Species interactions and the formation of novel annual plant communities following rapid environmental change

Abstract

Recent environmental change associated with human activities has given rise to ecological communities that have no historical counterpart. In particular, introductions of non-native plant species have in many cases altered the structure and functioning of resident plant communities through changes in species composition and the surrounding environment within which species interact. As a result, new combinations of species are forming “novel” communities across an increasingly large portion of the earth’s land surface. Because novel plant communities differ in configuration from original native-dominated communities, they present unique challenges to management, restoration, and conservation efforts. Thus, there is a growing need to understand how novel communities function differently from the original communities they replace. In this thesis, I investigate a variety of interactions in original and novel plant communities. Using a diverse annual plant system that persists within a fragmented agricultural landscape in Western Australia, I focus on the role of local-scale species interactions, an important biotic component of plant community assembly. I explore the complexities of local-scale interactions between native and non-native invasive species in light of coexistence theory, community assembly, and conservation of native floral diversity. This thesis comprises seven chapters. The first chapter serves as a general introduction which places the thesis within the larger context of multispecies coexistence in novel plant communities. The second chapter serves as a description of the York gum (Eucalyptus loxophleba subsp. loxophleba) – jam (Acacia acuminata) woodland annual flora, the study system for the data chapters (3 – 6) which are based on laboratory and field experiments. Chapters 3 and 4 are experimental evaluations of frequency-dependent and density-dependent performance of native and invasive species that co-occur in York gum-jam annual plant communities. Chapter 5 reports on a field experiment, which investigates the performance of common native and non-native invasive annuals experiencing inter- and intraspecific competition in natural York gum-jam annual assemblages. Chapter 5 also assesses changes in community-level functioning due to compositional differences by evaluating diversity effects in novel and original annual communities. The final data chapter, Chapter 6, experimentally investigates how local-scale environmental gradients and a non-native invasive annual grass impacts annual plant community structure in the field. Finally, I conclude with a discussion of my results in Chapter 7, which unites the previous chapters, addresses limitations of the thesis, and presents suggestions for future research. Overall, my results suggest that the local-scale impacts of non-native invasive species on native species may be more variable than those often reported in the literature on plant invasions (i.e. competitive exclusion). Consistent with previous studies, I did indeed observe negative interactions among invasive non-native and native annual plant species in field and laboratory settings. Specifically, I found that some species of invasive annual grass have the potential to negatively impact native populations over very short timescales through direct competition as well as interference from litter. These negative interactions, however, were not representative of interactions in novel communities as a whole. Notably, I found interactions between native annual forbs and an invasive non-native annual grass that ranged from neutral to positive. The direction and magnitude of invader impacts were highly dependent on species identity and the composition of the community, but were generally consistent across community densities and natural environmental gradients. Species interactions play a potentially complex role in the assembly of annual plant communities post-invasion. My results lend empirical support to the notion that species in these novel communities should be considered according to their impacts rather than their origins. These studies serve as some of the first investigations into the processes that stabilize interactions among native and invasive non-native species and contribute to novel community formation and maintenance. When considered alongside large-scale patterns from observational studies, my findings demonstrate that interactions that occur over small spatial and temporal scales have the potential to influence large-scale plant community dynamics. In total, this thesis represents a valuable contribution to the community ecology and biological invasions literature, and has the potential to inform future restoration and conservation efforts in this threatened woodland ecosystem and beyond.