Individual choices, community consequences: How individual differences shape biodiversity in fragmented landscapes

The unprecedented loss of species and genetic diversity due to anthropogenic pressures such as climate change, and in particular habitat destruction, calls for urgent conservation action. However, efforts to conserve biodiversity are complicated by a long-standing scientific debate: Are more species supported in a single large habitat reserve or in several small protected areas? Conservation has traditionally focused on large, continuous habitats, while empirical evidence is increasingly suggesting that multiple small patches can support greater species richness, calling for greater appreciation of remaining small habitat patches. A deeper understanding of the mechanisms that drive biodiversity patterns in fragmented landscapes, helping us to understand why, when and how fragmentation negatively or positively affects biodiversity, is crucial to resolve this debate and inform effective conservation strategies. By elucidating potential mechanisms by which fragmentation affects diversity, this thesis aims to reconcile seemingly contradictory observations and predictions. I argue that ubiquitous intraspecific trait variation, often dismissed as noise but increasingly recognised as a crucial bottom-up driver of population and community dynamics, is an important ecological dimension for understanding diversity responses to fragmentation. The core question of my research is how individual variability will affect species diversity in fragmented landscapes. In particular, my research investigates how individual behavioural differences within species, especially in movement and habitat selection determined by animal personality (in particular risk-taking tendencies), alter species interactions within competitive communities and ultimately shape biodiversity patterns in fragmented landscapes. Because behaviour, especially movement decisions, strongly connects individuals to their biotic and abiotic environment, it significantly determines their persistence under landscape and resource fragmentation. This will be addressed through a series of individual-based modelling studies, which are particularly suited to investigating the emergent community-level properties of individual variability. Each modelling study addresses specific challenges faced by organisms in fragmented landscapes, such as foraging at risky habitat edges, navigating unfavourable matrix environments, and making informed dispersal decisions. The first study examines how risk-taking behaviour influences the impact of edge effects on community dynamics in fragmented landscapes. By simulating differences in risk-taking behaviour that affect an individual’s foraging activity at risky habitat edges, the model reveals that the effects of fragmentation on biodiversity strongly depend on the behavioural composition of communities. While risk-averse communities suffered negative effects, risk-diverse communities showed increased diversity with fragmentation. Regardless of behavioural composition, fragmentation consistently expanded foraging ranges, highlighting movement as a key coping mechanism under resource fragmentation. This has informed the second study, which shifts the focus to the matrix, a critical element influencing movement and survival in fragmented landscapes. This model contrasts a baseline scenario of no matrix costs with direct mortality risk and deferred matrix costs, and examines how individual variation in matrix crossing affects community patterns under fragmentation. This chapter illustrates that fragmentation and matrix costs influence not only species diversity but also behavioural diversity through eco-evolutionary dynamics. The third study explores how prospecting for dispersal destinations, a key aspect of making informed dispersal decisions, shapes metacommunity biodiversity. This model examines the role of prospecting in coexistence versus competitive exclusion, combining local resource competition and regional dispersal dynamics, varying prospecting intensity, dispersal propensity and prospecting costs. Informed settlement significantly alters the relationship between dispersal and diversity. For example, high prospecting intensity accelerates colonisation of high quality patches, increases competition and reduces establishment success, especially for resource demanding species. This can be mediated by individual variation in prospecting effort. In addition, the paper introduces the visual ODD (vODD), a novel method for visualising individual-based models that my co-authors and I have developed to improve model communication and understanding. The vODD pictorially summarises key elements of the written ODD protocol – initialisation, processes, outputs and spatio-temporal resolution/extent – and captures the model’s narrative in a single image. This structured visualisation approach enhances model comprehension and provides a concise overview linked to the written ODD protocol, specifically designed for manuscripts, talks and posters. Overall, the core studies of this dissertation link personality-dependent spatial ecology to biodiversity research, highlighting the potential of animal personalities as drivers of species responses to habitat fragmentation. The mechanistic approach of individual-based modelling has allowed the identification of several underlying mechanisms by which individual variability affects community dynamics, such as reduced competition through individual niche specialisation or the buffering mechanism of divergent behaviours, which are discussed further in the final chapter of the thesis. The general discussion also considers the wider implications of this theoretical research for applied conservation efforts, such as the importance of small habitat patches for biodiversity through their contribution to overall landscape heterogeneity, and the potential for matrix management to shape biodiversity in already fragmented landscapes. It also argues for conserving biodiversity from the individual up, recognising that individual variation significantly influences communities and ecosystems, and that human activities strongly influence individual variation.

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