The ground beetle fauna of ancient and recent woodlands in the lowlands of north-west Germany (Coleoptera, Carabidae)

In north-west Germany the ground beetle fauna was investigated using pitfall traps at 79 sampling sites in ancient woodlands and recent woodlands (with and without direct contact to old stands). Two woodland types were considered: The Quercion robori-petraea-woodlands (oak-beech-type) on mainly sandy soils and the Stellario-Carpinetum-woodlands (hornbeam-type) on mainly loamy soils. The number of recorded ground beetle species inhabiting exclusively or predominantly woodlands in the investigation area is significantly higher in ancient stands of both woodland types than in recent ones. No statistically substantiated relation between habitat size (both about 1800 and in 1990) and the number of characteristic woodland ground beetle species could be ascertained. Carabus glabratus and Abax parallelus show a distinct focus in ancient woodlands. Significantly more records of Carabus violaceus and Abax parallelepipedus are known from ancient woodlands than from recent ones. Twelve of the 16 ground beetle species, for which no difference in the colonisation of ancient and recent woodlands was ascertained, are macropterous. Half of the eight brachypterous woodland species is exclusively or predominantly found in ancient woodlands, suggesting that power of dispersal is an important factor which determines the species number in woodland fragments of different age.     

Recent habitat fragmentation caused by major roads leads to reduction of gene flow and loss of genetic variability in ground beetles

Although habitat fragmentation is suspected to jeopardize the long-term survival of many species, few data are available on its impact on the genetic variability of invertebrates. We assess the genetic population structure of the flightless ground beetle Carabus violaceus L., 1758 in a Swiss forest, which is divided into several fragments by a highway and two main roads. Eight samples were collected from different forest fragments and analysed at six microsatellite loci. The largest genetic differentiation was observed between samples separated by roads and in particular by the highway. The number of roads between sites explained 44% of the variance in pairwise F(ST) estimates, whereas the age of the road and the geographical distance between locations were not significant factors. Furthermore, a comparison of allelic richness showed that the genetic variability in a small forest fragment isolated by the highway was significantly lower than in the rest of the study area. These findings strongly support the hypothesis that large roads are absolute barriers to gene flow in C. violaceus, which may lead to a loss of genetic variability in fragmented populations.     

Microsatellites and allozymes as the genetic memory of habitat fragmentation and defragmentation in populations of the ground beetle Carabus auronitens (Col., Carabidae)

Aim This paper has three aims: (1) to reconstruct the population history of a flightless silvicolous (forest) ground beetle in a region where strong anthropogenic activity has altered the connectivity of the landscape; (2) to estimate the effects of both contemporary and historical landscape structure on the genetics of populations; and (3) to investigate the reasons for clinal variation in one gene locus found in an earlier study carried out in the same geographical location. Location Münster (Westphalia), north‐west Germany. Methods We investigated 26 populations of the ground beetle Carabus auronitens Fabricius, 1792 by analysing seven polymorphic microsatellite loci and an allozyme locus. Samples of at least 16 individuals per site were studied. These were obtained from dry pitfall traps placed at 23 sites and from three additional (refuge) populations. We used regression and correlation analyses to investigate the effects of both historical and contemporary landscape structure on the allele frequency distributions of the investigated loci. Spatial autocorrelation analysis was used to study possible clinal variations, and admixture rates were calculated in order to assess the genetic influence of populations from possible refuges. Possible reasons for the development of the cline were examined using simulation models. Results The allele frequency distributions at the investigated loci could not be explained by selection or isolation by distance. We found clinal variation in 50% of the investigated loci and our simulations indicated that this was unlikely to have developed by chance. Admixture rates show significant influences of the investigated refuge populations on the populations under study. Main conclusions The findings strongly suggest that the clinal variation is secondary. It results from recolonization of the area by C. auronitens from multiple refuges after anthropogenic landscape changes caused forest fragmentation and led to species isolation.     

Repeated sampling detects gene flow in a flightless ground beetle in a fragmented landscape

Secondary clines level down in the course of time if the gene flow is not interrupted. Temporally repeated sampling of populations in a cline allows the investigation not only of its occurrence but also of the estimation of the amount of ongoing gene flow. We reinvestigated an allozyme gradient in Carabus auronitens populations in the Westphalian Lowlands (northwestern Germany) 15 to 20 years after it had originally been recorded. A total of 977 individuals of this flightless woodland species from 29 sample sites were genotyped at the diallelic Est-1 locus in 2005-2006 and compared to former findings, collected in 1985-1994 from the same populations. Both data sets showed clinal variation. Pairwise differences between the samples of both data sets indicated significant decrease in the steepness of the cline during the past 15 to 20 years. The estimated average gene flow per generation is 0.6% of each beetle population. Ongoing gene flow in the flightless ground beetle C. auronitens led to a less pronounced cline despite a stable degree of fragmentation (and connectivity) of the landscape. Migration and gene flow were obviously enabled by the numerous hedgerows. The corridors are seen to be a prerequisite for migration between populations and for possible future range shifts of forest insect species.     

Cork-oak woodlands as key-habitats for biodiversity conservation in Mediterranean landscapes: a case study using rove and ground beetles (Coleoptera: Staphylinidae, Carabidae)

Land-use intensification in Mediterranean agro-forest systems became a pressure on biodiversity, concerning particularly the woodland sensitive species. In 2001, the effects of a land-use gradient from old-growth cork-oak forest to a homogeneous agricultural area were assessed using rove beetles as indicators in a Mediterranean landscape. The aim was to find which species were negatively affected by land-use intensification at the landscape level and whether they benefited from cork-oak patches occurring along the land-use gradient. A total of 3,196 rove beetles from 88 taxa were sampled from all landscape types. Agricultural area recorded significantly higher numbers of abundance and species richness in relation to the cork-oak mosaics, i.e. the old-growth forest and the managed agro-forest landscapes (montados). Moreover, 70% of rove beetle indicator species common enough to be tested by IndVal displayed their highest indicator value for agriculture, showing a lower number of woodland indicators in comparison to ground beetles. Nevertheless, one rove beetle taxon was considered a specialist of closed woodland mosaics while no specialist ground beetle was found for that landscape typology. Some rare rove beetle species were also important in typifying diversity patterns of old-growth cork-oak forests. Hence, future management in Mediterranean landscapes should take into account not only indicator species common enough to be tested by IndVal, but also rare and endemic species. Considering the added value of cork-oak woodland cover for sensitive rove and ground beetle diversity, the strengthening of cork-oak woodland connectivity seems to be a crucial management that is required in agricultural Mediterranean landscapes.     

Temporally dynamic habitat suitability predicts genetic relatedness among caribou

Landscape heterogeneity plays a central role in shaping ecological and evolutionary processes. While species utilization of the landscape is usually viewed as constant within a year, the spatial distribution of individuals is likely to vary in time in relation to particular seasonal needs. Understanding temporal variation in landscape use and genetic connectivity has direct conservation implications. Here, we modelled the daily use of the landscape by caribou in Quebec and Labrador, Canada and tested its ability to explain the genetic relatedness among individuals. We assessed habitat selection using locations of collared individuals in migratory herds and static occurrences from sedentary groups. Connectivity models based on habitat use outperformed a baseline isolation-by-distance model in explaining genetic relatedness, suggesting that variations in landscape features such as snow, vegetation productivity and land use modulate connectivity among populations. Connectivity surfaces derived from habitat use were the best predictors of genetic relatedness. The relationship between connectivity surface and genetic relatedness varied in time and peaked during the rutting period. Landscape permeability in the period of mate searching is especially important to allow gene flow among populations. Our study highlights the importance of considering temporal variations in habitat selection for optimizing connectivity across heterogeneous landscape and counter habitat fragmentation.     

Wing-dimorphism and population expansion of Pterostichus melanarius (Illiger, 1798) at small and large scales in central Alberta, Canada (Coleoptera, Carabidae, Pterostichini)

A study spanning ten years revealed changes in wing-morph ratios corroborating the hypothesis that the wing-dimorphic introduced carabid, Pterostichus melanarius Ill.,is spreading through flight, from the city of Edmonton, Canada and establishing populations in natural aspen forest of more rural areas 45-50 km to the East. Comparison of wing-morph ratios between Pterostichus melanarius and the native wing dimorphic species Agonum retractum LeConte suggests that the spatial variation in ratios for Pterostichus melanarius does not reflect underlying environmental variation, but instead the action of selective forces on this wing-dimorphic species. About ten years after its earliest detection in some rural sites the frequency of macropterous individuals in Pterostichus melanarius has decreased c. five-fold, but it is still above the level seen in European populations in which the two wing-morphs are thought to exist in equilibrium. Pterostichus melanarius is expanding its range in native aspen forest much faster than three other introduced species Clivina fossor L.), Carabus granulatus O.F. Müllerand Clivina fossor L also encountered in this study. The two Carabus species are flightless, but Carabus fossor can be dimorphic. Although these four non-native ground beetle species comprise >85% of the carabids collected at sites in urban Edmonton, activity-density of native carabids was similar across the urban-rural gradient, suggesting little direct impact of introduced species on the local abundance of native species. In a second study conducted at a smaller scale near George Lake, Alberta, macropterous individuals of Pterostichus melanarius have penetrated furthest and most rapidly into native aspen forest. Furthermore, the percentage of micropterous individuals has increased markedly in areas first colonized a decade previously. Overall, these studies support the idea that macropterous beetles in wing-d dimorphic species are important vanguards for early colonization of unexploited territory, but that flightless individuals replace the flying morph relatively rapidly once populations are established.     

Species coherence in the face of karyotype diversification in holocentric organisms: the case of a cytogenetically variable sedge (Carex scoparia,Cyperaceae)

Background and Aims

The sedge genus Carex, the most diversified angiosperm genus of the northern temperate zone, is renowned for its holocentric chromosomes and karyotype variability. The genus exhibits high variation in chromosome numbers both among and within species. Despite the possibility that this chromosome evolution may play a role in the high species diversity of Carex, population-level patterns of molecular and cytogenetic differentiation in the genus have not been extensively studied.

Methods

Microsatellite variation (11 loci, 461 individuals) and chromosomal diversity (82 individuals) were investigated in 22 Midwestern populations of the North American sedge Carex scoparia and two Northeastern populations.

Key Results

Among Midwestern populations, geographic distance is the most important predictor of genetic differentiation. Within populations, inbreeding is high and chromosome variation explains a significant component of genetic differentiation. Infrequent dispersal among populations separated by >100 km explains an important component of molecular genetic and cytogenetic diversity within populations. However, karyotype variation and correlation between genetic and chromosomal variation persist within populations even when putative migrants based on genetic data are excluded.

Conclusions

These findings demonstrate dispersal and genetic connectivity among widespread populations that differ in chromosome numbers, explaining the phenomenon of genetic coherence in this karyotypically diverse sedge species. More generally, the study suggests that traditional sedge taxonomic boundaries demarcate good species even when those species encompass a high range of chromosomal diversity. This finding is important evidence as we work to document the limits and drivers of biodiversity in one of the world''s largest angiosperm genera.     

Landscape connectivity influences gene flow in a roe deer population inhabiting a fragmented landscape: an individual-based approach

Changes in agricultural practices and forest fragmentation can have a dramatic effect on landscape connectivity and the dispersal of animals, potentially reducing gene flow within populations. In this study, we assessed the influence of woodland connectivity on gene flow in a traditionally forest-dwelling species--the European roe deer--in a fragmented landscape. From a sample of 648 roe deer spatially referenced within a study area of 55 x 40 km, interindividual genetic distances were calculated from genotypes at 12 polymorphic microsatellite loci. We calculated two geographical distances between each pair of individuals: the Euclidean distance (straight line) and the 'least cost distance' (the trajectory that maximizes the use of wooded corridors). We tested the correlation between genetic pairwise distances and the two types of geographical pairwise distance using Mantel tests. The correlation was better using the least cost distance, which takes into account the distribution of wooded patches, especially for females (the correlation was stronger but not significant for males). These results suggest that in a fragmented woodland area roe deer dispersal is strongly linked to wooded structures and hence that gene flow within the roe deer population is influenced by the connectivity of the landscape.     

Degree of landscape fragmentation influences genetic isolation among populations of a gliding mammal

Forests and woodlands are under continuing pressure from urban and agricultural development. Tree-dependent mammals that rarely venture to the ground are likely to be highly sensitive to forest fragmentation. The Australian squirrel glider (Petaurus norfolcensis) provides an excellent case study to examine genetic (functional) connectivity among populations. It has an extensive range that occurs in a wide band along the east coast. However, its forest and woodland habitat has become greatly reduced in area and is severely fragmented within the southern inland part of the species' range, where it is recognised as threatened. Within central and northern coastal regions, habitat is much more intact and we thus hypothesise that genetic connectivity will be greater in this region than in the south. To test this we employed microsatellite analysis in a molecular population biology approach. Most sampling locations in the highly modified south showed signatures of genetic isolation. In contrast, a high level of genetic connectivity was inferred among most sampled populations in the more intact habitat of the coastal region, with samples collected 1400 km apart having similar genetic cluster membership. Nonetheless, some coastal populations associated with urbanisation and agriculture are genetically isolated, suggesting the historic pattern observed in the south is emerging on the coast. Our study demonstrates that massive landscape changes following European settlement have had substantial impacts on levels of connectivity among squirrel glider populations, as predicted on the basis of the species' ecology. This suggests that landscape planning and management in the south should be focused on restoring habitat connectivity where feasible, while along the coast, existing habitat connectivity must be maintained and recent losses restored. Molecular population biology approaches provide a ready means for identifying fragmentation effects on a species at multiple scales. Such studies are required to examine the generality of our findings for other tree-dependent species.     

Landscape genetics of a pollinator longhorn beetle [Typocerus v. velutinus (Olivier)] on a continuous habitat surface

Landscape connectivity, the degree to which the landscape structure facilitates or impedes organismal movement and gene flow, is increasingly important to conservationists and land managers. Metrics for describing the undulating shape of continuous habitat surfaces can expand the usefulness of continuous gradient surfaces that describe habitat and predict the flow of organisms and genes. We adopted a landscape gradient model of habitat and used surface metrics of connectivity to model the genetic continuity between populations of the banded longhorn beetle [Typocerus v. velutinus (Olivier)] collected at 17 sites across a fragmentation gradient in Indiana, USA. We tested the hypothesis that greater habitat connectivity facilitates gene flow between beetle populations against a null model of isolation by distance (IBD). We used next‐generation sequencing to develop 10 polymorphic microsatellite loci and genotype the individual beetles to assess the population genetic structure. Isolation by distance did not explain the population genetic structure. The surface metrics model of habitat connectivity explained the variance in genetic dissimilarities 30 times better than the IBD model. We conclude that surface metrology of habitat maps is a powerful extension of landscape genetics in heterogeneous landscapes.     

Woodland birds in patchy landscapes: the evidence base for strategic networks

Habitat creation and management within wooded networks is a potentially effective strategy to reduce ecological isolation and the deleterious effects of fragmentation. However, questions remain over the relative advantages of different approaches, e.g. buffering patches vs. increasing connectivity. Potential effects of woodland fragmentation include reduction in regional woodland cover, reduced patch size, edge effects with loss of core habitat, and increased isolation with disruption of dispersal and metapopulation dynamics. We adopt an evidence-based approach to review how each of these affects woodland birds with an emphasis on studies from the UK and use this to identify management priorities for mitigation. There is evidence for both patch area and composition effects: larger woodlands support more woodland bird species, and woods located within sparsely wooded landscapes are less valuable to specialist woodland species. Bird assemblages show a nested pattern with respect to area, and thus species found in small woods also occur in large woods but not vice versa. However, small woods may be preferred by a few edge species, while small woods also have greater variability in bird species composition. Consideration of the metapopulation dynamics of specialist species with poor dispersal shows that creating or buffering large woodlands is more efficient than a greater total area of small fragments. Connectivity appears most useful for widespread generalist species with almost continuous populations. Woodland structure and quality are of overwhelming importance: as well as mature woodland, young growth, scrub and edges are also key components. There is an urgent need to examine the relationship between nest predation and landscape structure within UK woodlands.     

Population ecology of the rare carabid beetle Carabus variolosus (Coleoptera: Carabidae) in north-west Germany

The declining European ground beetle Carabus variolosus, an indicator of undisturbed woodland brooks and swamps, is listed in the EU Species and Habitats Directive. Little is known about this flightless and endangered beetle, apart from its distribution range and habitat stenotopy. We present the results of a 2-year mark-release-recapture study on two neighbouring populations in Germany, which investigated basic parameters of population ecology that may be influential in the persistence of the beetle. Extremely small population sizes were the key findings. Since population densities corresponded to those of typical Carabus species, the size of the habitats was most probably the limiting factor. The risk of heavy losses during offspring development and hibernation may be alleviated by the considerable proportion of C. variolosus reproducing in more than 1 year. The fact that we did not recapture any marked specimens from the neighbouring population suggests a very limited dispersal power. The results imply a high vulnerability of the species, provide basic methodological information for an implementation of the EC Directive and for further research, and highlight the necessity to enlarge populated habitats.     

Landscape structure and the genetic effects of a population collapse

Both landscape structure and population size fluctuations influence population genetics. While independent effects of these factors on genetic patterns and processes are well studied, a key challenge is to understand their interaction, as populations are simultaneously exposed to habitat fragmentation and climatic changes that increase variability in population size. In a population network of an alpine butterfly, abundance declined 60–100% in 2003 because of low over-winter survival. Across the network, mean microsatellite genetic diversity did not change. However, patch connectivity and local severity of the collapse interacted to determine allelic richness change within populations, indicating that patch connectivity can mediate genetic response to a demographic collapse. The collapse strongly affected spatial genetic structure, leading to a breakdown of isolation-by-distance and loss of landscape genetic pattern. Our study reveals important interactions between landscape structure and temporal demographic variability on the genetic diversity and genetic differentiation of populations. Projected future changes to both landscape and climate may lead to loss of genetic variability from the studied populations, and selection acting on adaptive variation will likely occur within the context of an increasing influence of genetic drift.     

Preferred habitat and effective population size drive landscape genetic patterns in an endangered species

Landscape genetics provides a framework for pinpointing environmental features that determine the important exchange of migrants among populations. These studies usually test the significance of environmental variables on gene flow, yet ignore one fundamental driver of genetic variation in small populations, effective population size, N_e. We combined both approaches in evaluating genetic connectivity of a threatened ungulate, woodland caribou. We used least-cost paths to calculate matrices of resistance distance for landscape variables (preferred habitat, anthropogenic features and predation risk) and population-pairwise harmonic means of N_e, and correlated them with genetic distances, F_ST and D_c. Results showed that spatial configuration of preferred habitat and N_e were the two best predictors of genetic relationships. Additionally, controlling for the effect of N_e increased the strength of correlations of environmental variables with genetic distance, highlighting the significant underlying effect of N_e in modulating genetic drift and perceived spatial connectivity. We therefore have provided empirical support to emphasize preventing increased habitat loss and promoting population growth to ensure metapopulation viability.     

Explaining the saproxylic beetle diversity of a protected Mediterranean area

Saproxylic beetle diversity is high at the Cabañeros National Park (central Spain), where woodland habitats exhibit remarkable heterogeneity. Our aim was to explain the diversity of saproxylic beetles, focusing on species turnover among mature woodland types. We surveyed five woodland types that represented the heterogeneity of the park’s woodland habitats. Beetles were collected using window traps over a period of 20 months. The Jaccard Similarity Index was used as indirect value of beta diversity among woodlands and to test the relation between species turnover and geographical distance. We also identified the contribution of species turnover to landscape diversity by using a partitioning model. Moreover, the presence of mixed woodlands (more than one tree species) allowed us to attempt to valorise the effect of tree species (coupled with their historical management) on species turnover among woodlands. Finally, we looked for different saproxylic beetle preferences for habitat and tree species using an indicator value method. We found that saproxylic beetle species composition varied significantly among the studied woodlands. The variation in species turnover was independent from the distance among woodlands, which suggested that beetle dispersal abilities could not explain this high turnover. Tree species within woodlands were a key factor that increased diversity turnover in woodlands and, consequently, the diversity of the park. Moreover, we found saproxylic beetle species that had different habitat and tree species preferences. We conclude that woodland heterogeneity (highly affected by woodland composition) seems to be the driving force for saproxylic beetle diversity in this protected area.     

Interspecific gene flow in a multispecies oak hybrid zone in the Sierra Tarahumara of Mexico

Background and Aims

Interspecific gene flow can occur in many combinations among species within the genus Quercus, but simultaneous hybridization among more than two species has been rarely analysed. The present study addresses the genetic structure and morphological variation in a triple hybrid zone formed by Q. hypoleucoides, Q. scytophylla and Q. sideroxyla in north-western Mexico.

Methods

A total of 247 trees from ten reference and 13 presumed intermediate populations were characterized using leaf shape variation and geometric morphometrics, and seven nuclear microsatellites as genetic markers. Discriminant function analysis was performed for leaf shape variation, and estimates of genetic diversity and structure, and individual Bayesian genetic assignments were obtained.

Key Results

Reference populations formed three completely distinct groups according to discriminant function analysis based on the morphological data, and showed low, but significant, genetic differentiation. Populations from the zone of contact contained individuals morphologically intermediate between pairs of species in different combinations, or even among the three species. The Bayesian admixture analysis found that three main genetic clusters best fitted the data, with good correspondence of reference populations of each species to one of the genetic clusters, but various degrees of admixture evidenced in populations from the contact area.

Conclusions

The three oak species have formed a complex hybrid zone that is geographically structured as a mosaic, and comprising a wide range of genotypes, including hybrids between different species pairs, backcrosses and probable triple hybrids.     

Historical habitat connectivity affects current genetic structure in a grassland species

Many recent studies have explored the effects of present and past landscape structure on species distribution and diversity. However, we know little about the effects of past landscape structure on distribution of genetic diversity within and between populations of a single species. Here we describe the relationship between present and past landscape structure (landscape connectivity and habitat size estimated from historical maps) and current genetic structure in a perennial herb, Succisa pratensis. We used allozymes as co‐dominant markers to estimate genetic diversity and deviation from Hardy–Weinberg equilibrium in 31 populations distributed within a 5 km² agricultural landscape. The results showed that current genetic diversity of populations was related to habitat suitability, habitat age, habitat size and habitat connectivity in the past. The effects of habitat age and past connectivity on genetic diversity were in most cases also significant after taking the current landscape structure into account. Moreover, current genetic similarity between populations was affected by past connectivity after accounting for current landscape structure. In both cases, the oldest time layer (1850) was the most informative. Most populations showed heterozygote excess, indicating disequilibrium due to recent gene flow or selection against homozygotes. These results suggest that habitat age and past connectivity are important determinants of distribution of genetic diversity between populations at a scale of a few kilometres. Landscape history may significantly contribute to our understanding of distribution of current genetic structure within species and the genetic structure may be used to better understand landscape history, even at a small scale.     

Orchard and riparian habitats enhance ground dwelling beetle diversity in Mediterranean agro-forestry systems

The relative contribution of mixed orchard and riparian vegetation patches to local and regional diversity of Mediterranean landscapes dominated by cork-oak woodlands was tested in 2006–2007 using ground, rove and darkling beetles (Coleoptera: Carabidae, Staphylinidae, Tenebrionidae). Mixed orchard and riparian gallery habitats recorded higher values of abundance and species richness for overall beetle species, although most darkling beetle species were associated with the sclerophyllous cork-oak woodlands. Ground and rove beetle community structure changed from the orchard and riparian habitats to samples placed 100 m away into the surrounding cork-oak woodland, i.e., non-cork-oak patches enhanced beta diversity within the landscape mosaic. Analysis of ground beetle traits concerning moisture preferences revealed a higher proportion of hygrophilous species in mixed orchard and riparian gallery habitats while xerophilous species were dominant within the cork-oak woodlands. The results of this study suggested that land-use management promoting the maintenance of habitat heterogeneity enhances biodiversity conservation of important hygrophilous and xerophilous species, and subsequently the sustainable use of Mediterranean agro-forest mosaics.     

Aquatic insects in a sea of desert: population genetic structure is shaped by limited dispersal in a naturally fragmented landscape

Habitat requirements and landscape features can exert strong influences on the population structure of organisms. For aquatic organisms in particular, hydrologic requirements can dictate the extent of available habitat, and thus the degree of genetic connectivity among populations. We used a landscape genetics approach to evaluate hypotheses regarding the influence of landscape features on connectivity among populations of the giant water bug Abedus herberti (Hemiptera: Belostomatidae). Abedus herberti is restricted to naturally‐fragmented, perennial stream habitats in arid regions of North America. This species is exceptional because it is flightless at all life stages. Thus, we hypothesized a high degree of population genetic structure in A. herberti due to hydrologic constraints on habitat and low dispersal ability of the organism. A total of 617 individuals were sampled from 20 populations across southeastern Arizona, USA and genotyped at 10 microsatellite loci. We used a Bayesian clustering method to delineate genetic groups among populations. To determine which of six landscape variables (representing hypotheses of landscape‐level connectivity) has the strongest association with genetic connectivity in A. herberti, we used information‐theoretic model selection. Strong population structure was evident among A. herberti populations, even at small spatial scales. At a larger scale, A. herberti populations were hierarchically structured across the study region, with groups of related populations generally occurring in the same mountain range, rather than in the same major watershed. Surprisingly, stream network connectivity was not important for explaining among‐population patterns. Only the Curvature landscape variable was identified as having an association with genetic connectivity in A. herberti. The Curvature variable hypothesizes that gene flow tends to occur where local topography is concave, such as within stream drainages and dry gullies. Thus, our results suggest that population connectivity may depend on the shape of local overland topography rather than direct connectivity within stream drainage networks.