratory experiment using a “multiple-predator effects” design tested the hypothesis that increased settler mortality in the presence of adults (via enhanced predation risk or cannibalism) contributed to the observed negative relationship between juveniles and adults. Settler mortality did not differ between controls and treatments that contained either one (p = 0.08) or two (p = 0.09) adults. However, post hoc analyses revealed a significant positive correlation between the mean length of juveniles used in experimental trials and survival of juveniles in these treatments, suggesting that smaller juveniles may be vulnerable to cannibalism. There was no evidence for risk enhancement or predator interference when adults were present alongside a heterospecific predator (F. varium). These results highlight the complex nature of intercohort relationships in shaping recruitment patterns and add to the growing body of literature recognizing the importance of age class interactions.     

Species-specific antipredatory behaviours: effects on prey choice in different habitats

Summary Prey species may use many different behaviours to avoid predation. In this study, the antipredator behaviours of juvenile roach (Rutilus rutilus) and juvenile perch (Perca fluviatilis) were studied in wading pools with three kinds of structural complexity: no structure, structure simulating vegetation and structure simulating bottom crevices. Predation experiments with piscivorous perch and habitat choice experiments with the prey were performed, and the foraging success and prey choice of the predators were related to the type of structure. Predator foraging success was lower in the vegetation than in the other treatments. In the absence of structure and with vegetation structure, predators preferred perch over roach, while the preference was reversed in the crevice treatment. Roach and perch differed in their antipredatory behaviours. Roach responded to the presence of predators by schooling, moving fast and remaining at the surface, and escaped from attacks by jumping out of the water. In contrast, perch moved more slowly, dispersed after attacks and tried to hide at the bottom. Perch always preferred the vegetation structure to the non-structured part of the pool, while roach showed preference for the vegetation structure only when predators were present. Roach never occurred in crevices, whereas perch used crevices when predators where present. Predator pursuit speed was lower in the vegetation structure than in the non-structured treatment, but prey escape speed was unaffected. The results suggest that both the quantity and quality of structural complexity interacting with species-specific antipredator behaviours are important for predator-prey dynamics. It is also suggested that the presence of structure can have substantial effects on the structure of North Eurasian fish communities, by affecting relative and absolute predation pressures from piscivorous perch on prey species. Correspondence to: B. Christensen     

Experimental examination of behavioural interactions between free-ranging wild and domestic canids

The structure of mammalian carnivore communities is strongly influenced by both intraguild competition and predation. However, intraguild interactions involving the world’s most common carnivore, the domestic dog (Canis familiaris), have rarely been investigated. We experimentally examined the behavioural responses of a small canid, the Indian fox (Vulpes bengalensis), to the presence of dogs and dog odours. Resource competition between dogs and Indian foxes is low, so it is unclear whether foxes perceive dogs as interference competitors. To test this, we exposed foxes to neutral, live dog, and animal odour stimuli at food trays, and recorded the time spent at food trays, the amount of food eaten, and vigilance and non-vigilance behaviours. When dogs were visible, foxes continued to visit the food trays, but reduced the amount of time spent and food eaten at those trays. Foxes were more vigilant during dog trials than during neutral and odour trials and also exhibited lower levels of non-vigilance behaviour (resting and playing). In contrast, dog odours did not affect fox foraging and activity. These results show that vigilance/foraging trade-offs due to interference competition can occur between native and domestic carnivores despite low dietary overlap. These negative effects of dogs on a smaller member of the carnivore guild raise conservation concerns, especially for endangered carnivores. In many parts of the world, free-ranging dog densities are high due to human subsidies, and these subsidized predators have the potential to exacerbate the indirect effects of human presence.     

Foraging behavior of juvenile Carcinus maenas (L.) and Cancer pagurus L.

Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.     

The use of simulation modeling to evaluate the mechanisms responsible for the nutritional benefits of food-for-protection mutualisms

In some mutualisms, a plant or insect provides a food resource in exchange for protection from herbivores, competitors or predators. This food resource can benefit the consumer, but the relative importance of different mechanisms responsible for this benefit is unclear. We used a colony-level simulation model to test the relative importance of increased larval production, increased worker foraging and increased worker survival for colony growth of fire ants, Solenopsis invicta, that consume plant-based foods. Increased food for larvae had the largest effect on colony growth of S. invicta followed by decreased worker mortality. Increased foraging rate had a small effect in the simulation model but data from a small laboratory experiment and another published study suggest that plant-based foods have little or no effect on foraging rates of S. invicta. Colony growth steadily increased the longer plant-based food was available and colonies were most responsive to plant-based food in the early summer (i.e., June). Our results demonstrate that population level simulation modeling can be a useful tool for examining the ecology of mutualistic interactions and the mechanisms through which species interact.     

Behavioral aspects of competition in a three-species rodent guild of coastal southern California

Summary The influence of vole cycles on the demography, spatial organization, and abundance of potential rodent competitors was studied in three California rodents in the laboratory and in the field. Population densities ofMicrotus californicus were inversely correlated with reproductive success in two potential competitors (Reithrodontomys megalotis andMus musculus). Additionally, high-density vole populations forced these species into suboptimal habitats. During low-density vole populations,Reithrodontomys andMus left their refugia, expanded their usage of habitat, were more active, and their reproductive success was greater. Thus, this work suggests that during vole population peaks, competition with sympatric species of mice is severe. Conversely, when vole populations decline, competition is relaxed. The temporal and spatial organization of the three-species complex supports models of nonequilibrium and fugitive species coexistence.     

Patch use as an indicator of habitat preference,predation risk,and competition

Summary A technique for using patch giving up densities to investigate habitat preferences, predation risk, and interspecific competitive relationships is theoretically analyzed and empirically investigated. Giving up densities, the density of resources within a patch at which an individual ceases foraging, provide considerably more information than simply the amount of resources harvested. The giving up density of a forager, which is behaving optimally, should correspond to a harvest rate that just balances the metabolic costs of foraging, the predation cost of foraging, and the missed opportunity cost of not engaging in alternative activities. In addition, changes in giving up densities in response to climatic factors, predation risk, and missed opportunities can be used to test the model and to examine the consistency of the foragers' behavior. The technique was applied to a community of four Arizonan granivorous rodents (Perognathus amplus, Dipodomys merriami, Ammospermophilus harrisii, and Spermophilus tereticaudus). Aluminum trays filled with 3 grams of millet seeds mixed into 3 liters of sifted soil provided resource patches. The seeds remaining following a night or day of foraging were used to determine the giving up density, and footprints in the sifted sand indicated the identity of the forager. Giving up densities consistently differed in response to forager species, microhabitat (bush versus open), data, and station. The data also provide useful information regarding the relative foraging efficiencies and microhabitat preferences of the coexisting rodent species.