Structure and pattern of native and alien plant communities across local and global scales

Abstract

Identifying and quantifying the abiotic and biotic factors that influence the structure and pattern of native and alien plant species communities is of crucial importance in invasion ecology. There is little knowledge of how these factors covary with dominant environmental gradients and affect the plant communities especially in heavily modified landscape. In my thesis, I focus on how these factors covary with dominant environmental gradients and influence the composition and structure and richness patterns of native and alien plant communities on Banks Peninsula (New Zealand). The invasion of alien plant species frequently shows an alteration in the native community composition and structure (e.g. increasing biotic homogenization). The native-alien species richness relationship (NARR) is used here as an ecological indicator for assessing community invasibility. Factors underlying NARR are crucial for understanding community assembly and are here investigated across local scale (i.e. species communities), regional-landscape scale (i.e. Banks Peninsula) and global scale. Using Banks Peninsula as a model system, I tested the relative importance of factors that drive the invasion process on the Peninsula and set this within a global context, using GIS, multivariate statistical techniques and spatial regressions to analyse how these factors interact with native and alien plant communities to determine composition and structure and richness patterns. The species richness relationship at local and regional-landscape scales is contextualised on a global scale using a meta-analytical approach to the considerable body of literature on NARR and so to clarify the so-called "invasion paradox", which has not been adequately quantified so far. Across a heterogeneous environment such as Banks Peninsula, native and alien species communities were found to be spatially and ecologically segregated according to different responses, firstly, to land-use and -management and, secondly, to climate and environmental factors, both of which covary with elevational gradients. Both positive and negative relationships between native and alien species richness can arise, with this outcome moderated by the effects of land-use history and management. Globally, a significant positive NARR was found with plot size consistently the best predictor. For studies sub-grouped by plot size, NARR increased positively and significantly with increasing plot size. Across Banks Peninsula, high levels of human-related disturbance offer the best conditions for the spread of alien species and for an increased homogenization effect of the native community. In areas characterized by high intensity levels of landmanagement, biotic homogenization is stronger and a more serious problem than in less managed areas. Across Banks Peninsula, negative NARR can be attributed, not to biotic resistance, but mainly to the shift in community types as a consequence of land-use history and management along elevational gradients. Globally, NARR is overall positive and scale dependent. However, negative and positive NARR can occur within the same plot size and across the same landscape, depending on the plant community and the underlying gradients examined. It is therefore important to take into account human-related factors or proxy measures (e.g. land-use history and management) when studying drivers of plant invasions, because these are the primary factors in explaining composition and structure and richness patterns of native and alien species communities at local and regional-landscape scale.