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What makes a species vulnerable to climate change?

Written by Dylan Harrison-Atlas, 2015-2016 Sustainability Leadership Fellow and PhD Candidate in the Graduate Degree Program in Ecology.

In a complex and dynamic world, how do we identify those things that are most vulnerable to change? Economists, public health experts, and social scientists often think about these issues of vulnerability as they relate to how climate change affects different segments of the population. Increasingly, so do ecologists.

A major burden facing ecologists is understanding how climate change will affect individual species. It is an important question because it has profound implications for the economy, society, and the health of our ecosystems. To answer that question, ecologists consider four unique but integral components of vulnerability.

Note that although climate change is a global issue affecting a diverse array of species and ecosystems, here we focus on fish that occupy rivers and streams of the Southwestern region of the United States as a case study for understanding vulnerability. The first component, Exposure, describes the types and magnitudes of changes that are taking place. For example, average temperatures in the Southwestern United States are projected to increase by up to 8 degrees Celsius by 2100. That number alone may raise eyebrows, but without additional information it is hard to evaluate which species will be vulnerable and where. Fish species may also be exposed to other types of changes to their environment. For example, changes in seasonal precipitation can alter streamflow patterns that are important for maintaining suitable habitat conditions. In addition, the introduction and spread of non-native fish species may push native trout out of preferred habitats and further expose them to climate change. A second piece of information that ecologists must consider in evaluating the vulnerability of a species is Sensitivity, which looks at how susceptible or responsive a species is to a given level of exposure.  Does a species have a narrow or wide range of temperature over which it can exist? Certain species of fish including members of the iconic salmon and trout family cannot survive in warm waters – they are highly sensitive to increases in temperature (Figure 2). In a simple world, knowing exposure and sensitivity would be enough to predict Potential Impact, the third component of vulnerability. Fortunately for fish and other species, their fate also depends on a fourth component of vulnerability termed Adaptive Capacity. Adaptive Capacity, enables species to cope with impacts that might otherwise occur given their exposure and sensitivity. It includes intrinsic factors like genetic diversity that may allow them to cope with changes through time and extrinsic factors like their environment that can provide refugia and opportunities to escape from areas of high exposure.

An important measure of Adaptive Capacity relates to how freely species can move through their environment. If species are able to move without restriction to avoid high temperatures, then chances are they have relatively high Adaptive Capacity. In cases where opportunity for movement is constrained, then local conditions will dictate outcomes. With over 80,000 dams in existence across the country and more likely to be built (Figure 3), many rivers are already highly fragmented ecosystems meaning that they are structurally impaired. In addition, climate change models estimate an increase in the frequency and severity of droughts in the Southwestern United States, which may further sever connections among streams.

Understanding the interplay between exposure, sensitivity, potential impact and adaptive capacity is critical to understanding the vulnerability of species and to informing effective management strategies that will allow species to persist in the face of climate change. By focusing on these key components of vulnerability, ecologists are able to gain insight into a complex and dynamic world.

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