Spatial patterns of vulnerability in terrestrial mammals. Socioeconomic, land use and species-specific correlates of extinction risk at a global scale

Résumé

We are living an era of great and accelerated global changes. Biodiversity as a whole is exposed to human activities in the entire Earth surface; consequently, a generalized deterioration of its conservation status is taking place. Besides, the impact of humans on the biosphere keeps increasing given the present demand for food, fuels and other natural resources, resulting from population and consumption growth. Understanding which species and areas are most affected by these activities, and which are the main drivers of their current status is a crucial step to avoid further damages and preserve some of the remaining natural values. In the present doctoral thesis I expand the concept of vulnerability to explore the distribution of extinction risk at a global scale. A vulnerable species is one that has a greater chance of becoming extinct given its intrinsic characteristics and the external conditions to which it is exposed. A vulnerable area is one that is more likely to disappear as such, given its combination of species and environmental (including human) features. The present thesis focuses on spatial vulnerability incorporating knowledge at the species level to improve the understanding of global changes. Terrestrial mammals are selected to investigate the different factors associated to vulnerability, because they are a widely distributed and charismatic group, for which information on intrinsic characteristics and main threats is largely available. At the species level, predictors of vulnerability for terrestrial mammals have been widely identified; these can broadly be separated into species intrinsic traits and extrinsic human pressures. A spatial synthesis of both groups of factors is presented in chapter 1, identifying areas where both intrinsic and extrinsic vulnerabilities present high values (double-susceptibility areas), areas where the intrinsic is high and the extrinsic is low (intrinsic-susceptibility areas), areas where the extrinsic is high and the intrinsic is low (extrinsic-susceptibility areas), and areas where both show relatively low values (low-susceptibility areas). Instead of prioritizing one type of areas over the others, specific conservation actions are proposed to be defined according to the particularities of each area. For example, in extrinsic-susceptibility areas the emphasis should be put in controlling human activities, whereas in intrinsic-susceptibility areas, concentrating in particular species would be more advisable. Subsequent chapters focus exclusively on spatial vulnerability, first exploring the socioeconomic context (chapter 2) and then analyzing in depth the main proximate threat for mammals: human land use, explicitly considering its multiple facets (chapters 3 and 4). A country’s socioeconomic context has an important role in conservation biology, given that many indirect factors impose a global pressure on species and ecosystems (e.g. growth in world trade, demand for timber); besides, many environmental regulations are proposed at national –or international– level. Therefore, being able to portray the situation at this scale may serve to inform decision-making. Results from chapter 2 show that countries harboring more threatened mammals are generally: rural, predominantly exporters of goods and services, intermediately dependent on receiving international tourism and have relatively high human life expectancy. On the other hand, countries without threatened mammals are primarily those that already lost the most vulnerable fauna long ago and with means to maintain their remaining sensitive mammals. These findings highlight the importance of transboundary impacts and the fact that not having threatened mammals is not necessarily a sign of good environmental conservation status. Human land use is by far the main global change driver. There are many relevant aspects associated with its impact on biodiversity, such as land-use extent, intensity and history. Chapter 3 shows how including different metrics of agricultural land use and separating the world into regions with a historical and biogeographic common history can improve the understanding of the distribution of threatened species. Threatened mammals are not always found in zones where the most impacting human activities take place; instead, this pattern varies across biogeographic realms. Realms where agricultural expansion/intensification is currently taking place show a refuge pattern (e.g. Indomalay), with more threatened species concentrated in relatively low used areas (in terms of extent and/or intensity). On the other hand, regions with a long history of human settlement and a deeply modified territory show a threat pattern (e.g. Europe), with more threatened mammals co-occurring in highly human-used areas. Past land use data offer the opportunity to learn lessons from the past that can inform present and future actions. Chapter 4 explores past land use data spanning from around B.C.6000 (~establishment of agriculture) to A.D.2000. These data reveal three different general types of regions in the world, based on their trajectory of land use: low-, recently- and steadily-used areas. These three groups do not present net differences in terms of mammalian diversity, but they do differ in the way past and present land-use metrics relate to total richness or numbers of threatened mammals. In general, indicators of past human land use extent and rate of changes are the most important predictors. Interestingly, present land use values are generally less relevant to explain global patterns of mammalian distribution than past data. In conclusion, looking at the different dimensions of human activities on Earth offers the necessary perspective to tackle global conservation problems. Together with the traditional approach of prioritizing areas that most deserve conservation funds, disentangling the particularities of each region –put into context– helps design better conservation actions. This thesis synthesizes available global data, mapping patterns of threat for terrestrial mammals and proposing tools that could be applied to other taxonomic groups.