Bethany Bradley | University of Massachusetts Amherst (original) (raw)

Papers by Bethany Bradley

Key Points: • Many plant and animal species dependent on sagebrush ecosystems are declining and /... more Key Points: • Many plant and animal species dependent on sagebrush ecosystems are declining and / or endangered. • About 50% of the original distribution of sagebrush has been lost. • Research suggests climate change will negatively impact big sagebrush in the hottest portions of its current range but that climate change will have weak or even positive effects in cooler regions. • Concerns about climate change should not preclude investments in sagebrush conservation and restoration. • In cooler areas, land management should focus on indirect threats to sagebrush habitat, such as cheatgrass and fire frequency. • In hotter areas, land managers should focus on protecting sites with cooler or wetter microclimates where big sagebrush is more likely to thrive.

Ecosphere, 2018

Spatial abundance information is a critical component of invasive plant risk assessment. While sp... more Spatial abundance information is a critical component of invasive plant risk assessment. While spatial occurrence data provide important information about potential establishment, abundance data are necessary to understand invasive species' populations, which ultimately drive environmental and economic impacts. In recent years, the collective efforts of numerous management agencies and public participants have created unprecedented spatial archives of invasive plant occurrence, but consistent information about abundance remains rare. Here, we develop guidelines for the collection and reporting of abundance information that can add value to existing data collection efforts and inform spatial ecology research. In order to identify the most common methods used to report abundance, we analyzed over 1.6 million invasive plant records in the Early Detection and Distribution Mapping System (EDDMapS). Abundance data in some form are widely reported, with 58.9% of records containing qualitative or quantitative information about invasive plant cover, density, or infested area, but records vary markedly in terms of standards for reporting. Percent cover was the most commonly reported metric of abundance, typically collected in bins of trace (<1%), low (1-5%), moderate (5-25%), and high (>25%). However, percent cover data were rarely reported along with an estimate of area, which is critical for ensuring accurate interpretation of reported abundance data. Infested area is typically reported as a number with associated units of square feet or acres. Together, an estimate of both cover and infested area provides the most robust and interpretable information for spatial research and risk assessment applications. By developing consistent metrics of reporting for abundance, collectors can provide much needed information to support spatial models of invasion risk.

Biological Invasions, 2017

Cheatgrass (Bromus tectorum) is an invasive grass pervasive across the Intermountain Western US a... more Cheatgrass (Bromus tectorum) is an invasive grass pervasive across the Intermountain Western US and linked to major increases in fire frequency. Despite widespread ecological impacts associated with cheatgrass, we lack a spatially extensive model of cheatgrass invasion in the Intermountain West. Here, we leverage satellite phenology predictors and thousands of field surveys of cheatgrass abundance to create regional models of cheatgrass distribution and percent cover. We compare cheatgrass presence to fire probability, fire seasonality and ignition source. Regional models of percent cover had low predictive power (34% of variance explained), but distribution models based on a threshold of 15% cover to differentiate high abundance from low abundance had an overall accuracy of 74%. Cheatgrass achieves C 15% cover over 210,000 km 2 (31%) of the Intermountain West. These lands were twice as likely to burn as those with low abundance, and four times more likely to burn multiple times between 2000 and 2015. Fire probability increased rapidly at low cheatgrass cover (1-5%) but remained similar at higher cover, suggesting that even small amounts of cheatgrass in an ecosystem can increase fire risk. Abundant cheatgrass Electronic supplementary material The online version of this article (

Ecosphere, 2017

Measuring and predicting invasive plant abundance is critical for understanding impacts on ecosys... more Measuring and predicting invasive plant abundance is critical for understanding impacts on ecosystems and economies. Although spatial abundance datasets remain rare, occurrence datasets are increasingly available across broad regional scales. We asked whether the frequency of these point occurrences can be used as a proxy for abundance of invasive plants. We compiled both occurrence and abundance data for 13 regionally important invasive plants in the northeast United States from herbarium records and several contributed distribution datasets. We integrated all available abundance information based on infested area, stem count, percent cover, or qualitative descriptions into abundance rankings ranging from 0 (absent) to 4 (highly abundant). Within equal-area grid cells of 800 m, we counted numbers of occurrence points and used ordinal regression to test whether higher densities of occurrence points increased the odds of a higher abundance ranking. We compiled a total of 86,854 occurrence points in 34,596 grid cells, of which 26,114 points (30%) within 11,976 cells (35%) had some form of abundance information. Eleven of the 13 species had a slight but significantly positive odds ratio; that is, more occurrence points of a species increased the odds that the species was abundant within the grid cell. However, the predictive ability of the models was poor (j < 0.2) for the majority of species. Additionally, most grid cells contained only one or two occurrence points, making it impossible to infer abundance in all but a few locations. These results suggest that currently available occurrence datasets do not effectively represent abundance, which could explain why many distribution models based on occurrence data are poor predictors of abundance. Increased efforts to consistently collect and report invasive species abundance, ideally estimating both infested area and average cover, are strongly needed for regional-scale assessments of potential abundance and associated impact.

Global Change Biology, 2017

A number of modeling approaches have been developed to predict the impacts of climate change on s... more A number of modeling approaches have been developed to predict the impacts of climate change on species distributions, performance, and abundance. The stronger the agreement from models that represent different processes and are based on distinct and independent sources of information, the greater the confidence we can have in their predictions. Evaluating the level of confidence is particularly important when predictions are used to guide conservation or restoration decisions. We used a multi-model approach to predict climate change impacts on big sagebrush (Artemisia tridentata), the dominant plant species on roughly 43 million hectares in the western United States and a key resource for many endemic wildlife species. To evaluate the climate sensitivity of A. tridentata, we developed four predictive models, two based on empirically derived spatial and temporal relationships, and two that applied mechanistic approaches to simulate sagebrush recruitment and growth. This approach enabled us to produce an aggregate index of climate change vulnerability and uncertainty based on the level of agreement between models. Despite large differences in model structure, predictions of sagebrush response to climate change were largely consistent. Performance, as measured by change in cover, growth, or recruitment, was predicted to decrease at the warmest sites, but increase throughout the cooler portions of sagebrush's range. A sensitivity analysis indicated that sagebrush performance responds more strongly to changes in temperature than precipitation. Most of the uncertainty in model predictions reflected variation among the ecological models, raising questions about the reliability of forecasts based on a single modeling approach. Our results highlight the value of a multimodel approach in forecasting climate change impacts and uncertainties and should help land managers to maximize the value of conservation investments.

Nature Communications, 2016

Invasive alien species (IAS) threaten human livelihoods and biodiversity globally. Increasing glo... more Invasive alien species (IAS) threaten human livelihoods and biodiversity globally. Increasing globalization facilitates IAS arrival, and environmental changes, including climate change, facilitate IAS establishment. Here we provide the first global, spatial analysis of the terrestrial threat from IAS in light of twenty-first century globalization and environmental change, and evaluate national capacities to prevent and manage species invasions. We find that one-sixth of the global land surface is highly vulnerable to invasion, including substantial areas in developing economies and biodiversity hotspots. The dominant invasion vectors differ between high-income countries (imports, particularly of plants and pets) and low-income countries (air travel). Uniting data on the causes of introduction and establishment can improve early-warning and eradication schemes. Most countries have limited capacity to act against invasions. In particular, we reveal a clear need for proactive invasion ...

Ecological Applications, 2016

Humans have a profound effect on fire regimes by increasing the frequency of ignitions. Although ... more Humans have a profound effect on fire regimes by increasing the frequency of ignitions. Although ignition is an integral component of understanding and predicting fire, to date fire models have not been able to isolate the ignition location, leading to inconsistent use of anthropogenic ignition proxies. Here, we identified fire ignitions from the Moderate Resolution Imaging Spectrometer (MODIS) burned area product (2000-2012) to create the Accepted Article This article is protected by copyright. All rights reserved. first remotely sensed, consistently derived, and regionally comprehensive fire ignition data set for the western United States. We quantified the spatial relationships between several anthropogenic land use/disturbance features and ignition for ecoregions within the study area, and used hierarchical partitioning to test how the anthropogenic predictors of fire ignition vary among ecoregions. The degree to which anthropogenic features predicted ignition varied considerably by ecoregion, with the strongest relationships found in the Marine West Coast Forest and North American Desert ecoregions. Similarly, the contribution of individual anthropogenic predictors varied greatly among ecoregions. Railroad corridors and agricultural presence tended to be the most important predictors of anthropogenic ignition while population density and roads were generally poor predictors. Although human population has often been used as a proxy for ignitions at global scales, it is less important at regional scales when more specific land uses (e.g., agriculture) can be identified. The variability of ignition predictors among ecoregions suggests that human activities have heterogeneous impacts in altering fire regimes within different vegetation types and geographies.

Diversity and Distributions, 2016

Aim Biogeographers have long known that plant species do not fully encompass their fundamental ni... more Aim Biogeographers have long known that plant species do not fully encompass their fundamental niche. Nonetheless, in practice, species distribution modelling assumes that plant distributions represent a reasonable approximation of their environmental tolerance. For ecological forecasting, projections of habitat loss due to climate change assume that many species will be unable to tolerate climate conditions outside of those found within their current distributional ranges. We aim to test how well occurrences in the native range approximate the climatic conditions in which plant species can survive. Location Continental USA. Methods We compared the climatic conditions between occurrences in the US native versus US non-native ranges using 144 non-invasive plant species. We quantified differences in January minimum temperature, July maximum temperature and annual precipitation as indicators of climatic tolerance. We also compared modelled potential distributions throughout the US based on native and total ranges to test how expanded climatic tolerance translates into predicted geographical range. Results Most species (86%) had non-native occurrences in climates outside those described by their native distributions. For the 80 species with lower minimum temperatures at non-native occurrences, the median expansion of minimum temperature tolerance was À2.9°C. Similarly, for the 90 species with lower precipitation at non-native occurrences, the median expansion of minimum annual precipitation was À23 cm. Broader climatic conditions at nonnative occurrences expanded the modelled potential geographical range by a median of 35%, with smaller range species showing larger expansions of potential geographical range. Main conclusions Our results show that plants' native ranges strongly underestimate climatic tolerance, leading species distribution models to underpredict potential range. The climatic tolerance of species with narrow native ranges appears most prone to underestimation. These findings suggest that many plants will be able to persist in situ with climate change for far longer than projected by species distribution models.

Springer Series on Environmental Management, 2016

A prominent goal of invasive plant management is to prevent or reduce the spread of invasive spec... more A prominent goal of invasive plant management is to prevent or reduce the spread of invasive species into uninvaded landscapes and regions. Monitoring and control efforts often rely on scientifi c knowledge of suitable habitat for the invasive species. However, rising temperatures and altered precipitation projected with climate change are likely to shift the geographic range of that suitable habitat. Here, we review experimental and modeling studies of climatic limits to exotic annual Bromus (Bromus hereafter) distribution in the Intermountain West in the context of projections of future climate change. We update empirical models of range shifts to test whether Bromus rubens L. (red brome) is likely to expand into ranges that become less suitable for Bromus tectorum L. (cheatgrass or downy brome). Warming temperatures are likely to create an advantage for Bromus species throughout much of the Intermountain West, potentially enhancing invasion into formerly resistant ecosystems if native species mortality increases with warming and drought. Bromus rubens is likely to expand into areas of the Southern Great Basin and Colorado Plateau as warmer winters reduce range constraints caused by cold intolerance. However, a primary limitation to exotic annual Bromus invasion and expansion is growing season precipitation. Projections for precipitation change are uncertain, but increased precipitation during periods critical for exotic annual Bromus germination and growth is forecast for the Northern Great Basin. Increased Bromus reproduction and biomass may exacerbate B. tectorum invasion and associated fi re risk, especially if coupled with longer fi re seasons and more extreme

Global change biology, Jan 24, 2015

Understanding the mechanisms underlying ecosystem resilience - why some systems have an irreversi... more Understanding the mechanisms underlying ecosystem resilience - why some systems have an irreversible response to disturbances while others recover - is critical for conserving biodiversity and ecosystem function in the face of global change. Despite the widespread acceptance of a positive relationship between biodiversity and resilience, empirical evidence for this relationship remains fairly limited in scope and localized in scale. Assessing resilience at the large landscape and regional scales most relevant to land management and conservation practices has been limited by the ability to measure both diversity and resilience over large spatial scales. Here, we combined tools used in large scale studies of biodiversity (remote sensing, trait databases) with theoretical advances developed from small scale experiments to ask if the functional diversity within a range of woodland and forest ecosystems influences the recovery of productivity after wildfires across the four-corners regio...

Landscape Ecology, 2014

The spatial distribution of non-native, invasive plants on the landscape is strongly influenced b... more The spatial distribution of non-native, invasive plants on the landscape is strongly influenced by human action. People introduce non-native species to new landscapes and regions (propagule pressure) as well as increase ecosystem invasibility through disturbance of native ecosystems. However, the relative importance of different landscape drivers of invasion may vary with landscape context (i.e., the types and amounts of surrounding land cover and land use). If so, data collected in one context may not be appropriate for predicting invasion risk across a broader landscape. To test whether independent occurrence datasets suggest similar landscape drivers of invasion, we compared landscape models based on data compiled by the Invasive Plant Atlas of New England (IPANE), which are contributed opportunistically by trained citizen scientists, to models based on Forest Stewardship plans (FSPs), which are located in privately owned and relatively undisturbed forests. We evaluated 16 landscape variables related to propagule pressure and/or disturbance for significant predictors of invasive plant presence based on presence/absence and count regression models. Presence and richness of invasive plants within FSPs was most influenced by proportion of open land and proximity to residential areas, which are both sources of propagules in forest interiors. In contrast, IPANE invasive plant presence and richness for the same area was influenced by distance to roads and streams. These results suggest that landscape drivers of invasion vary considerably depending on landscape context, and the choice of occurrence dataset will strongly influence model results. Keywords Berberis thunbergii Á Celastrus orbiculatus Á Disturbance Á Euonymus alatus Á Frangula alnus Á Plant invasion Á Propagule pressure Á Rosa multiflora Á Species distribution model Electronic supplementary material The online version of this article (

Global Ecology and Biogeography, 2013

Aim Intercomparison of mechanistic and empirical models is an important step towards improving pr... more Aim Intercomparison of mechanistic and empirical models is an important step towards improving projections of potential species distribution and abundance. We aim to compare suitability and productivity estimates for a well-understood crop species to evaluate the strengths and weaknesses of mechanistic versus empirical modelling. Location South Africa. Methods We compared four habitat suitability models for dryland maize based on climate and soil predictors. Two were created using maximum entropy (MAXENT), the first based on national crop distribution points and the second based only on locations with high productivity. The third approach used a generalized additive model (GAM) trained with continuous productivity data derived from the satellite normalized difference vegetation index (NDVI). The fourth model was a mechanistic crop growth model (DSSAT) made spatially explicit. We tested model accuracy by comparing the results with observed productivity derived from MODIS NDVI and with observed suitability based on the current spatial distribution of maize crop fields. Results The GAM and DSSAT results were linearly correlated to NDVI-measured yield (R 2 = 0.75 and 0.37, respectively). MAXENT suitability values were not linearly related to yield (R 2 = 0.08); however, a MAXENT model based on occurrences of high-productivity maize was linearly related to yield (R 2 = 0.62). All models produced crop suitability maps of similarly good accuracy (Kappa = 0.73-75). Main conclusions These findings suggest that empirical models can achieve the same or better accuracy as mechanistic models for predicting both suitability (i.e. species range) and productivity (i.e. species abundance). While MAXENT could not predict productivity across the species range when trained on all occurrences, it could when trained with a high-productivity subset, suggesting that ecological niche models can be adjusted to better correlate with species abundance.

Global Change Biology, 2012

Non-native, invasive grasses have been linked to altered grass-fire cycles worldwide. Although a ... more Non-native, invasive grasses have been linked to altered grass-fire cycles worldwide. Although a few studies have quantified resulting changes in fire activity at local scales, and many have speculated about larger scales, regional alterations to fire regimes remain poorly documented. We assessed the influence of large-scale Bromus tectorum (hereafter cheatgrass) invasion on fire size, duration, spread rate, and interannual variability in comparison to other prominent land cover classes across the Great Basin, USA. We compared regional land cover maps to burned area measured using the Moderate Resolution Imaging Spectroradiometer (MODIS) for 2000-2009 and to fire extents recorded by the USGS registry of fires from 1980 to 2009. Cheatgrass dominates at least 6% of the central Great Basin (650 000 km 2). MODIS records show that 13% of these cheatgrass-dominated lands burned, resulting in a fire return interval of 78 years for any given location within cheatgrass. This proportion was more than double the amount burned across all other vegetation types (range: 0.5-6% burned). During the 1990s, this difference was even more extreme, with cheatgrass burning nearly four times more frequently than any native vegetation type (16% of cheatgrass burned compared to 1-5% of native vegetation). Cheatgrass was also disproportionately represented in the largest fires, comprising 24% of the land area of the 50 largest fires recorded by MODIS during the 2000s. Furthermore, multi-date fires that burned across multiple vegetation types were significantly more likely to have started in cheatgrass. Finally, cheatgrass fires showed a strong interannual response to wet years, a trend only weakly observed in native vegetation types. These results demonstrate that cheatgrass invasion has substantially altered the regional fire regime. Although this result has been suspected by managers for decades, this study is the first to document recent cheatgrass-driven fire regimes at a regional scale.

Frontiers in Ecology and the Environment, 2012

www.frontiersinecology.org E cosystems are experiencing not only gradual shifts in mean climate c... more www.frontiersinecology.org E cosystems are experiencing not only gradual shifts in mean climate conditions but also dramatic changes in climate variability and prevalence of extreme climatic events (ECEs). ECEs such as droughts, floods, severe storms, and heat waves are changing in frequency, magnitude, timing, and duration, depending on the region and the specific climate event (WebPanel 1; Easterling et al. 2000; Karl et al. 2008). These extreme events can strongly influence terrestrial and aquatic ecosystems (Parmesan et al. 2000; Thibault and Brown 2008) and may combine synergistically with other agents of environmental change, such as species invasions, to dramatically change ecosystems. Previous studies have shown that changes in broad climatic conditions may influence the probability of species invasions, while highlighting that the effects of changing climate are likely to be diverse and context-dependent (Rahel and Olden 2008; Walther et al. 2009; Bradley et al. 2010). However, the potential for ECEs in particular to promote species invasions has not been systematically assessed. Here, we use theory from community ecology and invasion biology to identify mechanisms and pathways by which ECEs may affect the establishment and spread of introduced species in recipient ecosystems. We then review current evidence for ECE impacts on invasions and assess the potential for changes in the frequency and magnitude of ECEs to affect species introductions in the future. By focusing on mechanisms of invasion following an ECE, we assess whether generalizations about invasion risk are possible across species, ecosystems, and ECE type. Finally, we highlight critical areas of research necessary for a better understanding of these processes, and suggest strategies for anticipating and subsequently managing invasions associated with ECEs. n What is an extreme climatic event? Climatic events can be defined as "extreme" according to two broad perspectives. First, extreme events are episodes that fall within the statistical tails of a climate parameter's historical range; a commonly used threshold is the most extreme 1% of annual values, equivalent to an annual event happening once per 100 years (eg a "100year flood"). The frequency of such events will shift as the mean or variance of a climate variable changes (Meehl et al. 2000). Second, an extreme event can be defined by organism-based criteria, such as the conditions

Ecological Applications, 2014

As the main witnesses of the ecological and economic impacts of invasions on ecosystems around th... more As the main witnesses of the ecological and economic impacts of invasions on ecosystems around the world, ecologists seek to provide the relevant science that informs managers about the potential for invasion of specific organisms in their region(s) of interest. Yet, the assorted literature that could inform such forecasts is rarely integrated to do so, and further, the diverse nature of the data available complicates synthesis and quantitative prediction. Here we present a set of analytical tools for synthesizing different levels of distributional and/or demographic data to produce meaningful assessments of invasion potential that can guide management at multiple phases of ongoing invasions, from dispersal to colonization to proliferation. We illustrate the utility of data-synthesis and data-model assimilation approaches with case studies of three well-known invasive species-a vine, a marine mussel, and a freshwater crayfish-under current and projected future climatic conditions. Results from the integrated assessments reflect the complexity of the invasion process and show that the most relevant climatic variables can have contrasting effects or operate at different intensities across habitat types. As a consequence, for two of the study species climate trends will increase the likelihood of invasion in some habitats and decrease it in others. Our results identified and quantified both bottlenecks and windows of opportunity for invasion, mainly related to the role of human uses of the landscape or to disruption of the flow of resources. The approach we describe has a high potential to enhance model realism, explanatory insight, and predictive capability, generating information that can inform management decisions and optimize phase-specific prevention and control efforts for a wide range of biological invasions.

Diversity and Distributions, 2012

Climate change directly threatens a significant fraction of the Earth's species. Changing precipi... more Climate change directly threatens a significant fraction of the Earth's species. Changing precipitation and temperature conditions likely will alter the geographical ranges of species and may drive many to extinction (Parmesan & Yohe, 2003; Thomas et al., 2004). Climate change is also likely to trigger a wide range of secondary impacts, whereby efforts by people to mitigate and

Invasive species and global climate change, 2014

Correlative models between species occurrences and climate (here referred to as 'habitat suit... more Correlative models between species occurrences and climate (here referred to as 'habitat suitability models') have become increasingly popular for forecasting risk from invasive plants under current and future climate scenarios. These models have the potential to inform management and monitoring efforts by prioritizing landscapes considered at highest risk under a changing climate. However, a wide range of choices regarding climatic predictor variables, model ling approaches and even distributional data sets influences the resulting projections. The effects of these choices are seldom defined explicitly, which reduces their utility for scientists and managers alike. This chapter reviews common practices of habitat suitability modelling as they apply to invasive plants. The chapter also reviews major findings of recent projections of range shifts in invasive plants. In both cases, the aim is to explore how different choices of predictors, models and input data can influence c...

Frontiers in Ecology and the Environment, 2012

The Department of Plant Pathology and Microbiology invites applications for a 9-month tenure-trac... more The Department of Plant Pathology and Microbiology invites applications for a 9-month tenure-track faculty position (research and teaching in the Agricultural Experiment Station), emphasizing the invasion and impacts of microbial (such as bacteria, fungi, viruses) pathogen or symbiont species into agricultural or wildland ecosystems. Applicants studying microbes that regulate invasive plants will also be considered. Approaches could include genetics, genomics, population ecology/evolution, biochemical, bioinformatics, ecoinformatics and/or modeling. The successful candidate will join a vibrant community of researchers studying microbe-host and microbe-environment interactions, have opportunities to collaborate with researchers in UC's Division of Agriculture and Natural Resources, the Center for Conservation Biology, the Center for Invasive Species Research, the Institute for Integrative Genome Biology, and have access to modern campus facilities in genomics, proteomics, microscopy, ecological sensing technologies and field stations and facilities. Consult www.plantpath.ucr.edu for details about the department.

… Journal of Remote …, May 5, 2011

Land-use/cover change (LUCC) has emerged as a crucial component of applied research in remote sen... more Land-use/cover change (LUCC) has emerged as a crucial component of applied research in remote sensing. This work compares two methodologies, based on two data sources, for assessing amounts of land transformed from open to built space in three regions in Israel. We use a ...

Ecosphere, 2018

Biological Invasions, 2017

Ecosphere, 2017

Global Change Biology, 2017

Nature Communications, 2016

Ecological Applications, 2016

Diversity and Distributions, 2016

Springer Series on Environmental Management, 2016

Global change biology, Jan 24, 2015

Landscape Ecology, 2014

Global Ecology and Biogeography, 2013

Global Change Biology, 2012

Frontiers in Ecology and the Environment, 2012

Ecological Applications, 2014

Diversity and Distributions, 2012

Invasive species and global climate change, 2014

Frontiers in Ecology and the Environment, 2012

… Journal of Remote …, May 5, 2011