Community assembly in human-modified habitats

Abstract

Globally, only 22% of ice-free land is free of human populations or land use activities, leading to losses in the diversity of life. Novel abiotic and biotic conditions encountered within human-modified habitats can alter species interactions, however the associated consequences remain unclear. Previous studies have demonstrated that trophic interactions are vital to biodiversity maintenance and community properties (e.g. resistance to disturbance). Thus, my thesis examines how trophic interactions have been altered in human-modified habitats, as well as the potential consequences for community assembly, in 3 different study systems and scales. First, using leaf-litter ant communities in secondary forests and exotic tree plantations in Hong Kong as a study system, I tested how species diversity and trophic interactions have been altered in human-modified habitats. While ant diversity was similar between habitats, and common forest species persisted in plantations, stable isotope analysis revealed that average trophic positions for species found in both habitats were significantly lower within exotic plantations. Furthermore, cafeteria experiments also demonstrated that conspecifics in plantations had increased preferences for protein resources relative to conspecifics in forests. These results showed that while trophic flexibility allows species to persist in human-modified habitats, such flexibility may alter the identity or extent of limiting nutrition for species, thus changing community assembly processes. Second, I investigated the relative importance of trophic interactions and other processes in shaping communities in human-modified habitats through the sampling of butterflies and measurements of environmental characteristics in 18 Hong Kong urban parks. Floral abundance was included in the best model explaining butterfly species composition between parks, indicating the importance of trophic interactions in community assembly. However, the combined effects of abiotic and biotic factors were weak compared to other studies, which can be explained by increased disturbance in urban areas inhibiting population establishments in urban parks and therefore reducing the importance of abiotic and biotic factors in shaping communities. Finally, I examined how trophic interactions affect invasion resistance of communities. Analyzing a global urban bird dataset, I found communities dominated by native dietary generalists had lower levels of invasion compared with those dominated by native dietary specialists. This result suggests that native dietary generalists can disproportionately confer invasion resistance in urban ecosystems, potentially through including anthropogenic food in their diet and consequently reducing resource availability for exotics. Overall, my thesis demonstrates how habitat modifications alter trophic interactions, which consequently can affect ecological communities through altering community assembly processes. These findings contribute to a better understanding of community assembly in human-modified habitats and how to improve models forecasting the impact of future habitat transformation on ecological communities.