Influence of elevation on bark beetle (Coleoptera: Curculionidae, Scolytinae) community structure and flight periodicity in ponderosa pine forests of Arizona

We examined abundance and flight periodicity of five Ips and six Dendroctonus species (Coleoptera: Curculionidae, Scolytinae) among three different elevation bands in ponderosa pine (Pinus ponderosa Douglas ex. Lawson) forests of northcentral Arizona. Bark beetle populations were monitored at 10 sites in each of three elevation bands (low: 1,600-1,736 m; middle: 2,058-2,230 m; high: 2,505-2,651 m) for 3 yr (2004-2006) using pheromone-baited Lindgren funnel traps. Trap contents were collected weekly from March to December. We also studied temperature differences among the elevation bands and what role this may play in beetle flight behavior. Bark beetles, regardless of species, showed no consistent elevational trend in abundance among the three bands. The higher abundances of Ips lecontei Swaine, I. calligraphus ponderosae Swaine, Dendroctonus frontalis Zimmerman, and D. brevicomis LeConte at low and middle elevations offset the greater abundance of I. knausi Swaine, D. adjunctus Blandford, D. approximatus Dietz, and D. valens LeConte at high elevations. I. pini (Say) and I. latidens LeConte were found in similar numbers across the three bands. Flight periodicity of several species varied among elevation bands. In general, the flight period shortened as elevation increased; flight initiated later and terminated earlier in the year. The timing, number, and magnitude of peaks in flight activity also varied among the elevation bands. These results suggest that abundance and flight seasonality of several bark beetles are related to elevation and the associated temperature differences. The implications of these results are discussed in relation to bark beetle management and population dynamics.     

The effect of sampling density and study area size on landscape genetics inferences for the Mississippi slimy salamander (Plethodon mississippi)

In landscape genetics, it is largely unknown how choices regarding sampling density and study area size impact inferences upon which habitat features impede vs. facilitate gene flow. While it is recommended that sampling locations be spaced no further apart than the average individual''s dispersal distance, for low‐mobility species, this could lead to a challenging number of sampling locations, or an unrepresentative study area. We assessed the effects of sampling density and study area size on landscape genetic inferences for a dispersal‐limited amphibian, Plethodon mississippi, via analysis of nested datasets. Microsatellite‐based genetic distances among individuals were divided into three datasets representing sparse sampling across a large study area, dense sampling across a small study area, or sparse sampling across the same small study area. These datasets were a proxy for gene flow (i.e., the response variable) in maximum‐likelihood population effects models that assessed the nature and strength of their relationship with each of five land‐use classes (i.e., potential predictor variables). Comparisons of outcomes were based on the rank order of effect, sign of effect (i.e., gene flow resistance vs. facilitation), spatial scale of effect, and functional relationship with gene flow. The best‐fit model for each dataset had the same sign of effect for hardwood forests, manmade structures, and pine forests, indicating the impacts of these land‐use classes on dispersal and gene flow in P. mississippi are robust to sampling scheme. Contrasting sampling densities led to a different inferred functional relationship between agricultural areas and gene flow. Study area size appeared to influence the scale of effect of manmade structures and the sign of effect of pine forests. Our findings provided evidence for an influence of sampling density, study area size, and sampling effort upon inferences. Accordingly, we recommend iterative subsampling of empirical datasets and continued investigation into the sensitivities of landscape genetic analyses using simulations.     

Fine-scale genetic population structure of southern pine beetle (Coleoptera: Curculionidae) in Mississippi forests

The southern pine beetle, Dendroctonus frontalis Zimmerman, is the most destructive insect pest of pine forests in the southeastern United States, Mexico, and Central America. Southern pine beetle aggressively attacks pine trees, and when in epidemic stages, they are capable of killing even the most healthy pine trees in a short period of time. Despite the amount of destruction caused by the southern pine beetle and the amount of monetary loss faced by the timber industry and recreation, the population genetics of this species has been limited to comparisons among distant geographic locations. This study investigates the fine-scale genetic population structure of the southern pine beetle in Mississippi. Very little genetic differentiation was observed among samples. Bayesian assignment testing failed to detect multiple groups within all samples; estimates of genetic differentiation and genetic distance were very low in magnitude; and a Mantel test did not reveal a significant relationship between genetic distance and geographic distance. These results suggest that management of the southern pine beetle needs to consider the potential movements of individuals within and among national forests and should be focused on a large scale, at least as big as continuously forested areas and possibly even multiple forests. These results further suggest that removal of beetle-infested trees is important.     

Landscape-scale genetic variation in a forest outbreak species, the mountain pine beetle (Dendroctonus ponderosae)

The mountain pine beetle Dendroctonus ponderosae is a native species currently experiencing large-scale outbreaks in western North American pine forests. We sought to describe the pattern of genetic variation across the range of this species, to determine whether there were detectable genetic differences between D. ponderosae occupying different host trees in common localities, and to determine whether there was molecular evidence for a past demographic expansion. Using a combination of amplified fragment length polymorphism (AFLP) and mitochondrial sequencing analyses, we found evidence of genetic structuring among populations that followed a broad isolation-by-distance pattern. Our results suggest that the geographical pattern of gene flow follows the core distribution of the principal D. ponderosae host species, around rather than across the Great Basin and Mojave Deserts. Patterns of haplotype diversity and divergence were consistent with a range-wide population expansion. This signal was particularly pronounced in the northern part of the species' range, where outbreak activity is currently increasing. Using AFLP markers, we were unable to detect significant differences among groups of insects sampled from different host trees in common locations. Incidentally, we found that a large proportion of the polymorphic AFLP markers were gender-specific, occurring only in males. While we did not include these markers in our analyses, this finding warrants further investigation.     

Spatial genetic structure of the mountain pine beetle (Dendroctonus ponderosae) outbreak in western Canada: historical patterns and contemporary dispersal

Environmental change has a wide range of ecological consequences, including species extinction and range expansion. Many studies have shown that insect species respond rapidly to climatic change. A mountain pine beetle epidemic of record size in North America has led to unprecedented mortality of lodgepole pine, and a significant range expansion to the northeast of its historic range. Our goal was to determine the spatial genetic variation found among outbreak population from which genetic structure, and dispersal patterns may be inferred. Beetles from 49 sampling locations throughout the outbreak area in western Canada were analysed at 13 microsatellite loci. We found significant north-south population structure as evidenced by: (i) Bayesian-based analyses, (ii) north-south genetic relationships and diversity gradients; and (iii) a lack of isolation-by-distance in the northernmost cluster. The north-south structure is proposed to have arisen from the processes of postglacial colonization as well as recent climate-driven changes in population dynamics. Our data support the hypothesis of multiple sources of origin for the outbreak and point to the need for population specific information to improve our understanding and management of outbreaks. The recent range expansion across the Rocky Mountains into the jack/lodgepole hybrid and pure jack pine zones of northern Alberta is consistent with a northern British Columbia origin. We detected no loss of genetic variability in these populations, indicating that the evolutionary potential of mountain pine beetle to adapt has not been reduced by founder events. This study illustrates a rapid range-wide response to the removal of climatic constraints, and the potential for range expansion of a regional population.     

Landscape genetics of the key African acacia species Senegalia mellifera (Vahl)- the importance of the Kenyan Rift Valley

Acacias across Africa have enormous ecological and economic importance, yet their population genetics are poorly studied. We used seven microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in the widespread acacia, Senegalia (Acacia) mellifera. We quantified variation among 791 individuals from 28 sampling locations, examining patterns at two spatial scales: (i) across Kenya including the Rift Valley, and (ii) for a local subset of 11 neighbouring locations on Mpala Ranch in the Laikipia plateau. Our analyses recognize that siblings can often be included in samples used to measure population genetic structure, violating fundamental assumptions made by these analyses. To address this potential problem, we maximized genetic independence of samples by creating a sibship-controlled data set that included only one member of each sibship and compared the results obtained with the full data set. Patterns of genetic structure and barriers to gene flow were essentially similar when the two data sets were analysed. Five well-defined geographic regions were identified across Kenya within which gene flow was localized, with the two strongest barriers to dispersal splitting the Laikipia Plateau of central Kenya from the Western and Eastern Rift Valley. At a smaller scale, in the absence of geographic features, regional habitat gradients appear to restrict gene flow significantly. We discuss the implications of our results for the management of this highly exploited species.     

Evidence for female-biased dispersal in the protandrous hermaphroditic Asian Seabass, Lates calcarifer

Movement of individuals influences individual reproductive success, fitness, genetic diversity and relationships among individuals within populations and gene exchange among populations. Competition between males or females for mating opportunities and/or local resources predicts a female bias in taxa with monogamous mating systems and a male-biased dispersal in polygynous species. In birds and mammals, the patterns of dispersal between sexes are well explored, while dispersal patterns in protandrous hermaphroditic fish species have not been studied. We collected 549 adult individuals of Asian seabass (Lates calcarifer) from four locations in the South China Sea. To assess the difference in patterns of dispersal between sexes, we genotyped all individuals with 18 microsatellites. Significant genetic differentiation was detected among and within sampling locations. The parameters of population structure (F(ST)), relatedness (r) and the mean assignment index (mAIC), in combination with data on tagging-recapture, supplied strong evidences for female-biased dispersal in the Asian seabass. This result contradicts our initial hypothesis of no sex difference in dispersal. We suggest that inbreeding avoidance of females, female mate choice under the condition of low mate competition among males, and male resource competition create a female-biased dispersal. The bigger body size of females may be a cause of the female-biased movement. Studies of dispersal using data from DNA markers and tagging-recapture in hermaphroditic fish species could enhance our understanding of patterns of dispersal in fish.     

Impacts of silvicultural thinning treatments on beetle trap captures and tree attacks during low bark beetle populations in ponderosa pine forests of northern Arizona

Our research used a combination of passive traps, funnel traps with lures, baited trees, and surveys of long-term thinning plots to assess the impacts of different levels of stand basal area (BA) on bark beetle tree attack and on trap captures of Ips spp., Dendroctonus spp., and their predators. The study occurred at two sites in ponderosa pine, Pinus ponderosa Dougl. ex Laws., forests, from 2004 to 2007 during low bark beetle populations. Residual stand BA ranged from 9.0 to 37.0 m2/ha. More predators and bark beetles were collected in passive traps in stands of lower BA than in stands of higher BA; however, significance varied by species and site, and total number of beetles collected was low. Height of the clear panel passive traps affected trap catches for some species at some sites and years. When pheromone lures were used with funnel traps [Ips pini (Say) lure: lanierone, +03/-97 ipsdienol], we found no significant difference in trap catches among basal area treatments for bark beetles and their predators. Similarly, when trees were baited (Dendroctonus brevicomis LeConte lure: myrcene, exo-brevicomin and frontalin), we found no significant difference for days to first bark beetle attack. Surveys of long-term thinning treatments found evidence of bark beetle attacks only in unthinned plots (approximately 37 m2/ha basal area). We discuss our results in terms of management implications for bark beetle trapping and control.     

Molecular Markers Reveal Limited Population Genetic Structure in a North American Corvid,Clark’s Nutcracker (Nucifraga columbiana)

The genetic impact of barriers and Pleistocene glaciations on high latitude resident species has not been widely investigated. The Clark’s nutcracker is an endemic North American corvid closely associated with Pinus-dominated forests. The nutcracker’s encompasses known barriers to dispersal for other species, and glaciated and unglaciated areas. Clark’s nutcrackers also irruptively disperse long distances in search of pine seed crops, creating the potential for gene flow among populations. Using the highly variable mitochondrial DNA control region, seven microsatellite loci, and species distribution modeling, we examined the effects of glaciations and dispersal barriers on population genetic patterns and population structure of nutcrackers. We sequenced 900 bp of mitochondrial control region for 169 individuals from 15 populations and analysed seven polymorphic microsatellite loci for 13 populations across the Clark’s nutcracker range. We used species distribution modeling and a range of phylogeographic analyses to examine evolutionary history. Clark’s nutcracker populations are not highly differentiated throughout their range, suggesting high levels of gene flow among populations, though we did find some evidence of isolation by distance and peripheral isolation. Our analyses suggested expansion from a single refugium after the last glacial maximum, but patterns of genetic diversity and paleodistribution modeling of suitable habitat were inconclusive as to the location of this refugium. Potential barriers to dispersal (e.g. mountain ranges) do not appear to restrict gene flow in Clark’s nutcracker, and postglacial expansion likely occurred quickly from a single refugium located south of the ice sheets.     

Evaluation of funnel traps for characterizing the bark beetle (Coleoptera: Scolytidae) communities in ponderosa pine forests of north-central Arizona

Lindgren funnel traps baited with aggregation pheromones are widely used to monitor and manage populations of economically important bark beetles (Coleoptera: Scolytidae). This study was designed to advance our understanding of how funnel trap catches assess bark beetle communities and relative abundance of individual species. In the second year (2005) of a 3-yr study of the bark beetle community structure in north-central Arizona pine (Pinus spp.) forests, we collected data on stand structure, site conditions, and local bark beetle-induced tree mortality at each trap site. We also collected samples of bark from infested (brood) trees near trap sites to identify and determine the population density of bark beetles that were attacking ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the area surrounding the traps. Multiple regression models indicated that the number of Dendroctonus and Ips beetles captured in 2005 was inversely related to elevation of the trap site, and positively associated with the amount of ponderosa pine in the stand surrounding the site. Traps located closer to brood trees also captured more beetles. The relationship between trap catches and host tree mortality was weak and inconsistent in forest stands surrounding the funnel traps, suggesting that trap catches do not provide a good estimate of local beetle-induced tree mortality. However, pheromone-baited funnel trap data and data from gallery identification in bark samples produced statistically similar relative abundance profiles for the five species of bark beetles that we examined, indicating that funnel trap data provided a good assessment of species presence and relative abundance.     

Temporal genetic dynamics among mosquitofish (<Emphasis Type="Italic">Gambusia holbrooki</Emphasis>) populations in invaded watersheds

The temporal components of genetic diversity and geographical structure of invasive mosquitofish populations are poorly known. Through the genetic monitoring of four consecutive cohorts of Gambusia holbrooki from three different river basins we aimed to determine temporal patterns of regional genetic variation and dispersal rates within invasive populations. Despite showing evidence of strong population size fluctuations, genetic diversity levels were maintained among local cohorts. We only detected temporal allele frequency changes associated with seasonal flooding that did not modify major trends on population structure among cohorts. Downstream gene flow coupled with increased connectivity at lowland locations to increase genetic diversity levels in these areas. A large proportion of local fish (up to 50 %) were dispersers, often originated from locations within the same river basin. High dispersal capability, ecological tolerance, and reproductive traits likely promote river colonization. Finally, our results also confirmed that human-assisted translocations promote within and among basin gene flow and maintained levels of genetic diversity, particularly in upstream locations.     

Landbird community composition varies among seasons in a heterogeneous ponderosa pine forest

ABSTRACT.   There is growing recognition of the need to conserve areas used by birds during migration, including forest and upland habitats. Because extensive thinning and burning treatments are planned for ponderosa pine ( Pinus ponderosa ) forests in the southwestern United States, information on the use of these forests by landbirds during migration is needed for conservation planning. We compared species richness among spring, breeding, and fall seasons at 69 points in a ponderosa pine forest to assess changes in landbird communities and the role of different ponderosa pine cover types in habitat selection among seasons. We detected a total of 64 bird species. Bird community similarity was lowest between the breeding and fall seasons and highest between the spring and breeding seasons. Twenty percent of the species detected were present exclusively in the fall and, of these, over half were Neotropical migrants. Only two species (3%) were detected exclusively during the spring. Although we found little difference in bird species similarity among vegetative cover types during the breeding season, forests that contained a deciduous component exhibited higher bird species similarity with each other than with habitats that did not include a deciduous component in spring and fall. In addition, foliage foragers dominated the community in spring and fall, and all Neotropical migrants detected exclusively in fall were found in ponderosa pine forests with a deciduous component. Our results indicate that ponderosa pine forests may be important to migrating or dispersing landbirds in autumn, especially if there is a deciduous component.     

Integrated pest management of Ips pini (Col., Scolytidae) populations in south-eastern Montana

Second growth ponderosa pine forests of the Northern Cheyenne Reservation of south-eastern Montana sustained a chronic Ips pini outbreak between 1978 and 1994. Studies determined the emergence and seasonal flight patterns of the scolytids with regards to temporal and spatial distribution of host material as well as population changes during the year. These changes related to the dry-down rate of host material (thinning and logging slash) and the availability of slash; an index of population change (IPC) was developed to estimate seasonal changes in the Ips population from spring to autumn. Principal management components of an integrated pest management system included (1) an emphasis on creating thinning and logging slash between August and December, a time that provides quickly degrading host material for the overwintering Ips population while not providing suitable host material for the subsequent spring flight; (2) restrictions on winter logging as slash produced during this period provided suitable hosts for the first spring flights of I. pini; and (3) restrictions on the use of trap logs or pheromones to lower Ips populations as these materials provide kairomones for the predators ( Enoclerus lecontei , Enoclerus spegeus and Temnochila chlorodia ) that partition their I. pini host across the spring, summer and autumn.     

ALLOZYME AND MORPHOLOGICAL DIFFERENTIATION OF MOUNTAIN PINE BEETLES DENDROCTONUS PONDEROSAE HOPKINS (COLEOPTERA: SCOLYTIDAE) ASSOCIATED WITH HOST TREE

The purpose of this study was to determine whether mountain pine beetles utilizing different host species were differentiated for either morphological or protein variation. Genetic differentiation among host species has been reported for the southern pine beetle, the Douglas-fir beetle, the jeffrey pine beetle, and the mountain pine beetle. However, in these studies, the host trees were sampled at separate sites, and hence geographic variation and variation due to host tree were confounded. The mountain pine beetle occasionally utilizes three host trees (ponderosa pine, lodgepole pine, and limber pine) at single sites in Colorado. Five polymorphic enzyme loci and six morphological characters were used to describe beetles resident in different hosts. Differentiation within a site among host trees was detected at two of five polymorphic proteins, and for both size and morphological shape. The magnitude of genetic differentiation among hosts within a site was approximately equivalent to the magnitude of differentiation among sites. These data suggest that the species of host tree may be an important biotic factor associated with the genetic structure of bark beetle communities. The results are discussed in terms of their potential role in the process of speciation by host race formation.     

Spatial–temporal analysis of species range expansion: the case of the mountain pine beetle, Dendroctonus ponderosae

Aim The spatial extent of western Canada’s current epidemic of mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae, Scolytinae), is increasing. The roles of the various dispersal processes acting as drivers of range expansion are poorly understood for most species. The aim of this paper is to characterize the movement patterns of the mountain pine beetle in areas where range expansion is occurring, in order to describe the fine‐scale spatial dynamics of processes associated with mountain pine beetle range expansion. Location Three regions of Canada’s Rocky Mountains: Kicking Horse Pass, Yellowhead Pass and Pine Pass. Methods Data on locations of mountain pine beetle‐attacked trees of predominantly lodgepole pine (Pinus contorta var. latifolia) were obtained from annual fixed‐wing aircraft surveys of forest health and helicopter‐based GPS surveys of mountain pine beetle‐damaged areas in British Columbia and Alberta. The annual (1999–2005) spatial extents of outbreak ranges were delineated from these data. Spatial analysis was conducted using the spatial–temporal analysis of moving polygons (STAMP), a recently developed pattern‐based approach. Results We found that distant dispersal patterns (spot infestations) were most often associated with marginal increases in the areal size of mountain pine beetle range polygons. When the mountain pine beetle range size increased rapidly relative to the years examined, local dispersal patterns (adjacent infestation) were more common. In Pine Pass, long‐range dispersal (> 2 km) markedly extended the north‐east border of the mountain pine beetle range. In Yellowhead Pass and Kicking Horse Pass, the extension of the range occurred incrementally via ground‐based spread. Main conclusions Dispersal of mountain pine beetle varies with geography as well as with host and beetle population dynamics. Although colonization is mediated by habitat connectivity, during periods of low overall habitat expansion, dispersal to new distant locations is common, whereas during periods of rapid invasion, locally connected spread is the dominant mode of dispersal. The propensity for long‐range transport to establish new beetle populations, and thus to be considered a driver of range expansion, is likely to be determined by regional weather patterns, and influenced by local topography. We conclude that STAMP appears to be a useful approach for examining changes in biogeograpical ranges, with the potential to reveal both fine‐ and large‐scale patterns.     

Frequent circumarctic and rare transequatorial dispersals in the lichenised agaric genus Lichenomphalia (Hygrophoraceae, Basidiomycota)

Species of the genus Lichenomphalia are mostly restricted to arctic-alpine environments with the exception of Lichenomphalia umbellifera which is also common in northern forests. Although Lichenomphalia species inhabit vast regions in several continents, no information is available on their genetic variation across geographic regions and the underlying population-phylogenetic patterns. We collected samples from arctic and subarctic regions, as well as from newly discovered subantarctic localities for the genus. Phylogenetic, nonparametric permutation methods, and coalescent analyses were used to assess phylogeny and population divergence and to estimate the extent and direction of gene flow among distinct geographic populations. All known species formed monophyletic groups, supporting their morphology-based delimitation. In addition, we found two subantarctic phylogenetic species (Lichenomphalia sp. and Lichenomphalia aff. umbellifera), of which the latter formed a well-supported sister group to L. umbellifera. We found no significant genetic differentiation among conspecific North American and Eurasian populations in Lichenomphalia. We detected high intercontinental gene flow within the northern polar region, suggesting rapid (re)colonisation of suitable habitats in response to climatic fluctuations and preventing pronounced genetic differentiation. On the other hand, our phylogenetic analyses suggest that dispersal between northern circumpolar and subantarctic areas likely happened very rarely and led to the establishment and subsequent divergence of lineages. Due to limited sampling in the Southern Hemisphere, it is currently uncertain whether the northern lineages occur in Gondwanan regions. On the other hand, our results strongly suggest that the southern lineages do not occur in the circumpolar north. Although rare transequatorial dispersal and subsequent isolation may explain the emergence of at least two subantarctic phylogenetic species lineages in Lichenomphalia, more samples from the Southern Hemisphere are needed to better understand the phylogeographic history of the genus.     

Identifying populations for management: fine-scale population structure in the New Zealand alpine rock wren (<Emphasis Type="Italic">Xenicus gilviventris</Emphasis>)

Examining the spatial genetic structure of cryptic species occupying challenging terrain can afford otherwise unattainable insights into ecological and evolutionary processes, such as population dynamics and dispersal patterns; information important for optimising conservation management. Using 13 microsatellite markers, we evaluated patterns of fine-scale gene flow and the spatial extent of genetic structuring of rock wren (Xenicus gilviventris), a threatened alpine passerine endemic to mountainous regions of the South Island, New Zealand. Through spatial autocorrelation analysis, we found that the ‘genetic patch size’, i.e. the distance over which individuals were not genetically independent, was surprisingly large (c. 70 km), given the rock wren’s limited flight ability. By estimating recent migration rates among sampling locations we also found asymmetries in gene flow indicative of source–sink dynamics. An area with intensive deer and predator control, in the Murchison Mountains, Fiordland, appears to be a particularly important source of migrants for other populations. These findings suggest that management to maintain connectivity is required across relatively large spatial scales and source populations may be those where introduced mammals are managed.     

Mountain pine beetle attack associated with low levels of 4-allylanisole in ponderosa pine

Mountain pine beetle (Dendroctonus ponderosae) is the most important insect pest in southern Rocky Mountain ponderosa pine (Pinus ponderosa) forests. Tree mortality is hastened by the various fungal pathogens that are symbiotic with the beetles. The phenylpropanoid 4-allylanisole is an antifungal and semiochemical for some pine beetle species. We analyzed 4-allylanisole and monoterpene profiles in the xylem oleoresin from a total of 107 trees at six sites from two chemotypes of ponderosa pine found in Colorado and New Mexico using gas chromatography-mass spectroscopy (GC-MS). Although monoterpene profiles were essentially the same in attacked and nonattacked trees, significantly lower levels of 4-allylanisole were found in attacked trees compared with trees that showed no evidence of attack for both chemotypes.     

Testing hypotheses of Pleistocene population history using coalescent simulations: phylogeography of the pygmy nuthatch (Sitta pygmaea)

In this paper, we use mitochondrial NADH dehydrogenase subunit 2 sequences to test Pleistocene refugial hypotheses for the pygmy nuthatch (Sitta pygmaea). Pygmy nuthatches are a common resident of long-needle pine forests in western North America and demonstrate a particular affinity with ponderosa pine (Pinus ponderosa). Palaeoecological and genetic data indicate that ponderosa pine was isolated in two Pleistocene refugia corresponding to areas in the southern Sierra Nevada in the west and southern Arizona and New Mexico in the east. We use coalescent simulations to test the hypothesis that pygmy nuthatches tracked the Pleistocene history of their preferred habitat and persisted in two refugia during the periods of glacial maxima. Coalescent simulation of population history does not support the hypothesis of two Pleistocene refugia for the pygmy nuthatch. Instead, our data are consistent with a single refuge model. Nucleotide diversity is greatest in the western populations of southern and coastal California. We suggest that the pygmy nuthatch expanded from a far western glacial refuge into its current distribution since the most recent glacial maximum.     

Population structure on breeding grounds of Lake Malawi’s ‘twilight zone’ cichlid fishes

Aim Free‐ranging benthopelagic fishes often have large population sizes and high rates of dispersal. These traits can act to homogenize population structure across the distributional range of a species and to reduce the likelihood of allopatric speciation. The apparent absence of any barriers to gene flow among populations, together with prior molecular evidence for panmixia across the ranges of three species, has resulted in Diplotaxodon, a genus of benthopelagic cichlid fishes of Lake Malawi, being proposed as a candidate case of sympatric speciation. Our aim was to further investigate this possibility by testing for intraspecific genetic subdivision among breeding populations, and intraspecific differences in breeding habitat. Location Lake Malawi, central‐east Africa. Methods We analysed eight microsatellite DNA loci to test for spatial genetic differences among populations on breeding grounds of eight Diplotaxodon species. We also tested for temporal population genetic differences within breeding grounds of three species. Records of ripe Diplotaxodon encountered during sampling were analysed to test if spatial variation in assemblage structure was linked to nearshore water depth and geographic proximity of sampling sites. Results Consistent with previous molecular evidence, within four of the eight species tested we found no evidence of spatial genetic structuring among breeding populations. However, within the other four species we found slight yet significant spatial genetic differences, indicating restricted gene flow among breeding grounds. There was no evidence of temporal genetic differences within sites. Analyses of the distributions of ripe Diplotaxodon revealed differences in assemblage structure linked to nearshore water depth. Main conclusions Together, these results demonstrate both the evolution of fidelity to deep‐water breeding locations in some Diplotaxodon species, and differences in breeding habitat among species. These findings are consistent with a role for divergence of breeding habitat in speciation of these cichlids, possibly promoted by dispersal limitation among geographically segregated spawning aggregations.