Why do some areas have more threatened species than others?

​To protect biodiversity effectively, we need to identify where to focus conservation actions and what those actions should be. This requires an understanding of the distribution and drivers of threatened species richness. We have a fairly good idea of where threatened species currently occur, but we don’t necessarily know why some areas are home to larger numbers of threatened species than others.  Are these areas subject to the pressures of extensive human activities? Or, conversely, might they be providing refuges for species away from human activities? Are the species present in an area more prone to extinction than species elsewhere? Or is it purely a question of numbers; do areas with large numbers of threatened species simply have more species overall?
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Our new paper in Conservation Letter addresses these questions, identifying the features of an area that influence how many threatened species occur there.  To do this, we collaborated with Curt Flather from the USDA Forest service to examine the distribution of threatened birds and mammals across the contiguous United States (Fig.1). We found that environmental factors, such as temperature and precipitation, are of far greater importance in determining threatened species richness than either the effects of human activities or the biological characteristics of the species present (e.g. their body mass, life span etc.).

Fig 1: The distribution of total and threatened bird and mammal species richness across the contiguous United States. Notice that areas with more threatened species are not always the same places that you find high overall species richness.
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​Significantly, we found that the effects of a number of variables on the richness of threatened species differed substantially from their effects on species richness as a whole (Fig. 2). For example, we found that whilst the number of threatened birds and mammals increases with the amount of land dedicated to anthropogenic activities (i.e. agriculture, urban development), the total number of species decreases. 

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​Figure 2: The effects of different environmental and anthropogenic factors on the total number (yellow lines) and number of threatened (blue lines) bird (panels a-f) and mammal (panels g-l) species.

By identifying the factors that drive the distribution of threatened species, we are better able to establish the costs and benefits of different conservation actions. So for example, we now have a better understanding of where new protected areas should be established in order to maximise their benefits for nature conservation. Our findings can also yield recommendations for the management of existing protected lands. In particular, some protected areas in the US operate under multiple-use mandates, allowing for timber harvesting, livestock grazing and mining. We can use our results to identify how much of an area needs to be dedicated to the protection of at-risk species, and how much could contribute to the provisioning of ecosystem services.

​Finally, and importantly, the results of our study can be used to help the USDA forest service with their scenario planning. By pairing these results with information on possible environmental change, we can now identify where threatened species may occur in future. This information can then feed into conservation planning to ensure that planned interventions are future-proof, maximising the benefits for nature conservation.