Habitat complexity dampens selection on prey activity level

Conspecific prey individuals often exhibit persistent differences in behavior (i.e., animal personality) and consequently vary in their susceptibility to predation. How this form of selection varies across environmental contexts is essential to predicting ecological and evolutionary dynamics, yet remains currently unresolved. Here, we use three separate predator–prey systems (sea star–snail, wolf spider–cricket, and jumping spider–cricket) to independently examine how habitat structural complexity influences the selection that predators impose on prey behavioral types. Prior to conducting staged predator–prey interaction encounters, we ran prey individuals through multiple behavioral assays to determine their average activity level. We then allowed individual predators to interact with groups of prey in either open or structurally complex habitats and recorded the number and individual identity of prey that were eaten. Habitat complexity had no effect on overall predation rates in any of the three predator–prey systems. Despite this, we detected a pervasive interaction between habitat structure and individual prey activity level in determining individual prey survival. In open habitats, all predators imposed strong selection on prey behavioral types: sea stars preferentially consumed sedentary snails, while spiders preferentially consumed active crickets. Habitat complexity dampened selection within all three systems, equalizing the predation risk that active and sedentary prey faced. These findings suggest a general effect of habitat complexity that reduces the importance of prey activity level in determining individual predation risk. We reason this occurs because activity level (i.e., movement) is paramount in determining risk within open environments, whereas in complex habitats, other behavioral traits (e.g., escape ability to a refuge) may take precedence.     

The fear diet: Risk,refuge, and biological control by omnivorous weed seed predators

Weed seed biocontrol by omnivorous mice and insects can limit weed seedbanks, but this ecosystem service can be difficult to predict given the broad diet breadth of seed predators and their potential for intraguild predation. Seed foraging behavior is further modified by fluctuating cues of predation risk from higher trophic levels and the availability of refuge habitat. Uncertainty about whether co-occurring insects and mice additively contribute to weed biocontrol or interfere with each other via intraguild predation limits our ability to recommend habitat management strategies that reliably promote seed destruction. Using seed removal assays, fluorescent powder tracking, and stable isotope analyses, we assessed effects of a predation risk cue (moonlight) on mouse foraging patterns in a patchwork of vegetated and exposed plots in a cultivated field. Mouse foraging activity decreased on exposed ground during the full moon, compared to dark nights, yet foraging movements were unaffected by moon cycle within refuge patches. Weed seed consumption was more than three times higher in cover than exposed soil, and 78% of that difference was attributable to invertebrate granivores. Mice and invertebrate granivores both exhibited higher foraging activity in cover, indicating co-occurrence of intraguild predators and prey. However, stable isotope analyses of fecal samples revealed that mice captured in refuge habitats fed at slightly lower trophic levels than those in exposed habitats (suggesting minimal intraguild predation in refuge habitat), and mouse diet was unaffected by moonlight. Despite increased availability of invertebrate prey in cover patches, mice do not appear to preferentially exploit prey when avoiding their own predators or interfere with weed seed predation. Therefore, functional redundancy of mice and invertebrate seed predators in cover crops and other refuge habitats may strengthen and stabilize weed seed biocontrol.     

When perception reflects reality: Non‐native grass invasion alters small mammal risk landscapes and survival

Modification of habitat structure due to invasive plants can alter the risk landscape for wildlife by, for example, changing the quality or availability of refuge habitat. Whether perceived risk corresponds with actual fitness outcomes, however, remains an important open question. We simultaneously measured how habitat changes due to a common invasive grass (cheatgrass, Bromus tectorum) affected the perceived risk, habitat selection, and apparent survival of a small mammal, enabling us to assess how well perceived risk influenced important behaviors and reflected actual risk. We measured perceived risk by nocturnal rodents using a giving‐up density foraging experiment with paired shrub (safe) and open (risky) foraging trays in cheatgrass and native habitats. We also evaluated microhabitat selection across a cheatgrass gradient as an additional assay of perceived risk and behavioral responses for deer mice (Peromyscus maniculatus) at two spatial scales of habitat availability. Finally, we used mark‐recapture analysis to quantify deer mouse apparent survival across a cheatgrass gradient while accounting for detection probability and other habitat features. In the foraging experiment, shrubs were more important as protective cover in cheatgrass‐dominated habitats, suggesting that cheatgrass increased perceived predation risk. Additionally, deer mice avoided cheatgrass and selected shrubs, and marginally avoided native grass, at two spatial scales. Deer mouse apparent survival varied with a cheatgrass–shrub interaction, corresponding with our foraging experiment results, and providing a rare example of a native plant mediating the effects of an invasive plant on wildlife. By synthesizing the results of three individual lines of evidence (foraging behavior, habitat selection, and apparent survival), we provide a rare example of linkage between behavioral responses of animals indicative of perceived predation risk and actual fitness outcomes. Moreover, our results suggest that exotic grass invasions can influence wildlife populations by altering risk landscapes and survival.     

Predation risk predicts use of a novel habitat

Predator–prey interactions are often highly co‐evolved, with selection over time for prey with morphological and behavioral traits that minimize predation risk. Consequently, in many environments prey choose among potential habitats according to their refuge value. It is unclear, however, when presented with new habitats, if prey are able to evaluate the predation risk of these relative to familiar habitats and utilize these in accordance with their value. We tested whether, along the east coast of the USA, native mud crabs Panopeus herbstii utilize the non‐native alga Gracilaria vermiculophylla according to its relative refuge value. Experiments examining predation by blue crabs Callinectes sapidus on mud crabs revealed that the non‐native alga had an intermediate refuge value relative to native oysters, which were the most protective, and unvegetated sediment, which was the least. In subsequent choice experiments, mud crabs selected oysters over alga over unvegetated sediment, in accordance with habitat refuge values. Further, in field experiments, the use of Gracilaria by mud crabs was inversely related to the proximity of the alga to the preferred habitat type, oysters, and was reduced by the presence of a blue crab predator. Consequently, mud crabs are utilizing the non‐native alga Gracilaria in accordance with its intermediate refuge value. The relative refuge value of non‐native vs native habitat‐forming species may provide a baseline expectation against which to measure the speed of learning and opportunism in the response of native prey to novel protective habitats.     

Direct and indirect effects of predation on mosquitofish behavior and survival

Predation can have strong direct and indirect effects on the behavior of prey. We investigated whether predation by chain pickerel (Esox niger) caused adult eastern mosquitofish (Gambusia holbrooki) to alter their habitat use and whether pickerel predation influenced survival of adult and neonate mosquitofish. The number of adult mosquitofish using the riskier of three habitats was lowest when two predators occupied the risky habitat, intermediate in the treatment with one predator, and highest when no predators occurred there. More mosquitofish neonates survived high predation treatments than treatments lacking pickerel. We conclude that pickerel predation causes adult mosquitofish to shift to refuge habitats. The pattern of neonate survival suggests that adult habitat use may create a refuge from cannibalism for neonate mosquitofish, resulting in higher neonate survival in treatments with more pickerel. Hence, pickerel predation has a direct effect on adult mosquitofish behavior and a strong indirect effect on neonate survival. Both interspecific and intraspecific predation can effect prey populations and can interact to produce important indirect effects.     

Habitat overlap of enemies: temporal patterns and the role of spatial complexity

Environmental heterogeneity can promote coexistence of conflicting species by providing spatial or temporal refuges from strong interactions (e.g., intraguild predation, competition). However, in many systems, refuge availability and effectiveness may change through time and space because of variability in habitat use by either species. Here I consider how the intensity of intraguild predation risk varies from day to night for aquatic insects that use both vegetated and open water habitats. Large (1,265 l) and small (42 l) mesocosms were used to test the hypothesis that Buenoa would choose an open-water habitat that minimized predation by the ambush predator Notonecta during the day, but that at night Buenoa would safely use both vegetated and open water. Regardless of container size, Notonecta remained in vegetated water during the day and exploited both habitats at night, despite exhibiting greatest instantaneous predation rates in open water during the day. In contrast, Buenoa maintained an even distribution throughout the mesocosms during day and night, even though habitat-specific predation risks were fivefold lower in open waters than in vegetation during the day and habitat-specific predation risk would have been reduced threefold by fully exploiting open waters. Thus, temporal heterogeneity was both beneficial and detrimental to Buenoa; darkness of night reduced predation, but spatial refuges also disappeared. Together, these patterns suggest that while environmental heterogeneity can dampen intense biotic interactions, enemies do not select habitats solely on the basis of conflict avoidance. Instead, it appears that habitat-specific variation in other biotic (e.g., visual predators) or physical factors (e.g., UV radiation) may also mediate species interactions by influencing habitat selection.     

Inter‐ and intraspecific effects of body size on habitat use among sexually‐dimorphic macropodids

Body size affects key life‐history parameters including dietary requirements and predation risk. We examined these effects on diel habitat use in a community of three sexually‐dimorphic macropodid marsupial species: western grey kangaroo Macropus fuliginosus, red‐necked wallaby M. rufogriseus and swamp wallaby Wallabia bicolor. In particular, our study seeks evidence of these effects operating concurrently at the intra‐ and interspecific levels. We used radio‐tracking to quantify habitat use and characterised each used location by recording the cover of plant functional groups and the presence of plant species. During nocturnal foraging periods we predicted that smaller animals (between and within species) should use habitats with higher‐quality forage, which is often less abundant, than larger animals, as metabolic demand scales with body size. During diurnal resting periods we predicted that smaller animals (between and within species), being more vulnerable to predation, should use greater concealment cover than larger animals. Western grey kangaroos and swamp wallabies behaved as predicted during foraging periods, but red‐necked wallabies did not, using more open, poorer‐quality habitats than expected. Only western grey kangaroos showed a within‐species effect on habitat use: the relatively smaller females foraged in higher‐quality patches. Habitats used by animals during the resting period generally offered greater concealment cover than those used during the foraging period, but there were no clear body size effects on the density of vegetation used. In our system, body size alone could not explain all of the observed patterns, suggesting that there may also be individual differences in habitat requirements influenced by factors such as reproductive costs, predation risk and social facilitation.     

Mechanisms of species coexistence: a field test of theoretical models using intertidal snails

Competitor coexistence is often facilitated by spatial segregation. Traditionally, spatial segregation is predicted to occur when species differ in the habitat in which they are either superior at competing for resources or less susceptible to predation. However, predictions from a behavioural model demonstrate that spatial segregation and coexistence can also occur in the absence of such interspecific trade‐offs in competitive ability and vulnerability to predation. Unlike other models of competitor coexistence this model predicts that when species rank both habitat productivity and ‘riskinesses’ similarly, but differ slightly in their habitat‐specific vulnerabilities to predators, they will tend to segregate across habitats, with the species experiencing the higher ratio of mortality risk across the habitats occurring primarily in the safer habitat. Here, we investigate the hypothesis that intraspecific trade‐offs between resource availability and mortality risk can lead to spatial segregation of competing species by (1) documenting the spatial (i.e. intertidal) distribution of two marine snails, Littorina sitkana and L. subrotundata and (2) performing field experiments to quantify growth and mortality rates of each species at ‘low’ and ‘high’ intertidal heights. Our results indicate that both species agree on the rankings of habitat riskiness and productivity, experiencing higher predation and higher growth in low‐ than in high‐intertidal habitats. However, L. sitkana and L. subrotundata experienced differences in their habitat‐specific mortality risks and growth rates. Despite both species being similarly at risk of predation in high‐intertidal habitats (where mortality was lower), L. subrotundata was subject to significantly higher mortality than L. sitkana at the low‐intertidal height. In contrast, growth rate differences between habitats were greater for L. sitkana than for L. subrotundata. Whereas both species grew at the same rate at the high‐intertidal level (where growth was lower), L. sitkana individuals grew more rapidly than L. subrotundata snails at the low‐intertidal level. As predicted by the behavioural model, the species that experienced the higher ratio of mortality across habitats (i.e. L. subrotundata) occurred exclusively in the safer, high‐intertidal habitat. Taken together, these results provide support for the hypothesis that spatial segregation, and potentially competitor coexistence, can occur in the absence of interspecific trade‐offs in resource acquisition ability or vulnerability to predation.     

Ecological effects of fear: How spatiotemporal heterogeneity in predation risk influences mule deer access to forage in a sky‐island system

Forage availability and predation risk interact to affect habitat use of ungulates across many biomes. Within sky‐island habitats of the Mojave Desert, increased availability of diverse forage and cover may provide ungulates with unique opportunities to extend nutrient uptake and/or to mitigate predation risk. We addressed whether habitat use and foraging patterns of female mule deer (Odocoileus hemionus) responded to normalized difference vegetation index (NDVI), NDVI rate of change (green‐up), or the occurrence of cougars (Puma concolor). Female mule deer used available green‐up primarily in spring, although growing vegetation was available during other seasons. Mule deer and cougar shared similar habitat all year, and our models indicated cougars had a consistent, negative effect on mule deer access to growing vegetation, particularly in summer when cougar occurrence became concentrated at higher elevations. A seemingly late parturition date coincided with diminishing NDVI during the lactation period. Sky‐island populations, rarely studied, provide the opportunity to determine how mule deer respond to growing foliage along steep elevation and vegetation gradients when trapped with their predators and seasonally limited by aridity. Our findings indicate that fear of predation may restrict access to the forage resources found in sky islands.     

Quantifying habitat structure: surface convolution and living space for species in complex environments

Habitat complexity is often used to explain the distribution of species in environments, yet the ability to predict outcomes of structural differences between habitats remains elusive. This stems from the difficulty and lack of consistency in measuring and quantifying habitat structure, making comparison between different habitats and systems problematic. For any measure of habitat structure to be useful it needs to be applicable to a range of habitats and have relevance to their associated fauna. We measured three differently‐shaped macrophyte analogues with nine indices of habitat structure to determine which would best distinguish between their shape and relate to the abundance and rarefied species richness of their associated macroinvertebrate assemblages. These indices included the physical, whole‐plant attributes of surface area (SA) and plant volume (PV), the interstitial space attributes of average space size and frequency (ISI), average refuge space from predation (Sp/Pr), and total refuge space (FFV), and the degree of surface convolution at a range of scales (i.e. the fractal dimension at four spatial scales: 7.5×, 5×, 2.5× and 1× magnification). We found a high degree of inter‐correlation between the structural indices such that they could be organised into two suites: one group describing interstitial space and surface convolution at coarse scales, the other describing whole‐plant attributes and surface convolution at fine scales. Two of these indices fell into both suites: the average refuge space from predation (Sp/Pr) and the fractal dimension at 5× magnification. These two measures were also strongly related to macroinvertebrate abundance and rarefied species richness, which points to their usefulness in quantifying habitat structure and illustrates that habitat structure depends not just on shape, but on the space associated with shape.     

Color change and color-dependent behavior in response to predation risk in the salamander sister species<Emphasis Type="Italic"> Ambystoma barbouri</Emphasis> and<Emphasis Type="Italic"> Ambystoma texanum</Emphasis>

Although many organisms show multiple types of trait responses to predation risk (e.g., shifts in behavior, morphology, color, chemistry or life history), relatively few studies have examined how prey integrate these multiple responses. We studied the joint expression of color and behavioral responses to predation risk in two sister species of salamander larvae that live in habitats with different selection pressures. We examined responses to predation risk in three situations that differed in availability of refuge and substrate color heterogeneity, and thus availability of behavioral options for reducing risk. Relative to Ambystoma texanum, A. barbouri larvae were darker in color and showed a greater range of color change. With no variation in background color or refuge available, both species exhibited color change to better match the available background. The degree of color change showed by both species, however, did not depend on predation risk. Given the option to choose between light and dark substrates, A. texanum exhibited behavioral background matching (i.e., they preferred substrates that matched their own body color), while A. barbouri's substrate preferences did not depend on their initial body color. Instead, A. barbouri responded to risk by showing a strong preference for dark substrates, followed by a change to a darker body color. With refuge available, A. texanum's refuge use was color-dependent; larvae that were well camouflaged spent less time in refuge. In contrast, A. barbouri showed strong refuge use in response to risk, regardless of their body color. Overall, these results reflect how conflicting selection pressures (predation risk, habitat ephemerality, risk of UV damage) and species differences in mean color and ability to change color can govern the interplay of complementary and compensatory behavioral and color responses to predation risk.     

Effects of Habitat on Competition Between Kit Foxes and Coyotes

Abstract: San Joaquin kit foxes (Vulpes macrotis mutica) are an endangered species with a narrow geographic range whose natural populations are limited by predation by coyotes (Canis latrans). In the warm, arid grassland and shrubland habitats where kit foxes occur, coyotes are more cover dependent than kit foxes, creating the possibility of habitat segregation. Effects of habitat variation on coyote and kit fox competition are unknown. We assessed exploitation and interference competition between coyotes and kit foxes in grassland and shrubland habitats to determine if such competition varies among habitats. With respect to exploitation competition, we evaluated habitat and spatial partitioning, diet, prey abundance, and survival for kit foxes and coyotes at the Lokern Natural Area in central California, USA, from January 2003 through June 2004. Kit foxes partitioned habitat, space, and diet with coyotes. Coyotes primarily used shrubland habitats whereas kit foxes selectively used burned grasslands. Kit foxes and coyotes had high dietary overlap with regards to items used, but proportional use of items differed between the 2 species. Kit foxes selected for Heermann's kangaroo rats (Dipodomys heermanni), which were closely tied to shrub habitats. With respect to interference competition, predation was the primary source of mortality for kit foxes, and survival of individual kit foxes was inversely related to proportion of shrub habitat within their home ranges. Our results suggest that a heterogeneous landscape may benefit kit foxes by providing habitat patches where predation risk may be lower.     

Habitat formation prevails over predation in influencing fouling communities

Coastal human‐made structures, such as marinas and harbors, are expanding worldwide. Species assemblages described from these artificial habitats are novel relative to natural reefs, particularly in terms of the abundance of nonindigenous species (NIS). Although these fouling assemblages are clearly distinctive, the ecosystem functioning and species interactions taking place there are little understood. For instance, large predators may influence the fouling community development either directly (feeding on sessile fauna) or indirectly (feeding on small predators associated with these assemblages). In addition, by providing refuges, habitat complexity may modify the outcome of species interactions and the extent of biotic resistance (e.g., by increasing the abundance of niche‐specific competitors and predators of NIS). Using experimental settlement panels deployed in the field for 2.5 months, we tested the influence of predation (i.e., caging experiment), artificial structural complexity (i.e., mimics of turf‐forming species), and their interactions (i.e., refuge effects) on the development of sessile and mobile fauna in two marinas. In addition, we tested the role of biotic complexity—arising from the habitat‐forming species that grew on the panels during the trial—on the richness and abundance of mobile fauna. The effect of predation and artificial habitat complexity was negligible, regardless of assemblage status (i.e., native, cryptogenic, and nonindigenous). Conversely, habitat‐forming species and associated epibionts, responsible for biotic complexity, had a significant effect on mobile invertebrates (richness, abundance, and community structure). In particular, the richness and abundance of mobile NIS were positively affected by biotic complexity, with site‐dependent relationships. Altogether, our results indicate that biotic complexity prevails over artificial habitat complexity in determining the distribution of mobile species under low predation pressure. Facilitation of native and non‐native species thus seems to act upon diversity and community development: This process deserves further consideration in models of biotic resistance to invasion in urban marine habitats.     

根田鼠栖息地选择的影响因素

2002 年7～9 月在中国科学院海北高寒草甸生态系统定位站地区对根田鼠的栖息地选择进行了研究。对根田鼠栖息地利用强度变量(一定面积内的跑道长度、跑道分叉数、洞口数) 与12 个栖息地特征变量进行多元线性逐步回归分析表明: 显著影响跑道长度的变量为灌丛高度、竞争性啮齿类、电线杆、早熟禾、围栏、灌丛间距和双子叶植物生物量; 显著影响跑道分叉数的变量为灌丛高度、竞争性啮齿类、电线杆、早熟禾和围栏; 显著影响洞口数的变量为灌丛高度、土壤含水率和围栏。对影响利用强度变量的因素进行综合生态学分析表明,在小生境尺度上, 研究期间影响根田鼠栖息地选择的因素主要为捕食风险和种间竞争, 食物资源和土壤硬度也有一定程度的影响。     

Influence of Habitat and Intrinsic Characteristics on Survival of Neonatal Pronghorn

Increased understanding of the influence of habitat (e.g., composition, patch size) and intrinsic (e.g., age, birth mass) factors on survival of neonatal pronghorn (Antilocapra americana) is a prerequisite to successful management programs, particularly as they relate to population dynamics and the role of population models in adaptive species management. Nevertheless, few studies have presented empirical data quantifying the influence of habitat variables on survival of neonatal pronghorn. During 2002–2005, we captured and radiocollared 116 neonates across two sites in western South Dakota. We documented 31 deaths during our study, of which coyote (Canis latrans) predation (n = 15) was the leading cause of mortality. We used known fate analysis in Program MARK to investigate the influence of intrinsic and habitat variables on neonatal survival. We generated a priori models that we grouped into habitat and intrinsic effects. The highest-ranking model indicated that neonate mortality was best explained by site, percent grassland, and open water habitat; 90-day survival (0.80; 90% CI = 0.71–0.88) declined 23% when grassland and water increased from 80.1 to 92.3% and 0.36 to 0.40%, respectively, across 50% natal home ranges. Further, our results indicated that grassland patch size and shrub density were important predictors of neonate survival; neonate survival declined 17% when shrub density declined from 5.0 to 2.5 patches per 100 ha. Excluding the site covariates, intrinsic factors (i.e., sex, age, birth mass, year, parturition date) were not important predictors of survival of neonatal pronghorns. Further, neonatal survival may depend on available land cover and interspersion of habitats. We have demonstrated that maintaining minimum and maximum thresholds for habitat factors (e.g., percentages of grassland and open water patches, density of shrub patches) throughout natal home ranges will in turn, ensure relatively high (>0.50) neonatal survival rates, especially as they relate to coyote predation. Thus, landscape level variables (particularly percentages of open water, grassland habitats, and shrub density) should be incorporated into the development or implementation of pronghorn management plans across sagebrush steppe communities of the western Dakotas, and potentially elsewhere within the geographic range of pronghorn.     

Diel variation in predator abundance, predation risk and prey distribution in shallow-water estuarine habitats

Predation by visual predators is often affected by light conditions and may therefore exhibit strong diel variation. The dominant predators on grass shrimp, Palaemonetes pugio, are finfish predators that are thought to locate their prey by visual cues. We examined the response of grass shrimp to diel variation in predation risk in the nearshore shallow waters of the Chesapeake Bay. We used diel shoreline seines to assess the relative abundance of predators. We assessed the relative risk of predation with shrimp tethered at refuge (30 cm) and nonrefuge (60 cm) depths. To measure grass shrimp response to predation risk, we used dipnets to monitor habitat use. Four predominantly visual predators dominated the shoreline seine catches, Fundulus heteroclitus, Micropogonias undulatus, Morone americana and Morone saxatilis. Total predator abundance had a diel component, with dramatic nighttime decreases in total abundance, whereas guild composition and relative abundance remained unchanged. Relative predation risk for tethered shrimp exhibited significant time by habitat interaction. During the day, depth negatively affected survivorship of tethered shrimp while at night overall survivorship increased and there was no effect of depth. Shrimp habitats use reflected diel predation risks. Abundances in the near shore were highest during the day with decreased abundances at night. Together, the seine and tethering data highlight the importance for a refuge (e.g., shallow water) from predation during the daytime and a relaxation of predation pressure at night.     

Refuge or predation risk? Alternate ways to perceive hiker disturbance based on maternal state of female caribou

Human presence in natural environments is often a source of stress that is perceived by large ungulates as an increased risk of predation. Alternatively, disturbance induced by hikers creates a relatively predator‐free space that may serve as a refuge. We measured the behavioral responses of female caribou to disturbance associated with the presence of hikers during summer in the Gaspésie National Park. We used those data to determine whether caribou responded negatively to human activity (i.e., the predation risk hypothesis) or whether human activity resulted in a decrease in the magnitude of perceived risk (i.e., the refuge hypothesis). Female caribou with a calf spent nearly half of their time feeding, regardless of the presence of a trail or the number of hikers. They also decreased their vigilance near trails when the number of hikers increased. Conversely, lone females fed less frequently and almost doubled the time invested in vigilance under the same circumstances. However, both groups of females moved away from trails during the day, especially in the presence of hikers. We demonstrated that risk avoidance was specific to the maternal state of the individual. Lactating females accommodated the presence of hikers to increase time spent foraging and nutritional intake, providing support for the refuge hypothesis. Alternatively, lone females with lower energetic requirements and no maternal investment in a vulnerable calf appeared less tolerant to risk, consistent with the predation risk hypothesis. Synthesis and applications: Hikers influenced the vigilance–feeding trade‐off in caribou, underlining the importance of appropriate management of linear structures and human activities, especially across the critical habitat of endangered species. Even if some individuals seemed to benefit from human presence, this behavioral adaptation was not sufficient to reduce annual calf mortality associated with predation.     

Different nest predator faunas and nest predation risk on ground and shrub nests at forest ecotones: an experiment and a review

This study examined predator faunas of artificial ground and shrub nests and whether nest predation risk was influenced by nest site, proximity to forest edge, and habitat structure in 38 grassland plots in south-central Sweden. There was a clear separation of predator faunas between shrub and ground nests as identified from marks in plasticine eggs. Corvids accounted for almost all predation on shrub nests whereas mammals mainly depredated ground nests. Nest predation risk was significantly greater for shrub than for ground nests at all distances (i.e. 0, 15 and 30 m) from the forest edge. However, nest predation risk was not significantly related to distance to forest edge, but significantly increased with decreasing distance to the nearest tree. Different corvid species robbed nests at different distances from the forest edge, with jays robbing nests closest to edges. We conclude that the relationship between the predation risk of grassland bird nests and distance to the forest edge mainly depends on the relative importance of different nest predator species and on the structure of the forest edge zone. A review of published articles on artificial shrub and ground nest predation in the temperate zone corroborated the results of our own study, namely that shrub nests experienced higher rates of depredation in open habitats close to the forest edge and that avian predators predominantly robbed shrub nests. Furthermore, the review results showed that predation rates on nests in general are highest <50 m inside the forest and lower in open as well as forest interior habitats (≥50 m from the edge). Received: 16 March 1998 / Accepted: 30 July 1998     

The influence of habitat on travel speed, intermittent locomotion, and vigilance in a diurnal rodent

We studied effects of habitat structure on routine travel velocities, intermittent locomotion, and vigilance by the degu (Octodondegus), a diurnal rodent of central Chile. We predicted thattravel speed, pauses during locomotion, and vigilance wouldbe greater in open (riskier) than in shrub (safer) habitats.Video recordings of marked individuals in the wild were used to measure speed and other variables of spontaneous locomotionnot triggered by predatory attack or any other noticeable stimulusduring nonforaging periods. Time spent vigilant while foragingwas also measured. Because degus use bare-ground runways fordistant movements (e.g., between burrow openings and/or foodpatches), data on locomotion decisions were not confounded by effects of obstructive vegetation cover and/or resource abundance.When moving across the habitat between different feeding places,degus showed an intermittent pattern of locomotion, interruptingrunning events with short pauses. As predicted, travel speedand the duration of pauses between locomotion bursts were significantlygreater in open habitats. Further, the duration of locomotionbursts between feeding sites or between feeding sites and burrowswas significantly longer in open habitats. Our assumption that pauses and velocities are independent decisions was supportedby the lack of correlation between pauses and speeds duringlocomotion events. During foraging, degus devoted more timeto vigilance in open than in shrub habitats. The static positionadopted by degus during pauses, the speeds attained during movements, and the concordance between pausing behavior andvigilance across habitats suggest that pausing has an antipredatoryrole and is not limited to orientation and/or physiologicalrecovery. Our results support the view that degus perceivehigher predation risk in open areas and that flexible movement behavior reflects an adaptive antipredator response.     

Head ornaments in owls: what are their functions?

We studied head ornamentation in owls by focussing on the hypothesis that it may serve both intra- and interspecific communication. We carried out pairwise comparisons of morphological, ecological and behavioural traits on a subset of closely related species with contrasting head ornamentation patterns. We found that the degree of head ornamentation was associated with habitat and activity rhythm, highly ornamented species (e.g., most of the genus Bubo ) living preferentially in open habitats and being active by day, but not with body size. Although our results do not exclude the possibility that head ornaments may represent a warning mechanism to startle predators and mobbers, they nevertheless suggest that such signals have evolved for visual communication with conspecifics, being more expressed in species that may use them more effectively without incurring the risk of predation or disturbance.