Refuge Quality Research Articles

Refuge identification as a climate adaptation strategy to promote fish persistence during drought

AbstractClimate change is leading to global increases in extreme events, such as drought, that threaten the persistence of freshwater biodiversity. Identification and management of drought refuges, areas that promote resistance and resilience to drought, will be critical for preserving and recovering aquatic biodiversity in the face of climate change and increasing human water use. Although several reviews have addressed the effects of droughts and highlighted the role of refuges, a need remains on how to identify functional refuges that can be used in a drought management framework to support fish assemblages. We synthesize literature on drought refuges and propose a framework to identify and manage functional refuges that incorporate species physiological tolerances, behaviours and life‐history strategies. Stream pools, perennial reaches and off‐channel habitat were identified as important drought refuges for fish. The ability of refuges to improve species resistance and resilience to drought requires careful consideration of the biology of the target species and targeted management to promote persistence, quality and connectivity of refuges. Case studies illustrate that management of drought refuges can be challenging because of competing demands for water, incomplete knowledge of ecological requirements for target species and the increasing occurrence of multi‐year droughts. Climate adaptation is increasingly important, and drought refuges can increase fish resistance and resilience to climate‐related drought across the riverscape.

Risk of migration: not all gregarious locust nymphs reach appropriate refuges.

Sedentary animals choose appropriate refuges against predators, while migratory ones may not necessarily do so. In ectotherms, refuge selection is critical during low temperatures, because they cannot actively evade predators. To understand how migratory ectotherms alter their defensive behaviors depending on refuge quality in cold temperatures, we evaluated migratory gregarious desert locust nymphs (Schistocerca gregaria) in the Sahara Desert, where daily thermal constraints occur. We recorded how roosting plant type (bush/shrub) and its height influenced two alternative defense behaviors (dropping/stationary) during cold mornings, in response to an approaching simulated ground predator. Most locusts in bushes dropped within the bush and hid irrespective of their height, whereas those roosting > 2 m height in shrubs remained stationary. These defenses are effective and match with refuge plant types because dynamic locomotion is not required. When nymphs roosted on shrubs < 1.5-m height, which was an unsafe position, nearly half showed both defensive behaviors, indicating that escaping decisions become ambiguous when the refuges are inappropriate. These results suggest that locusts display flexible defensive behaviors when finding appropriate refuges and selecting refuge before daily thermal limitations occur could be critical for migratory ectotherms, which is a risk associated with migration.

Quality of thermal refuges influences use by the cold intolerant Florida manatee

Quality of thermal refuges influences use by the cold intolerant Florida manatee

Intrinsic traits, social context, and local environment shape home range size and fidelity of sleepy lizards

AbstractHome ranges (HRs), the regions within which animals interact with their environment, constitute a fundamental aspect of their ecology. HR sizes and locations commonly reflect costs and benefits associated with diverse social, biotic, and abiotic factors. Less is known, however, about how these factors affect intraspecific variation in HR size or fidelity (the individual's tendency to maintain the same HR location over time) or whether variation in these features emerge from consistent differences among individuals or among the sites they occupy. To address this knowledge gap, we used an extensive GPS‐tracking data set of a long‐lived lizard, the sleepy lizard (Tiliqua rugosa), which included repeated observations of multiple individuals across years. We tested how three categories of predictors—(1) lizard characteristics (sex, aggressiveness, and parasitic tick counts), (2) environmental characteristics (precipitation, food, and refuge quality), and (3) social conditions (conspecific overlap and number of neighbors)—affected HR size and fidelity. We found that individuals differed consistently in the size and fidelity of annual HRs (with a repeatability of 0.58 and 0.33, respectively), and that all three categories of predictors affected both HR size and fidelity. For example, HRs were smaller in areas with more food, and males had larger HRs than females. In addition, more aggressive lizards tended to have larger HRs. Conspecific overlap and number of individuals that a lizard interacted with (social network degree) had an interactive effect on HR size where individuals whose HRs overlapped more with neighbors had larger HRs, and this effect was particularly strong for individuals that interacted with more neighbors. HR fidelity declined over time (HR locations drifted from year to year), but individuals differed consistently in this rate of drift. The fact that HR size was consistent despite drifting locations suggests that lizard HRs reflect individual traits (e.g., habitat choice criteria that differ among individuals), rather than simple heterogeneity among sites. Overall, these findings demonstrate (1) both strong, long‐term, within‐individual consistency and between‐individual differences in space use and (2) combined effects of individual traits, social conditions, and environmental characteristics on animal HRs, with implications for diverse ecological processes.

The anthropocene landscape and ecosystem services in the closure of sand mining: Paraíba do Sul River basin – Brazil

Geodiversity is a determinant of mining patterns, because its nature defines the interaction with humans, type of exploitation, and, consequently, ecosystem services (ES) of the final landscape. The environmental recovery obligations defined in the legislation serve as a good guidance, but sand mining companies are not always able to finalize their activity entirely and to comply with the laws. To take this into account, the present study was developed with an objective of analyzing the changes observed in the ES after 70 years of sand exploitation in the Paraíba do Sul River Valley, Brazil. The services of the floodplain soil formation and associated components, such as the quality and quantity of water and refuges provided by the riparian forest, are highlighted in this study. To evaluate the different ESs modified by mining, socio-environmental variables were studied, and the analytic hierarchy process (AHP) method was applied to assign significance to each variable in the phase of closing activities, thus creating a compound index for ES evaluation at mine closure. The closure conditions were analyzed against the UN Sustainable Development Goals (SDGs) 6, 8, 9, and 11. The results showed that the companies are small, that they met the legislation in equal criteria of environmental rehabilitation, and that they were induced to a closure standard, reflecting the reforestation with native forest variable, related to the ES of provisioning of habitat to wild fauna. However, the mine facilities variable, associated with the cultural ES of recreation, predominated, because the buildings and access roads were preserved, even though the movable assets were demobilized quickly. It was concluded from the analysis that the scenario of the ES at mine closure is generally linked to geodiversity, company size, and compliance with legal duties.

The positive effect of coexisting ecosystem engineers: a unique seaweed-mussel association provides refuge for native mud crabs against a non-indigenous predator.

In marine sedimentary bottoms, mussels and macroalgae have long been recognized as important autogenic engineers that create habitat and modify abiotic conditions. The structural complexity added by bivalves and macroalgae may also mediate intraguild predation amongst marine decapod crustaceans. While spatial distributions of these ecosystem engineers frequently overlap, there is limited understanding of compounded effects when more than one engineer is present. Here we demonstrate that the coexistence of two ecosystem engineers may create habitat valuable for the survival of a small native species, the Atlantic mud crab (Panopeus herbstii), in the presence of the invasive green crab (Carcinus maenas). Using laboratory and field habitat mimics, we measured mud crab survival rates as a proxy for refuge quality. We compared the refuge provided by a unique association between shells of blue mussels (Mytilus edulis) and the giant strain of Irish moss (Chondrus crispus) to that provided by bare substrate, and by each engineer alone. These experiments revealed that the association of giant Irish moss with blue mussel shells positively and non-additively increased mud crab survival compared to the other less complex habitat mimics. In contrast, parallel experiments revealed that high habitat complexity was less important for young green crabs to survive predation from large conspecifics. These results suggest that the impact of ecosystem engineers on trophic dynamics should be considered in a broader, whole-community context encompassing multiple habitat-forming species present.

The lizard abides: cold hardiness and winter refuges ofLiolaemus pictus argentinusin Patagonia, Argentina

In environments where the temperature periodically drops below zero, it is remarkable that some lizards can survive. Behaviorally, lizards can find microsites for overwintering where temperatures do not drop as much as the air temperature. Physiologically, they can alter their biochemical balance to tolerate freezing or avoid it by supercooling. We evaluated the cold hardiness of a population of Liolaemus pictus argentinus Müller and Hellmich, 1939 in the mountains of Esquel (Patagonia, Argentina) during autumn. Additionally, we assessed the thermal quality (in degree-days) of potential refuges in a mid-elevation forest (1100 m above sea level (asl)) and in the high Andean steppe (1400 m asl). We analyzed the role of urea, glucose, total proteins, and albumin as possible cryoprotectants, comparing a group of lizards gradually exposed to temperatures lower than 0 °C with a control group maintained at room temperature. However, we found no evidence to support the presence of freeze tolerance or supercooling mechanisms in this species as related to the analyzed metabolites. Instead, the low frequency of degree-days below 0 °C and temperatures never lower than −3 °C in potential refuges suggest that L. p. argentinus might avoid physiological investments (such as supercooling and freeze tolerance) by behaviorally selecting appropriate refuges to overcome cold environmental temperatures.

Refuge quality to cope with UV radiation affects energy allocation in an intertidal fish

Ultraviolet (UV) radiation is a primary environmental stressor for marine species inhabiting intertidal pools. Thus, the use of microhabitats as refuges is key to protect organisms against this stressor. In this study, we compared the quality of rocky and algae as refuges for the intertidal fish Girella laevifrons exposed to UV radiation. Refuge quality was studied by evaluating oxygen consumption and weight gain in control and UV-exposed fish. Rocky-refuge fish consumed less oxygen and gained significantly more weight than algal-refuge fish. The obtained results support the importance of refuge quality on energetic balance of intertidal organisms, where energy can be differentially allocated towards key life processes such as protection/repair or growth. Energy trade-offs need to be considered in research concerning animals inhabiting stressful habitats.

Three-dimensional interstitial space mediates predator foraging success in different spatial arrangements.

Identifying and quantifying the relevant properties of habitat structure that mediate predator-prey interactions remains a persistent challenge. Most previous studies investigate effects of structural density on trophic interactions and typically quantify refuge quality using one or two-dimensional metrics. Few consider spatial arrangement of components (i.e., orientation and shape) and often neglect to measure the total three-dimensional (3D) space available as refuge. This study tests whether the three-dimensionality of interstitial space, an attribute produced by the spatial arrangement of oyster (Crassostrea virginica) shells, impacts the foraging success of nektonic predators (primary blue crab, Callinectes sapidus) on mud crab prey (Eurypanopeus depressus) in field and mesocosm experiments. Interstices of 3D-printed shell mimics were manipulated by changing either their orientation (angle) or internal shape (crevice or channel). In both field and mesocosm experiments, under conditions of constant structural density, predator foraging success was influenced by 3D aspects of interstitial space. Proportional survivorship of tethered mud crabs differed significantly as 3D interstitial space varied by orientation, displaying decreasing prey survivorship as angle of orientation increased (0°=0.76, 22.5°=0.13, 45°=0.0). Tethered prey survivorship was high when 3D interstitial space of mimics was modified by internal shape (crevicesurvivorship =0.89, channelsurvivorship =0.96) and these values did not differ significantly. In mesocosms, foraging success of blue crabs varied with 3D interstitial space as mean proportional survivorship (± SE) of mud crabs was significantly lower in 45° (0.27±0.06) vs. 0° (0.86±0.04) orientations and for crevice (0.52±0.11) vs. channel shapes (0.95±0.02). These results suggest that 3D aspects of interstitial space, which have direct relevance to refuge quality, can strongly influence foraging success in our oyster reef habitat. Our findings highlight the importance of spatial arrangement in mediating consumptive pathways in hard-structured habitats and demonstrate how quantifying the three-dimensionality of living space captures aspects of habitat structure that have been missing from previous empirical studies of trophic interactions and structural complexity.

Sharing refuges on arid islands: ecological and social influence on aggregation behaviour of wall geckos.

BackgroundThe extent of social behaviour among reptiles is underappreciated. Two types of aggregations are recognized in lizards: ecological and social, i.e., related to the attraction to a site or to animals of the same species, respectively. As most lizards are territorial, aggregations increase the probability of aggressive interactions among individuals, a density-dependent behaviour.MethodsAfter some spurious observations of aggregation behaviour in the endemic Cabo Verde nocturnal gecko Tarentola substituta, we conducted a field-based study in order to thoroughly characterize it. We sampled 48 transects and 40 10 × 10 m quadrats on São Vicente Island to describe the incidence, size and composition of aggregations and to study the effect of gecko and refuge density, plus refuge quality, on refuge sharing. We hypothesize that when density of animals and scarcity of high-quality refuges is higher, lizards have increased probability of aggregating. We also predict a consistent pattern of size and composition of groups (male–female pairs, only one adult male per group) throughout the year if there is a selected behaviour to avoid agonistic interactions, and low thermal advantage to aggregating individuals.ResultsWe present one of the first evidences of aggregation for Phyllodactylidae geckos. We found that T. substituta forms aggregations around 30–40% of the time, and that refuges are almost always shared by a female-male pair, sometimes with a juvenile, probably a mechanism to avoid aggressive interactions. We also observed that refuge sharing is dependent on refuge quality, as medium–large (thermally more stable and positively selected) rocks are shared much more frequently than small ones, but independent of adult sizes. Refuge sharing is also directly related to the density of geckos and inversely related to the density of high-quality refuges. We found no relation between body temperatures of geckos and refuge sharing when controlling the effect of rock/air temperature, suggesting that huddling does not improve thermoregulation.DiscussionOur results suggest that in this harsh environment (rocks reach 46 °C) aggregation incidence is mainly driven by an ecological factor (scarcity of high-quality refuges) and its intersexual composition by social factors (avoidance of agonistic interactions by males, and possible increased reproductive success of the pair). This study sheds some light on the little explored gecko aggregation behaviour and other studies should follow.

Refuge quality impacts the strength of nonconsumptive effects on prey.

Prey often retreat into the safety of refuges for protection from predators. This shift into refuge can reduce foraging opportunities, escalating the costs of risk and the strength of nonconsumptive effects. Such costs, however, may be shaped by the variation in resources that refuges harbor for prey foraging (i.e., refuge quality), and change dynamically via impacts on prey state. Despite its potential importance, we lack an explicit understanding of how refuge quality impacts prey performance under risk. Using a rocky intertidal food chain, we examined the interaction between predation risk and the amount of resources available for prey in refuge. We found that refuges with more resources greatly reduce the costs of refuge use, and that nonconsumptive effects are thereby weakened by as much as one-half, with especially strong impacts on prey growth and growth efficiency. These results suggest that failure to consider refuge quality could result in overestimation of the negative effects associated with prey refugeuse.

The role of refuges in the persistence of Australian dryland mammals.

Irruptive population dynamics are characteristic of a wide range of fauna in the world's arid (dryland) regions. Recent evidence indicates that regional persistence of irruptive species, particularly small mammals, during the extensive dry periods of unpredictable length that occur between resource pulses in drylands occurs as a result of the presence of refuge habitats or refuge patches into which populations contract during dry (bust) periods. These small dry-period populations act as a source of animals when recolonisation of the surrounding habitat occurs during and after subsequent resource pulses (booms). The refuges used by irruptive dryland fauna differ in temporal and spatial scale from the refugia to which species contract in response to changing climate. Refuges of dryland fauna operate over timescales of months and years, whereas refugia operate on timescales of millennia over which evolutionary divergence may occur. Protection and management of refuge patches and refuge habitats should be a priority for the conservation of dryland-dwelling fauna. This urgency is driven by recognition that disturbance to refuges can lead to the extinction of local populations and, if disturbance is widespread, entire species. Despite the apparent significance of dryland refuges for conservation management, these sites remain poorly understood ecologically. Here, we synthesise available information on the refuges of dryland-dwelling fauna, using Australian mammals as a case study to provide focus, and document a research agenda for increasing this knowledge base. We develop a typology of refuges that recognises two main types of refuge: fixed and shifting. We outline a suite of models of fixed refuges on the basis of stability in occupancy between and within successive bust phases of population cycles. To illustrate the breadth of refuge types we provide case studies of refuge use in three species of dryland mammal: plains mouse (Pseudomys australis), central rock-rat (Zyzomys pedunculatus), and spinifex hopping-mouse (Notomys alexis). We suggest that future research should focus on understanding the species-specific nature of refuge use and the spatial ecology of refuges with a focus on connectivity and potential metapopulation dynamics. Assessing refuge quality and understanding the threats to high-quality refuge patches and habitat should also be a priority. To facilitate this understanding we develop a three-step methodology for determining species-specific refuge location and habitat attributes. This review is necessarily focussed on dryland mammals in continental Australia where most refuge-based research has been undertaken. The applicability of the refuge concept and the importance of refuges for dryland fauna conservation elsewhere in the world should be investigated. We predict that refuge-using mammals will be widespread particularly among dryland areas with unpredictable rainfall patterns.

Winter Conditions and Land Cover Structure the Subnivium, A Seasonal Refuge beneath the Snow.

In seasonally snow-covered environments, many organisms endure winter by using the subnivium, a below-snow thermally stable seasonal refugium. Because the insulation of snow is dependent on snow depth and density, the stability of temperatures within the subnivium varies across land cover types. Additionally, across much of the Northern Hemisphere snow extent, depth and duration are generally decreasing while snow density is increasing due to climate change. These changes are likely to destabilize the thermal profile of the subnivium, although they have not yet been quantified. To explore the effects of land cover and climate change on the subnivium, we measured snow pack characteristics (depth and density), and ambient and subnivium temperatures from three different land cover types (prairie, deciduous forest, and coniferous forest) and within a micro-greenhouse (2.5 x 2.5 x 2 m) that maintained a temperature of 5°C warmer than outdoor ambient temperatures, and automatically opened during snow events throughout the winter of 2013/14. We found that the mean daily subnivium temperature was significantly colder in the deciduous cover type than the prairie cover type, and that prairie had higher maximum subnivium temperatures than both of the other cover types. Our climate change simulation revealed that, although ambient temperatures within the micro-greenhouse were 5°C warmer than outside the greenhouse, the daily minimum subnivium temperature was significantly lower inside the greenhouse. Our findings suggest that climate change could have considerable effects on the refuge quality of the subnivium, and that some cover types appear to be more susceptible to these effects than others.

Vertical heterogeneity in predation pressure in a temperate forest canopy

The forest canopy offers a vertical gradient across which variation in predation pressure implies variation in refuge quality for arthropods. Direct and indirect experimental approaches were combined to assess whether canopy strata differ in ability to offer refuge to various arthropod groups. Vertical heterogeneity in impact of avian predators was quantified using exclosure cages in the understory, lower, mid, and upper canopy of a north-temperate deciduous forest near Montreal, Quebec. Bait trials were completed in the same strata to investigate the effects of invertebrate predators. Exclusion of birds yielded higher arthropod densities across all strata, although treatment effects were small for some taxa. Observed gradients in predation pressure were similar for both birds and invertebrate predators; the highest predation pressure was observed in the understory and decreased with height. Our findings support a view of the forest canopy that is heterogeneous with respect to arthropod refuge from natural enemies.

Predation constrains host choice for a marine mesograzer

For small sedentary herbivores that inhabit seaweeds, choosing a host that provides adequate nutrition and refuge should be favored by natural selection. Yet, the relative importance of seaweed nutritional value versus habitat quality in driving mesograzer host choice remains poorly understood for most herbivores. Previous work in coastal North Carolina, USA, and 2 tropical locations suggests that amphipods often utilize host seaweeds that offer superior refuge from both omni vorous and carnivorous consumers. Our study was conducted in New Zealand, where carnivores alone are the major consumers of seaweed-dwelling amphipods. We show that the herbivorous amphipod Aora typica preferentially utilizes the dictyotalean seaweed Dictyota kunthii over a dominant canopy-forming kelp, Ecklonia radiata, and that this preferred seaweed host provides a superior refuge from predators in both laboratory and field experiments. There was no difference in A. typica feeding preference between D. kunthii and E. radiata, but A. typica grew faster when reared on E. radiata. These results suggest that seaweed refuge quality, not nutritional quality, drives A. typica host preference and distribution in this system.

Refuge Use in a Patagonian Nocturnal Lizard, Homonota darwini: The Role of Temperature

The thermal quality of diurnal refuges is important to the performance and survival of nocturnal reptiles. We studied refuge use on both slopes of an east–west-oriented hill by the thigmothermic gecko Homonota darwini, the southernmost-distributed nocturnal lizard in the world, in the vicinity of Bariloche, Rio Negro, in the Patagonia of Argentina. Because of the harsh climatic conditions in Patagonia, suitable refuges are limited, and retreat-site use is important for these geckos. Homonota darwini used refuges significantly more frequently on the warmer western slope in our study site. Geckos on the western slope used those refuges with higher temperatures regardless of size and thickness of rocks that acted as retreats. We tested whether refuge temperature affected locomotor performance of these lizards. Performance experiments showed that maximum sprint speed was affected by the temperature of the refuges. Refuges at 22.5°C allowed lizards to achieve their fastest sprint performance. Unexpectedly, sprint performance of lizards that used refuges with temperatures >32°C was the lowest among all tested refuge temperatures (18°, 22.5°, 27.5°, and 33°C). Our data illustrate the importance of the thermal quality of refuges for reptiles living in extreme environments.

Microclimatic buffering and resource‐based habitat in a glacial relict butterfly: significance for conservation under climate change

AbstractIn contrast to several organisms that have already shown range shifts to the north as a response to climate change, southern populations of relict species are trapped in isolated altitudinal habitats. Therefore, there is a growing interest to better understand their habitat use, with particular attention to the thermal aspects and associated significance for their habitat management. We address this issue by a study of larval habitat use relative to vegetation structure and microclimate in a glacial relict butterfly of peat bog ecosystems, using a functional, resource‐based habitat approach. We analysed caterpillar presence and density relative to vegetation composition (reflecting gradients of humidity, temperature, and natural succession of the peat bog) and to the availability and quality of thermal refuges for caterpillars (i.e., structures provided by Sphagnum hummocks). We also tested caterpillar survival rates under different temperature and humidity treatments. We found that (1) Boloria aquilonaris was a specialist butterfly of early successional stages with very humid zones of peat bog, (2) the lack of Sphagnum hummocks reduced larval habitat suitability, and hence the population density, and (3) a reduction of the thermal buffering ability of Sphagnum hummocks was observed in less humid, degraded parts, or late‐successional stages of peat bog. A larval rearing experiment showed a significant impact of temperature on caterpillar survival; survival being higher at lower temperature. Our field and laboratory results support the idea that the thermal environment exploited by caterpillars should be considered as a functional resource and included in a population‐specific habitat definition. Appropriate management of the peat bog habitat of this glacial relict species should not exclusively focus on the larval and adult feeding resources, but also on the quality of thermal refuges provided by Sphagnum hummocks in humid zones of the peat bog, especially in the current critical context of climate warming.

The effects of thermal biology and refuge availability on the restricted distribution of an alpine lizard

AbstractAim In an effort to disentangle the ecological processes that confine ectotherms to alpine environments, we studied the thermoregulatory and microhabitat selection behaviours of the rock lizard Iberolacerta cyreni, which is endemic to some mountains of central Spain, and of the wall lizard Podarcis muralis, which is a potential competitor of rock lizards.Location We chose three areas in the Sierra de Guadarrama (central Spain) that differed in their thermal quality [mean deviation of environmental operative temperatures from the lizards’ preferred thermal range (PTR)] and refuge availability: a pine forest (1770 m a.s.l.) in which P. muralis was the only species found, and two mixed shrub and rock sites (1770 and 1900 m a.s.l.) where both species were present.Methods In the field we collected data on refuge availability, sun exposure, body temperature (Tb) and operative temperature (Te). Thus, we estimated the thermal habitat quality of the areas sampled and the thermoregulation accuracy and effectiveness of both species.Results The pine forest had the lowest thermal quality and refuge availability. The lower‐elevation shrub site offered the best thermal quality, but refuges were much scarcer than at the higher‐elevation site. Both species thermoregulated accurately, because mean deviations of body temperature (Tb) from PTR were considerably smaller than those of Te. Podarcis muralis had higher Tb values than did I. cyreni, which had similar Tb values at both shrub sites, whereas P. muralis had lower Tb values at higher elevation. Overall, the thermoregulatory effectiveness (extent to which Tb values are closer to the PTR than are Te values) of both species was similar, but whereas I. cyreni thermoregulated more efficiently at higher elevation, the opposite was true for P. muralis. At the lower‐elevation shrub site, I. cyreni remained closer to refuges than did P. muralis.Main conclusions Our results suggest that the pine forest belt might prevent the expansion of rock lizards towards lower elevations as a result of its low thermal quality and scarcity of refuges, that the thermoregulatory effectiveness of rock lizards in alpine environments depends more on refuge availability than on thermal habitat quality, and that competition with wall lizards is unlikely to explain either the distribution or the thermoregulatory effectiveness of rock lizards.

Diurnal Refuge Competition in the Freshwater Prawn, Macrobrachium Australiense

This study examined important stimuli influencing agonistic conspecific pairs of Macrobrachium australiense and how these affected diurnal refuge usage. The results demonstrated key behaviours underlying refuge competition, and have ramifications for how we view the link between conspecific competition and exposure to predation. Different levels of vegetation stalk density were employed to examine the effects of increased refuge quality on the hierarchical interaction. The data analysis demonstrated that the prawns were responding to nearby stimuli which altered detectability, e.g., vegetation density, conspecific size, conspecific movement. Dominants and subordinates both responded to the same stimuli, however, the behavioural response was different. Increasing subordinate size lead to increased dominant activity in all refuges, irrespective of vegetation density. Increasing dominant size caused an increase in subordinate activity in high density refugia but a reduction in low density refugia. The increased activity lead to an overall increase in time spent outside of the refuge for all individuals. Subordinates spent substantially more time outside of the refuge than the dominant individuals. As such refuge competition biases the exposure of individuals to risk, localised changes to this bias, i.e., the size of neighbouring conspecifics, can influence this risk and determine the vulnerability of specific demographics, i.e., smaller individuals, to predation.

Determinants of overwintering habitat quality for beetles and spiders on arable farmland

The provision of overwintering refuges has important implications for both conservation of invertebrate biodiversity on farmland and biological control of crop pests. In this study we carried out a systematic investigation of the effects of habitat type and age on the quality of overwintering refuges for Coleoptera and Araneae on an arable farm in the UK. Hedgerow habitats contained taller and more diverse vegetation with a greater proportion of bare ground compared with field margins. Vegetation height and diversity, and the richness and cover of forbs all increased significantly with habitat age. Soil organic matter content also increased with habitat age, suggesting a more favourable structure for burrowing Coleoptera. Total abundance and richness of Coleoptera and Araneae were significantly higher in hedgerow habitats compared with the field margins. This largely reflected the response of staphylinid beetles, the most abundant group recorded. There was no effect of habitat age on overall abundance or diversity, suggesting the good dispersal ability of many farmland invertebrates, but also reflecting the close relationship between habitat age and type. The response of individual invertebrate species was more mixed: 10 species were recorded in significantly higher densities in the hedge base compared with one in the field margin. Similarly, eight species were recorded in significantly higher densities in mature habitats compared with four in recently established vegetation. There were significant interactions between habitat type and age for six species, of which four showed a marked preference for the base of mature hedgerows. These included both important predators of cereal aphids and crop pests. It was concluded that hedgerows provided the highest quality overwintering habitat for invertebrates emphasising the importance of strategies directed at conserving and enhancing this habitat. However, newly created field margins fulfilled a useful role in rapidly providing overwintering refuges for a more limited group of invertebrate species.