Biodiversity patterns and dynamics across spatial scales : the roles of climate and land use
Keywords: Biology.

Understanding the distribution, abundance, and dynamics of species across landscapes, as well as the processes that generate these patterns, is foundational to ecological study. Such enquiries seek to determine the evolutionary, ecological, and environmental drivers of the presence and abundance of populations, species, and communities, and change therein. During the early years of modern ecological research, these studies were typically focused on conditions and processes that operated generally outside of human influence, such as natural selection, macroenvironmental patterns such as topography and climate, and species interactions. As human activity has left an increasingly large mark on biological communities, however, it has become virtually impossible to study biodiversity patterns and dynamics in the absence of human activity. Consideration of human impacts is essential not only for a holistic understanding of ecological processes but is for informing conservation interventions in the face of human-induced global change, which has now imperiled nearly one million species. In this dissertation, I focus on two of the dominant drivers of human-induced global change -- climate change and land-use intensification -- on the distribution and abundance of species and communities. I do so across spatial scales and taxonomic groups, to move towards a more mechanistic understanding of when one might expect climate change or land-use intensification to be more impactful for the stability of populations or communities. After a brief introduction, I begin in Chapter 2 with a systematic, global analysis of the concept ecoregions. Specifically, I ask whether ecoregions do in fact delineate distinct biological communities from one another. I show compelling evidence that -- across plants, fungi, arthropods, and vertebrate animals -- ecoregions do in fact divide unique communities of species from one another. However, there is a great deal of variability that ecoregional boundaries fail to explain. Therefore, in Chapter 3 I investigate how various environmental conditions, including climate and land use, interact with each taxonomic and functional group of interest, to determine how robust ecoregional classification schemes are expected to be. I show evidence that ecoregions (i) are better suited to describe species in tropical zones and (ii) perform better for small-bodied species that feed lower on trophic food webs. In Chapters 4 and 5, I begin to narrow the spatial scale at which I study these biogeographic patterns and home in on birds and insects as focal taxa. Advancing understanding at both global and local scales is crucial to opening coherent understanding that is supported by consideration of processes relevant at different scales. In Chapter 4, I examine the relative roles of climate and land use on local bird abundance. In this chapter, I compare traditional species distribution models to a new hybrid model that incorporates elements of traditional species distribution models with elements of local field studies. I demonstrate that the new hybrid model shows a much larger impact of land use than do traditional species distribution models. Moreover, these hybrid models place the potential impacts of even modest land-use change scenarios on par with climate change in terms of their total impact on bird biodiversity. Finally, in Chapter 5 I measure insect biodiversity in a small region of northwest Costa Rica, where gradients of human land use and precipitation lend themselves exceptionally well to study their independent effects. I show that while local species richness is influenced by both land use and precipitation, there is no interaction between these forces on local species richness. There is, however, an interactive effect of climate and land use on community composition, with wet agricultural areas more closely resembling dry forest areas than wet forest areas. There remains a large amount of work to do to arrive at a full understanding of how climate change and land-use change will affect biodiversity independently and in concert. My dissertation work shows that, depending on the spatial scale of analysis, the relative importance of these two drivers seems to shift, with climate being more impactful at large spatial scales and land use being more important at fine spatial scales. These patterns appear to persist across taxonomic and functional groups, though the underlying biological mechanisms are almost certainly different. As researchers continue to tackle these questions over the coming decades, we will gain not only a better understanding of basic ecological mechanisms, but also the ability to provide ever more meaningful and specific science-based guidance to decision-makers for safeguarding biodiversity through the Anthropocene