PhD Thesis

Anthropogenic biodiversity changes and altered biogeographic patterns on islands

Anna Walentowitz (10/2019-11/2022)

Support: Carl Beierkuhnlein, Lisa Hülsmann

Current biodiversity patterns on islands are the result of a complex interplay of natural and anthropogenic biogeographic drivers. Island ontogeny, area, topography, isolation, and latitudinal location on the planet, to name just a few, play a central role in determining insular species occurrences. Over millennia and millions of years, islands have assembled unique species communities shaped by their typical environmental characteristics. However, since the emergence and planetary-wide settlement of humans, islands have experienced an additional dimension of change. Natural land was converted into agricultural fields, meadows, and settlements. Infrastructure cuts across ecosystems, natural resources are exploited, and climate is changing at immense speed. Plant and animal species are transported deliberately or accidentally outside their natural ranges and that way also reach islands. Given the ongoing changes to natural ecosystems in the Anthropocene, it is fundamental to differentiate between impacts of natural and human drivers of changes to biodiversity patterns. In the context of global biodiversity transformation and loss, disentanglement of precise impact mechanisms and causalities affecting insular biodiversity is essential for a better understanding of natural long-term ecological and evolutionary principles in our rapidly changing world. This thesis is devoted to study biogeographic patterns and their emergence in the Anthropocene in multifaceted ways. The manuscripts presented built upon the legacy of island biogeographic research. Islands with their discrete boundaries are used as microcosms or natural laboratories to develop, test, and challenge biogeographic theory. The presented research thereby extends our knowledge on specific aspects of natural as well as human-mediated biodiversity. This thesis thereby addresses the so-called Hookerian shortfall of biodiversity research that focuses on the challenge of using islands as discrete ecosystems to understand biodiversity changes in times of heavy anthropogenic alterations of entire landscapes. The first part of the underlying work comprises multi-island studies and incorporates anthropogenic aspects to advance classic theory of island biogeography. The analysed reference systems are barrier, continental, and oceanic islands around the globe. A novelty is the linkage of functional aspects, precisely plant dispersal syndromes, to island biogeographic theory while also accounting for human-mediated change of island biodiversity. A global palaeoecological study on long-term trajectories of temporal nonnative species development unveils an earlier onset of such species (1000 years ago) than has been deduced from existent censuses and scientific records (500 years ago). This study relates to ongoing discussions about novel ecosystems and provides baseline information for restoration and conservation projects on islands. The development of specific case studies, presented in the second part of this thesis, enables a detailed view on specific aspects of extant insular biodiversity. The Canary and Galápagos Islands serve as reference systems as these archipelagos were visited by the author of this doctoral thesis for field work. It is a privilege to follow the scientific routes of Alexander von Humboldt and Charles Darwin, whose ground-breaking advances in ecology and evolutionary biology were inspired by these particular island systems. The presented case studies analyse biogeographic patterns of non-native plant species and their impacts on native vegetation. The findings aid understanding of invasions and ecosystem restoration to mitigate adverse impacts of such introduced biota on island endemics. Furthermore, local arks of outstanding insular biodiversity, that constitute important reference sites as opposed to heavily humanimpacted areas in the Canary Islands, are studied. A comprehensive floristic baseline established for this archipelago to advance and support biogeographic, ecological, and evolutionary studies complements this section. This thesis contributes to the general understanding of insular biodiversity patterns in the Anthropocene. Thereby, advancements from the inclusion of the anthropogenic dimensions into assessments of extant species’ distributions, incorporation of functional, dispersal-related aspects into island biogeographic theory, and the development of biodiversity timelines are achieved. Contemporizing island biogeography is a future challenge. This doctoral thesis is a contribution to a better understanding of past and current biodiversity on islands.