Assessing biodiversity responses to changes in climate and land use

Biodiversity loss due to changes in climate and land use has been assessed recently. The earliest biodiversity assessments already showed that species are declining faster than at any time in the past and that ecosystems are rapidly deteriorating. Moreover, these assessments indicated that the projected changes in climate and land use likely drive further biodiversity losses in the 21st century, both directly and in synergy with each other. This accumulated evidence positions climate change and land-use change among the major human-induced direct drivers of biodiversity loss. Climate change affects biodiversity as climate variables, such as temperature and precipitation, largely determine the geographical distributions of species. Hence, in areas where climate is less suitable, species shift their geographical ranges and go extinct locally. Land-use change poses immediate threats to biodiversity as the conversion of natural habitats (e.g. forests, wetlands and grasslands) into agricultural land results in populations decline and extinctions become more likely. These adverse effects consequently change ecosystems functioning and potentially affect the supply of ecosystems services and thus human well-being. Although research on climate and land-use change impacts on biodiversity and the consequent implications was repeatedly conducted, the range of estimates for these impacts remains disturbingly large. Moreover, such research relied on climate-change scenarios that depict relatively small increases in global mean temperatures (i.e. <2°C). Nowadays, the plausibility of climate-change scenarios which overshoot the 2°C policy target from The Paris Agreement, is rapidly increasing. Advances are thus needed to better understand how biodiversity will respond to such larger changes, including quantifications of the expected biodiversity decline at different climate and land-use change levels, and the effect derived from interaction mechanisms between these drivers. Furthermore, the global efforts to combat climate change and to keep global average temperature to well-below 2°C will require large mitigation commitments from the land sector with potentially both positive and negative consequences for biodiversity. These implications of land-based mitigation efforts have to be further assessed. My PhD thesis therefore aimed to <em>explore future biodiversity trends under projected direct and synergistic changes in climate and land use and to advance understanding of climate-change mitigation consequences for biodiversity</em>. In this thesis, climate change was indicated by global mean temperature increase (°C) and land-use change by land-use intensity levels (i.e. grazing and cropland levels) and land-cover type transitions. In Chapter 2, I assessed the magnitude of expected changes of biodiversity by systematically reviewing studies and performing a meta-analysis of the responses of species distributions to climate change. I proposed two indicators to quantify the local response of terrestrial biodiversity to climate change: the fraction of remaining species (FRS) and the fraction of remaining area (FRA) with suitable habitat for each species. The FRS and FRA calculate deviations from the original biodiversity state and both they indicate biodiversity intactness. The biodiversity response was quantified for different intervals of global mean temperature increase and for different taxonomic groups and ecosystems. The results showed that projected climate-change impacts likely cause changes to the distribution of many plants and animals and this leads to severe range contractions and local extinction of some species (i.e. decreasing biodiversity). The FRS and FRA were projected to gradually decrease with significant reductions of 14% and 35% between 1°C and 2°C increases in global mean temperature, and 32% and 54% beyond 4°C increase. This chapter showed that already at moderate temperature increases the original biodiversity significant decreased. In Chapter 3, I estimated biodiversity decline from changes in climate and land use in grassland ecosystems, which are among the most extensive ecosystems in the world. The analysis was conducted in the Central Asian grasslands, which are nowadays transforming by changes in land use and climate. I used a scenario analysis based on the latest Shared Socio-Economic Pathways (SSPs) and Representative Concentration Pathways (RCPs) (i.e. SSP-RCP scenario framework) and further detail land-use scenarios for the region. I selected contrasting socio-economic and climate conditions (i.e. SSP1-RCP4.5, SSP3-RCP8.5, SSP4-RCP4.5 and SSP5-RCP8.5). In this analysis, the climate-change impact for the selected RCP4.5 and RCP8.5 was indicated by the FRS for grasslands as estimated in Chapter 2; the land-use change impact was indicated by changes in land-use intensity derived from the land-use scenarios; and the future biodiversity was indicated by the Mean Species Abundance (MSA). The MSA expresses the mean abundance of originally occurring species in disturbed conditions (e.g. after climate change) relative to their original abundance in undisturbed habitats. The contrasting scenario combinations showed that grasslands’ biodiversity remained under continuous threat and will further decline under each scenario. The strongest impact on biodiversity was projected in SSP5-RCP8.5, where half of the grasslands will likely undergo a large decrease in their species abundance by 2100. This chapter stressed the potential vulnerability of the Central Asian grasslands to increasing land-use intensity and climate change. In Chapter 4, I explored interaction mechanisms between climate and land-use change effects on biodiversity. Climate change and land-use change are often addressed as drivers that interact synergistically in several ways and alter their mutual effects on biodiversity. I identified interaction mechanisms in which species in heavily modified landscapes may respond differently to climate change than species in pristine landscapes. These interactions arise if 1) species adapted to modified landscapes differ in their sensitivity to climate change from species adapted to natural landscapes and if 2) land-use composition restricts climate-change induced dispersal of species in fragmented landscapes. To verify these conditions, I performed systematic reviews and a meta-analysis of bioclimatic studies on species distributions in landscapes with varying proportions of cropland (first condition) and species’ dispersal under climate change in fragmented landscapes (second condition). I used the FRS as the effect-size metric in this meta-analysis. Based on the results of this analysis, I found no significant interaction effect for the first condition. This indicates that the influence of global mean temperature increase on the FRS did not change with different cropland levels. No quantitative studies were found to verify the second condition for climate-change induced dispersal of species. This chapter emphasized the need to assess interactions between land-use and climate-change effects on biodiversity, integrating other conditions, such as spatial location, adaptive capacity and time lags. In Chapter 5, I assessed carbon-dioxide-removal options in the Agriculture, Forestry and Other Land Use sectors (i.e. land-based mitigation options) implemented in different mitigation pathways that keep global temperature increase to well-below 2°C for their biodiversity impacts using the MSA indicator. Land-based mitigation options may preserve, increase or deteriorate biodiversity, because of their land-use impact. In this chapter, I reviewed climate change mitigation studies that assessed each of the selected land-based mitigation options and indicated the land transition needed to achieve a significant climate change mitigation (i.e. potential land-cover and/or land-use change). I found that reforestation of cultivated and managed areas together with restoration of wetlands deliver the largest increase of MSA, if provided the opportunity to reach mature states over time. Contrary, intensification of agricultural areas and bioenergy with carbon capture and sequestration decreased MSA locally. Options such as afforestation and reduced deforestation, either positively or negatively affect MSA. This depends on their spatial implementation and the precise forest conservation schemes. This chapter provided insights on possible synergies that emerge from certain scenarios and their benefits for current and future biodiversity conservation in regions with large land-based mitigation potential. My PhD thesis advanced scientific understanding of climate and land-use change impacts on biodiversity that can feed into the current UN Conventions on Biological Diversity and Climate Change agendas. It showed future biodiversity trends and proposed methods that translate relevant information of socio-economic and climate-change drivers to assess interactions between climate and land-use change effects on biodiversity. Such knowledge is quickly becoming an important element to develop strategies for regional and global biodiversity conservation and thus to minimize biodiversity loss. I stress the importance of holding climate change well-below 2°C as this helps to maintain the composition of local communities and their climatically suitable areas, while seeking for the desired combinations that will reduce the use of detrimental land-based mitigation options to biodiversity.