Several studies have highlighted the importance of quantifying the factors that deter- mine the arrival, initial dispersal, and naturalization of self-sustaining populations, and their spread (Pimentelet al.,2001,2005;Viláet al.,2009b;Oreska and Aldridge,
2011) if we are to fully understand and tease apart the drivers of invasion and their ecological and economic implications. Although increasing attention has been given to this matter, it is generally accepted that there is little current knowledge of the factors promoting the naturalization of alien plant species (Duncan and Williams,
2002;Bellinghamet al.,2004;Theoharides and Dukes,2007;Hayes and Barry,2008;
Milbau and Stout,2008; Diezet al.,2009b). Distribution of alien, and also of native species, are influenced by a number of abiotic (mainly climate) and biotic factors (e.g. water, nutrients or livestock grazing;Pettitet al.,1995;Richardsonet al.,2000;Pyšek
et al.,2002;Arévaloet al.,2005;Prober and Wiehl,2012). Successful plant invasion
depends not only on the attributes of the invaders (Noble,1989;Bellingham and Coomes,2003) but also on aspects on the invaded ecosystem (Catfordet al.,2008;
Foxcroftet al.,2011;Pyšeket al.,2012). Studies have long determined that some ecosystems contain few alien species (e.g. tropical forests), while others contain many (e.g. warm-temperate regions;Lonsdale,1999;Pyšek and Richardson,2006). Focussing on the characteristics of the invaded ecosystem rather than on those of the invader is likely to be a more effective research strategy for understanding the processes of invasion (Hobbs and Humphries,1995), althoughFoxcroftet al.(2011) have suggested that attention should be paid to both.
Recent studies have advanced hypotheses about the changes in land-use and -management as they covary with climate and which are likely to be major drivers of alien species distribution patterns (Parkeret al.,2010), in contrast with native species distribution patterns. However, in a study byDidhamet al.(2005) that follows
MacDougall and Turkington (2005), the authors pointed out how human-related disturbance (e.g. habitat modification) may be the primary driver of native species loss with invasive alien plant species just a passenger. The impact of land-use history and management on the distribution of native and alien species has recently been discussed within the context of invasion ecology in areas such as the United States (Parkeret al.,2010;Boughtonet al.,2011) and Europe (Polceet al.,2011). Recent biogeographical research has indicated, in fact, that the likelihood of biological invasions at large extents might be reasonably well predicted from an analysis of climatic and bioclimatic factors and human-related impacts, although the interaction
between these two sets of drivers remains unclear (Evans and Gaston,2005;Evans
et al.,2005;Mariniet al.,2009;Polceet al.,2011;Mariniet al.,2012). Mariniet al.
(2009) tested whether the relationships between species and climatic/bioclimatic
factors and between species and human-related factors vary between native and alien species when other environmental variables are taken into account. They concluded that alien species richness was higher in areas with the most rich and diverse assemblages of native species and that there was a stronger response of aliens than natives in currently warm, urbanized, low-altitude areas than in cold, high- altitude areas where human population density was low. Evidence also indicated that the distribution of native and alien plant species across habitats was not similar, with invasive alien species being found more frequently in anthropogenically disturbed habitats (Chytrˇy et al.,2008b). In a heavily modified landscape, past and recent land use and management may be the primary factors in explaining not only alien species distribution patterns but also those for native species (Chapter 4). These human-related factors may, in fact, promote the establishment of alien species via alterations of the disturbance regime through fire and grazing (Hobbs and Huenneke,
1992; D’Antonio, 2000; Keeley et al., 2003), changes in soil nutrient status as a consequence of fertilization (Dukes and Mooney,1999;Radfordet al.,2010) and increased colonization pressure (sensu Lockwood et al., 2009) often by species that are associated with similar human-related disturbances. Hulme(2008,2009) demonstrated that native and alien plant species distributions may not respond similarly to environmental change. Hulme(2008) highlighted how this outcome is mainly due to the scale dependence of native and alien species distribution according to related climate and environmental factors. Within a region, local environmental drivers (e.g. habitat, propagule pressure and introduction history) influence native and alien species richness differently, while when the spatial grain increases (i.e. across regions), it is climate that influences both native and alien species richness (Hulme,2008).
My related study therefore seeks to determine how the structure and pattern of native and alien plant communities may be influenced by abiotic and biotic drivers that covary with dominant environmental gradients (i.e. land-use history and management and climate-environmental factors) on the Ecological Region of Banks Peninsula (New Zealand). Disentangling these drivers is an important part of general invasion ecology and is the focus of my particular work.
Geographic Information Systems (GIS) and Remote Sensing (RS) are useful tools in invasion ecology for determining the spatial distribution of (native+ alien) plant
species in a heterogeneous landscape such as Banks Peninsula and how this may be best described according to available GIS data layers. In Chapter 2, I use these tools and statistical techniques (i.e. data exploration and validation) to verify both the quality and adequacy of available plant species data and the explanatory variables as GIS data layers which are then used in Chapter 3 and Chapter 4.
Having established in Chapter 2 a body of verified data along with the creation of a geodatabase, I then, in Chapter 3, use multivariate statistical techniques and specific analyses of species composition and structure to determine the interaction of native and alien plant species communities with climate, environmental and human- related factors along the environmental gradients of Banks Peninsula. Understanding how and why native and alien plant species distribution and community structure differ along these gradients with associated human-related factors is the focus of my research in this field of invasion ecology. In order to clarify issues of ordination and classification analysis and to understand if levels of invasion vary across communities in low or highly managed areas, two related questions need to be asked: (1) does invasion lead to distinct native and alien plant communities? and (2) do native and alien dominated communities segregate along abiotic (i.e. climate) or biotic (i.e. land-use history and management) gradients? As species invasions may alter community composition and structure, I then ask if and how alien species influence levels of homogenization and community dissimilarity within and across different plant communities, and how different environmental tolerances of native and alien species shape community structure and nestedness?
The native and alien plant species communities thus established are analysed in Chapter 4 using spatial regression methods to determine the relationship between species richness on Banks Peninsula and abiotic and biotic drivers such as climate, environmental and human-related factors. In this context, it is important to consider the following issues: (1) is the relationship between native and alien plant species richness shaped by variation in anthropogenic and environmental gradients and, if so, how strongly?; (2) do similar native and alien relationships hold in plant communities that have either experienced relatively high or low human impact and are respectively dominated by either alien or native species? and (3) what is the relative contribution of environmental and anthropogenic gradients to the relationship between native and alien plant species richness?
Numerous studies have found that the positive sign of the native-alien species richness relationship (NARR) can be explained by the biotic acceptance hypothesis whereas the negative sign can be explained by the biotic resistance hypothesis
(Stohlgrenet al.,2006;Fridleyet al.,2007;Bartomeus et al.,2012). However, other studies have suggested that NARR sign and magnitude can be explained by the shift from biotic to abiotic drivers of plant community structure and that this is related to scale dependence of NARR (Fridley et al.,2007). Using the considerable body of international literature on NARR allowed me to examine this apparent contradiction and to examine, in Chapter 5, to what extent NARR on Banks Peninsula might mirror that observed worldwide. Using a meta-analytical approach combined with a multi-model inference within an information-theoretic approach, I also attempt to clarify and explain one of the fundamental questions in plant invasion ecology, that is the variability in the sign and magnitude of NARR at a global scale, the so-called "invasion paradox".