Assessing the effects of landscape structure on the relationship between species diversity and functional diversity

Abstract

Promoting the conservation of multiple aspects of biodiversity in transformed landscapes is a fundamental challenge. Researchers have become increasingly interested in understanding not only how landscape structure affects the number of species that coexist in an area, but also the distribution of functional traits (i.e. functional diversity) that determine the relationship between species diversity and ecosystem functioning. As fragmented landscapes are becoming increasingly common, controlling landscape structure (i.e. both landscape configuration and composition) may help to promote the conservation of both taxonomic and functional diversity. Although we have a relatively good understanding of how landscape composition impact species richness over large spatial scales, less is known about how landscape configuration drives the relationship between species richness and functional diversity simultaneously. Furthermore, despite the conceptual support for trait-based frameworks, we still lack basic information on how different species traits explain species’ responses and their potential to provide functions at different spatial scales. In this thesis, I addressed these issues to generate new insights around how landscape structure can be managed to potentially maximize the conservation of both taxonomic and functional diversity in human-dominated landscapes.  First, I conducted a systematic review on current approaches to evaluate how the influence of species traits on the relationship between environmental variables and ecological responses varies among scales (i.e. the scale-dependent role of traits; chapter 2). I show that there is a lack of studies comparing the effect of species traits on ecological responses at multiple scales. In addition, several ecological responses related to ecosystem functioning and species interactions such as seed dispersal, predation and multi-trophic networks have been widely overlooked. Importantly, the effects of landscape structure is also often ignored, and this makes it difficult to disentangle to what degree the responses of ecological systems across habitats, patches and landscapes are dependent on species traits. To address this gap, I then used data on bird ensembles collected in Brisbane (Australia) to test how landscape structure affects the distribution of species traits at different scales (chapter 3). I measured landscape composition and landscape configuration at two spatial resolutions: 100 m X 100 m and 1 km X 1 km. I found that depending on the scale of analysis, the strength of the association between species traits and landscape composition or configuration varies. At landscape scales, habitat configuration and the percentage of tree cover played a key role shaping the distribution of body sizes and dispersal capacities. Conversely, at local scales associations between the distribution of species traits and environmental variables were weak. Furthermore, the interaction between tree cover and fragmentation was important explaining trait distributions only at landscape scales. When tree cover was low at landscape scales, the presence of small-bodied species with low dispersal capacities depended mostly on having low levels of fragmentation. Although effects of fragmentation are stronger at landscape scales, habitat configuration is important determining trait distributions at local scales. Accounting for spatial scale can thus help to find more general models of the effect of traits for predicting species responses to landscape change.Then, I developed a spatially explicit meta-community model to quantify how the relationship between species diversity and functional diversity is driven by both habitat amount and fragmentation (chapter 4). I focus on the correlation between “response traits” (traits involved in species responses to environmental change) and “effect traits” (traits associated with species effects on ecosystem functioning). My model shows that the strength at which fragmentation modifies functional diversity in modified landscapes depends on the distribution of response traits and the correlation between response and effect traits. Finally, in chapter 5 I used my empirical data from Brisbane to disentangle the independent direct and indirect effects of landscape composition and configuration on bird functional diversity and species richness simultaneously. I evaluated the pathways through which landscape structure affects functional diversity in two main ways: 1) indirect effects through changes in species richness or 2) direct effects through redistribution in the abundances of particular traits. I show evidence that landscape structure acts differently on species richness compared to functional diversity. Landscape structure effects on species richness are consistent among different functional groups. However, patterns of functional diversity are more complex, vary among functional groups, and their dependence on the level of tree cover and fragmentation changes depending on the functional component evaluated. Importantly, although fragmentation does not always influence directly functional diversity, it can act indirectly through changes in species richness.This thesis integrates different concepts of community and landscape ecology to understand how species richness and different components of functional diversity may respond to landscape structure. Crucially, the hypotheses presented here would simplify forecasting and understanding the mechanisms that drive functional diversity in different landscapes and represent an advance for generality in landscape ecology. In addition, it provides new evidence about the importance of evaluating species diversity–functional diversity relationships at different spatial scales. My study suggests that a focus on conserving species with particular traits at the local scale may be ineffective if attributes of landscape structure are ignored. In addition, the results presented in this thesis are key for understanding how urban growth might best be done to maximize the conservation of functional and taxonomic diversity.