Understanding the factors that lead to the successful establishment of organisms outside their native range is a fundamental, but complex, goal of invasion biologists. Although only a fraction of introduced species make the transition to become naturalized and even fewer become invasive, those species capable of successful integration into new habitats can have devastating effects on native ecosystems. Invasive plant species have been linked to reductions in native species richness (Greenwood, O’Dowd & Lake 2004; Lake & Leishman 2004; Fridley et al. 2007), changes in soil nutrient dynamics (Mack, D’Antonio & Ley 2001; Allison & Vitousek 2004; Yelenik, Stock & Richardson 2004; Ashton et al. 2005), declines in limiting resources such as light (Asner et al. 2008; Iponga, Milton & Richardson 2008), and global and regional biotic homogenization (Qian & Ricklefs 2006; Schwartz, Thorne & Viers 2006; La Sorte, McKinney & Pysek 2007). Hence, invasive plants are recognized as one of the most pervasive agents of global change, and escalating rates of introduction, linked to the expansion of global trade, suggest they will continue to pose conservation challenges in the future.
The invasion process is characterized by three distinct, but not discrete, stages: introduction, naturalization and spread (Richardson et al. 2000). Naturalization is defined as the ability to establish self-sustaining populations following introduction into a new range, whereas invasion is only achieved by a subset of naturalized species that spread away from founding populations to become widespread and abundant (Pyšek et al. 2008). Species progress through these stages by overcoming a series of abiotic and biotic barriers, and the suitability of the climate in the introduced range is cited as one of the main abiotic barriers to the establishment of exotic plants (Williamson 2006; Hayes & Barry 2008).
The term ‘climate matching’ refers to the match between native and exotic regions based on either a single variable such as temperature (Chown, Gremmen & Gaston 1998), a suite of climatic variables (Thuiller et al. 2005; Richardson & Thuiller 2007), or on indirect measures of climate such as latitude (Pyšek 1998; Blackburn & Duncan 2001; Maron 2006; Jimenez et al. 2008). This technique has been applied to a range of issues in invasive species management. These include the identification of high-risk source areas (Richardson & Thuiller 2007), pre-emptive projection of the spatial extent and magnitude of risk posed by introductions both now (Panetta & Mitchell 1991; Thuiller et al. 2005; Ficetola, Thuiller & Miaud 2007; Mgidi et al. 2007; Crossman & Bass 2008) and under future climates (Broennimann & Guisan 2008; Beaumont et al. 2009), prioritization of search areas for potential biocontrol agents (Senaratne, Palmer & Sutherst 2006; Robertson, Kriticos & Zachariades 2008) and the identification of suitable areas for release of biological control agents across invasive species ranges (Rafter et al. 2008).
To be a useful predictor of invasion potential, climate matching relies on the conservation of climate niches across both space and time. While there are numerous examples of correspondence between the native climate of exotic species and their introduced range (see Wiens & Graham 2005), not all exotic species show high levels of climate matching (e.g. see Broennimann et al. 2007; Fitzpatrick et al. 2007; Loo, Mac Nally & Lake 2007; Beaumont et al. 2009) due to both ecological and evolutionary processes. Historic and geographic constraints may prevent a species from occupying the entire fundamental climatic niche within its native range. Introduction to a new environment may also result in changes to a realized niche as the species is ‘released’ from the biotic constraints imposed by enemies and competitors on its native range margin. Rapid evolutionary adjustments to novel environments may also contribute to the successful spread of invasive species (Broennimann et al. 2007; Lavergne & Molofsky 2007) and can occur within < 20 generations (Prentis et al. 2008). Results of studies exploring these issues have implications beyond invasion ecology, as they can inform us about the factors that constrain the fundamental niche of species. In this way, studies that compare exotic species in their native and novel ranges are analogous to large-scale transplant experiments.
Thus, the assumption that species will maintain the same climate niche across their exotic ranges as they do in their native range does not always hold true. Across multi-dimensional climate space, five patterns of realized native and exotic climate niches can be conceptualized, as illustrated in Fig. 1: (i) both niches may overlap almost completely; (ii) the exotic climate niche may form a subset of the native climate niche; (iii) the native climate niche may form a subset of the exotic climate niche; (iv) there may be a partial overlap between the two niches; or (v) the two niches may be completely disjunct.
While climate matching assumes either near-complete overlap or that the exotic niche represents a subset of the native niche, equivocal evidence from single-species studies indicates that we cannot yet assess how common the ability to shift climatic niche is in species introduced to novel regions. This information is critical for quantifying the uncertainty inherent to climate matching exercises used in invasive species management, such as niche modelling. It also has wider implications for niche theory in its ability to delimit the bounds of the elusive fundamental niche. Using a comparative approach that examines a suite of species and their traits invading into a common location may further our understanding of the need to account for climatic niche shifts when applying climate matching techniques. In this study, we quantified the extent to which the current realized climatic range of naturalized or invasive exotic species overlaps with that of their native range, for 26 plant species introduced to Australia. We then tested hypotheses about the role of species traits, introduction history and attributes of the distribution of invasive species in determining the degree of climate matching between the native and exotic ranges.
Consistent with climate niche theory, invasive species are also generally expected to show strong biome conservatism between their native and exotic ranges (Rejmánek et al. 2005). Biomes are broad-scale ecological regions defined by vegetation structure and climatic conditions, in particular rainfall and temperature. They often closely approximate climatic zones and are used to divide the Earth into areas that share a similar functional or ecological role. Biome conservatism between species’ native and exotic ranges has previously been shown to be associated with invasion success across a variety of taxa (Peterson 2003; Thuiller et al. 2005), but the ability to shift into new biomes following introduction remains unexplored. Therefore, we also examined how often the 26 species used in this study have shifted into a novel biome not occupied in the native range, when introduced to Australia.