UC Merced

Islands have long captivated evolutionary and ecological biologists, especially since Darwin’s pivotal insights on natural selection during his visit to the Galapagos Islands. While islands account for only 6% of the Earth’s land area, they harbor 20% of global species diversity, hosting many endemic species and unique morphological diversity, providing textbook examples of adaptive radiation. However, island biodiversity has been severely affected during the Anthropocene, with three out of four island species becoming extinct, with the disappearance of the dodo (Raphus cucullatus) in the 17th century being one of the first recognized human caused extinctions on islands. The extinction of a species can profoundly affect an entire ecosystem, leading to changes in both ecological and evolutionary dynamics. Focusing on the evolution of morphological traits, I combined theoretical and empirical approaches to explore the ecological and evolutionary consequences of extinctions on island communities. In Chapter 1, I explored the interplay between colonization, extinction, and coevolution, and how these intersecting dynamics shape species’ traits and the structure of mutualistic networks on islands. For that, I used a stochastic mathematical model, integrating Island Biogeography Theory with coevolutionary dynamics. My results show that as extinction rates increase, the number of interactions needed for a species to achieve maximum persistence on the islands also rises, but only up to a threshold. Moreover, islands with higher extinction rates have species with greater trait similarity. In Chapter 2, I used data from historical Hawaiian honeycreepers, a group of Hawaii-endemic birds that have lost several species in the last century. I test whether greater morphological variation and larger niche breadth increases resilience to extinction in each species. Using geometric morphometrics and stable isotope ratios, I show that within guilds, species that have gone extinct exhibited lower morphological variation. Lastly, in Chapter 3, I used phylogenetic comparative methods to investigate the evolution of beak shape in the Hawaiian honeycreepers and the loss of unique adaptive peaks with species extinctions. I show that the extinction of Hawaiian honeycreepers is leading to a drastic reduction in the occupied morphospace, resulting in a homogenization in their trait space, which can directly impact ecosystem functioning and the provision of ecosystem services, such as seed dispersal. Taken together, my dissertation broadens the understanding of how extinction can affect both ecological and evolutionary dynamics of island communities.