L. Scott Mills

University of Montana, Missoula, MT, United States

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Publications (64)217.52 Total impact

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    ABSTRACT: Hybridization drives the evolutionary trajectory of many species or local populations, and assessing the geographic extent and genetic impact of interspecific gene flow may provide invaluable clues to understand population divergence or the adaptive relevance of admixture. In North America, hares (Lepus spp.) are key species for ecosystem dynamics and their evolutionary history may have been affected by hybridization. Here we reconstructed the speciation history of the three most widespread hares in North America - the snowshoe hare (Lepus americanus), the white-tailed jackrabbit (L. townsendii) and the black-tailed jackrabbit (L. californicus) - by analyzing sequence variation at eight nuclear markers and one mitochondrial DNA (mtDNA) locus (6 240 bp; 94 specimens). A multilocus-multispecies coalescent-based phylogeny suggests that L. americanus diverged ~2.7 Mya and that L. californicus and L. townsendii split more recently (~1.2 Mya). Within L. americanus a deep history of cryptic divergence (~2.0 Mya) was inferred, which coincides with major speciation events in other North American species. While the isolation-with-migration model suggested that nuclear gene flow was generally rare or absent among species or major genetic groups, coalescent simulations of mtDNA divergence revealed historical mtDNA introgression from L. californicus into the Pacific Northwest populations of L. americanus. This finding marks a history of past reticulation between these species, which may have affected other parts of the genome and influence the adaptive potential of hares during climate change.This article is protected by copyright. All rights reserved.
    Molecular Ecology 08/2014; · 6.28 Impact Factor
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    ABSTRACT: With climate warming, the ranges of many boreal species are expected to shift northward and to fragment in southern peripheral ranges. To understand the conservation implications of losing southern populations, we examined range-wide genetic diversity of the snowshoe hare (Lepus americanus), an important prey species that drives boreal ecosystem dynamics. We analyzed microsatellite (8 loci) and mitochondrial DNA sequence (cytochrome-b and control region) variation in almost 1000 snowshoe hares. A hierarchical structure analysis of the microsatellite data suggests initial subdivision in two groups, Boreal and southwestern. The southwestern group further splits into Greater Pacific Northwest and U.S. Rockies. The genealogical information retrieved from mtDNA is congruent with the three highly differentiated and divergent groups of snowshoe hares. These groups can correspond with evolutionarily significant units that might have evolved in separate refugia south and east of the Pleistocene ice sheets. Genetic diversity was highest at mid-latitudes of the species’ range and genetic uniqueness was greatest in southern populations, consistent with sub-structuring inferred from both mtDNA and microsatellite analyses at finer levels of analysis. Surprisingly, snowshoe hares in the Greater Pacific Northwest mtDNA lineage were more closely related to black-tailed jackrabbits (Lepus californicus) than to other snowshoe hares, which may result from secondary introgression or shared ancestral polymorphism. Given the genetic distinctiveness of southern populations and minimal gene flow with their northern neighbors, fragmentation and loss of southern boreal habitats could mean loss of many unique alleles and reduced evolutionary potential.This article is protected by copyright. All rights reserved.
    Molecular Ecology 05/2014; · 6.28 Impact Factor
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    ABSTRACT: As duration of snow cover decreases owing to climate change, species undergoing seasonal colour moults can become colour mismatched with their background. The immediate adaptive solution to this mismatch is phenotypic plasticity, either in phenology of seasonal colour moults or in behaviours that reduce mismatch or its consequences. We observed nearly 200 snowshoe hares across a wide range of snow conditions and two study sites in Montana, USA, and found minimal plasticity in response to mismatch between coat colour and background. We found that moult phenology varied between study sites, likely due to differences in photoperiod and climate, but was largely fixed within study sites with only minimal plasticity to snow conditions during the spring white-to-brown moult. We also found no evidence that hares modify their behaviour in response to colour mismatch. Hiding and fleeing behaviours and resting spot preference of hares were more affected by variables related to season, site and concealment by vegetation, than by colour mismatch. We conclude that plasticity in moult phenology and behaviours in snowshoe hares is insufficient for adaptation to camouflage mismatch, suggesting that any future adaptation to climate change will require natural selection on moult phenology or behaviour.
    Proceedings of the Royal Society B: Biological Sciences 01/2014; 281(1782):20140029. · 5.68 Impact Factor
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    ABSTRACT: Most examples of seasonal mismatches in phenology span multiple trophic levels, with timing of animal reproduction, hibernation, or migration becoming detached from peak food supply. The consequences of such mismatches are difficult to link to specific future climate change scenarios because the responses across trophic levels have complex underlying climate drivers often confounded by other stressors. In contrast, seasonal coat color polyphenism creating camouflage against snow is a direct and potentially severe type of seasonal mismatch if crypsis becomes compromised by the animal being white when snow is absent. It is unknown whether plasticity in the initiation or rate of coat color change will be able to reduce mismatch between the seasonal coat color and an increasingly snow-free background. We find that natural populations of snowshoe hares exposed to 3 y of widely varying snowpack have plasticity in the rate of the spring white-to-brown molt, but not in either the initiation dates of color change or the rate of the fall brown-to-white molt. Using an ensemble of locally downscaled climate projections, we also show that annual average duration of snowpack is forecast to decrease by 29-35 d by midcentury and 40-69 d by the end of the century. Without evolution in coat color phenology, the reduced snow duration will increase the number of days that white hares will be mismatched on a snowless background by four- to eightfold by the end of the century. This novel and visually compelling climate change-induced stressor likely applies to >9 widely distributed mammals with seasonal coat color.
    Proceedings of the National Academy of Sciences 04/2013; · 9.81 Impact Factor
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    ABSTRACT: The non-uniformity of the distribution of biodiversity makes allocation of the limited resources available for conservation of biodiversity a difficult task. Approaches such as biodiversity hotspot identification, endemic bird areas, crisis ecoregions, global 200 ecoregions, and the Last of the Wild are used by scientists and international conservation agencies to prioritize conservation efforts. As part of the biodiverse Eastern Himalayan region, Bhutan has been identified as a conservation priority area by all these different approaches, yet data validating these assessments are limited. To examine whether Bhutan is a biodiversity hot spot for a key taxonomic group, we conducted camera trapping in the lower foothills of Bhutan, in Royal Manas National Park, from November 2010 to February 2011. We recorded six species of wild felids of which five are listed on the IUCN Red List: tiger Panthera tigris, golden cat Pardofelis temminckii, marbled cat Pardofelis marmorata, leopard cat Prionailurus bengalensis, clouded leopard Neofelis nebulosa and common leopard Panthera pardus. Our study area of 74 km2 has c. 16% of felid species, confirming Bhutan as a biodiversity hot spot for this group.
    Oryx 04/2013; 47(02). · 1.62 Impact Factor
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    ABSTRACT: Species of conservation concern are increasingly threatened by multiple, anthropogenic stressors which are outside their evolutionary experience. Greater sage-grouse are highly susceptible to the impacts of two such stressors: oil and gas (energy) development and West Nile virus (WNv). However, the combined effects of these stressors and their potential interactions have not been quantified. We used lek (breeding ground) counts across a landscape encompassing extensive local and regional variation in the intensity of energy development to quantify the effects of energy development on lek counts, in years with widespread WNv outbreaks and in years without widespread outbreaks. We then predicted the effects of well density and WNv outbreak years on sage-grouse in northeast Wyoming. Absent an outbreak year, drilling an undeveloped landscape to a high permitting level (3.1 wells/km(2)) resulted in a 61% reduction in the total number of males counted in northeast Wyoming (total count). This was similar in magnitude to the 55% total count reduction that resulted from an outbreak year alone. However, energy-associated reductions in the total count resulted from a decrease in the mean count at active leks, whereas outbreak-associated reductions resulted from a near doubling of the lek inactivity rate (proportion of leks with a last count = 0). Lek inactivity quadrupled when 3.1 wells/km(2) was combined with an outbreak year, compared to no energy development and no outbreak. Conservation measures should maintain sagebrush landscapes large and intact enough so that leks are not chronically reduced in size due to energy development, and therefore vulnerable to becoming inactive due to additional stressors.
    PLoS ONE 01/2013; 8(8):e71256. · 3.53 Impact Factor
  • Cynthia Hartway, L Scott Mills
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    ABSTRACT: Management strategies for the recovery of declining bird populations often must be made without sufficient data to predict the outcome of proposed actions or sufficient time and resources necessary to collect these data. We quantitatively reviewed studies of bird management in Canada and the United States to evaluate the relative efficacy of 4 common management interventions and to determine variables associated with their success. We compared how livestock exclusion, prescribed burning, removal of predators, and removal of cowbirds (Molothrus ater) affect bird nest success and used meta-regression to evaluate the influence of species and study-specific covariates on management outcomes. On average, all 4 management interventions increased nest success. When common species and threatened, endangered, or declining species (as defined by long-term trend data from the North American Breeding Bird Survey) were analyzed together, predator removal was the most effective management option. The difference in mean nest success between treatment and control plots in predator-removal experiments was more than twice that of either livestock exclusion or prescribed burning. However, when we considered management outcomes from only threatened, endangered, or declining species, livestock exclusions resulted in the greatest mean increase in nest success, more than twice that of the 3 other treatments. Our meta-regression results indicated that between-species variation accounted for approximately 86%, 40%, 35%, and 7% of the overall variation in the results of livestock-exclusion, prescribed-burn, predator-removal, and cowbird-removal studies, respectively. However, the covariates we tested explained significant variation only in outcomes among prescribed-burn studies. The difference in nest success between burned and unburned plots displayed a significant, positive trend in association with time since fire and was significantly larger in grasslands than in woodlands. Our results highlight the importance of comparative studies on management effects in developing efficient and effective conservation strategies.
    Conservation Biology 06/2012; 26(4):657-66. · 4.36 Impact Factor
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    ABSTRACT: Evidence of inbreeding depression is commonly detected from the fitness traits of animals, yet its effects on population growth rates of endangered species are rarely assessed. We examined whether inbreeding depression was affecting Sierra Nevada bighorn sheep (Ovis canadensis sierrae), a subspecies listed as endangered under the U.S. Endangered Species Act. Our objectives were to characterize genetic variation in this subspecies; test whether inbreeding depression affects bighorn sheep vital rates (adult survival and female fecundity); evaluate whether inbreeding depression may limit subspecies recovery; and examine the potential for genetic management to increase population growth rates. Genetic variation in 4 populations of Sierra Nevada bighorn sheep was among the lowest reported for any wild bighorn sheep population, and our results suggest that inbreeding depression has reduced adult female fecundity. Despite this population sizes and growth rates predicted from matrix-based projection models demonstrated that inbreeding depression would not substantially inhibit the recovery of Sierra Nevada bighorn sheep populations in the next approximately 8 bighorn sheep generations (48 years). Furthermore, simulations of genetic rescue within the subspecies did not suggest that such activities would appreciably increase population sizes or growth rates during the period we modeled (10 bighorn sheep generations, 60 years). Only simulations that augmented the Mono Basin population with genetic variation from other subspecies, which is not currently a management option, predicted significant increases in population size. Although we recommend that recovery activities should minimize future losses of genetic variation, genetic effects within these endangered populations-either negative (inbreeding depression) or positive (within subspecies genetic rescue)-appear unlikely to dramatically compromise or stimulate short-term conservation efforts. The distinction between detecting the effects of inbreeding depression on a component vital rate (e.g., fecundity) and the effects of inbreeding depression on population growth underscores the importance of quantifying inbreeding costs relative to population dynamics to effectively manage endangered populations.
    Conservation Biology 12/2011; 25(6):1240-9. · 4.36 Impact Factor
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    ABSTRACT: Despite decades of field research on greater sage-grouse, range-wide demographic data have yet to be synthesized into a sensitivity analysis to guide management actions. We reviewed range-wide demographic rates for greater sage-grouse from 1938 to 2011 and used data from 50 studies to parameterize a 2-stage, female-based population matrix model. We conducted life-stage simulation analyses to determine the proportion of variation in population growth rate (λ) accounted for by each vital rate, and we calculated analytical sensitivity, elasticity, and variance-stabilized sensitivity to identify the contribution of each vital rate to λ. As expected for an upland game bird, greater sage-grouse showed marked annual and geographic variation in several vital rates. Three rates were demonstrably important for population growth: female survival, chick survival, and nest success. Female survival and chick survival, in that order, had the most influence on λ per unit change in vital rates. However, nest success explained more of the variation in λ than did the survival rates. In lieu of quantitative data on specific mortality factors driving local populations, we recommend that management efforts for greater sage-grouse first focus on increasing female survival by restoring large, intact sagebrush-steppe landscapes, reducing persistent sources of human-caused mortality, and eliminating anthropogenic habitat features that subsidize species that prey on juvenile, yearling, and adult females. Our analysis also supports efforts to increase chick survival and nest success by eliminating anthropogenic habitat features that subsidize chick and nest predators, and by managing shrub, forb, and grass cover, height, and composition to meet local brood-rearing and nesting habitat guidelines. We caution that habitat management to increase chick survival and nest success should not reduce the cover or height of sagebrush below that required for female survival in other seasons (e.g., fall, winter). The success or failure of management actions for sage-grouse should be assessed by measuring changes in vital rates over long time periods to avoid confounding with natural, annual variation. © 2011 The Wildlife Society.
    Journal of Wildlife Management 11/2011; 76(2):336 - 347. · 1.64 Impact Factor
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    ABSTRACT: Habitat fragmentation often separates and reduces populations of vertebrates, but the relative effects of habitat attributes within remnant patches versus the matrix surrounding the patches are less clear. For snowshoe hares (Lepus americanus) lower densities and disrupted cycles in their southern range have been ascribed to habitat fragmentation, although relevant scales of landscape influence remain unknown. In a fragmented forest in north-central Washington we counted fecal pellets of snowshoe hares to examine the extent to which relative snowshoe hare densities within stands of suitable habitat changed with the composition of surrounding habitats. Pellet densities were associated primarily with density of large shrubs and saplings and medium trees within a stand. Pellet densities also were correlated positively with the amount of moist forest (dominated by Engelmann spruce [Picea engelmannii] and subalpine fir [Abies lasiocarpa]) and correlated negatively with the amount of open-structured habitat within 300 m of the stand perimeter. These results suggest that forest managers will have positive impacts on hare densities by managing both focal stands and the surrounding stands for the higher densities of large shrubs and saplings and medium trees that hares select.
    Journal of Mammalogy 06/2011; 92:561-567. · 2.31 Impact Factor
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    PAUL C. GRIFFIN, L. SCOTT MILLS
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    ABSTRACT: ABSTRACT Management of young forests is not often considered in conservation plans, but young forests provide habitat for some species of conservation concern. Snowshoe hares (Lepus americanus), critical prey of forest carnivores including the United States federally threatened Canada lynx (Lynx canadensis), can be abundant in young montane and subalpine forests with densely spaced saplings and shrub cover. Precommercial thinning (PCT) is a silvicultural technique that reduces sapling and shrub density on young forest stands. We tested for effects of PCT on snowshoe hare abundance for 2 years after experimental treatment at 3 replicate study areas. We also tested the effectiveness of a precommercial thinning with reserves (PCT-R) prescription, where 20% of the total stand was retained in uncut quarter-hectare patches. All stands were in montane—subalpine coniferous forests of western Montana, USA, where there is a persistent population of Canada lynx. Posttreatment changes in abundance were strongly negative on stands treated with standard PCT prescriptions (100% of the stand was treated), relative to both controls and stands treated with PCT-R. Trapping, snowtrack, and winter fecal-pellet indices indicated that snowshoe hares used the quarter-ha retention patches more than thinned portions of the PCT-R-treated stands in winter. We suggest that managing forest landscapes for high snowshoe hare abundance will require adoption of silvicultural techniques like PCT-R for stands that will be thinned, in addition to conservation of structurally valuable early and late-successional forest stands.
    Journal of Wildlife Management 12/2010; 71(2):559 - 564. · 1.64 Impact Factor
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    ABSTRACT: Age ratios (e.g., calf:cow for elk and fawn:doe for deer) are used regularly to monitor ungulate populations. However, it remains unclear what inferences are appropriate from this index because multiple vital rate changes can influence the observed ratio. We used modeling based on elk (Cervus elaphus) life-history to evaluate both how age ratios are influenced by stage-specific fecundity and survival and how well age ratios track population dynamics. Although all vital rates have the potential to influence calf:adult female ratios (i.e., calf:cow ratios), calf survival explained the vast majority of variation in calf:adult female ratios due to its temporal variation compared to other vital rates. Calf:adult female ratios were positively correlated with population growth rate (Λ) and often successfully indicated population trajectories. However, calf:adult female ratios performed poorly at detecting imposed declines in calf survival, suggesting that only the most severe declines would be rapidly detected. Our analyses clarify that managers can use accurate, unbiased age ratios to monitor arguably the most important components contributing to sustainable ungulate populations, survival rate of young and Λ. However, age ratios are not useful for detecting gradual declines in survival of young or making inferences about fecundity or adult survival in ungulate populations. Therefore, age ratios coupled with independent estimates of population growth or population size are necessary to monitor ungulate population demography and dynamics closely through time.
    Journal of Wildlife Management 12/2010; 72(5):1143 - 1151. · 1.64 Impact Factor
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    ABSTRACT: Probably no conservation genetics issue is currently more controversial than the question of whether grey wolves (Canis lupus) in the Northern Rockies have recovered to genetically effective levels. Following the dispersal-based recolonization of Northwestern Montana from Canada, and reintroductions to Yellowstone and Central Idaho, wolves have vastly exceeded population recovery goals of 300 wolves distributed in at least 10 breeding pairs in each of Wyoming, Idaho and Montana. With >1700 wolves currently, efforts to delist wolves from endangered status have become mired in legal battles over the distinct population segment (DPS) clause of the Endangered Species Act (ESA), and whether subpopulations within the DPS were genetically isolated. An earlier study by vonHoldt et al. (2008) suggested Yellowstone National Park wolves were indeed isolated and was used against delisting in 2008. Since then, wolves were temporarily delisted, and a first controversial hunting season occurred in fall of 2009. Yet, concerns over the genetic recovery of wolves in the Northern Rockies remain, and upcoming District court rulings in the summer of 2010 will probably include consideration of gene flow between subpopulations. In this issue of Molecular Ecology, vonHoldt et al. (2010) conduct the largest analysis of gene flow and population structure of the Northern Rockies wolves to date. Using an impressive sampling design and novel analytic methods, vonHoldt et al. (2010) show substantial levels of gene flow between three identified subpopulations of wolves within the Northern Rockies, clarifying previous analyses and convincingly showing genetic recovery.
    Molecular Ecology 10/2010; 19(20):4383-5. · 6.28 Impact Factor
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    ABSTRACT: Summary1. To successfully manipulate populations for management and conservation purposes, managers must be able to track changes in demographic rates and determine the factors driving spatial and temporal variation in those rates. For populations of management concern, however, data deficiencies frequently limit the use of traditional statistical methods for such analyses. Long-term demographic data are often piecemeal, having small sample sizes, inconsistent methodologies, intermittent data, and information on only a subset of important parameters and covariates.2. We evaluated the effectiveness of Bayesian state-space models for meeting these data limitations in elucidating dynamics of federally endangered Sierra Nevada bighorn sheep Ovis canadensis sierrae. We combined ground count, telemetry, and mark–resight data to: (1) estimate demographic parameters in three populations (including stage-specific abundances and vital rates); and (2) determine whether density, summer precipitation, or winter severity were driving variation in key demographic rates.3. Models combining all existing data types increased the precision and accuracy in parameter estimates and fit covariates to vital rates driving population performance. They also provided estimates for all years of interest (including years in which field data were not collected) and standardized the error structure across data types.4. Demographic rates indicated that recovery efforts should focus on increasing adult and yearling survival in the smallest bighorn sheep population. In evaluating covariates we found evidence of negative density dependence in the larger herds, but a trend of positive density dependence in the smallest herd suggesting that an augmentation may be needed to boost performance. We also found that vital rates in all populations were positively associated with summer precipitation, but that winter severity only had a negative effect on the smallest herd, the herd most strongly impacted by environmental stochasticity.5. Synthesis and applications. For populations with piecemeal data, a problem common to both endangered and harvested species, obtaining precise demographic parameter estimates is one of the greatest challenges in detecting population trends, diagnosing the causes of decline, and directing management. Data on Sierra Nevada bighorn sheep provide an example of the application of Bayesian state-space models for combining all existing data to meet these objectives and better inform important management and conservation decisions.
    Journal of Applied Ecology 09/2010; 47(5):1083 - 1093. · 4.74 Impact Factor
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    ABSTRACT: To develop effective management strategies for the recovery of threatened and endangered species, it is critical to identify those vital rates (survival and reproductive parameters) responsible for poor population performance and those whose increase will most efficiently change a population's trajectory. In actual application, however, approaches identifying key vital rates are often limited by inadequate demographic data, by unrealistic assumptions of asymptotic population dynamics, and of equal, infinitesimal changes in mean vital rates. We evaluated the consequences of these limitations in an analysis of vital rates most important in the dynamics of federally endangered Sierra Nevada bighorn sheep (Ovis canadensis sierrae). Based on data collected from 1980 to 2007, we estimated vital rates in three isolated populations, accounting for sampling error, variance, and covariance. We used analytical sensitivity analysis, life-stage simulation analysis, and a novel non-asymptotic simulation approach to (1) identify vital rates that should be targeted for subspecies recovery; (2) assess vital rate patterns of endangered bighorn sheep relative to other ungulate populations; (3) evaluate the performance of asymptotic vs. non-asymptotic models for meeting short-term management objectives; and (4) simulate management scenarios for boosting bighorn sheep population growth rates. We found wide spatial and temporal variation in bighorn sheep vital rates, causing rates to vary in their importance to different populations. As a result, Sierra Nevada bighorn sheep exhibited population-specific dynamics that did not follow theoretical expectations or those observed in other ungulates. Our study suggests that vital rate inferences from large, increasing, or healthy populations may not be applicable to those that are small, declining, or endangered. We also found that, while asymptotic approaches were generally applicable to bighorn sheep conservation planning; our non-asymptotic population models yielded unexpected results of importance to managers. Finally, extreme differences in the dynamics of individual bighorn sheep populations imply that effective management strategies for endangered species recovery may often need to be population-specific.
    Ecological Applications 09/2010; 20(6):1753-65. · 3.82 Impact Factor
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    ABSTRACT: ABSTRACT  The Mahalanobis distance statistic (D2) has emerged as an effective tool to identify suitable habitat from presence data alone, but there has been no mechanism to select among potential habitat covariates. We propose that the best combination of explanatory variables for a D2 model can be identified by ranking potential models based on the proportion of the entire study area that is classified as potentially suitable habitat given that a predetermined proportion of occupied locations are correctly classified. In effect, our approach seeks to minimize errors of commission, or maximize specificity, while holding the omission error rate constant. We used this approach to identify potentially suitable habitat for the Olympic marmot (Marmota olympus), a declining species endemic to Olympic National Park, Washington, USA. We compared models built with all combinations of 11 habitat variables. A 7-variable model identified 21,143 ha within the park as potentially suitable for marmots, correctly classifying 80% of occupied locations. Additional refinements to the 7-variable model (e.g., eliminating small patches) further reduced the predicted area to 18,579 ha with little reduction in predictive power. Although we sought a model that would allow field workers to find 80% of Olympic marmot locations, in fact, <3% of 376 occupied locations and <9% of abandoned locations were >100 m from habitat predicted by the final model, suggesting that >90% of occupied marmot habitat could be found by observant workers surveying predicted habitat. The model comparison procedure allowed us to identify the suite of covariates that maximized specificity of our model and, thus, limited the amount of less favorable habitat included in the final prediction area. We expect that by maximizing specificity of models built from presence-only data, our model comparison procedure will be useful to conservation practitioners planning reintroductions, searching for rare species, or identifying habitat for protection.
    Journal of Wildlife Management 06/2010; 74(5):1112 - 1121. · 1.64 Impact Factor
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    ABSTRACT: Estimation of a population trend from a time series of abundance data is an important task in ecology, yet such estimation remains logistically and conceptually challenging in practice. First, the extent to which unequal intervals in the time series, due to missing observations or irregular sampling, compromise trend estimation is not well-known. Furthermore, the predominant trend estimation method (loglinear regression of abundance data against time) ignores the possibility of process noise, while an alternative method (the ‘diffusion approximation’) ignores observation error in the abundance data. State-space models that account for both process noise and observation error exist but have been little used. We study an adaptation of the exponential growth state-space (EGSS) model for use with missing data in the time series, and we compare its trend estimation to the status quo methods. The EGSS model provides superior estimates of trend across wide ranges of time series length and sources of variation. The performance of the EGSS model even with half of the counts in the time series missing implies that trend estimates may be improved by diverting effort away from annual monitoring and towards increasing time series length or improving precision of the abundance estimates for years that data are collected.
    Oikos 11/2009; 118(12):1940 - 1946. · 3.33 Impact Factor
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    Paul C. Griffin, L. Scott Mills
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    ABSTRACT: A full understanding of population dynamics of wide-ranging animals should account for the effects that movement and habitat use have on individual contributions to population growth or decline. Quantifying the per-capita, habitat-specific contribution to population growth can clarify the value of different patch types, and help to differentiate population sources from population sinks. Snowshoe hares, Lepus americanus, routinely use various habitat types in the landscapes they inhabit in the contiguous US, where managing forests for high snowshoe hare density is a priority for conservation of Canada lynx, Lynx canadensis. We estimated density and demographic rates via mark–recapture live trapping and radio-telemetry within four forest stand structure (FSS) types at three study areas within heterogeneous managed forests in western Montana. We found support for known fate survival models with time-varying individual covariates representing the proportion of locations in each of the FSS types, with survival rates decreasing as use of open young and open mature FSS types increased. The per-capita contribution to overall population growth increased with use of the dense mature or dense young FSS types and decreased with use of the open young or open mature FSS types, and relatively high levels of immigration appear to be necessary to sustain hares in the open FSS types. Our results support a conceptual model for snowshoe hares in the southern range in which sink habitats (open areas) prevent the buildup of high hare densities. More broadly, we use this system to develop a novel approach to quantify demographic sources and sinks for animals making routine movements through complex fragmented landscapes.
    Oikos 09/2009; 118(10):1487 - 1498. · 3.33 Impact Factor
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    ABSTRACT: Snowshoe hares (Lepus americanus) undergo remarkable cycles and are the primary prey base of Canada lynx (Lynx canadensis), a carnivore recently listed as threatened in the contiguous United States. Efforts to evaluate hare densities using pellets have traditionally been based on regression equations developed in the Yukon, Canada. In western Montana, we evaluated whether or not local regression equations performed better than the most recent Yukon equation and assessed whether there was concordance between pellet-based predictions and mark–recapture density estimates of hares. We developed local Montana regression equations based on 224 data points consisting of mark-recapture estimates and pellet counts, derived from 38 sites in 2 different areas sampled for 1 to 5 years using 2 different pellet plot shapes. We evaluated concordance between estimated density and predicted density based on pellet counts coupled with regression equations at 436 site-area-season combinations different from those used to develop the regression equations. At densities below 0.3 hares/ha, predicted density based on pellets tended to be greater than for mark–recapture; the difference was usually <1 hare per ha on an absolute scale, but at low densities this translated to proportional differences of 1,000% or greater. At densities above 0.7 hares/ha, pellet regressions tended to predict lower density than mark–recapture. Because local regression equations did not outperform the Yukon equation, we see little merit in further development of local regression equations unless a study is to be conducted in a formal double-sampling framework. We recommend that widespread pellet sampling be used to identify areas with very low hare densities; subsequent surveys using mark–recapture methodology can then focus on higher density areas where density inferences are more reliable.
    Journal of Wildlife Management 09/2009; · 1.64 Impact Factor
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    ABSTRACT: Background/Question/Methods A common problem in the conservation of endangered species is that critical management decisions must be made without complete demographic information. When demographic data do exist they are often piecemeal, with different field methods used in different years, data that have been collected intermittently, small sample sizes, and information on only a subset of important parameters. New quantitative approaches, such as bayesian state-space models, meet many of these limitations by allowing multiple data types to be “integrated” into a single demographic analysis. The benefits of such models are that they generate more precise parameter estimates, standardize the error structure across data types, mechanistically link vital rate information to population abundance data, and estimate “hidden” parameters that are not necessarily measured in the field but can be extracted from the data. We applied this bayesian state-space approach to data on endangered Sierra Nevada bighorn sheep, the rarest subspecies of mountain sheep in North America. Our objective was to use all data available on this subspecies to generate annual estimates of population size and stage-specific survival and reproductive rates. We combined data from minimum counts (collected intermittently 1981-2009), mark-resight surveys (collected 2004-2009), and telemetry (collected 2002-2009) into a single demographic model to estimate key demographic parameters and simultaneously evaluate the influence of different covariates (i.e. weather, predation and density) on those parameters. Results/Conclusions We demonstrate the utility of this approach for maximizing information obtained from piecemeal data indicative of many endangered populations. Our bayesian state-space models, integrating minimum count, mark-resight, and telemetry data, increased precision in demographic parameter estimates over those obtained from any single data type alone. In addition, the models derived parameter values for years with missing data, standardized the error structure across data types, accurately estimated trends in key parameters through time, and fit covariates explaining population change. We found that the importance of weather, predation, and density on bighorn sheep demography varied among our subpopulations, a result with important implications for the conservation and management of this subspecies.
    94th ESA Annual Convention 2009; 08/2009

Publication Stats

2k Citations
217.52 Total Impact Points

Institutions

  • 1996–2013
    • University of Montana
      • • Wildlife Biology Program
      • • Division of Biological Sciences
      Missoula, MT, United States
  • 2009
    • University of Idaho
      • Department of Fish and Wildlife Resources
      Moscow, ID, United States
    • North Carolina State University
      • Department of Forestry and Environmental Resources
      Raleigh, North Carolina, United States
  • 2000
    • US Forest Service
      • Rocky Mountain Research Station
      Washington, Washington, D.C., United States
  • 1995
    • University of California, Berkeley
      • Department of Integrative Biology
      Berkeley, MO, United States