Range-edge Populations Research Articles

Elevated Genetic Structure in the Coastal Tailed Frog (Ascaphus truei) in Managed Redwood Forests

Landscape alterations have dramatic impacts on the distribution of genetic variation within and among populations and understanding these effects can guide contemporary and future conservation strategies. We initiated a landscape-scale genetic study of the coastal tailed frog (Ascaphus truei) on commercial timberlands within the southern range of the species in Mendocino County (CA, USA). In total, 294 individuals from 13 populations were analyzed at 9 microsatellite loci. None of the sampled populations departed from mutation-drift equilibrium, indicating recent population bottlenecks were not detected in contemporary samples. Fine-scale analysis indicated sampled populations were structured at the watershed level (mean F (ST) = 0.077 and mean G'(ST) = 0.425). Landscape analyses suggested wet and moist areas may serve as significant corridors for gene flow within watersheds in this region (r (2) = 0.32-0.54 for moisture-related features). Results indicate populations of frogs may have persisted at this scale through intense periods of timber harvest, making southern range edge populations of coastal tailed frogs resilient to past land use practices.

Interactions between soil habitat and geographic range location affect plant fitness.

Populations are often found on different habitats at different geographic locations. This habitat shift may be due to biased dispersal, physiological tolerances or biotic interactions. To explore how fitness of the native plant Chamaecrista fasciculata depends on habitat within, at and beyond its range edge, we planted seeds from five populations in two soil substrates at these geographic locations. We found that with reduced competition, lifetime fitness was always greater or equivalent in one habitat type, loam soils, though early-season survival was greater on sand soils. At the range edge, natural populations are typically found on sand soil habitats, which are also less competitive environments. Early-season survival and fitness differed among source populations, and when transplanted beyond the range edge, range edge populations had greater fitness than interior populations. Our results indicate that even when the optimal soil substrate for a species does not change with geographic range location, the realized niche of a species may be restricted to sub-optimal habitats at the range edge because of the combined effects of differences in abiotic and biotic effects (e.g. competitors) between substrates.

Rapid evolution of parasite life history traits on an expanding range‐edge

Parasites of invading species undergoing range advance may be exposed to powerful new selective forces. Low host density in range-edge populations hampers parasite transmission, requiring the parasite to survive longer periods in the external environment before encountering a potential host. These conditions should favour evolutionary shifts in offspring size to maximise parasite transmission. We conducted a common-garden experiment to compare life history traits among seven populations of the nematode lungworm (Rhabdias pseudosphaerocephala) spanning from the parasite population core to the expanding range-edge in invasive cane toads (Rhinella marina) in tropical Australia. Compared to conspecifics from the population core, nematodes from the range-edge exhibited larger eggs, larger free-living adults and larger infective larvae, and reduced age at maturity in parasitic adults. These results support a priori predictions regarding adaptive changes in offspring size as a function of invasion history, and suggest that parasite life history traits can evolve rapidly in response to the selective forces exerted by a biological invasion.

Can limited dispersal or biotic interaction explain the declining abundance of the whelk, Morula marginalba, at the edge of its range?

The southern range limit of the planktotrophic, intertidal whelk, Morula marginalba, occurs immediately to the north of the south-east Australian biogeographical barrier and we predict that its range may be limited by dispersal and biotic interactions. The barrier includes several hundred kilometers without suitable habitat and is the convergence zone of major ocean currents. Within the barrier region, along-shore larval transport is expected to be erratic, with water moving offshore to form long-lived eddies. Through comparison of central and range edge populations, we test for evidence of recruitment limitation and interspecific competition with the whelk, Haustrum vinosum, which occurs sympatrically at the range edge. We found similar levels of recruitment and allozyme genetic variation within centre and edge populations where H. vinosum was absent, implying that populations are strongly interconnected. Nevertheless, recruitment and abundance of M. marginalba were lower at its extreme range limit, where it occurs with H. vinosum. Indeed, we detected significant inverse correlations between the abundance of these species, although their habitat use was similar and they consumed similar prey. These lines of evidence suggest a restricted recruitment across the barrier limits the range of M. marginalba but emphasize that multiple factors may act to set species range limits. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 849–862.

High clonal diversity in threatened peripheral populations of the yellow bird's nest (Hypopitys monotropa; syn. Monotropa hypopitys).

Peripheral populations of plant species are often characterized by low levels of genetic diversity as a result of genetic drift, restricted gene flow, inbreeding and asexual reproduction. These effects can be exacerbated where range-edge populations are fragmented. The main aim of the present study was to assess the levels of genetic diversity in remnant populations of Hypopitys monotropa (syn. Monotropa hypopitys; yellow bird's nest) at the edge of the species' European range in Northern Ireland, since these remnant populations are small and highly fragmented. Every plant found through surveys of 21 extant populations was genotyped for eight microsatellite loci to estimate levels and patterns of genetic diversity and clonality. Levels of genetic diversity were relatively high in the populations studied, and the incidence of clonal reproduction was generally low, with a mean of only 14·45 % of clonal individuals. Clones were small and highly spatially structured. Levels of inbreeding, however, were high. The observed low levels of clonality suggest that the majority of genets in the populations of H. monotropa studied are fertile and that reproduction is predominantly sexual. As the species is highly self-compatible, it is likely that the high levels of inbreeding observed in the populations in the present study are the result of self-pollination, particularly given the small numbers of individuals in most of the patches. Given this extent of inbreeding, further genetic monitoring would be advisable to ensure that genetic diversity is maintained.

Long-term approaches to native woodland restoration: Palaeoecological and stakeholder perspectives on Atlantic forests of Northern Europe

The long timeframes involved in woodland regeneration and adaptation introduce considerable uncertainty into management and conservation planning as most ecological datasets span only a small part of ecosystem dynamics. This is a particular concern in vulnerable habitats, such as Atlantic birch-oak woods in north-western Britain, where range edge populations are at risk from herbivory and climate change. This study combines historical palaeoecology and stakeholder observations to assess how multiple perspectives can inform existing models, expectations and goals for Atlantic woodland management. Long-term evidence for changing woodland composition addresses stakeholder uncertainties over the currently restricted distribution of oak, alder and hazel. Oak has remained a secondary component of birch-dominated woods and habitat definitions based on oak are too narrow to ensure sustainability. Birch has survived numerous periods of climatic and biotic adversity and shows the strongest positive response to historic reductions in grazing pressure. This has led to a pulse of regeneration since c. AD 1900, so current restoration efforts are taking place within the strongest period of woodland expansion in the last c. 500 years. Positive and negative cultural legacies are evident and intervention is considered a necessary restoration tool. Managed grazing at key stages of habitat development can support continued recruitment and regeneration. A more flexible and integrated management approach is recommended. Long-term ecology can provide direction and address uncertainties, while ecological and stakeholder evidence provide the detail necessary to develop effective management that incorporates ecosystem perspectives. This can help shape management strategies that balance possible conflicts over perceived short-term ‘damage’ to secure longer-term processes. Learning based on multiple temporal perspectives has the potential to contribute to conservation and restoration planning and practice. These principles are more widely applicable in adaptive management.

Similar performance in central and range-edge populations of a Eurasian steppe grass under different climate and soil pH regimes

The abundant centre hypothesis predicts that changing environmental conditions are detrimental to a species' abundance and performance towards the periphery of its range. We tested these predictions for the perennial grass Stipa capillata, a species that is commonly found in steppes of Asia but is rare at its north-western range edge, in central Europe. We compared 21 populations in dry grassland fragments in central Europe and 20 populations in steppe habitats of Kazakhstan. We studied 15 plant performance traits both in situ and under laboratory and common greenhouse conditions, including local density, plant size and biomass production, seed size, weight and viability. To assess environmental conditions, we assembled data on topography, soil properties and climatic parameters. Using variance components analysis and multivariate methods we analyzed whether plant performance and environmental attributes differed more, as predicted, between the core and peripheral regions or whether they differed more among their subregions or populations. Additionally, we tested whether performance was affected by the same set of environmental predictors in each region. Contrary to our expectations, plant performance traits were virtually unchanged between the range periphery and centre. As expected, macroclimatic conditions showed a significant difference between the two regions (annual mean temperature, annual precipitation). The only other measured environmental variable that differed significantly between the two regions was soil pH, which was lower in core-range populations. Our findings thus do not support the notion of reduced performance at the range edge. Instead, our data lend support to earlier theories of relative habitat constancy, suggesting that peripheral populations can shift to other habitats through plasticity or adaptation.

Range edges and the molecular divergence of Greya moth populations

Aim We investigated the geographical pattern of genetic divergence and demographic history in the prodoxid moth Greya obscura throughout its entire geographical range in far western North America and compared it to the geographical patterns found in a previously studied species, Greya politella, which co-occurs over the same range, in the same habitats, and on the same host plants. Location The study included sites distributed throughout the California Floristic Province. Methods We used analysis of cytochrome c oxidase subunit I (COI) and amplified fragment length polymorphisms to evaluate the pattern and history of genetic continuity among populations. Results Greya obscura populations show a history of spatial expansion with considerable haplotype diversity in the centre of the geographical range. As with G. politella, some range-edge populations of G. obscura are sufficiently divergent (6.7% in COI) to be considered as potentially cryptic species. Greya obscura and G. politella, however, differ in the specific range-edge sites showing greatest genetic divergence and cryptic speciation. Main conclusions These results corroborate the view that range edges are important cradles of divergence and speciation. In addition, the results indicate that the geographical pattern of divergence at edges may differ even among closely related species occupying the same habitats and using the same hosts.

Unidirectional hybridization at a species’ range boundary: implications for habitat tracking

AbstractAim Introgressive hybridization between a locally rare species and a more abundant congener can drive population extinction via genetic assimilation, or the replacement of the rare species gene pool with that of the common species. To date, however, few studies have assessed the effects of such processes at the limits of species’ distribution ranges. In this study, we have examined the potential for hybridization between range‐edge populations of the wintergreenPyrola minorand sympatric populations ofPyrola grandiflora.Location Qeqertarsuaq, Greenland and Churchill, Manitoba, Canada.Methods Genetic analysis of samples from Greenland and Canada was carried out using a combination of nuclear and chloroplast single nucleotide polymorphisms (SNPs).Results Analysis of nuclear SNPs confirmed hybridization in populations of morphologically intermediate individuals, as well as revealing the existence of cryptic hybrids in ostensibly morphologically pureP. minorpopulations. Analysis of chloroplast SNPs revealed that this hybridization is unidirectional and suggests that hybrids originate via pollen swamping ofP. minorby the more commonP. grandiflora.Main conclusions Extensive unidirectional hybridization may lead to the extinction of peripheral populations ofP. minorwhere the two species grow sympatrically. Extinction could occur as a result of genetic assimilation where F1s are fertile, or via the removal of unidirectionally pollinated sterile F1s, or by a combination of these processes. This could compromise the ability of species to respond to climate change via habitat tracking, although the final outcome of these processes may ultimately depend on the rate of global climate change and its effect on the species’ distributions.

Characterisation of major histocompatibility complex class II alleles in the natterjack toad, Bufo calamita

Major histocompatibility complex (MHC) class II genes play a major role in the vertebrate immune response, and are useful indicators of adaptive variation in wild populations. We developed PCR primers for part (114 bp excluding primers, coding for 38 amino acids) of the second exon of two expressed MHC class II genes (designated A and B) in the natterjack toad Bufo calamita. We detected five alleles in 200 individuals from range-edge populations in Britain, two of which we ascribed to locus A and three to locus B. Seven out of 10 variable amino acid sites were associated with antigen binding. In a discrete study population we detected a significant excess of non-synonymous substitutions at antigen binding sites, and a heterozygote excess, at locus A indicating that this gene is under positive selection. However, there were no significant excesses of non-synonymous substitutions or heterozygotes at locus B.

Recent introduction or ancient ancestry? Use of genetic evidence to investigate the origins of range edge populations in natterjack toads (Bufo calamita)

The ability to distinguish between native and translocated populations is important in conservation biology, but can be problematic, especially near range edges. We describe here the use of variable genetic markers as an approach to resolving this issue. Natterjack toads Bufo calamita have a very restricted distribution in south-west Ireland. Populations in the main distribution area are well documented, but there are also some recently discovered ones at a widely separated site. We used genetic evidence to investigate whether the recently discovered populations are newly established or of ancient origin, using three criteria: genetic diversity, genetic structure and estimated time since divergence. The results highlighted some difficulties of using genetic data from range edge populations, where such issues are most likely to arise but where diversity (and thus statistical power) is often low. On balance the genetic data weakly supported the hypothesis that the newly discovered populations are not recent translocations. It will always be important in such investigations to use genetic inferences in combination with other independent lines of evidence (such as fossils) wherever possible.

Competition at the range boundary in the slimy salamander: using reciprocal transplants for studies on the role of biotic interactions in spatial distributions

1. Determining the factors that influence the distribution of species has been a longstanding goal in the field of ecology. New techniques such as ecological niche modelling have the potential to aid in addressing many broad questions in ecology, evolutionary biology, and behavioural ecology. 2. This study combines broad-scale ecological niche models with fine-scaled studies of biotic interactions to examine how abiotic and biotic interactions affect the spatial distribution of the terrestrial salamander species Plethodon glutinosus (northern slimy salamander), in a potential contact zone shared with Plethodon mississippi (Mississippi slimy salamander). 3. The core habitat in the interior portion of the range of P. glutinosus and the contact zone are distributed in unique environmental niche space. 4. The form of competition, inter- or intraspecific, significantly affected mass loss of adult salamanders. Salamanders lost more mass when interacting with a heterospecific. 5. Abiotic conditions strongly influenced the impact of competition on salamanders. Under stressful environmental conditions at the field site located in the contact zone, salamanders lost more mass than at the field site located in the interior of the range. 6. Furthermore, adult salamanders from range-edge populations and core populations (from the interior of the range) differed in their respective abilities to compete under the abiotic conditions in the contact zone.

Cirsium species show disparity in patterns of genetic variation at their range-edge, despite similar patterns of reproduction and isolation.

• Genetic variation was assessed across the UK geographical range of Cirsium acaule and Cirsium heterophyllum. A decline in genetic diversity and increase in population divergence approaching the range edge of these species was predicted based on parallel declines in population density and seed production reported seperately. Patterns were compared with UK populations of the widespread Cirsium arvense. • Populations were sampled along a latitudinal transect in the UK and genetic variation assessed using microsatellite markers. • Cirsium acaule shows strong isolation by distance, a significant decline in diversity and an increase in divergence among range-edge populations. Geographical structure is also evident in C. arvense, whereas no such patterns are seen in C. heterophyllum. • There is a major disparity between patterns of genetic variation in C. acaule and C. heterophyllum despite very similar patterns in seed production and population isolation in these species. This suggests it may be misleading to make assumptions about the geographical structure of genetic variation within species based solely on the present-day reproduction and distribution of populations.

Natal dispersal and personalities in great tits (Parus major).

Dispersal is a major determinant of the dynamics and genetic structure of populations, and its consequences depend not only on average dispersal rates and distances, but also on the characteristics of dispersing and philopatric individuals. We investigated whether natal dispersal correlated with a predisposed behavioural trait: exploratory behaviour in novel environments. Wild great tits were caught in their natural habitat, tested the following morning in the laboratory using an open field test and released at the capture site. Natal dispersal correlated positively with parental and individual exploratory behaviour, using three independent datasets. First, fast-exploring parents had offspring that dispersed furthest. Second, immigrants were faster explorers than locally born birds. Third, post-fledging movements, comprising a major proportion of the variation in natal dispersal distances, were greater for fast females than for slow females. These findings suggest that parental behaviour influenced offspring natal dispersal either via parental behaviour per se (e.g. via post-fledging care) or by affecting the phenotype of their offspring (e.g. via their genes). Because this personality trait has a genetic basis, our results imply that genotypes differ in their dispersal distances. Therefore, the described patterns have profound consequences for the genetic composition of populations.