Papers by Thomas Stohlgren
Modern biogeographers recognize that humans are seen as constituents of ecosystems, drivers of si... more Modern biogeographers recognize that humans are seen as constituents of ecosystems, drivers of significant change, and perhaps, the most invasive species on earth. We found it instructive to model humans as invasive organisms with the same environmental factors. We present a preliminary model of the spread of modern humans in the conterminous United States between 1992 and 2001 based on a subset of National Land Cover Data (NLCD), a time series LANDSAT product. We relied on the commonly used Maxent model, a species-environmental matching model, to map urbanization. Results: Urban areas represented 5.1% of the lower 48 states in 2001, an increase of 7.5% (18,112 km 2 ) in the nine year period. At this rate, an area the size of Massachusetts is converted to urban land use every ten years. We used accepted models commonly used for mapping plant and animal distributions and found that climatic and environmental factors can strongly predict our spread (i.e., the conversion of forests, shrub/grass, and wetland areas into urban areas), with a 92.5% success rate (Area Under the Curve). Adding a roads layer in the model improved predictions to a 95.5% success rate. 8.8% of the 1-km 2 cells in the conterminous U.S. now have a major road in them. In 2001, 0.8% of 1-km 2 cells in the U.S. had an urbanness value of > 800, (>89% of a 1-km 2 cell is urban), while we predict that 24.5% of 1-km 2 cells in the conterminous U.S. will be > 800 eventually. Main conclusion: Humans have a highly predictable pattern of urbanization based on climatic and topographic variables. Conservation strategies may benefit from that predictability.
We examined species-area curves, species composition and similarity (Jaccard's coefficients), and... more We examined species-area curves, species composition and similarity (Jaccard's coefficients), and species richness in 17 vegetation types to develop a composite index of a vegetation type's contribution to regional species richness. We collected data from 1 to 1000 m 2 scales in 147 nested plots in Rocky Mountain National Park, Colorado, USA to compare three species-area curve models' abilities to estimate the number of species observed in each vegetation type. The log(species)-log(area) curve had the largest adjusted coefficients of determination (r 2 values) in 12 of the 17 types, followed by the species-log(area) curve with five of the highest values. When the slopes of the curves were corrected for species overlap among plots with Jaccard's coefficients, the species-log(area) curves estimated values closest to those observed. We combined information from speciesarea curves and measures of heterogeneity with information on the area covered by each vegetation type and found that the types making the greatest contributions to regional biodiversity covered the smallest areas. This approach may provide an accurate and relatively rapid way to rank hotspots of plant diversity within regions of interest. #
Modern biogeographers recognize that humans are seen as constituents of ecosystems, drivers of si... more Modern biogeographers recognize that humans are seen as constituents of ecosystems, drivers of significant change, and perhaps, the most invasive species on earth. We found it instructive to model humans as invasive organisms with the same environmental factors. We present a preliminary model of the spread of modern humans in the conterminous United States between 1992 and 2001 based on a subset of National Land Cover Data (NLCD), a time series LANDSAT product. We relied on the commonly used Maxent model, a species-environmental matching model, to map urbanization. Results: Urban areas represented 5.1% of the lower 48 states in 2001, an increase of 7.5% (18,112 km 2 ) in the nine year period. At this rate, an area the size of Massachusetts is converted to urban land use every ten years. We used accepted models commonly used for mapping plant and animal distributions and found that climatic and environmental factors can strongly predict our spread (i.e., the conversion of forests, shrub/grass, and wetland areas into urban areas), with a 92.5% success rate (Area Under the Curve). Adding a roads layer in the model improved predictions to a 95.5% success rate. 8.8% of the 1-km 2 cells in the conterminous U.S. now have a major road in them. In 2001, 0.8% of 1-km 2 cells in the U.S. had an urbanness value of > 800, (>89% of a 1-km 2 cell is urban), while we predict that 24.5% of 1-km 2 cells in the conterminous U.S. will be > 800 eventually. Main conclusion: Humans have a highly predictable pattern of urbanization based on climatic and topographic variables. Conservation strategies may benefit from that predictability.
Environmental monitoring programs must efficiently describe state shifts. We propose using maximu... more Environmental monitoring programs must efficiently describe state shifts. We propose using maximum entropy modeling to select dissimilar sampling sites to capture environmental variability at low cost, and demonstrate a specific application: sample site selection for the Central Plains domain (453,490 km 2 ) of the National Ecological Observatory Network (NEON). We relied on four environmental factors: mean annual temperature and precipitation, elevation, and vegetation type. A "sample site" was defined as a 20 km × 20 km area (equal to NEON's airborne observation platform [AOP] footprint), within which each 1 km 2 cell was evaluated for each environmental factor. After each model run, the most environmentally dissimilar site was selected from all potential sample sites. The iterative selection of eight sites captured approximately 80% of the environmental envelope of the domain, an improvement over stratified random sampling and simple random designs for sample site selection. This approach can be widely used for cost-efficient selection of survey and monitoring sites.
Ensemble species distribution models combine the strengths of several species environmental match... more Ensemble species distribution models combine the strengths of several species environmental matching models, while minimizing the weakness of any one model. Ensemble models may be particularly useful in risk analysis of recently arrived, harmful invasive species because species may not yet have spread to all suitable habitats, leaving species-environment relationships difficult to determine. We tested five individual models (logistic regression, boosted regression trees, random forest, multivariate adaptive regression splines (MARS), and maximum entropy model or Maxent) and ensemble modeling for selected nonnative plant species in Yellowstone and Grand Teton National Parks, Wyoming; Sequoia and Kings Canyon National Parks, California, and areas of interior Alaska. The models are based on field data provided by the park staffs, combined with topographic, climatic, and vegetation predictors derived from satellite data. For the four invasive plant species tested, ensemble models were the only models that ranked in the top three models for both field validation and test data. Ensemble models may be more robust than individual species-environment matching models for risk analysis.
Species distribution models are increasing in popularity for mapping suitable habitat for species... more Species distribution models are increasing in popularity for mapping suitable habitat for species of management concern. Many investigators now recognize that extrapolations of these models with geographic information systems (GIS) might be sensitive to the environmental bounds of the data used in their development, yet there is no recommended best practice for "clamping" model extrapolations. We relied on two commonly used modeling approaches: classification and regression tree (CART) and maximum entropy (Maxent) models, and we tested a simple alteration of the model extrapolations, bounding extrapolations to the maximum and minimum values of primary environmental predictors, to provide a more realistic map of suitable habitat of hybridized Africanized honey bees in the southwestern United States. Findings suggest that multiple models of bounding, and the most conservative bounding of species distribution models, like those presented here, should probably replace the unbounded or loosely bounded techniques currently used [Current Zoology 57 : 642-647, 2011].
Scale and plant invasions: A theory of biotic acceptance. -Preslia 78: 405-426.
Natural area parks managed by the United States National Park Service were established to protect... more Natural area parks managed by the United States National Park Service were established to protect native species and historical (but living) landscapes and scenery, and to provide public enjoyment of the same, as long as the natural area remained "unimpaired for future generations." A growing human population, and a 40-fold increase global trade and transportation may provide the most significant challenge to Park Service management: the invasion of alien plants, animals, and diseases into so called 'protected areas'. General ecological theory suggests that an increase in biological diversity should increase the overall stability of an ecosystem. We provide examples of plant invasions in U.S. National Parks to show that, despite increases in plant diversity, alien plant species are capable of greatly affecting the native species that the parks were established to protect. Furthermore, alien plant species are affecting natural patterns of grazing, disturbance, and nutrient cycling, resulting in decreased habitat quality, perhaps providing less resistance and resilience to natural and anthropogenic stresses such as climate change, land use change, recreation, and future invasions. The primary mission of the National Park Service, protecting native species and ecosystems for present and future generations, may be increasingly difficult due to the continuing invasions of alien organisms. Key elements of an effective invasive plant management programme are identified.
BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting... more BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.
Ensemble species distribution models combine the strengths of several species environmental match... more Ensemble species distribution models combine the strengths of several species environmental matching models, while minimizing the weakness of any one model. Ensemble models may be particularly useful in risk analysis of recently arrived, harmful invasive species because species may not yet have spread to all suitable habitats, leaving species-environment relationships difficult to determine. We tested five individual models (logistic regression, boosted regression trees, random forest, multivariate adaptive regression splines (MARS), and maximum entropy model or Maxent) and ensemble modeling for selected nonnative plant species in Yellowstone and Grand Teton National Parks, Wyoming; Sequoia and Kings Canyon National Parks, California, and areas of interior Alaska. The models are based on field data provided by the park staffs, combined with topographic, climatic, and vegetation predictors derived from satellite data. For the four invasive plant species tested, ensemble models were the only models that ranked in the top three models for both field validation and test data. Ensemble models may be more robust than individual species-environment matching models for risk analysis.
Managers of natural landscapes need cost-efficient, accurate, and precise systems to inventory pl... more Managers of natural landscapes need cost-efficient, accurate, and precise systems to inventory plant diversity. We investigated a nested-intensity sampling design to assess local and landscape-scale heterogeneity of plant species richness in aspen stands in southern Colorado, USA. The nested-intensity 2 design used three vegetation sampling techniques: the Modified-Whittaker, a 1000-m multiple-scale plot 2 2 (n 5 8); a 100-m multiple-scale Intensive plot (n 5 15); and a 100-m single-scale Extensive plot (n 5 2 28). The large Modified-Whittaker plot (1000 m) recorded greater species richness per plot than the 2 other two sampling techniques (P , 0.001), estimated cover of a greater number of species in 1-m 2 subplots (P , 0.018), and captured 32 species missed by the smaller, more numerous 100-m plots of the other designs. The Intensive plots extended the environmental gradient sampled, capturing 17 species missed by the other techniques, and improved species–area calculations. The greater number of Extensive plots further expanded the gradient sampled, and captured 18 additional species. The multi-scale Modified-Whittaker and Intensive designs allowed quantification of the slopes of species–area curves in the single-scale Extensive plots. Multiple linear regressions were able to predict the slope of 2 species–area curves (adj R 5 0.64, P , 0.001) at each Extensive plot, allowing comparison of species richness at each sample location. Comparison of species–accumulation curves generated with each technique suggested that small, single-scale plot techniques might be very misleading because they underestimate species richness by missing locally rare species at every site. A combination of large and small multi-scale and single-scale plots greatly improves our understanding of native and exotic plant diversity patterns.
This study explored the impacts of sustained heavy grazing in six vegetation types across an alti... more This study explored the impacts of sustained heavy grazing in six vegetation types across an altitudinal and rainfall gradient in the Kamiesberg mountain range in Namaqualand. The study was carried out across the fence separating the Leliefontein communal area and surrounding privately owned farms. The communal area has been stocked over decades at approximately twice the government recommended stocking rate, while adjacent privately owned farms have generally adhered to recommended rates. Plant community data were collected from 66, 0.1 ha modified Whittaker plots and analysed for diversity and compositional changes. Consideration of community-wide responses through NMDS ordination showed that heavy grazing did not result in the dominance of a few widespread , weedy species in communal areas. Species richness at the 0.1 ha scale was also not affected by different land use practices. However, there was a significant compositional shift away from large woody and succulent shrubs, and an associated increase in dwarf shrubs and herbaceous perennial plants on the communal areas. This shift was only evident on the sandy lowland habitats, while a reduction in perennial grass was recorded in the rocky upland habitats on the communal areas. Compositional shifts towards smaller and more ephemeral species in the communal area are indicative of a system more closely dependent on rainfall. This has
We are using parallel geostatistical codes to study spatial relationships among biospheric resour... more We are using parallel geostatistical codes to study spatial relationships among biospheric resources in the Cerro Grande Wildfire Site, Los Alamos, New Mexico, and Rocky Mountain National Park, Colorado. For example, spatial statistical models based on large-and small-scale variability have been used to predict species richness of both native and exotic plants (hot spots of diversity) and patterns of exotic plant invasion. However, broader use of geostastics in natural resource modeling, especially at landscape and regional scales, has been limited due to the large computing requirements of these applications. To address this problem, we implemented an MPI version of kriging on a Beowulf cluster that shows nearly linear scaling and a 31x speedup on 32 processors. These techniques are proving effective and provide the basis for a national decision support capability for invasive species management that is being jointly developed by NASA and the US Geological Survey.
Geographical Information Systems (GIS) are powerful tools in the field of invasive species manage... more Geographical Information Systems (GIS) are powerful tools in the field of invasive species management. GIS can be used to create potential distribution maps for all manner of taxa, including plants, animals, and diseases. GIS also performs well in the early detection and rapid assessment of invasive species. Here, we used GIS applications to investigate species richness and invasion patterns in fish in the United States (US) at the 6-digit Hydrologic Unit Code (HUC) level. We also created maps of potential spread of the cane toad (Bufo marinus) in the southeastern US at the 8-digit HUC level using regression and environmental envelope techniques. Equipped with this potential map, resource managers can target their field surveys to areas most vulnerable to invasion. Advances in GIS technology, maps, data, and many of these techniques can be found on websites such as the National Institute of Invasive Species Science (www.NIISS.org). Such websites provide a forum for data sharing and analysis that is an invaluable service to the invasive species community.
Understanding coupled climatic and ecosystem responses to global climate change in the Central Grasslands
Bulletin of the Ecological Society of America
A long-term National Park Service research program to assess the potential effect of global clima... more A long-term National Park Service research program to assess the potential effect of global climate change on the Central Grasslands Biogeographic Area is underway. The program consists of two integrated projects: Projecting climate and vegetation change at regional to landscape scales; and Predicting the effect of global change on vegetation in park landscapes at the plot to landscape scales. Together, these integrated field and modeling studies establish a means to validate mesoscale and landscape vegetation models, a central goal of the NPS Global Change Research Program. Vegetation life form modeling suggests that under climate change scenarios the distribution of grassland vegetation zones will undergo major shifts. Results indicate that climate change impacts that reduce water availability will more severely depress productivity of C[sub 3] grass communities relative to C[sub 4] grass communities.
Scaling NEON biological data from points to the continent
Background/Question/Methods The NEON framework provides the scientific community with an unpreced... more Background/Question/Methods The NEON framework provides the scientific community with an unprecedented opportunity, and an unprecedented challenge, to scale biological data from points to the continent. We tested multi-scale vegetation plots in the Central Plains Experiment Station (core area for the Central Plains Domain) to assess the scalability of invasive plant species data. One preliminary question we asked was whether the ratio of non-native to native plant species was consistent and predictable across spatial scales. Results/Conclusions We stratified vegetation types based on the National Land Cover Data, and used a standard multi-scale plot method consisting of a 7.3-m radius plot, with three nested 1-m2 subplots in summer, 2008. We compared data to county-level species data provided by the Biota of North America Program (www.BONAP.org). Preliminary analysis demonstrated that the percentages of non-native plant species (of total plant species) is fairly consistent across sp...
Potential habitat modeling for a new climate
Background/Question/Methods There is a general need in invasive species research to quantify the ... more Background/Question/Methods There is a general need in invasive species research to quantify the potential habitat of invasive plant species. We were interested in modeling the shifts in suitable habitat over time and environmental space, especially in the face of climate change. Many readily available climate scenarios are only available at very coarse spatial and temporal resolutions (e.g., 0.5° model outputs for the year 2100), which are not fine enough to be useful to local resource managers. We used 1-km climate scenarios projected to the years 2020 and 2035 for the continental United States, to get a finer look at potential invasive species habitat distributions. We chose 10 species with a broad range of characteristics to model, some more recently introduced, some well established, some habitat-generalists, and some habitat-specialist species. We harvested most of the data from on-line sources, especially the National Institute of Invasive Species Science (www.NIISS.org). We ...
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Papers by Thomas Stohlgren