Urbanisation versus agriculture: a comparison of local genetic diversity and gene flow between wood mouse Apodemus sylvaticus populations in human‐modified landscapes

Urbanisation and agriculture dramatically modify the landscapes available for use by wildlife, affecting key aspects of their ecology such as survival, foraging, predation, competition and reproductive success. Relatively little is known about the effects of urbanisation and agriculture on the genetic structure, gene flow and genetic diversity of wild species. Here, landscape genetic techniques were applied to compare local genetic diversity and gene flow between wood mouse populations in urban and arable landscapes. Using nine microsatellite markers, individuals were genotyped from six arable and seven urban sample sites. Inter‐population genetic differentiation was significantly greater in urban than arable habitat, while allele richness, private allele richness and heterozygosity were higher for arable sample sites, with varying degrees of significance. These suggest that urban habitat was sufficiently fragmented to limit gene flow. To test the effect of landscape features on gene flow, several cost‐distance measures were generated. Overland distance and Euclidean distance correlated best with inter‐population genetic differentiation in arable habitat, whereas distances that accommodated differences in habitat quality better explained differentiation in urban habitat. There was no evidence that margins adjacent to roads, rivers or railways facilitated gene flow. Together, the results indicate that urban landscapes expose wood mice to greater fragmentation in habitat quality than arable areas, leading to greater population isolation that is not mitigated by the presence of dispersal corridors.     

A multiscale analysis of gene flow for the New England cottontail,an imperiled habitat specialist in a fragmented landscape

Landscape features of anthropogenic or natural origin can influence organisms' dispersal patterns and the connectivity of populations. Understanding these relationships is of broad interest in ecology and evolutionary biology and provides key insights for habitat conservation planning at the landscape scale. This knowledge is germane to restoration efforts for the New England cottontail (Sylvilagus transitionalis), an early successional habitat specialist of conservation concern. We evaluated local population structure and measures of genetic diversity of a geographically isolated population of cottontails in the northeastern United States. We also conducted a multiscale landscape genetic analysis, in which we assessed genetic discontinuities relative to the landscape and developed several resistance models to test hypotheses about landscape features that promote or inhibit cottontail dispersal within and across the local populations. Bayesian clustering identified four genetically distinct populations, with very little migration among them, and additional substructure within one of those populations. These populations had private alleles, low genetic diversity, critically low effective population sizes (3.2–36.7), and evidence of recent genetic bottlenecks. Major highways and a river were found to limit cottontail dispersal and to separate populations. The habitat along roadsides, railroad beds, and utility corridors, on the other hand, was found to facilitate cottontail movement among patches. The relative importance of dispersal barriers and facilitators on gene flow varied among populations in relation to landscape composition, demonstrating the complexity and context dependency of factors influencing gene flow and highlighting the importance of replication and scale in landscape genetic studies. Our findings provide information for the design of restoration landscapes for the New England cottontail and also highlight the dual influence of roads, as both barriers and facilitators of dispersal for an early successional habitat specialist in a fragmented landscape.     

Spatial genetic patterns indicate mechanism and consequences of large carnivore cohabitation within development

Patterns of human development are shifting from concentrated housing toward sprawled housing intermixed with natural land cover, and wildlife species increasingly persist in close proximity to housing, roads, and other anthropogenic features. These associations can alter population dynamics and evolutionary trajectories. Large carnivores increasingly occupy urban peripheries, yet the ecological consequences for populations established entirely within urban and exurban landscapes are largely unknown. We applied a spatial and landscape genetics approach, using noninvasively collected genetic data, to identify differences in black bear spatial genetic patterns across a rural‐to‐urban gradient and quantify how development affects spatial genetic processes. We quantified differences in black bear dispersal, spatial genetic structure, and migration between differing levels of development within a population primarily occupying areas with >6 houses/km² in western Connecticut. Increased development disrupted spatial genetic structure, and we found an association between increased housing densities and longer dispersal. We also found evidence that roads limited gene flow among bears in more rural areas, yet had no effect among bears in more developed ones. These results suggest dispersal behavior is condition‐dependent and indicate the potential for landscapes intermixing development and natural land cover to facilitate shifts toward increased dispersal. These changes can affect patterns of range expansion and the phenotypic and genetic composition of surrounding populations. We found evidence that subpopulations occupying more developed landscapes may be sustained by male‐biased immigration, creating potentially detrimental demographic shifts.     

Habitat fragmentation in forests affects relatedness and spatial genetic structure of a native rodent,Rattus lutreolus

Habitat fragmentation can have a range of negative demographic and genetic impacts on disturbed populations. Dispersal barriers can be created, reducing gene flow and increasing population differentiation and inbreeding in isolated habitat remnants. Aggregated retention is a form of forestry that retains patches of forests as isolated island or connected edge patches, with the aim of ‘lifeboating’ species and processes, retaining structural features and improving connectivity. Swamp rats (Rattus lutreolus) are a cover‐dependent species that are sensitive to habitat removal. We examined the effects of aggregated retention forestry and forestry roads in native wet Eucalyptus forests on swamp rat gene flow and population genetic structure. We characterized neighbourhood size in unlogged forest to provide a natural state for comparison, and examined population structure at a range of spatial scales, which provided context for our findings. Tests of pairwise relatedness indicated significant differentiation between island and edge populations in aggregated retention sites, and across roads in unlogged sites. Spatial autocorrelation suggested a neighbourhood size of 42–55 m and revealed male‐biased dispersal. We found no genetic isolation by geographical distance at larger (>2.3 km) scales and populations were all significantly differentiated. Our results suggest that removal of mature forest creates barriers for swamp rat dispersal. In particular, roads may have long‐term impacts, while harvesting of native forests is likely to create only short‐term dispersal barriers at the local scale, depending on the rate of regeneration.     

Genetic structure of the marsh frog (Pelophylax ridibundus) populations in urban landscape

Urbanization is a pervasive process causing habitat fragmentation, spatial isolation of populations, and reduction of biological diversity. In this study, we applied 11 microsatellite loci and Bayesian analyses to investigate genetic diversity and population structure in marsh frogs (Pelophylax ridibundus) living in two types of environment—highly fragmented urban landscapes, and landscapes characterized by the presence of a river and artificial canals. Our results show reduced genetic diversity, lower effective population sizes, and higher genetic differentiation for spatially isolated urban populations in comparison with populations outside intensely urbanized areas. Reduction of allelic diversity in urban localities isolated for 13–37 generations is more conspicuous than reduction of expected heterozygosity. Populations living close to the River Danube, its branches, and artificial canals are genetically more homogenous. Our results also suggest that the Danube in Bratislava is not a natural barrier to gene flow. In contrast, it acts as a natural corridor for water frog dispersal. Population structure of P. ridibundus also shows higher genetic connectivity within water paths than between them, suggesting limited overland dispersal, and reflects the historical landscape structure associated with the distribution of the lost river branches.     

Where movement happens: scale‐dependent landscape effects on genetic differentiation in the European tree frog

Functional connectivity among fragmented populations depends on the landscape matrix between occupied habitat patches and its effect on the frequency of animal movement and gene flow. The quantification of landscape effects on gene flow should therefore be scale‐dependent. Here, we explored the impact of different spatial scales in a landscape genetic analysis of the European tree frog Hyla arborea in a fragmented landscape in Switzerland. We examined the effects of landscape elements and geographic distance on genetic differentiation at three distance classes reflecting varying frequencies of tree frog movement. We calculated pairwise F_ST‐values and assembled 16 landscape elements within 1 km wide corridors between all pairs of tree frog breeding sites. Per distance class, we computed a multiple regression model with stepwise backward elimination and permutation testing. At distances of<2 km, only a larger river acted as a barrier to gene flow. At distances>2 km, geographic distance had a negative effect on gene flow as had landscape elements such as forests and roads. In general, hedgerows and various structure‐rich landscape elements positively affected gene flow. As we found distinct scale‐dependent landscape effects on gene flow, future landscape genetic studies should analyse the effects of landscape variables at different spatial dimensions relevant for the movement and dispersal of the study organisms. Corresponding studies should also carefully consider relevant correlations among the landscape elements tested and should preferentially replicate their analysis at the landscape‐level in order to avoid idiosyncratic results owing to the particular scale and landscape studied.     

Directed dispersal by rotational shepherding supports landscape genetic connectivity in a calcareous grassland plant

Directed dispersal by animal vectors has been found to have large effects on the structure and dynamics of plant populations adapted to frugivory. Yet, empirical data are lacking on the potential of directed dispersal by rotational grazing of domestic animals to mediate gene flow across the landscape. Here, we investigated the potential effect of large‐flock shepherding on landscape‐scale genetic structure in the calcareous grassland plant Dianthus carthusianorum, whose seeds lack morphological adaptations to dispersal to animals or wind. We found a significant pattern of genetic structure differentiating population within grazed patches of three nonoverlapping shepherding systems and populations of ungrazed patches. Among ungrazed patches, we found a strong and significant effect of isolation by distance (r = 0.56). In contrast, genetic distance between grazed patches within the same herding system was unrelated to geographical distance but significantly related to distance along shepherding routes (r = 0.44). This latter effect of connectivity along shepherding routes suggests that gene flow is spatially restricted occurring mostly between adjacent populations. While this study used nuclear markers that integrate gene flow by pollen and seed, the significant difference in the genetic structure between ungrazed patches and patches connected by large‐flock shepherding indicates the potential of directed seed dispersal by sheep across the landscape.     

Anthropogenic landscape change promotes asymmetric dispersal and limits regional patch occupancy in a spatially structured bird population

1. Local extinctions in habitat patches and asymmetric dispersal between patches are key processes structuring animal populations in heterogeneous environments. Effective landscape conservation requires an understanding of how habitat loss and fragmentation influence demographic processes within populations and movement between populations. 2. We used patch occupancy surveys and molecular data for a rainforest bird, the logrunner (Orthonyx temminckii), to determine (i) the effects of landscape change and patch structure on local extinction; (ii) the asymmetry of emigration and immigration rates; (iii) the relative influence of local and between-population landscapes on asymmetric emigration and immigration; and (iv) the relative contributions of habitat loss and habitat fragmentation to asymmetric emigration and immigration. 3. Whether or not a patch was occupied by logrunners was primarily determined by the isolation of that patch. After controlling for patch isolation, patch occupancy declined in landscapes experiencing high levels of rainforest loss over the last 100 years. Habitat loss and fragmentation over the last century was more important than the current pattern of patch isolation alone, which suggested that immigration from neighbouring patches was unable to prevent local extinction in highly modified landscapes. 4. We discovered that dispersal between logrunner populations is highly asymmetric. Emigration rates were 39% lower when local landscapes were fragmented, but emigration was not limited by the structure of the between-population landscapes. In contrast, immigration was 37% greater when local landscapes were fragmented and was lower when the between-population landscapes were fragmented. Rainforest fragmentation influenced asymmetric dispersal to a greater extent than did rainforest loss, and a 60% reduction in mean patch area was capable of switching a population from being a net exporter to a net importer of dispersing logrunners. 5. The synergistic effects of landscape change on species occurrence and asymmetric dispersal have important implications for conservation. Conservation measures that maintain large patch sizes in the landscape may promote asymmetric dispersal from intact to fragmented landscapes and allow rainforest bird populations to persist in fragmented and degraded landscapes. These sink populations could form the kernel of source populations given sufficient habitat restoration. However, the success of this rescue effect will depend on the quality of the between-population landscapes.     

Does long‐distance pollen dispersal preclude inbreeding in tropical trees? Fragmentation genetics of Dysoxylum malabaricum in an agro‐forest landscape

Tropical trees often display long‐distance pollen dispersal, even in highly fragmented landscapes. Understanding how patterns of spatial isolation influence pollen dispersal and interact with background patterns of fine‐scale spatial genetic structure (FSGS) is critical for evaluating the genetic consequences of habitat fragmentation. In the endangered tropical timber tree Dysoxylum malabaricum (Meliaceae), we apply eleven microsatellite markers with paternity and parentage analysis to directly estimate historic gene flow and contemporary pollen dispersal across a large area (216 km²) in a highly fragmented agro‐forest landscape. A comparison of genetic diversity and genetic structure in adult and juvenile life stages indicates an increase in differentiation and FSGS over time. Paternity analysis and parentage analysis demonstrate high genetic connectivity across the landscape by pollen dispersal. A comparison between mother trees in forest patches with low and high densities of adult trees shows that the frequency of short‐distance mating increases, as does average kinship among mates in low‐density stands. This indicates that there are potentially negative genetic consequences of low population density associated with forest fragmentation. Single isolated trees, in contrast, frequently receive heterogeneous pollen from distances exceeding 5 km. We discuss the processes leading to the observed patterns of pollen dispersal and the implications of this for conservation management of D. malabaricum and tropical trees more generally.     

Effects of structural connectivity on fine scale population genetic structure of muskrat,Ondatra zibethicus

In heterogeneous landscapes, physical barriers and loss of structural connectivity have been shown to reduce gene flow and therefore lead to population structuring. In this study, we assessed the influence of landscape features on population genetic structure and gene flow of a semiaquatic species, the muskrat. A total of 97 muskrats were sampled from three watersheds near Sudbury, Ontario, Canada. We estimated population genetic structure using 11 microsatellite loci and identified a single genetic cluster and no genetic differences were found among the watersheds as a result of high levels of gene flow. At finer scales, we assessed the correlation between individual pairwise genetic distances and Euclidean distance as well as different models of least cost path (LCP). We used a range of cost values for the landscape types in order to build our LCP models. We found a positive relationship between genetic distance and least cost distance when we considered roads as corridors for movements. Open landscapes and urban areas seemed to restrict but not prevent gene flow within the study area. Our study underlines the high‐dispersal ability of generalist species in their use of landscape and highlights how landscape features often considered barriers to animal movements are corridors for other species.     

The influence of landscape on gene flow in the eastern massasauga rattlesnake (Sistrurus c. catenatus): insight from computer simulations

Understanding how gene flow shapes contemporary population structure requires the explicit consideration of landscape composition and configuration. New landscape genetic approaches allow us to link such heterogeneity to gene flow within and among populations. However, the attribution of cause is difficult when landscape features are spatially correlated, or when genetic patterns reflect past events. We use spatial Bayesian clustering and landscape resistance analysis to identify the landscape features that influence gene flow across two regional populations of the eastern massasauga rattlesnake, Sistrurus c. catenatus. Based on spatially explicit simulations, we inferred how habitat distribution modulates gene flow and attempted to disentangle the effects of spatially confounded landscape features. We found genetic clustering across one regional landscape but not the other, and also local differences in the effect of landscape on gene flow. Beyond the effects of isolation‐by‐distance, water bodies appear to underlie genetic differentiation among individuals in one regional population. Significant effects of roads were additionally detected locally, but these effects are possibly confounded with the signal of water bodies. In contrast, we found no signal of isolation‐by‐distance or landscape effects on genetic structure in the other regional population. Our simulations imply that these local differences have arisen as a result of differences in population density or tendencies for juvenile rather than adult dispersal. Importantly, our simulations also demonstrate that the ability to detect the consequences of contemporary anthropogenic landscape features (e.g. roads) on gene flow may be compromised when long‐standing natural features (e.g. water bodies) co‐exist on the landscape.     

Urbanization drives community shifts towards thermophilic and dispersive species at local and landscape scales

The increasing conversion of agricultural and natural areas to human‐dominated urban landscapes is predicted to lead to a major decline in biodiversity worldwide. Two conditions that typically differ between urban environments and the surrounding landscape are increased temperature, and high patch isolation and habitat turnover rates. However, the extent and spatial scale at which these altered conditions shape biotic communities through selection and/or filtering on species traits are currently poorly understood. We sampled carabid beetles at 81 sites in Belgium using a hierarchically nested sampling design wherein three local‐scale (200 × 200 m) urbanization levels were repeatedly sampled across three landscape‐scale (3 × 3 km) urbanization levels. First, we showed that communities sampled in the most urbanized locations and landscapes displayed a distinct species composition at both local and landscape scale. Second, we related community means of species‐specific thermal preferences and dispersal capacity (based on European distribution and wing morphology, respectively) to the urbanization gradients. We showed that urban communities consisted on average of species with a preference for higher temperatures and with better dispersal capacities compared to rural communities. These shifts were caused by an increased number of species tolerating higher temperatures, a decreased richness of species with low thermal preference, and an almost complete depletion of species with very low‐dispersal capacity in the most urbanized localities. Effects of urbanization were most clearly detected at the local scale, although more subtle effects could also be found at the scale of entire landscapes. Our results demonstrate that urbanization may fundamentally and consistently alter species composition by exerting a strong filtering effect on species dispersal characteristics and favouring replacement by warm‐dwelling species.     

Simulation modeling reveals the evolutionary role of landscape shape and species dispersal on genetic variation within a metapopulation

Different shapes of landscape boundaries can affect the habitat networks within them and consequently the spatial genetic-patterns of a metapopulation. In this study, we used a mechanistic framework to evaluate the effects of landscape shape, through watershed elongation, on genetic divergence among populations at the metapopulation scale. Empirical genetic data from four, sympatric stream-macroinvertebrates having aerial adults were collected from streams in Japan to determine the roles of species-specific dispersal strategies on metapopulation genetics. Simulation results indicated that watershed elongation allows the formation of river networks with fewer branches and larger topographic constraints. This results in decreased interpopulation connectivity but a lower level of spatial isolation of distal populations (e.g. those found in headwaters) occurring in the landscapes examined. Distal populations had higher genetic divergence when their downstream-biased dispersal (relative to upstream- and/or overland-biased dispersal) was high. This underscores the importance of distal populations influencing genetic divergence at the metapopulation scale for species having downstream-biased dispersal. In turn, lower genetic divergence was observed under watershed elongation when the genetic isolation of distal populations was decreased in such species. This strong association between landscape shape and evolutionary processes highlights the importance of natural, spatial architecture in assessing the effectiveness of conservation and management strategies.     

Connectivity of prairie dog colonies in an altered landscape: inferences from analysis of microsatellite DNA variation

Connectivity of populations influences the degree to which species maintain genetic diversity and persist despite local extinctions. Natural landscape features are known to influence connectivity, but global anthropogenic landscape change underscores the importance of quantifying how human-modified landscapes disrupt connectivity of natural populations. Grasslands of western North America have experienced extensive habitat alteration, fragmenting populations of species such as black-tailed prairie dogs (Cynomys ludovicianus). Population sizes and the geographic range of prairie dogs have been declining for over a century due to habitat loss, disease, and eradication efforts. In many places, prairie dogs have persisted in the face of emerging urban landscapes that carve habitat into smaller and smaller fragments separated by uninhabitable areas. In extreme cases, prairie dog colonies are completely bounded by urbanization. Connectivity is particularly important for prairie dogs because colonies suffer high probabilities of extirpation by plague, and dispersal permits recolonization. Here we explore connectivity of prairie dog populations using analyses of 11 microsatellite loci for 9 prairie dog colonies spanning the fragmented landscape of Boulder County, Colorado. Isolation-by-resistance modeling suggests that wetlands and high intensity urbanization limit movement of prairie dogs. However, prairie dogs appear to move moderately well through low intensity development (including roads) and freely through cropland and grassland. Additionally, there is a marked decline in gene flow between colonies with increasing geographic distance, indicating isolation by distance even in an altered landscape. Our results suggest that prairie dog colonies retain some connectivity despite fragmentation by urbanization and agricultural development.     

The genetic structure of populations of Metrioptera bicolor in a spatially structured landscape: effects of dispersal barriers and geographic distance

The stability and long-term survival of animal populations in fragmented landscapes largely depends on the colonisation of habitat patches and the exchange of individuals between patches. The degree of inter-patch dispersal, in turn, depends on the dispersal abilities of species and the landscape structure (i.e. the nature of the landscape matrix and habitat distribution). Here, we investigated the genetic structure of populations of Metrioptera bicolor, a wing-dimorphic bush cricket, in a spatially structured landscape with patches of suitable habitat distributed within a diverse matrix of different habitat types. Using six microsatellite markers, we assessed the effects of geographic distance and different matrix types on the extent of genetic differentiation among 24 sampling sites. We found that forest and a river running through the study area both impede inter-patch dispersal. The presence of these two matrix types was positively correlated with the extent of genetic differentiation between sites. In addition, we found a significant positive correlation between pairwise genetic and geographic distances for a subsample of sites which were separated only by arable land or settlements. For the complete data set, this correlation could not be found. This is most probably because the adverse effect of forest and river on gene flow dominates the effect of geographic distance in our limited set of patches. Our analyses clearly emphasize the differential resistance of different habitat types on dispersal and the importance of a more detailed view on matrix “quality” in metapopulation studies.     

Landscape configuration determines gene flow and phenotype in a flightless forest-edge ground-dwelling bush-cricket, Pholidoptera griseoaptera

Spatial configuration of habitats influences genetic structure and population fitness whereas it affects mainly species with limited dispersal ability. To reveal how habitat fragmentation determines dispersal and dispersal-related morphology in a ground-dispersing insect species we used a bush-cricket (Pholidoptera griseoaptera) which is associated with forest-edge habitat. We analysed spatial genetic patterns together with variability of the phenotype in two forested landscapes with different levels of fragmentation. While spatial configuration of forest habitats did not negatively affect genetic characteristics related to the fitness of sampled populations, genetic differentiation was found higher among populations from an extensive forest. Compared to an agricultural matrix between forest patches, the matrix of extensive forest had lower permeability and posed barriers for the dispersal of this species. Landscape configuration significantly affected also morphological traits that are supposed to account for species dispersal potential; individuals from fragmented forest patches had longer hind femurs and a higher femur to pronotum ratio. This result suggests that selection pressure act differently on populations from both landscape types since dispersal-related morphology was related to the level of habitat fragmentation. Thus observed patterns may be explained as plastic according to the level of landscape configuration; while anthropogenic fragmentation of habitats for this species can lead to homogenization of spatial genetic structure.     

Urban land use limits regional bumble bee gene flow

Potential declines in native pollinator communities and increased reliance on pollinator‐dependent crops have raised concerns about native pollinator conservation and dispersal across human‐altered landscapes. Bumble bees are one of the most effective native pollinators and are often the first to be extirpated in human‐altered habitats, yet little is known about how bumble bees move across fine spatial scales and what landscapes promote or limit their gene flow. In this study, we examine regional genetic differentiation and fine‐scale relatedness patterns of the yellow‐faced bumble bee, Bombus vosnesenskii, to investigate how current and historic habitat composition impact gene flow. We conducted our study across a landscape mosaic of natural, agricultural and urban/suburban habitats, and we show that B. vosnesenskii exhibits low but significant levels of differentiation across the study system (F_ST = 0.019, D_est = 0.049). Most importantly, we reveal significant relationships between pairwise F_ST and resistance models created from contemporary land use maps. Specifically, B. vosnesenskii gene flow is most limited by commercial, industrial and transportation‐related impervious cover. Finally, our fine‐scale analysis reveals significant but declining relatedness between individuals at the 1–9 km spatial scale, most likely due to local queen dispersal. Overall, our results indicate that B. vosnesenskii exhibits considerable local dispersal and that regional gene flow is significantly limited by impervious cover associated with urbanization.     

Gene flow and genetic drift in urban environments

Evidence is growing that human modification of landscapes has dramatically altered evolutionary processes. In urban population genetic studies, urbanization is typically predicted to act as a barrier that isolates populations of species, leading to increased genetic drift within populations and reduced gene flow between populations. However, urbanization may also facilitate dispersal among populations, leading to higher genetic diversity within, and lower differentiation between, urban populations. We reviewed the literature on nonadaptive urban evolution to evaluate the support for each of these urban fragmentation and facilitation models. In a review of the literature with supporting quantitative analyses of 167 published urban population genetics studies, we found a weak signature of reduced within‐population genetic diversity and no evidence of consistently increased between‐population genetic differentiation associated with urbanization. In addition, we found that urban landscape features act as barriers or conduits to gene flow, depending on the species and city in question. Thus, we speculate that dispersal ability of species and environmental heterogeneity between cities contributes to the variation exhibited in our results. However, >90% of published studies reviewed here showed an association of urbanization with genetic drift or gene flow, highlighting the strong impact of urbanization on nonadaptive evolution. It is clear that species biology and city heterogeneity obscure patterns of genetic drift and gene flow in a quantitative analysis. Thus, we suggest that future research makes comparisons of multiple cities and nonurban habitats, and takes into consideration species' natural history, environmental variation, spatial modelling and marker selection.     

Genetic discontinuities and disequilibria in recently established populations of the plant pathogenic fungus Mycosphaerella fijiensis

Dispersal processes of fungal plant pathogens can be inferred from analysis of spatial genetic structures resulting from recent range expansion. The relative importance of long‐distance dispersal (LDD) events vs. gradual dispersal in shaping population structures depends on the geographical scale considered. The fungus Mycosphaerella fijiensis, pathogenic on banana, is an example of a recent worldwide epidemic. Founder effects in this species were detected at both global and continental scale, suggesting stochastic spread of the disease through LDD events. In this study, we analysed the structure of M. fijiensis populations in two recently (∼1979–1980) colonized areas in Costa Rica and Cameroon. Isolates collected in 10–15 sites distributed along a ∼250‐ to 300‐ km‐long transect in each country were analysed using 19 microsatellite markers. We detected low‐to‐moderate genetic differentiation among populations in both countries and isolation by distance in Cameroon. Combined with historical data, these observations suggest continuous range expansion at the scale of banana‐production area through gradual dispersal of spores. However, both countries displayed specific additional signatures of colonization: a sharp discontinuity in gene frequencies was observed along the Cameroon transect, while the Costa Rican populations seemed not yet to have reached genetic equilibrium. These differences in the genetic characteristics of M. fijiensis populations in two recently colonized areas are discussed in the light of historical data on disease spread and ecological data on landscape features.     

Gap-crossing movements predict species occupancy in Amazonian forest fragments

In fragmented landscapes, species persistence within isolated habitat patches is governed by a myriad of species life‐history, habitat patch and landscape characteristics. We investigated the inter‐specific variation in non‐forest gap‐crossing abilities of an entire tropical forest‐dependent avifauna. We then related this measure of dispersal ability to species life‐history characteristics and occupancy data from 31 variable‐sized forest patches sampled within the same fragmented forest landscape. A total of 5436 gap‐crossing movements of 231 forest‐dependent bird species were observed across ten linear forest gaps of varying widths, adjacent to large areas of undisturbed forest. Species persistence in isolated fragments was strongly linked to gap‐crossing ability. The most capable gap‐crossers were medium to large‐bodied species in the large insectivore, frugivore and granivore guilds, matching the most prevalent subset of species in small forest patches. However, some competent gap‐crossing species failed to occur in small patches, and minimum forest‐patch area requirements were more important in determining patch occupancy for these species. Narrow forest gaps (4–70 m) created by roads and power‐lines may become territory boundaries, thereby eliminating home‐range gap‐crossing movements for many forest species, but permit rarer dispersal events. Wider gaps (>70 m) may inhibit gap‐crossing behaviour for all but the most vagile species. Although patch size and quality may be the most important factors in structuring species assemblages in forest fragments, our results show that the degree of patch isolation and permeability of the surrounding matrix also explain which species can persist in forest isolates. Reducing the number and width of forest‐dividing gaps; maintaining and/or creating forest corridors and increasing matrix permeability through the creation and maintenance of ‘stepping‐stone’ structures will maximise the species retention in fragmented tropical forest landscapes.