Metapopulation dynamics and conservation of the marsh fritillary butterfly: Population viability analysis and management options for a critically endangered species in Western Europe

This study investigates the dynamics and viability of a marsh fritillary butterfly Euphydryas aurinia metapopulation in a Belgian successional landscape. Based on capture-mark-recapture and winter nest census data, we first estimated demography (survival and recruitment rates, population size, density dependence) and dispersal parameters (emigration rate, effect of patch connectivity on dispersal, mortality during dispersal). Then using RAMAS/GIS platform, we parameterised a population viability analysis (PVA) model with these parameters to simulate the future of this metapopulation under different scenarios.The metapopulation does not seem viable even if natural reforestation is controlled by adequate management. In its present state, the patch system is not able to sustain enough individuals: due to the large temporal fluctuations in demographic parameters, a carrying capacity far higher than currently would be necessary to limit extinction risk to 1%, suggesting the existence of an extinction debt for the species in Belgium. The situation of E. aurinia appears much worse compared to two other fritillary species threatened in Belgium, for which similar PVA are available. It is therefore urgent to increase the carrying capacity of the patch system. How and where it is achieved are of secondary importance for the gain in viability: improvement of habitat quality through restoration, or increase of habitat quantity via enlargement of existing patches and/or creation of new habitat in the matrix. A regime of management based on regular re-opening and maintenance of habitat patches may be the only guarantee of long-term persistence for this critically endangered species in Belgium.     

An empirical evaluation of the area and isolation paradigm of metapopulation dynamics

Much of metapopulation theory assumes that the persistence of individual populations in a metapopulation, and persistence of the metapopulation as a whole, is best modeled by the area of habitat patches and their isolation. Estimates of isolation typically include a measure of geographic distance and a measure of either population size or patch area. This “area and isolation paradigm” assumes a functional relationship between the area of a patch and its extinction probability, and between isolation of a patch and its colonization probability. Although these assumptions are fundamental to use of incidence function models of metapopulation dynamics, the assumptions have been validated in only a small number of studies. We tested the ability of area and isolation to predict extinction and colonization patterns using multiple-year occupancy data for 10 species from three taxonomic groups (butterflies, amphibians, and birds). We examined 13 potential models of metapopulation dynamics. All models included four basic parameters: occupancy during the first year of the survey, probability of extinction, probability of colonization, and single-visit detection probability. In eight models, each parameter was either constant or time-dependent. Five models included a patch-level covariate of extinction probability (patch area or population size), colonization probability (connectivity, the inverse of isolation), or both. Extinction patterns generally were predicted more effectively as a function of local population size than as a function of patch area, a constant probability of extinction, or a time-dependent probability of extinction. In most cases, inclusion of connectivity as a patch-level covariate did not improve predictions of colonization patterns. We estimated single-visit detection probabilities for all species in our analyses, thus providing evidence-based guidelines for the refinement of future monitoring protocols.     

Are habitat-tracking metacommunities dispersal limited? Inferences from abundance-occupancy patterns of epiphylls in Amazonian forest fragments

In theory, habitat fragmentation alters plant community dynamics by influencing both local (within patch) and regional (among patch) processes. However, the lengthy generation times of plant taxa relative to the short duration of most experiments has precluded studies from assessing the impact of fragmentation at both local and regional scales. Due to their accelerated life cycles, high rates of local extinction, and naturally patchy substrates, epiphyllous bryophyte assemblages are an appropriate plant guild for empirically testing metacommunity-based predictions associated with habitat fragmentation. By examining the local abundance and regional distribution patterns of 67 epiphyllous (leaf-inhabiting) bryophyte species in an experimentally fragmented landscape in Amazonia, we demonstrate that changes in local abundance wrought by habitat fragmentation are best explained by fragment size rather than proximity to forest edge. Furthermore, evidence of a simultaneous inter-specific decline in epiphyll local abundance and regional distribution in small (1- and 10-ha) forest fragments corroborate with metapopulation-based predictions highlighting the importance of immigration in buffering from patch extinction risk (i.e., the rescue effect). Collectively, these results provide indirect evidence that dispersal limitation, rather than compromised habitat quality attributable to edge effects, likely account for species loss from small tropical forest fragments. Whether dispersal limitation is due to increased insularity from regional sources for epiphyll recolonization or rather to lowered within-fragment dispersal potential is unknown; nonetheless, the long-term persistence of these microscopic plant metacommunities depends on the preservation of rain forest reserves of at least 100-ha in size.     

Dynamics of a declining amphibian metapopulation: Survival, dispersal and the impact of climate

Climate can interact with population dynamics in complex ways. In this study we describe how climatic factors influenced the dynamics of an amphibian metapopulation over 12 years through interactions with survival, recruitment and dispersal. Low annual survival of great crested newts (Triturus cristatus) was related to mild winters and heavy rainfall, which impacted the metapopulation at the regional level. Consequently, survival varied between years but not between subpopulations. Despite this regional effect, the four subpopulations were largely asynchronous in their dynamics. Three out of the four subpopulations suffered reproductive failure in most years, and recruitment to the metapopulation relied on one source. Variation in recruitment and juvenile dispersal was therefore probably driving asynchrony in population dynamics. At least one subpopulation went extinct over the 12 year period. These trends are consistent with simulations of the system, which predicted that two subpopulations had an extinction risk of >50% if adult survival fell below 30% in combination with low juvenile survival. Intermittent recruitment may therefore only result in population persistence if compensated for by relatively high adult survival. Mild winters may consequently reduce the viability of amphibian metapopulations. In the face of climate change, conservation actions may be needed at the local scale to compensate for reduced adult survival. These would need to include management to enhance recruitment, connectivity and dispersal.     

Spatial population structure of the Cabrera vole in Mediterranean farmland: The relative role of patch and matrix effects

There is mounting evidence that both patch networks and the intervening matrix influence species persistence in fragmented landscapes, though the relative importance of each of these factors in determining spatial population structure remains poorly understood. This study examined this issue using a three-year data set on the distribution of Cabrera voles (Microtus cabrerae) in Mediterranean farmland. The spatial pattern appeared consistent with a metapopulation structure, as voles occupied discrete tall herb patches scattered across the agricultural landscape, where local extinctions and colonizations induced temporal changes in occupancy patterns. Patch dynamics determined deviations from classical metapopulation assumptions, with over half the extinctions resulting from agricultural disturbance or vegetation succession, and recolonizations often occurring after the recovery of suitable habitat conditions sometime after disturbance. Occupancy in undisturbed patches was more stable, with vole occurrence in one year strongly reflecting that in the previous year. Overall, occupancy increased with both patch size and connectivity, but the unique contribution of patch variables to explain variation in vole occurrence was far smaller than that of matrix attributes. Voles occurred more often in patches surrounded by natural pastures, while prevalence declined with increasing cover by shrubland, pine plantations, improved pastures and grazed cropland. It is hypothesised that unfavourable land uses may increase the effective isolation of habitat patches through increased predation risk of dispersing voles. Conservation of the Cabrera vole in Mediterranean farmland should thus strive to maintain lightly grazed fields surrounding well-connected networks of suitable habitat patches.     

Habitat succession, hardwood encroachment and raccoons as limiting factors for Lower Keys marsh rabbits

The Lower Keys marsh rabbit (LKMR, Sylvilagus palustris hefneri), a marsh rabbit subspecies endemic to the Lower Keys, Florida was protected in 1990, however, populations continue to decline despite recovery efforts. We hypothesized on-going habitat loss and fragmentation due to succession and hardwood encroachment has lead to increased edge, reduced habitat quality, and increased activity by native raccoons (Procyon lotor). These factors reduce the suitability of patches in a later successional state, thus threatening LKMR recovery and metapopulation persistence. We surveyed 150 LKMR patches in 2008, tallying adult and juvenile rabbit pellets, estimating measures of habitat succession and quality (woody and herbaceous ground cover, distribution of herbaceous species) and recording raccoon activity (number of raccoon signs). We calculated patch edge (patch shape index) using ArcGIS. We evaluated the relationship between patch and habitat attributes and LKMR using regression analysis and model selection. We found both adult and juvenile LKMR pellet counts were lower in patches with higher shape indices and higher in patches with greater occurrence of bunchgrasses and forbs. We also found adult LKMR pellet counts were lower in patches with higher raccoon activity. Our results suggest patch edge, habitat succession and quality, and raccoons pose a threat to the persistence and recovery of LKMR populations. Recovery efforts should focus on reducing these trends through habitat management and raccoon removal implemented in carefully controlled experiments with proper monitoring. Measures of patch and habitat attributes important to LKMR should be incorporated into long-term metapopulation monitoring and used to evaluate recovery actions.     

The relative importance of patch habitat quality and landscape attributes on a declining steppe-bird metapopulation

Metapopulation theory is one of the most popular approaches to identify the factors affecting the spatial and temporal dynamics of populations in fragmented habitat networks. Habitat quality, patch area and isolation are mainly focused on when analyzing distribution patterns in fragmented landscapes. The effects of landscape heterogeneity in the non-occupied matrix, however, have been largely neglected. Here, we determined the relative importance of patch quality and landscape attributes on the occurrence, density and extinction of the Dupont’s lark (Chersophilus duponti), an endangered steppe passerine whose habitat has been extremely reduced to highly isolated and fragmented patches embedded in a mainly unsuitable landscape matrix. Habitat patch quality, measured in terms of vegetation structure, grazing pressure, arthropod availability, predator abundance, and inter-specific competition, did not affect occurrence, density or extinction. At the landscape scale, however, the species’ occurrence was principally determined by the interactions among patch size, geographic isolation and landscape matrix. Isolation had the main independent contribution to explaining the probability of occurrence, followed by landscape matrix composition and patch size. The species’ density was negatively correlated to patch size, suggesting crowding effects in small fragments, while extinction events were exclusively related to isolation. Our findings suggest that landscape rather than local population characteristics are crucial in determining the patterns of distribution and abundance of non-equilibrium populations in highly fragmented habitat networks. Consequently, conservation measures for these species should simultaneously involve patch size, isolation and landscape matrix and apply to the entire metapopulation rather than to particular patches.     

Patch dynamics and the timing of colonization-abandonment events by male Kirtland’s Warblers in an early succession habitat

Habitat colonization and abandonment affects the distribution of a species in space and time, ultimately influencing the duration of time habitat is used and the total area of habitat occupied in any given year. Both aspects have important implications to long-term conservation planning. The importance of patch isolation and area to colonization-extinction events is well studied, but little information exists on how changing regional landscape structure and population dynamics influences the variability in the timing of patch colonization and abandonment events. We used 26 years of Kirtland’s Warbler (Dendroica kirtlandii) population data taken during a habitat restoration program (1979-2004) across its historical breeding range to examine the influence of patch attributes and temporal large-scale processes, specifically the rate of habitat turnover and fraction of occupied patches, on the year-to-year timing of patch colonization and abandonment since patch origin. We found the timing of patch colonization and abandonment was influenced by patch and large-scale regional factors. In this system, larger patches were typically colonized earlier (i.e., at a younger age) and abandoned later than smaller patches. Isolated patches (i.e., patches farther from another occupied patch) were generally colonized later and abandoned earlier. Patch habitat type affected colonization and abandonment; colonization occurred at similar patch ages between plantation and wildfire areas (9 and 8.5 years, respectively), but plantations were abandoned at earlier ages (13.9 years) than wildfire areas (16.4 years) resulting in shorter use. As the fraction of occupied patches increased, patches were colonized and abandoned at earlier ages. Patches were abandoned at older ages when the influx of new habitat patches was at low and high rates. Our results provide empirical support for the temporal influence of patch dynamics (i.e., patch destruction, creation, and succession) on local colonization and extinction processes that help explain large-scale patterns of habitat occupancy. Results highlight the need for practitioners to consider the timing of habitat restoration as well as total amount and spatial arrangement of habitat to sustain populations.     

Metapopulation dynamics and threatened species management: Why does the broad-toothed rat (Mastacomys fuscus) persist?

Many species that exist patchily across the landscape are declining due to incremental losses of their constituent sub-populations and increasing isolation of those that remain. For threatened species with such patchy metapopulation structure, it is particularly important to identify key habitat patches and understand what factors govern their occupancy so that their management can be targeted and effective. In this paper, we describe the spatial and temporal distribution of an endangered population of broad-toothed rats (Mastacomys fuscus) at Barrington Tops, New South Wales, and model its dynamics using metapopulation theory. The study population occurs patchily in swamps on the Barrington Plateau. Using faecal pellet searches, live-trapping and data from previous surveys, we identified 12 swamps where M. fuscus persists and 13 where there has been a history of colonisation and extinction. The species now appears to be entirely absent from seven of these latter swamps. Using logistic regression and model selection procedures, we found the strongest predictor of the presence of M. fuscus to be proximity to the nearest occupied swamp. Persistence declined strongly with swamp isolation, probably due to low success of individuals dispersing through the intervening habitat. These patterns support the interpretation that swamp patches at Barrington Tops contain a functioning but fragile metapopulation of M. fuscus. We predict continued loss of remaining sub-populations in peripheral swamps if current dynamics continue, and recommend research to identify the factors that are limiting dispersal and re-colonisation so that the species’ decline can be slowed and reversed.     

Viability and patch occupancy of a swamp rabbit metapopulation at the northern edge of its distribution

Swamp rabbits (Sylvilagus aquaticus) are state-endangered in Indiana, USA, and population decline has been attributed to habitat loss. We conducted pellet surveys as part of a long-term survey effort that has been conducted at approximate 10-year intervals over the last 40 years. We modeled patch occupancy and conducted a spatially-explicit population viability analysis (PVA). Although occupancy of individual patches varied over time, occupancy rate has been constant for the last 30 years, and Indiana swamp rabbits exist as a metapopulation that appears to be stable. Metapopulation dynamics were best characterized as being stationary, but area was an important factor in extinction rates; occupied patches (142 ± 37 ha) were significantly larger (P = 0.01) than unoccupied patches (79 ± 20 ha). We did not find strong support for models with colonization rates as a function of distance to neighboring patches, nor was distance to contiguous patches of habitat significantly different (P = 0.12) for occupied and unoccupied sites. Population viability analysis corroborated our findings based on occupancy modeling, and evaluation of the PVA model using occupancy data for the period 1985–2006 resulted in predictions that nearly matched our field observations (33% observed patch occupancy vs. 25% predicted patch occupancy). Population viability was most sensitive to reductions in survival and fecundity rates, but was otherwise robust to changes in parameters such as initial abundance and carrying capacity. Our findings provide novel insights into a poorly studied member of Sylvilagus and into species metapopulation dynamics at the edge of the range.     

The role of fragment area and isolation in the conservation of heathland species

Heathland area in the northwestern part of Belgium has been strongly reduced during the past 200 years, and the remaining heathland is forced back into several small and isolated relics. In this study, we investigated how the fragmentation of these heathlands affects the distribution patterns of heathland plant species. Furthermore, we tested whether differences in patch occupancy patterns could be explained in terms of life history traits related to dispersal capacity and persistence. Multivariate logistic regression models showed that the incidence of almost three quarters of the species was influenced by fragmentation. For the majority of these species, isolation was the most important factor determining their presence or absence in a heathland patch. Differences between the species in isolation-sensitivity could be attributed mainly to differences in seed bank characteristics, with species having a long living seed bank being less affected by isolation. In contrast, species having mechanisms facilitating long distance dispersal were as much affected by isolation as species lacking these mechanisms.Our results suggest that for the majority of the species extinction in a patch can be prevented by dispersal from neighbouring patches. Further isolation of the patches should therefore be prevented and connectivity between the patches needs to be assured. As almost none of the species is affected by a declining patch area, for most even small patches are important for their survival. Hence, conservation efforts should focus not only on large heathlands.     

The effects of fire and predators on the long-term persistence of an endangered shrub, Grevillea caleyi

Grevillea caleyi is an endangered plant species with a restricted range lying partly within Ku-ring-gai Chase and Garigal National Parks in NSW, Australia. The principle threatening processes affecting G. caleyi are habitat destruction and adverse fire regimes combined with high levels of seed predation. A stochastic, spatially explicit, individual-based model was constructed to investigate the population dynamics of small populations of the species and to determine the impact of a variety of management strategies. Results of model simulations indicate there is a high risk of population decline and local extinction in remnant sites with small populations under current management regimes. The most effective fire management strategy is to schedule fires that burn 20-100% of sub-populations every 5-15 years, in combination with reduced predation rates. When predation management strategies are employed in conjunction with a structured fire regime, then a 20-30% reduction in predation rates can improve the chance of long-term persistence substantially.     

Dynamics of a black bear population within a desert metapopulation

Understanding metapopulation dynamics in large carnivores with naturally fragmented populations is difficult because of the large temporal and spatial context of such dynamics. We coupled a long-term database of visitor sighting records with an intensive 3-year telemetry study to describe population dynamics of recolonization by black bears (Ursus americanus) of Big Bend National Park in Texas during 1988-2002. This population, which occurs within a metapopulation in western Texas and northern Mexico, increased from a single pair of known breeding-age animals in 1988 to 29 bears (including 6 females of breeding age) in March 2000 (λ = 1.25/year). A migration and dispersal event in August-December 2000 reduced the population to 2 adult females and as few as 5-7 individuals. One-way movement distances from the study area during this event averaged 76 km for females (n = 7) and 92 km for males (n = 4), and 3 animals conducted migrations of at least 154, 178, and 214 km, respectively. Our observations exemplify the importance of stochastic events on demographics of small populations and highlight the potential scale of bear movement among montane islands of southwestern North America. They also provide insight into the use of dispersal data in parameterizing metapopulation models for large carnivores.     

Small mammals, habitat patches and PVA models: a field test of model predictive ability

The results are described of comparisons between actual values for patch occupancy for two species of Australian small mammals (Bush Rat Rattus fuscipes and Agile Antechinus Antechinus agilis) determined from field sampling and predictions of patch occupancy made using VORTEX, a generic simulation model for Population Viability Analysis (PVA). The work focussed on a fragmented forest in south-eastern Australia comprised of a network of 39 patches of native eucalypt forest surrounded by extensive stands of exotic softwood Radiata Pine (Pinus radiata) plantation. A range of modelling scenarios were completed in which four broad factors were varied: (1) inter-patch variation in habitat quality; (2) the pattern of inter-patch dispersal; (3) the rate of inter-patch dispersal; and (4) the population sink effects of the Radiata Pine matrix that surrounded the eucalypt patches. Model predictions were made for the total number of animals, the distribution of animal density among patches, the total number of occupied patches, and the probability of patch occupancy. Predictions were then compared with observed values for these same measures based on extensive field surveys of small mammals in the patch system. For most models for the Bush Rat, the predicted relative density of animals per patch correlated well with the values estimated from field surveys. Predictions of patch occupancy were not significantly different from the actual value for the number of occupied patches in half the models tested. The better models explained 10-16% of the log-likelihood of the probability of patch occupancy. While some of the models gave reasonable forecasts of the number of occupied patches, even in these cases, they had only moderate ability to predict which patches were occupied. Field surveys revealed there was no relationship between patch area and population density for the Agile Antechinus—an outcome correctly predicted by only a few models. Five of the 18 scenarios completed for the Agile Antechinus gave predicted numbers of occupied patches not significantly different from the observed number. In each of these five cases, large standard deviations around the mean predicted value meant uncertainty generated by the simulation model limited the predictive power of the PVA. Some of the models gave reasonable predictions for the number of occupied patches, but those models were unable to predict which ones were actually occupied. The results of our study suggest that key processes influencing which specific patches would be occupied were not modelled appropriately. High levels of variability and fecundity drive the population dynamics of the Bush Rat and Agile Antechinus, making the patch system unpredictable and difficult to model accurately. Despite the fact that both the Bush Rat and the Agile Antechinus are two of the most studied mammals in Australia, there are attributes of their biology that are presently poorly understood (which were not included in the VORTEX model), but which could strongly influence patch occupancy. For example, local landscape features may be important determinants of inter-patch movement and habitat utilisation in the patch system. Further empirical studies are needed to explore this aspect of small mammal biology.     

Metapopulation viability analysis of the greater glider Petauroides volans in a wood production area

Successful forest wildlife management is dependent on information that estimates long-term viability of populations in response to different management practices. In this paper we couple information captured in a GIS database, relationships between habitat attributes and habitat quality, and the dynamics of those habitat attributes, to assess the long-term metapopulation viability of a forest-dependent arboreal marsupial, greater glider Petauroides volans Kerr, in the Ada Forest Block in south-eastern Australia. Estimates of the size and spatial distribution of populations in remnant patches of old-growth forest, and the dynamics of key elements of that habitat, are input to ALEX, a computer package for population viability analysis. The model is used to predict the probability of persistence of P. volans within the Ada Forest Block concentrating on scenarios that assess the value of different old-growth patches and the impact of wildfire. We conclude that small patches of old-growth forest ( <20 ha) make almost no contribution to the persistence of the species. In addition, control of wildfire will significantly increase the viability of the species in the remaining habitat.     

The influence of social behaviour, dispersal and landscape fragmentation on population structure in a sedentary bird

White-browed Babblers Pomatostomus superciliosus lived in groups of up to 13 birds in the highly fragmented landscape of the WA wheatbelt. Contacts between these groups and sexual differences in dispersal behaviour interacted with the landscape mosaic at a number of spatial scales to produce a hierarchically structured population with four levels of organization: (1) groups, which were the basic breeding unit; (2) social neighbourhoods, where group interactions were frequent, and male dispersal and female post-natal and breeding dispersal occurred; (3) local population neighbourhoods, which contained social neighbourhoods between which female natal dispersal was frequent; and (4) metapopulations, which contained local population neighbourhoods between which dispersal was infrequent. The boundaries of these structural units, with the exception of the group, were not discrete and were influenced by the structure of the landscape they occupied.Interactions between groups occupying different patches were rare, and the frequency of group interactions was lower in small patches. Male dispersal was restricted to groups within the same patch or in patches less than 1 km apart. Therefore, decreasing patch size and increasing patch isolation resulted in smaller social neighbourhoods. Males generally dispersed to smaller groups and these dispersals may have enhanced the productivity of these groups by increasing their size. Therefore, habitat loss and fragmentation are likely to disrupt social neighbourhoods resulting in lower levels of social interaction and reduced productivity.The size and configuration of local populations were dependent on female natal dispersal, which in turn depended on landscape connectivity. White-browed Babblers used remnant vegetation in preference to other landscape elements when dispersing, but were not dependent solely on corridors. The permeability to dispersal of the boundaries between remnants and agricultural vegetation was dependent on patch configuration. Changes in boundary permeability were found to alter connectivity between habitat patches in a complex and asymmetric manner. Therefore, it is essential to consider landscape connectivity in a spatially explicit context for species that use some elements of the landscape mosaic in preference to others when dispersing.Habitat loss and fragmentation impose a complex set of changes, at a number of different scales, to processes that affect aspects of a species' life history. In order to manage species in fragmented agricultural landscapes it is necessary to understand the hierarchical structure of their populations, and how processes affect the different organizational levels within this structure.     

Inbreeding and small population size reduce seed set in a threatened and fragmented plant species, Lupinus sulphureus ssp. kincaidii (Fabaceae)

Willamette Valley upland prairie in western Oregon, USA, has been reduced to less than 1% of its original historic range following European settlement in the 1850s. Lupinus sulphureus ssp. kincaidii (Kincaid's lupine), a threatened species and the primary larval host plant of the endangered Icaricia icarioides fenderi (Fender's blue butterfly), was historically a panmictic metapopulation. Habitat fragmentation may be causing many of the Kincaid's lupine colonies to display typical symptoms of inbreeding depression, such as low seed production. Hand outcrosses on bagged inflorescences significantly increased seed set and seed fitness compared to open pollination and within-colony pollen treatments. Natural seed set was positively correlated with an increase in the number of Kincaid's lupine patches, suggesting that population size limits seed set. An increase in fruit set was positively correlated with Kincaid's lupine raceme number, raceme density, and the number of lupine patches, demonstrating that floral display and population size increase pollinator service. Restoration of Kincaid's lupine populations should consider measures that lessen the effects of inbreeding depression, especially in small, isolated populations, for the long-term persistence of the species.     

Evaluating population persistence of Delmarva fox squirrels and potential impacts of climate change

In addition to the combined effects of forest fragmentation, habitat loss, and population isolation on the long-term persistence of many species including the endangered Delmarva fox squirrel (Sciurus niger cinereus), future changes in climate may make existing habitats less productive and more variable. The Palmer Drought Severity Index (PDSI) for the Delmarva Peninsula of the mid-Atlantic USA, reveals a trend for longer durations of potentially unfavorable conditions for fox squirrel population growth. We used a stochastic population matrix model and available life history information to assess population extinction risk for the Delmarva fox squirrel under a number of scenarios of landscape change and environmental variation, including uncertainties in the future range of climate patterns. Patch size (carrying capacity) was the most important factor influencing persistence of isolated populations. Extinction risk increased markedly across all patch sizes when year to year patterns in environmental variability were autocorrelated to match regional patterns in the PDSI. Increased autocorrelation matching the regional PSDI increased extinction risk, ranging from a factor of 5 to a factor of over 100 in some scenarios when compared to uncorrelated patterns in environmental variability. Increasing the range of variation in survival probabilities was less important to persistence, but its effect also increased when year-to-year changes were autocorrelated in time. Comparing model results with the size and landscape configuration of currently occupied patches on the Delmarva Peninsula showed that many existing populations are above the size threshold identified by these simulations for long-term persistence under current conditions, but these may become vulnerable should climate variability increase and adverse conditions persist for several years at a time.     

Grasslands as movement barriers for a forest-associated salamander: Migration behavior of adult and juvenile salamanders at a distinct habitat edge

The persistence of amphibian populations in fragmented landscapes requires dispersal and recolonization of habitat patches after local extinction. These processes entail individuals crossing habitat edges. Edge permeability integrates the behavior of individuals with the vegetative structure of the habitat edge and may influence the dispersal rates between habitat patches. We used drift fences, radio telemetry, and an experimental displacement to examine the movement behavior of juvenile and adult spotted salamanders (Ambystoma maculatum) at a pond located in continuous forest and a pond located on a distinct forest-grassland edge. At the pond on the habitat edge, adult salamanders migrated to and from the forested side of the pond. Resident adults with transmitters migrated to forested habitat without approaching the habitat edge. Displaced adults with transmitters halted emigration movements when they approached the habitat edge. None of the radio-tagged adults were observed more than a few meters into the grassland. Recently metamorphosed juveniles exiting the grassland side of the pond changed their direction of movement and migrated into the forested habitat. We conclude that salamanders oriented movements with respect to features in the terrestrial habitat, detected the habitat edge, and behaviorally avoided the grassland. Exploring the permeability of habitat edges will improve our understanding of dispersal within fragmented landscapes and enhance efforts to conserve regional populations of amphibians.     

Restricted gene flow in the clonal hepatic Trichocolea tomentella in fragmented landscapes

We studied the genetic diversity, gene flow and population structure among 18 populations of the clonal bryophyte Trichocolea tomentella located in Finland, Lithuania, the UK and Canada using DNA fingerprinting methods. T. tomentella is a habitat-limited, unisexual hepatic, which occupies spring and mesic habitats in woodland. The relatively small populations are increasingly fragmented with a high risk for extinction for extrinsic reasons. The presence of relatively high levels of genetic diversity regardless of population size highlights the role of even small remnant populations as important sources of genetic diversity in T. tomentella. The long-term accumulation of genotypes and somatic mutations may explain the observed levels of diversity. Gene flow among populations seems to be infrequent indicating dispersal limitation also on the relatively small spatial scale. Colonization within populations is not affected by isolation by distance suggesting the occurrence of random short-range dispersal of detached vegetative fragments. The population structure study confirmed the low mortality rates of shoots indicating a long life span of the clones in favourable conditions. Efficient ramet production by branching is likely to operate against interspecific competition. To conclude, T. tomentella appears to persist well in undisturbed habitats due to clonal regeneration, although restricted dispersal capacity is likely to prevent successful (re-)colonization in the potential habitat patches of recovering forest landscapes. The implications of the results for conservation are introduced.