Understanding the factors that contribute to the success of invasive species may facilitate the prediction of future invasions, determine the best ways to control invasive species, and elucidate the impact of invasive species on native communities (Pysek & Richardson 2007). To identify traits associated with invasiveness, researchers have primarily evaluated invasive and native or introduced, non-invasive species within large regional data sets (Sutherland 2004; Hamilton et al. 2005) and within plant genera or families (Grotkopp et al. 2002; Muth & Pigliucci 2006). A recent analysis of these two approaches concluded that plant height, vegetative growth, and early and extended flowering are strongly associated with invasiveness (Pysek & Richardson 2007). However, our understanding of how the dynamic nature of reproductive, life-history, physiological and morphological traits contributes to invasiveness is still limited.
It has been repeatedly suggested that invasive species possess high phenotypic plasticity (broadly defined as the ability of organisms to alter their morphology and/or physiology in response to varying environmental conditions) which may allow them to occupy a wide range of new environments (Baker 1965; Marshall & Jain 1968; Sultan 2001; Callaway et al. 2003; Daehler 2003; Pigliucci 2005; Rejmanek et al. 2005). While empirical studies generally support this idea (e.g. Rice & Mack 1991; Pattison et al. 1998; Gerlach Jr. & Rice 2003; Niinemets et al. 2003), the lack of phylogenetically and ecologically equivalent comparisons among invasive and native species limits the ability to explicitly link plasticity with invasiveness. Phylogenetic comparative designs are necessary to minimize trait differences associated with comparing unrelated species and disparate life forms (Burns & Winn 2006; Muth & Pigliucci 2006; Richards et al. 2006).
Because plasticity in physiological and morphological traits can provide greater access to limiting resources, invasive species may benefit from plasticity in low resource environments (where plant growth is strongly limited by water, nutrient or light availability). For example, invaders that adjust physiological or morphological traits to take advantage of spatially variable light availability (e.g. sunflecks) or temporally variable water and nutrient availability (e.g. storms, deposition) in low resource systems may outperform less plastic neighbouring species (Poorter & Lambers 1986; Davis et al. 2000). However, Chapin (1980) and Grime et al. (1986) argued that resource conservation (e.g. storage, retention of existing leaves) should be more advantageous than resource acquisition (e.g. the deployment of new leaves or roots) in low resource or stressful environments. Thus, native species adapted to low resource systems may exhibit low phenotypic plasticity. While recent work supports this idea (e.g. Balaguer et al. 2001; Steinger et al. 2003), it is not known if species invading low resource systems also demonstrate low trait plasticity or if they succeed in these systems due to increased trait plasticity.
I examined how light and nutrient availability influenced physiological and morphological plasticity among five phylogenetically related pairs of invasive and native species that occur in nitrogen-limited habitats in Hawai’i. Based on the theory of Chapin (1980) and Grime et al. (1986), I predicted that invaders would be similar to natives adapted to these low resource systems by displaying low trait plasticity. A recent field survey of species across low resource habitats in Hawai’i found that invasive species displayed similar or high resource-use efficiency (resource conservation traits) relative to neighbouring native species (Funk & Vitousek 2007). Thus, I also predicted that invasive species would display high resource-use efficiency and that, across species, resource-use efficiency and plasticity would be negatively correlated.
As trait plasticity will only influence the success of invasive species if traits are linked to plant fitness, I also explored the functional significance of the observed plasticity. This study examined species-level plasticity (plasticity among individuals within a population across different environments; Richards et al. 2006; Valladares et al. 2006) as opposed to genotype-level plasticity (the capacity of individual genotypes to produce different phenotypes across different environments; Sultan 2000). While both species- and genotype-level approaches are useful in the exploration of invasive species success (Richards et al. 2006), species-level approaches permit the measurement of more traits and species by sacrificing replication at the level of individual genotype. In the absence of genetic information required for the determination of adaptive plasticity by traditional selection analysis (e.g. Dudley 1996; Heschel et al. 2004), I evaluated the functional significance of species-level plasticity by correlating traits with plant performance across different environments (Donovan & Ehleringer 1994; Burton & Bazzaz 1995; Funk et al. 2007).