title: Atoll islands as ecosystem replicates : Organization of biodiversity and environmental impact of different human land uses
Creators: name: Steibl, Sebastian
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: atoll ecology
subject: island ecology
subject: anthropogenic disturbance
subject: community ecology
subject: global change biology
subject: 590

description: Biodiversity on our planet follows defined patterns. On a global scale, these patterns are organized by few parameters, like latitude and elevation. On a local scale, the organization of biodiversity becomes less predictable as various factors simultaneously determine species’ diversity and distribution. Understanding how biodiversity and species distribution are organized on small spatial scales (i.e., within ecosystems or habitats) is fundamental to ecological research. This relevance stems from the ongoing global change. The increasing necessity for understanding how biodiversity and species distribution is organized exists because the most prevalent threat for biodiversity worldwide is land conversion, which acts primarily on a local scale (i.e., within ecosystems or habitats). Various land uses drive the conversion of former pristine ecosystems into modified and degraded land. A plethora of research has described the loss of habitat area or species richness following different human land uses. However, few studies reach beyond merely describing species losses and investigated the underlying mechanisms by which human activities alter the organization biodiversity and distribution. For a thorough understanding of how different human land uses impact the organization of biodiversity within ecosystems, it is first necessary understand how natural abiotic and biotic factors organize and drive biodiversity and distribution in natural systems free of any direct human disturbance before investigating any human-driven changes. Empirical research aiming to analyse the organization of biodiversity and distribution in ecosystems is often hindered by the overall ecosystem complexity and difficulty of demarcating and replicating communities or ecosystems. In this PhD thesis, I introduce a methodical framework that uses small insular ecosystems for investigating how biotic, abiotic, and anthropogenic factors drive biodiversity and distribution. The small sizes and clear boundaries of islands, together with the possibility to use groups of islands as ecosystem replicates, pose an elegant solution to the abovementioned limitations. This PhD thesis uses this insular framework as a model system to study how natural biotic and abiotic factors drive the organization of biodiversity and disentangle the impacts of different human land uses by investigating islands that hold only one specific type of human land use. For this, I consulted a two-step approach. In the first chapter of this PhD thesis, I used a focal taxon approach, while in the second chapter, I used a community-wide sampling approach. Analysing how abiotic, biotic, and anthropogenic factors drive biodiversity and distribution on two ecological levels enabled a more thorough understanding of the relevant factors, as both sampling approaches each bring their methodical advantages. In chapter A, I identified the key natural abiotic drivers for the distribution patterns of the investigated focal taxon, terrestrial hermit crabs (genus Coenobita), and show which physical conditions predominantly influence its distribution. At the same time, interspecific competition does not drive its biodiversity and distribution patterns. Instead, intrinsic mechanisms of resource partitioning stabilize co-occurrence. The investigation of the focal taxon’s biodiversity and distribution patterns on islands used either solely for touristic purposes (‘tourist islands’) or as permanent settlings by the local population (‘local islands’) disentangled the environmental impact of two different human land uses. I demonstrate that two forms of human land use can impact different aspects of the same taxon in the same overall region. Single-taxon-approaches are limited in their generalizability and give less insight than community-wide analyses. Hence, chapter B focused on the ground-associated faunal community of the investigated insular ecosystem (section B.1-B.4 and B.5-B.6) and the avifauna (section B.4). I demonstrate that the biodiversity and distribution patterns of the ground-associated insular community are organized in a compartmentalized way, both concerning habitat niche occupation and trophic niche occupation. Closely related species cluster within the overall niche space and form compartments attributable to distinct insular habitats (e.g., a beach food web compartment and an inland food web compartment). I show that this compartmentalized organization is not occurring due to an overall depauperate insular fauna, but likely stabilized by relative productivity gradients between compartments. More mobile bird taxa likely also form no relevant link between the distinct compartments of the ground-associated infauna. I further show that both investigated forms of human land use have a negative impact on the insular biodiversity patterns. However, the drivers for the observed losses differ between the two land uses. Ultimately, I show that both land uses are related, at least partly, to significant shifts in the trophic niche occupation of the impacted species, which might indicate a species’ susceptibility to land-use-driven abundance declines. By using islands as the methodical framework to study how biodiversity and species distribution is organized on a local scale and how human land uses alter these patterns, this PhD thesis paves the way for future research in community and disturbance ecology. The novelty of this insular approach is further emphasized by the presentation of two species newly recorded in the investigated insular region in the final chapter of my thesis. This underlines the broad spectrum of possible scientific insights that islands as model systems offer for biodiversity research.