Defining ecological drought for the twenty-first century

Droughts of the twenty-first century are characterized by hotter temperatures, longer duration, and greater spatial extent, and are increasingly exacerbated by human demands for water. This situation increases the vulnerability of ecosystems to drought, including a rise in drought-driven tree mortality globally (Allen et al. 2015) and anticipated ecosystem transformations from one state to another—for example, forest to a shrubland (Jiang et al. 2013). When a drought drives changes within ecosystems, there can be a ripple effect through human communities that depend on those ecosystems for critical goods and services (Millar and Stephenson 2015). For example, the “Millennium Drought” (2002–10) in Australia caused unanticipated losses to key services provided by hydrological ecosystems in the Murray–Darling basin—including air quality regulation, waste treatment, erosion prevention, and recreation. The costs of these losses exceeded AUD $800 million, as resources were spent to replace these services and adapt to new drought-impacted ecosystems (Banerjee et al. 2013). Despite the high costs to both nature and people, current drought research, management, and policy perspectives often fail to evaluate how drought affects ecosystems and the “natural capital” they provide to human communities. Integrating these human and natural dimensions of drought is an essential step toward addressing the rising risk of drought in the twenty-first century.

Part of the problem is that existing drought definitions describing meteorological drought impacts (agricultural, hydrological, and socioeconomic) view drought through a human-centric lens and do not fully address the ecological dimensions of drought. Redmond (2002) posed the question, “Like the tree falling in the forest, does drought occur if there is no human to record or experience it?” (p. 1144). Redmond later answered his own question by arguing that drought indeed “extends to vegetation and ecosystems” (p. 1144). Yet, ecosystem responses to drought remain largely absent from many drought-planning efforts, resulting in debates that often pit the water needs of humans against the needs of ecosystems. Meanwhile, rapidly expanding human populations and anthropogenic climate change increase pressure on ecological water supplies and alter ecosystems in ways that can increase their vulnerability to drought, with real consequences for human communities through loss of ecosystem services. To prepare for the rising risk of drought in the twenty-first century, the drought conversation needs to be reframed by underscoring the value to human communities in sustaining ecosystems and the critical services they provide when water availability dips below critical thresholds. In particular, defining a new type of drought—ecological drought—that integrates the ecological, climatic, hydrological, socioeconomic, and cultural dimensions of drought.

To this end, this study defines the term ecological drought as an episodic deficit in water availability that drives ecosystems beyond thresholds of vulnerability, impacts ecosystem services, and triggers feedbacks in natural and/or human systems. It supports this definition with a novel, integrated framework for ecological drought that is organized along two dimensions—the components of vulnerability (exposure + sensitivity/adaptive capacity) and a continuum from human to natural factors.

The purpose of this framework is to help guide drought researchers and decision-makers to understand 1) the roles that both people and nature play as drivers of ecosystem vulnerability, 2) that ecological drought’s impacts are transferred to human communities via ecosystem services, and 3) these ecological and ecosystem service impacts will feed back to both natural and human systems. In addition, the framework will help identify important trade-offs and strategies for reducing the ecological drought risks facing both human and natural systems in the twenty-first century.