Habitat Gradient Research Articles

Ecophysiological variation across a forest‐ecotone gradient produces divergent climate change vulnerability within species

Climate change related risks and impacts on ectotherms will be mediated by habitats and their influence on local thermal environments. While many studies have documented morphological and genetic aspects of niche divergence across habitats, few have examined thermal performance across such gradients and directly linked this variation to contemporary climate change impacts. In this study, we quantified variation in thermal performance across a gradient from forest to gallery forest‐savanna mosaic in Cameroon for a skink species (Trachylepis affinis) known to be diverging genetically and morphologically across that habitat gradient. Based on these results, we then applied a mechanistic modelling approach (NicheMapR) to project changes in potential activity, as constrained by thermal performance, in response to climate change. As a complimentary approach, we also compared mechanistic projections with climate‐driven changes in habitat suitability based on species distribution models of forest and ecotone skinks. We found that ecotone skinks may benefit from warming and experience increased activity while forest skinks will likely face a drastic decrease in thermal suitability across the forest zone. Species distribution models projected that thermal suitability for forest skinks in coastal forests would decline but in other parts of the forest zone skinks are projected to experience increased thermal suitability. The results here highlight the utility of mechanistic approaches in revealing and understanding patterns of climate change vulnerability which may not be detected with species distribution models alone. This study also emphasizes the importance of intra‐specific physiological variation, and habitat‐specific thermal performance relationships in particular, in determining warming responses.

Methane fluxes from tree stems and soils along a habitat gradient

Forests are major sources of terrestrial CH4 and CO2 fluxes but not all surfaces within forests have been measured and accounted for. Stem respiration is a well-known source of CO2, but more recently tree stems have been shown to be sources of CH4 in wetlands and upland habitats. A study transect was established along a natural moisture gradient, with one end anchored in a forested wetland, the other in an upland forest and a transitional zone at the midpoint. Stem and soil fluxes of CH4 and CO2 were measured using static chambers during the 2013 and 2014 growing seasons, from May to October. Mean stem CH4 emissions were 68.8 ± 13.0 (mean ± standard error), 180.7 ± 55.2 and 567.9 ± 174.5 µg m−2 h−1 for the upland, transitional and wetland habitats, respectively. Mean soil methane fluxes in the upland, transitional and wetland were − 64.8 ± 6.2, 7.4 ± 25.0 and 190.0 ± 123.0 µg m−2 h−1, respectively. Measureable CH4 fluxes from tree stems were not always observed, but every individual tree in our experiment released measureable CH4 flux at some point during the study period. These results indicate that tree stems represent overlooked sources of CH4 in forested habitats and warrant investigation to further refine CH4 budgets and inventories.

Variation in Stomatal Characteristics of <i>Bursera simaruba</i> (L.) Sarg., a Dominant Tree Species of Tropical Hardwood Hammock Forest across a Habitat Gradient in the Florida Keys

Tree species in coastal forests may exhibit specialization or plasticity in coping with drought through changes in their stomatal morphology or activity, allowing for a balance between gas exchange and water loss in a periodically stressful environment. To examine these responses, we sought to answer two primary research questions: a) how is variation in B. simaruba’s stomatal traits partitioned across hierarchical levels, i.e., site, tree, and leaf; and b) is variation in stomatal traits an integrated response to physiological stress expressed across the habitat gradient of Florida Keys forests? At eight sites distributed throughout the Keys, five leaves were collected from three mature trees for stomatal analysis. Leaf carbon stable isotope ratio (δ13C) was determined to infer the changes in water use efficiency caused by physiological stress experienced by each tree. The results showed that substantial proportions of the total variance in three traits (stomatal density, stomatal size, and δ13C) were observed at all levels, suggesting that processes operating at each scale are important in determining trait values. A significant negative correlation between stomatal density and size across scales was observed. Path model analysis showed that environmental variables, distance to ground water and ground water salinity, affect leaf δ13C indirectly, via its effects on stomatal traits, not directly to leaf δ13C. Therefore, the combination of small and densely distributed stomata seems to represent a strategy that allows B. simaruba to conserve water under conditions of physiological drought induced by either higher ground water salinity or flooding stress at very low elevation.

Bird diversity along a gradient of fragmented habitats of the Cerrado.

Understanding the factors that affect biodiversity is of central interest to ecology, and essential to species conservation and ecosystems management. We sampled bird communities in 17 forest fragments in the Cerrado biome, the Central-West region of Brazil. We aimed to know the communities structure pattern and the influence of geographical distance and environmental variables on them, along a gradient of fragmented habitats at both local and landscape scales. Eight structural variables of the fragments served as an environmental distance measurement at the local scale while five metrics served as an environmental distance measurement at the landscape scale. Species presence-absence data were used to calculate the dissimilarity index. Beta diversity was calculated using three indices (βsim, βnes and βsor), representing the spatial species turnover, nestedness and total beta diversity, respectively. Spatial species turnover was the predominant pattern in the structure of the communities. Variations in beta diversity were explained only by the environmental variables of the landscape with spatial configuration being more important than the composition. This fact indicates that, in Cerrado of Goiás avian communities structure, deterministic ecological processes associated to differences in species responses to landscape fragmentation are more important than stochastic processes driven by species dispersal.

Traveling waves and spatial patterns from dispersal on homogeneous and gradient habitats

Models of dispersal for response to global climate change can refer to theory on traveling waves developed for invasions. The relevance of this theory may depend on its density- versus habitat-dependence, but it has not considered environmental gradients nor related feedbacks. An agent based simulation model (ABM) for a plant species or vegetation type is developed here, focusing on properties of traveling waves. The ABM simulated no-tailed, thin-tailed, and fat-tailed dispersal kernels (random uniform, Gaussian, and Cauchy kernels, respectively). Probabilities were based on either population density or habitat, and for the latter as homogeneous or on a gradient, and if a gradient positive feedback from populations to the habitat was added to represent process at ecotones such as a positive feedback switch or the stress gradient hypothesis (SGH) feedback. The uniform and Gaussian kernels produced traveling waves with constant speed, but the fat-tailed Cauchy kernel produced traveling waves that accelerated and also flattened as the low-probability portion of wave accelerated fastest. On a gradient, when populations approached the limit of habitability the waves steepened and stopped traveling. The effect of SGH feedback on traveling wave slopes depends on the dispersal kernel. With strong facilitation on the gradient, greater variety of spatial patterns were able to develop among the replicate simulations of the fat-tailed kernel when it flattened to a greater spatial extent of low probability presence. In addition to the heterogeneity of the habitat and its modification by facilitation, the details of dispersal can also affect the feedback between spatial pattern and the establishment process that will affect the rate of advance of a species or an ecotone into new territory.

Intra-colony disease progression induces fragmentation of coral fluorescent pigments

As disease spreads through living coral, it can induce changes in the distribution of coral’s naturally fluorescent pigments, making fluorescence a potentially powerful non-invasive intrinsic marker of coral disease. Here, we show the usefulness of live-imaging laser scanning confocal microscopy to investigate coral health state. We demonstrate that the Hawaiian coral Montipora capitata consistently emits cyan and red fluorescence across a depth gradient in reef habitats, but the micro-scale spatial distribution of those pigments differ between healthy coral and coral affected by a tissue loss disease. Naturally diseased and laboratory infected coral systematically exhibited fragmented fluorescent pigments adjacent to the disease front as indicated by several measures of landscape structure (e.g., number of patches) relative to healthy coral. Histology results supported these findings. Pigment fragmentation indicates a disruption in coral tissue that likely impedes translocation of energy within a colony. The area of fragmented fluorescent pigments in diseased coral extended 3.03 mm ± 1.80 mm adjacent to the disease front, indicating pathogenesis was highly localized rather than systemic. Our study demonstrates that coral fluorescence can be used as a proxy for coral health state, and, such patterns may help refine hypotheses about modes of pathogenesis.

DISPERSAL POTENTIAL OF HERBACEOUS SPECIES ACCORDING TO CLIMATE, LAND USE AND HABITAT CONDITIONS IN WEST AFRICAN SAVANNAH

According to environmental predictions, West Africa is becoming vulnerable to the adverse effects of climate change and land use dis- turbance. Herbaceous vegetation is the most sensitive to these effects. To assess the poten- tial of species to cope with these changes, this study investigated the dispersal potential of different herbaceous species. Data on her- baceous plant composition and environmen- tal parameters were collected along climate, land use and habitat gradients in West Afri- can savannah areas, as well as the diaspores of all herbaceous species encountered. Their traits were described in order to document the diversity of diaspore categories in herbaceous savannah vegetation. Based on an occurrence diagram, variations in dissemination patterns within families were identified. The dispersal potential of each species was determined on the basis of their patterns of dispersal. A hie- rarchical classification method was used to establish a dispersal typology, and principal component analysis was applied to identify the environmental conditions that account for their patterns of dispersal. The results show that the diaspores of herbaceous spe- cies can be classified according to fruit type, diaspore type, presence of heterodiaspory, exposure of diaspores, number of seeds per diaspore, diaspore morphology, shape of diaspores and pattern of dispersal. Poaceae, the most abundant family in savannah areas, have six patterns of dispersal. Most species have more than one pattern, and species with high dispersal potential (85.43%) dominate the herbaceous vegetation. Four groups of herbaceous species were identified according to their dispersal patterns. Potentially epizoo- chorous and anemochorous species (39.25% of the flora) were more related to village areas independently of climatic conditions and habi- tat types. Potentially endozoochorous and dysozoochorous species (31.06% of the flora) were more related to fresh and dry habitats in protected areas of the North and South Suda- nian zones. Potentially hydrochorous species (12.63% of the flora) were related to wet habi- tats but were more prominent in the Sahel, and potentially autochorous species (17.06% of the flora) were more related to bowé habi- tats in the southern Sudanian zone. Our study showed that all herbaceous species have good dispersal potential, which might enable them to persist in West African savannahs despite the severe climatic changes predicted.

Patterns and drivers of deep chlorophyll maxima structure in 100 lakes: The relative importance of light and thermal stratification

AbstractThe vertical distribution of chlorophyll in stratified lakes and reservoirs frequently exhibits a maximum peak deep in the water column, referred to as the deep chlorophyll maximum (DCM). DCMs are ecologically important hot spots of primary production and nutrient cycling, and their location can determine vertical habitat gradients for primary consumers. Consequently, the drivers of DCM structure regulate many characteristics of aquatic food webs and biogeochemistry. Previous studies have identified light and thermal stratification as important drivers of summer DCM depth, but their relative importance across a broad range of lakes is not well resolved. We analyzed profiles of chlorophyll fluorescence, temperature, and light during summer stratification from 100 lakes in the Global Lake Ecological Observatory Network (GLEON) and quantified two characteristics of DCM structure: depth and thickness. While DCMs do form in oligotrophic lakes, we found that they can also form in eutrophic to dystrophic lakes. Using a random forest algorithm, we assessed the relative importance of variables associated with light attenuation vs. thermal stratification for predicting DCM structure in lakes that spanned broad gradients of morphometry and transparency. Our analyses revealed that light attenuation was a more important predictor of DCM depth than thermal stratification and that DCMs deepen with increasing lake clarity. DCM thickness was best predicted by lake size with larger lakes having thicker DCMs. Additionally, our analysis demonstrates that the relative importance of light and thermal stratification on DCM structure is not uniform across a diversity of lake types.

Assessing astragalar morphology and biomechanics in western Palaearctic Bison populations with geometric morphometrics

Having arrived 1.8Ma ago, bison prevailed in the bovid assemblages of the European subcontinent for more than 1.5Ma. The current geometric morphometric study outlines a framework of ecomorphological differences among several Bison populations of the western Palaearctic, shown by inferences from the tibial and tarsal joint surfaces of their astragalus. Given the principal biomechanical role of this element in the locomotion mechanism, its anatomical features could be linked to diverse functional aspects. In terms of morphological affinity, it is possible to attribute the studied fossil Bison astragalar material to several morphological trends. Shape variation is not explained by size differences and is possibly associated with an open-close habitat gradient, as indicated by the presence of expanded or compressed astragali, respectively. This intragroup spatial and temporal phenotypic diversity among the examined populations could indicate a biogeographic segregation influenced by regional climatic and landscape heterogeneity in the European territory during Pleistocene. Furthermore, a relation to habitat-specific locomotor ecology could be supported, revealing forms with increased cursoriality, operating in open biomes and closed-country dwellers as well.

Genomic footprints of adaptation in a cooperatively breeding tropical bird across a vegetation gradient.

Identifying the genetic basis of phenotypic variation and its relationship with the environment is key to understanding how local adaptations evolve. Such patterns are especially interesting among populations distributed across habitat gradients, where genetic structure can be driven by isolation by distance (IBD) and/or isolation by environment (IBE). Here, we used variation in ~1,600 high-quality SNPs derived from paired-end sequencing of double-digest restriction site-associated DNA (ddRAD-Seq) to test hypotheses related to IBD and IBE in the Yucatan jay (Cyanocorax yucatanicus), a tropical bird endemic to the Yucatán Peninsula. This peninsula is characterized by a precipitation and vegetation gradient-from dry to evergreen tropical forests-that is associated with morphological variation in this species. We found a moderate level of nucleotide diversity (π=.008) and little evidence for genetic differentiation among vegetation types. Analyses of neutral and putatively adaptive SNPs (identified by complementary genome-scan approaches) indicate that IBD is the most reliable explanation to account for frequency distribution of the former, while IBE has to be invoked to explain those of the later. These results suggest that selective factors acting along a vegetation gradient can promote local adaptation in the presence of gene flow in a vagile, nonmigratory and geographically restricted species. The putative candidate SNPs identified here are located within or linked to a variety of genes that represent ideal targets for future genomic surveys.

Local shifts in floral biotic interactions in habitat edges and their effect on quantity and quality of plant offspring.

Spatial shifts in insect fauna due to ecological heterogeneity can severely constrain plant reproduction. Nonetheless, data showing effects of insect visit patterns and intensity of mutualistic and/or antagonistic plant-insect interactions on plant reproduction over structured ecological gradients remain scarce. We investigated how changes in flower-visitor abundance, identity and behaviour over a forest-open habitat gradient affect plant biotic interactions, and quantitative and qualitative fitness in the edge-specialist Dianthus balbisii. Composition and behaviour of the insects visiting flowers of D. balbisii strongly varied over the study gradient, influencing strength and patterns of plant biotic interactions (i.e. herbivory and pollination likelihood). Seed set comparison in free- and manually pollinated flowers suggested spatial variations in the extent of quantitative pollen limitation, which appeared more pronounced at the gradient extremes. Such variations were congruent to patterns of flower visit and plant biotic interactions. The analyses on seed and seedling viability evidenced that spatial variation in amount and type of pollinators, and frequency of herbivory affected qualitative fitness of D. balbisii by influencing selfing and outcrossing rates. Our work emphasizes the role of plant biotic interactions as a fine-scale mediator of plant fitness in ecotones, highlighting that optimal plant reproduction can take place into a restricted interval of the ecological gradients occurring at forest edges. Reducing the habitat complexity typical of such transition contexts can threat edge-adapted plants.

Urban environments are associated with earlier clutches and faster nestling feather growth compared to natural habitats

Urbanization creates new habitats with novel benefits and challenges not found in natural systems. How a species fares in urban habitats is largely dependent on its life history, yet predicting the response of individual species to urbanization remains a challenge. While some species thrive in urban areas, others do poorly or are not present at all. Mountain chickadees (Poecile gambeli) are year-round residents of montane regions of western North America. Commonly found in higher-elevation coniferous forests, these birds can also be found in urban areas where they will regularly visit bird feeders and nest in nest boxes. We monitored mountain chickadees nesting along a habitat gradient, from natural habitat to suburban areas, to determine if the degree of urbanization was associated with: clutch size and success; nestling growth rates; or variation in parental size and age. Females nesting in urbanized areas initiated clutches earlier in the breeding season than those in natural areas, but neither fledging success nor the rate of nestling mass-change differed between habitats. Nestling feather growth-rate increased with later first egg dates in both habitats, and the magnitude of this increase was greatest in urban habitats. We found no difference in the proportion of first-time breeders versus experienced breeders between habitat types, nor any differences in male or female mass or size. Our results indicate no detriment to nesting in urban habitats, suggesting mountain chickadees are able to adapt to moderate urbanization much like other members of the Paridae family.

Persistence versus extinction for two competing species under a climate change

This paper considers effects of a climate-induced range shift on outcomes of two competitive species, which is modeled by a reaction–diffusion system with the increasing growth rates of species along a shifting habitat gradient. Analytical conditions are established for the coexistence or competitive exclusion of two-competitors under the climate change, which present the control strategies to maintain the persistence of species.

Frugivore-Mediated Selection in A Habitat Transformation Scenario

Plant-animal interactions are strong drivers of phenotypic evolution. However, the extent to which anthropogenic habitat transformation creates new selective scenarios for plant-animal interactions is a little explored subject. We examined the effects of native forest replacement by exotic Eucalyptus trees on the frugivore-mediated phenotypic selection coefficients imposed by the relict marsupial Dromiciops gliroides upon traits involved in frugivore attraction and germination success of the mistletoe Tristerix corymbosus (Loranthaceae). We found significant gradients for seed weight and sugar content along the native - transformed habitat gradient. While selection for larger seed weight was more relevant in native habitats, fruits with intermediate sugar content were promoted in transformed habitats. The spatial habitat structure and microclimate features such as the degree of sunlight received influenced the natural selection processes, as they correlated with the phenotypic traits analysed. The response of this plant-frugivore interaction to human disturbance seemed to be context-dependent, in which extremely transformed habitats would offer new opportunities for natural selection on dispersal-related traits. Even in recent transformation events like this, human disturbance acts as a strong contemporary evolution driver.

Water-borne pharmaceuticals reduce phenotypic diversity and response capacity of natural phytoplankton communities.

Chemical micropollutants occur worldwide in the environment at low concentrations and in complex mixtures, and how they affect the ecology of natural systems is still uncertain. Dynamics of natural communities are driven by the interaction between individual organisms and their growth environment, which is mediated by the organisms’ expressed phenotypic traits. We tested whether exposure to a mixture of 12 pharmaceuticals and personal care products (PPCP) influences phenotypic trait diversity in lake phytoplankton communities and their ability to regulate biomass production to fit environmental changes (response capacity). We exposed natural phytoplankton assemblages to three mixture levels in permeable microcosms maintained at three depths in a eutrophic lake for one week, during which the environmental conditions were fluctuating. We studied individual-level traits, phenotypic diversity and community biomass. PPCP reduced individual-level trait variance and overall community phenotypic diversity, but maintained higher standing phytoplankton biomass compared to untreated controls. Estimated effect sizes of PPCP on traits and community properties were very large (partial Eta-squared > 0.15). The PPCP mixture antagonistically interacted with the natural environmental gradient in habitats offered by different depths and, at concentrations comparable to those in waste-water effluents, prevented communities from converging to the same phenotypic structure and total biomass of unexposed controls. We show that micropollutants can alter individual-level trait diversity of lake phytoplankton communities and therefore their capacity to respond to natural environmental gradients, potentially affecting aquatic ecosystem processes.

Ecological structure of the West Siberian forest-steppe spider community (Arachnida, Araneae) and its comparison with the ground-beetle (Coleoptera, Carabidae) community

An investigation of the spatiotemporal management of the ground-dwelling spider community has been carried out at the continuous habitat gradient from the forest outlier center to the meadowland center in the southern part of the forest-steppe zone of Novosibirsk oblast in 2007‒2008. We apply two sampling methods which complete each other: pitfall trapping and hand separation of soil samples. Eighty-three species of spiders from 14 families are determined. Spiders are most abundant in the early summer and less abundant in the middle of summer. Representatives of different families are of maximum abundance in different sites: Lycosidae and Gnaphosidae mainly in the forest and Philodromidae in the solonetz meadow. The distribution of diversity and abundance of spiders and ground beetles throughout the habitat gradient are generally similar. This is contradictory to the spatial interactions of these two groups in Arctic and extreme ecosystems. Niche distinguishing between ground beetles and spider species is not only topical. It includes the usage of different strata of vegetation, various seasonal activity, and trophic specialization.

Management implications of bird responses to variation in non-native/native tree ratios within central European forest stands

Plant invasions are a serious threat for global biodiversity and various studies have reported their impacts on forest bird populations. However, a majority of these studies involved comparisons of sites covered solely by invaded or native forest habitats, whereas the real world is dominated by habitat gradients. To address this issue, we counted birds in central European forest stands differing in the coverage of an invasive woody plant, Robinia pseudoacacia L., a major global tree invader. This species significantly affects the vegetation composition in invaded habitats, resulting in shifts in bird community compositions toward a dominance of generalist species at the expense of specialists. Here we ask: (i) how the bird species richness changes with the increasing relative coverage of R. pseudoacacia, recognizing several bird groups represented by all species, habitat specialists, habitat generalists and forest specialists, (ii) to what extent the alteration of vegetation composition accounts for these changes. By employing conditional autoregressive models, and by taking forest stand area and distance to the forest edge into account, we found that the numbers of all bird species, habitat specialists and forest specialists were highest in stands with intermediate coverage of R. pseudoacacia. This result indicates that the non-native tree was not detrimental for bird biodiversity, and that birds indeed benefited from its presence in focal forest stands, but only if native and non-native trees were present in approximately equal proportions, which was particularly true for forest specialists. The effect of R. pseudoacacia on bird species richness was significant even after taking changes in vegetation composition into account, suggesting that this invasive species affects birds not only by altering structural components of the habitat, but also by some other factors that might include food supply for birds or nest-holes. Our sampling was quite limited, particularly with respect to the range of structures and management types typically encountered in native trees, and results must be treated accordingly. However, our results appear to suggest that eradication of R. pseudoacacia from forest within our study area may not be necessary for achieving bird conservation aims, because this species does not markedly depress bird species richness, including specialists. However, the generality of our findings require further testing across other forests affected by R. pseuadoacacia invasion, to examine whether retention of this non-native tree is more widely justified.

Landscape‐scale life‐history gradients in New Zealand freshwater fish

Summary Landscape‐scale clines in environmental parameters can lead to life‐history gradients. Study of interspecific life‐history differences can provide evidence of evolutionary adaptations in response to environmental cues, whereas examination of intraspecific life‐history variation can identify the ecological mechanisms driving trait evolution. We examined egg size and fecundity variation in three species of closely related freshwater fish (Galaxiidae) which occur across a habitat gradient in South Island, New Zealand. Inter‐population trait variation was assessed in the species with the widest distribution to investigate the ecological mechanisms driving egg size evolution. Substantial interspecific egg size and fecundity differences were observed. Galaxias paucispondylus, which generally occurs in high altitude headwater streams, showed an average egg size an order of magnitude larger, and fecundity an order of magnitude lower than G. brevipinnis, a species which rears in pelagic environments. Galaxias vulgaris, which rears fluvial environments but is generally distributed in lower catchment reaches than G. paucispondylus, showed intermediate fecundity and egg size traits. This interspecific trait variation is likely to be driven by differences in the larval rearing habitat, i.e. rearing in cold high altitude locations requires large well‐developed larvae (and therefore large eggs) but these come at the cost of maternal fecundity, whereas in rearing in warmer lower catchment watercourses, and pelagic habitats, does not require such large well‐developed larvae. Hence, fitness is maximised by progressively increasing fecundity by reducing egg size across this habitat cline. Life history‐habitat trends were repeated when intraspecific patterns in the widely distributed G. vulgaris were assessed, with population egg size increasing, and fecundity decreasing, in relation to a suite of interrelated environmental variables up an approximate altitudinal cline. The results highlight how inter‐ and intraspecific variation in egg size–fecundity trade‐offs in closely related species can interact to drive the evolution of landscape‐scale life‐history gradients. Egg size differences are likely to be driven by differences in the quality of the larval rearing habitat, i.e. rearing in a challenging habitat requires a larger egg whereas in relatively benign larval rearing habitat, fecundity can be maximised by reducing egg size. The interspecific variation observed provides evidence of trait adaptation in response to an environmental cue, while the intraspecific inter‐population differences indicate the potential ecological mechanisms driving trait evolution.

Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird.

Understanding the processes that drive divergence within and among species is a long-standing goal in evolutionary biology. Traditional approaches to assessing differentiation rely on phenotypes to identify intra- and interspecific variation, but many species express subtle morphological gradients in which boundaries among forms are unclear. This intraspecific variation may be driven by differential adaptation to local conditions and may thereby reflect the evolutionary potential within a species. Here, we combine genetic and morphological data to evaluate intraspecific variation within the Nelson's (Ammodramus nelsoni) and salt marsh (Ammodramus caudacutus) sparrow complex, a group with populations that span considerable geographic distributions and a habitat gradient. We evaluated genetic structure among and within five putative subspecies of A.nelsoni and A.caudacutus using a reduced-representation sequencing approach to generate a panel of 1929 SNPs among 69 individuals. Although we detected morphological differences among some groups, individuals sorted along a continuous phenotypic gradient. In contrast, the genetic data identified three distinct clusters corresponding to populations that inhabit coastal salt marsh, interior freshwater marsh and coastal brackish-water marsh habitats. These patterns support the current species-level recognition but do not match the subspecies-level taxonomy within each species-a finding which may have important conservation implications. We identified loci exhibiting patterns of elevated divergence among and within these species, indicating a role for local selective pressures in driving patterns of differentiation across the complex. We conclude that this evidence for adaptive variation among subspecies warrants the consideration of evolutionary potential and genetic novelty when identifying conservation units for this group.

Daily energy expenditure in the face of predation: hedgehog energetics in rural landscapes

Failure to balance daily energy expenditure (DEE) with energy intake can have an impact on survival and reproduction, and therefore on the persistence of populations. Here we study the DEE of the European hedgehog (Erinaceus europaeus), which is declining in the UK. We hypothesise that there is a gradient of suitable habitat for hedgehogs in rural areas, which is a result of fewer food resources, a higher risk from predation by badgers (Meles meles) and colder ambient temperatures, as distance to the nearest building increases. We used the doubly labelled water method to obtain 44 measurements of DEE from hedgehogs on four predominately arable sites, to determine the energetic costs associated with proximity to buildings, on sites with and without badgers. The mean±s.e.m. DEE was 508.9±34.8 kJ day-1 DEE increased the further a hedgehog was from buildings during the study, possibly as they ranged larger distances on arable land, supporting the hypothesis that hedgehogs select villages owing to the lower energy demands in comparison to arable farmland. Hedgehogs had an approximately 30% lower DEE on sites with badgers. We speculate that on badger-occupied sites, hedgehogs may restrict movement and foraging in response to a threat from predation and thus have reduced DEE. Therefore, hedgehogs may also seek refuge in villages where the perceived threat of predation is lower and foraging is unrestricted. In a broader context, we demonstrate that individual differences in DEE can aid in understanding habitat selection in a patchily distributed species.