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Defining ecological drought for the twenty-
first century
Shelley D. Crausbay, Aaron R. Ramirez, Shawn L. 
Carter, Molly S. Cross, Kimberly R. Hall, Deborah J. 
Bathke, Julio L. Betancourt, Steve Colt, Amanda E. 
Cravens, Melinda S. Dalton, Jason B. Dunham, 
Lauren E. Hay, Michael J. Hayes, Jamie McEvoy, 
Chad A. McNutt, Max A. Moritz, Keith H. Nislow, 
Nejem Raheem, Todd Sanford
Crausbay, Shelley D., Aaron R. Ramirez, Shawn L. Carter, Molly S. Cross, Kimberly R. Hall, 
Deborah J. Bathke, Julio L. Betancourt et al. "Defining ecological drought for the twenty-first 
century." Bulletin of the American Meteorological Society 98, no. 12 (2017): 2543-2550.
Defining Ecological Drought  
for the Twenty-First Century
sHelley d. Crausbay, aaron r. ramirez, sHawn l. Carter, molly s. Cross, kimberly r. Hall, 
deboraH J. batHke, Julio l. betanCourt, steve Colt, amanda e. Cravens, melinda s. dalton,  
Jason b. dunHam, lauren e. Hay, miCHael J. Hayes, Jamie mCevoy, CHad a. mCnutt,  
max a. moritz, keitH H. nislow, neJem raHeem, and todd sanford
THE RISING RISK OF DROUGHT. Droughts quality regulation, waste treatment, erosion preven-
of the twenty-first century are characterized by hot- tion, and recreation. The costs of these losses exceeded 
ter temperatures, longer duration, and greater spatial AUD $800 million, as resources were spent to replace 
extent, and are increasingly exacerbated by human these services and adapt to new drought-impacted 
demands for water. This situation increases the vul- ecosystems (Banerjee et al. 2013). Despite the high 
nerability of ecosystems to drought, including a rise costs to both nature and people, current drought 
in drought-driven tree mortality globally (Allen et al. research, management, and policy perspectives often 
2015) and anticipated ecosystem transformations fail to evaluate how drought affects ecosystems and the 
from one state to another—for example, forest to a “natural capital” they provide to human communities. 
shrubland (Jiang et al. 2013). When a drought drives Integrating these human and natural dimensions of 
changes within ecosystems, there can be a ripple effect drought is an essential step toward addressing the ris-
through human communities that depend on those ing risk of drought in the twenty-first century.
ecosystems for critical goods and services (Millar Part of the problem is that existing drought defi-
and Stephenson 2015). For example, the “Millennium nitions describing meteorological drought impacts 
Drought” (2002–10) in Australia caused unanticipated (agricultural, hydrological, and socioeconomic) view 
losses to key services provided by hydrological eco- drought through a human-centric lens and do not 
systems in the Murray–Darling basin—including air fully address the ecological dimensions of drought. 
AFFILIATIONS: Crausbay and ramirez*—National Center mCnutt—National Integrated Drought Information System 
for Ecological Analysis and Synthesis, University of California, Program Office, Joint Office for Science Support, University 
Santa Barbara, Santa Barbara, California; Carter—National Corporation for Atmospheric Research, Boulder, Colorado; 
Climate Change and Wildlife Science Center, U.S. Geological moritz—Department of Environmental Science, Policy, and 
Survey, Reston, Virginia; Cross—Wildlife Conservation Society, Management, University of California, Berkeley, Berkeley, 
Bozeman, Montana; Hall—North America Region, The Nature California; nislow—Northern Research Station, U.S. Forest 
Conservancy, Haslett, Michigan; batHke and Hayes—National Service, University of Massachusetts, Amherst, Massachusetts; 
Drought Mitigation Center, University of Nebraska—Lincoln, raHeem—Department of Marketing Communication, Emerson 
Lincoln, Nebraska; betanCourt—National Research Program, College, Boston, Massachusetts; sanford—Climate Central, 
U.S. Geological Survey, Reston, Virginia; Colt—Business Boulder, Colorado
Administration Program, Alaska Pacific University, Anchorage, * These authors contributed equally
Alaska; Cravens—Fort Collins Science Center, U.S. Geological CORRESPONDING AUTHORS: Shelley D. Crausbay, shelley 
Survey, Fort Collins, Colorado; dalton—Georgia Water Science @csp-inc.org; Aaron R. Ramirez, ramireza@reed.edu
Center, U.S. Geological Survey, Atlanta, Georgia; dunHam—
DOI:10.1175/BAMS-D-16-0292.1
Forest and Rangeland Ecosystem Science Center, U.S. Geological 
Survey, Corvallis, Oregon; Hay—Denver Federal Center, U.S. ©2017 American Meteorological Society
Geological Survey, Denver, Colorado; mCevoy—Department of For information regarding reuse of this content and general  
Earth Sciences, Montana State University, Bozeman, Montana; copyright information, consult the AMS Copyright Policy.
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Fig. 1. Conceptual diagram of ecological drought in the twenty-first century. This diagram illustrates the 
key drivers of drought vulnerability and impacts in coupled natural–human systems. Vulnerability = expo-
sure + sensitivity + adaptive capacity. Curved arrows indicate feedbacks where ecological responses and 
changes in human behavior or institutions can alter ecological drought vulnerability. The yellow–blue color 
gradient represents the continuum of coupled natural–human systems.
Redmond (2002) posed the question, “Like the tree for the rising risk of drought in the twenty-first cen-
falling in the forest, does drought occur if there is no tury, we need to reframe the drought conversation 
human to record or experience it?” (p. 1144). Redmond by underscoring the value to human communities in 
later answered his own question by arguing that sustaining ecosystems and the critical services they 
drought indeed “extends to vegetation and ecosys- provide when water availability dips below critical 
tems” (p. 1144). Yet, ecosystem responses to drought thresholds. In particular, we need to define a new type 
remain largely absent from many drought-planning of drought—ecological drought—that integrates the 
efforts, resulting in debates that often pit the water ecological, climatic, hydrological, socioeconomic, and 
needs of humans against the needs of ecosystems. cultural dimensions of drought.
Meanwhile, rapidly expanding human populations To this end, we define the term ecological drought 
and anthropogenic climate change increase pressure as an episodic deficit in water availability that drives 
on ecological water supplies and alter ecosystems in ecosystems beyond thresholds of vulnerability, 
ways that can increase their vulnerability to drought, impacts ecosystem services, and triggers feedbacks 
with real consequences for human communities in natural and/or human systems. We support this 
through loss of ecosystem services. To prepare us definition with a novel, integrated framework for 
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ecological drought that is organized along two di- in the historical record (Cook et al. 2016). Similarly, 
mensions—the components of vulnerability (expo- the way drought spreads through a region is charac-
sure + sensitivity/adaptive capacity) and a continuum terized by an interaction between natural landscape 
from human to natural factors (Fig. 1). The purpose of features (e.g., topography and soils) and human modi-
this framework is to help guide drought researchers fications of hydrological processes (e.g., reservoirs and 
and decision-makers to understand 1) the roles that irrigation) (Haddeland et al. 2014; Van Loon et al. 
both people and nature play as drivers of ecosystem 2016). For example, the Millennium Drought was 
vulnerability, 2) that ecological drought’s impacts are largely driven by ENSO, but groundwater extraction 
transferred to human communities via ecosystem ser- and river regulation nearly doubled the reduction in 
vices, and 3) these ecological and ecosystem service river flows that led to costly ecological impacts (van 
impacts will feed back to both natural and human Dijk et al. 2013).
systems. In addition, our framework will help iden-
tify important trade-offs and strategies for reducing Sensitivity, adaptive capacity, and natural resource 
the ecological drought risks facing both human and management. As with drought exposure, sensitivity 
natural systems in the twenty-first century. to ecological drought and adaptive capacity are also 
driven by interactions between natural and human 
ECOLOGICAL DROUGHT VULNERABIL- systems. Sensitivity refers to how strongly a species 
ITY FRAMEWORK. The drought vulnerability of or ecosystem is affected by drought exposure and 
an ecological community, population, individual, or results from a combination of the basic life history 
process is determined by its exposure, sensitivity, and traits and physiology of species, population/com-
adaptive capacity (Glick et al. 2011) to reduced water munity structure (e.g., demographics and diversity), 
availability. In the twenty-first century, each of these and ecosystem-level processes (Glick et al. 2011). 
components of vulnerability arises from interactions Adaptive capacity is the ability to accommodate or 
between natural processes and human activities. Our cope with the effects of drought—for example, by 
novel framework clarifies these human and natural plants exhibiting phenotypic plasticity or animals 
dimensions of vulnerability to highlight opportuni- moving to a new location in response to reduced 
ties for mitigation of and/or adaptation to ecological ecological water supply (Fig. 1). These aspects of 
drought (Fig. 1). vulnerability are important because variability in 
a system’s sensitivity and ability to adapt can cause 
Ecologically available water and drought exposure. different drought responses to the same water defi-
The amount of water that is ultimately available to cit. For example, variations in mortality patterns in 
ecosystems during a drought—ecologically available southwestern U.S. piñon-juniper woodlands exposed 
water—is inf luenced by a combination of natural to the severe drought of 2002/03 were driven by 
and human-modified processes (Fig. 1). Historically, interactions between plant water-use traits, stand 
the geography, frequency, and duration of drought characteristics, and bark-beetle infestation (i.e., vari-
conditions were driven primarily by sea surface tem- able sensitivity) (McDowell et al. 2008). Similarly, 
peratures in major oceanic basins, ocean–atmosphere differences in genetic diversity of European silver fir 
interactions such as El Niño–Southern Oscillation (i.e., variable adaptive capacity) determine whether a 
(ENSO), internal atmospheric variability, and land– population’s growth is tightly controlled by drought 
atmosphere feedbacks (McCabe et al. 2008; Cook or largely unaffected by it (Bosela et al. 2016). Humans 
et al. 2016). However, anthropogenic climate change can influence drought sensitivity and adaptive capaci-
increasingly affects the frequency, intensity, and extent ty through natural resource management actions that 
of droughts (Trenberth et al. 2013), largely through manipulate these ecological and evolutionary char-
higher temperatures that drive higher evaporative acteristics (Fig. 1). For example, research in forests 
demand, as well as changes in precipitation type (snow shows that drought-induced tree mortality is higher 
versus rain) and timing, which can lead to increased in denser stands and points toward reducing basal 
dry-season length, particularly in the tropics. Climate area as a management strategy to reduce vulnerability 
change is also expected to increase the likelihood of of some forested ecosystems to drought (Bradford and 
multidecadal “megadroughts,” which were common Bell 2017). This strategy can be accomplished through 
during some time periods in the paleorecord, but silvicultural thinning or, for some species, through 
which far exceed the duration of any drought observed prescribed fire (van Mantgem et al. 2016).
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UNDERSTANDING DROUGHT IN COU- within a system’s adaptive capacity that fail to leave 
PLED NATURAL–HUMAN SYSTEMS. Types an ecological or social footprint (Fig. 2). Instead, we 
of ecological drought. Historically, droughts were define ecological drought as a disturbance that pushes 
natural events that shaped ecological processes and coupled natural–human systems beyond their adap-
evolutionary adaptations. Yet, changing conditions in tive capacity and triggers important socioecological 
the twenty-first century are resulting in an increased feedbacks (response arrows in Fig. 1; Fig. 2).
risk of megadisturbances—that is, widespread dis- This definition is flexible enough to include mul-
turbances that overwhelm the adaptive capacity of tiple types of ecological drought, differentiated based 
ecosystems and human communities, leading to on which part of the coupled natural–human system 
important ecological changes and ecosystem service is impacted and which set of feedbacks is triggered 
losses (Millar and Stephenson 2015). Drought impacts (Fig. 2). For example, an ecological drought may 
cover a wide spectrum of severity, from small-scale, result in ecological impacts that feed back to alter 
temporary responses (e.g., reduced productivity in natural systems—selection of drought-adapted traits 
plants or increased dehydration stress in wildlife) or species, range shifts, or ecoclimatic teleconnections 
to widespread and persistent ecosystem transfor- (e.g., Stark et al. 2016)—with little influence on the 
mations (e.g., vegetation type conversion or species ecosystem services provided (type I). Alternatively, 
range shifts). Our definition of ecological drought an ecological drought may produce only minor eco-
aims to exclude the small-scale, short-term effects logical effects that do not feed back to natural systems 
Fig. 2. Types of ecological drought are differentiated by which side of the coupled natural–human system 
crosses a threshold (as in Fig. 1) and experiences the strongest impacts and feedbacks. Ecological impacts 
(yellow) feed back to the natural system and ecosystem service losses (blue) feed back to the human system; 
AC = adaptive capacity, CNH = coupled natural–human.
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but result in larger effects on 
ecosystem services that alter 
connected human systems 
(type II). A third type of eco-
logical drought is defined by 
impacts and feedbacks in both 
human and natural systems 
(type III). Our definition also 
includes transformational 
ecological droughts (type IV), 
where ecological impacts and 
ecosystem service losses are 
extreme and drive a persis-
tent state change in human 
and natural systems, such as 
vegetation type conversion or 
mass human migrations (e.g., Fig. 3. Reframe the people vs nature debate. (a) Agricultural workers in Cali-
fornia’s Central Valley march in protest of state legislative action to reduce 
the Dust Bowl migration). water diversions and protect endangered fish populations. (b) Advocates 
for the Klamath and Trinity Rivers demand the release of reservoir water 
The importance of ecosystem slated for Central Valley irrigators in order to prevent a drought-induced fish 
services. A focus on ecosystem kill (Sacramento, 2014). (Photo credits: (a) www.redstate.com, (b) https://
services allows us to better lostcoastoutpost.com.)
appreciate that ecological im-
pacts of drought also have important implications for integrates human and ecological values and empha-
human communities. Pederson et al. (2006) identified sizes identification of innovative solutions with the 
that ecological impacts from drought in mountainous potential for mutual benefits.
areas of the western United States can affect a variety 
of ecosystem services including provisioning (e.g., A CALL TO ACTION. Our framing of ecologi-
declining fisheries), cultural (e.g., reduced forest- cal drought highlights opportunities to mitigate the 
related tourism), and regulating (e.g., increased threat risks of drought to both nature and people. But, ef-
and cost of fires and pest outbreaks) services. In the forts by drought researchers and decision-makers are 
twenty-first century, we increasingly understand that needed to operationalize the concepts presented here. 
ecosystem services are linked to human well-being Researchers can use our vulnerability framework to 
and, as a result, are beginning to address disparate evaluate the relative roles of exposure, sensitivity, and 
problems like poverty and biodiversity conserva- adaptive capacity, as well as parse out human versus 
tion with innovative mutually beneficial solutions natural drivers of ecosystem vulnerability to drought. 
for nature and people (Guerry et al. 2015). However, This exercise can be useful in linking ecological 
drought and its acute risks to both nature and people drought impacts to the most relevant drivers in a given 
can sometimes challenge this progress and create situ- system, which can lead to more targeted and effective 
ations where ecosystem and human water needs are management strategies. Our framework also encour-
viewed as competing demands for a limited resource ages decision-makers to use an ecosystem-services-
(Fig. 3). This perspective can cause us to ignore inter- based approach when considering trade-offs between 
dependence of ecosystems and human well-being and human and ecosystem water needs in drought policy 
thus bypass potential, mutually beneficial solutions. and management and may help identify strategies that 
Our framework for ecological drought encourages are mutually beneficial.
an integrated approach to considering human and There is a current groundswell of ecological 
ecosystem water needs that relies on the concept of drought research and synthesis, with important dis-
ecosystem services to better understand drought im- coveries regarding the drivers of ecological drought 
pacts and highlight potential strategies for integrative impacts, especially the role of hotter, climate-change-
drought management. Such an approach corrects the driven droughts and interacting disturbances (e.g., 
“nature vs. people” misperception because it explicitly Allen et al. 2015; Millar and Stephenson 2015; Vose 
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et al. 2016). However, the effects of human water more effective if there is a fundamental understand-
and land use on environmental water supplies are ing of the interdependence of human well-being and 
not always considered in current ecological drought ecosystem services. There are currently few organized 
research, monitoring, or prediction. The relative efforts to categorize or quantify the ecosystem services 
importance of natural climate variability, climate affected by drought (see van Dijk et al. 2013). However, 
change, and direct human influences on environ- recent work in drought-prone areas in Australia 
mental water supplies are likely to vary across regions (Banerjee et al. 2013) and the southwestern United 
and ecosystems, with the direct human influences States (Raheem et al. 2015) may serve as excellent 
outweighing the role of climate change in some situa- starting places for strengthening our understanding of 
tions (Haddeland et al. 2014). This argues for the need how ecological drought influences the goods and ser-
to focus more research on quantifying and separating vices people value and how those values vary through 
these aspects of drought exposure. space and time. Considering the value of ecosystem 
Additionally, the ecological characteristics that services at the outset of the planning process can in-
most inf luence drought sensitivity and adaptive tegrate human and natural water needs and move us 
capacity, as well as how proactive and anticipatory forward with the understanding that an investment 
resource management can target these traits to reduce in water for nature may ultimately be an investment 
drought vulnerability ahead of a drought needs to be in water for people.
more fully investigated. A growing body of literature Acting on these mutually beneficial solutions 
linking life history, physiology, and other functional requires a focus on drought adaptation—that is, 
traits to drought sensitivity in forests (Anderegg et al. actions taken to proactively reduce drought risk 
2016), shrublands (Venturas et al. 2016), and aquatic over short or long time scales. Ecological drought 
ecosystems (Lytle and Poff 2004) provides useful vulnerability may be successfully reduced through 
examples for other systems. Recent work has built proactive natural resource management strategies 
upon this ecological knowledge to show that direct (e.g., thinning the forest) or strategies that work with 
manipulation of ecological characteristics can reduce and support natural processes, rather than employing 
vulnerability to ecological drought through strategies engineered solutions that may degrade natural sys-
like prescribed fire and forest thinning (e.g., van Man- tems (e.g., high-elevation reservoirs). For example, in 
tgem et al. 2016; Bradford and Bell 2017). But, this field the Amazon, reducing deforestation would reduce the 
of study needs to keep expanding to determine which ecoclimatic teleconnections that increase drought in 
ecosystems and at what scales (temporal and spatial) the region (e.g., Stark et al. 2016) and could result in 
these kinds of proactive preparedness strategies are benefits to hydropower generation while simultane-
most effective. ously reducing drought-induced tree mortality. As 
Currently, research rarely integrates all aspects another example, in western North America, beaver 
of ecological drought vulnerability simultaneously. reintroduction is a drought adaptation strategy that 
Therefore, research that characterizes the human builds upon the natural role that these mammals 
and natural dimensions of exposure, sensitivity, and play in modifying hydrology in streams and wet-
adaptive capacity are needed to attribute the causes of lands (Pollock et al. 2014). Reintroducing beaver, or 
ecological impacts and their social implications. As a mimicking their structures, is a viable technique for 
start, researchers can use our framework and types of restoring the natural water storage capacity of the 
ecological drought as guides to develop questions and landscape—thereby reducing drought exposure—
conduct research that determines where the greatest for the benefit of both ecological and agricultural 
vulnerability lies in a given system and, therefore, systems. Such strategies, often referred to as “nature-
which strategies may be most effective. Advancing based solutions,” are investments in protecting and 
ecological drought research in these directions will restoring natural systems but also hold promise for 
help decision-makers identify proactive strategies that reducing risks associated with ecological drought. 
can directly lead to effective, place-based management However, such approaches are currently underuti-
for reducing vulnerability to droughts of the future. lized in the drought arena and their efficacy and cost 
Mitigating the impacts of ecological drought may is rarely quantified or compared to infrastructure-
be possible through various changes to policies, man- based mitigation techniques (Jones et al. 2012).
agement practices, and water infrastructure. However, Changing laws and policies that guide human 
these attempts to change human institutions will be modifications to water flows is another action that 
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could benefit both people and nature, particularly government. We dedicate this paper to Dr. Kelly Redmond, 
where human modifications contribute the most whose insights and thoughtful perspectives first inspired our 
to ecological drought. New policies that reallo- conceptualization of ecological drought. His work, generos-
cate water to the environment during times of low ity, and prescient insights continue to inspire work on this 
streamflow have proven successful, if sometimes topic, and many others. He will be missed.
difficult to achieve. A prime example of this success 
is in Australia’s Murray–Darling basin when during FOR FURTHER READING
the Millennium Drought, the proportion of f lows 
diverted for agriculture increased dramatically, Allen, C. D., D. D. Breshears, and N. G. McDowell, 2015: 
with a disproportionate impact on the environment. On underestimation of global vulnerability to tree 
Lakes and rivers acidified, lagoons salinized, and the mortality and forest die-off from hotter drought 
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response to this crisis, an active water market using .org/10.1890/ES15-00203.1.
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from irrigators, facilitated reallocation of water from Pellegrini, B. Choat, and S. Jansen, 2016: Meta-anal-
irrigated agriculture to the environment, and despite ysis reveals that hydraulic traits explain cross-species 
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constant through the Millennium Drought (Grafton https://doi.org/10.1073/pnas.1525678113.
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