1377 Limits to the niche and range margins of alien species Jake M. Alexander and Peter J. Edwards J. M. Alexander (jake.alexander@env.ethz.ch) and P. J. Edwards, Inst. of Integrative Biology, ETH Zürich, Universitätstrasse 16, ETH Zentrum CHN, CH–8092 Zürich, Switzerland. We discuss the apparent paradox that while introduced populations often adapt rapidly to conditions in the new range, it is normally assumed that the species’ niche remains unchanged. Focusing on plants, we argue that studies of the niche dynamics of alien species are useful for understanding the constraints acting on species in their native ranges, and vice versa. Most hypotheses about species ecological range margins are more consistent with there being a niche shift than niche stasis in the new range. After reviewing the evidence for niche shifts in alien species, we suggest that the probability of a shift occurring depends primarily upon the ecological and genetic processes limiting the species in its native range. For example, a fundamental niche shift might occur if introduced individuals are released from maladaptive gene flow from central populations, or if genetic diversity is increased by the mixing of individuals from different sources. In addition, other factors such as species characteristics, introduction history and conditions in the new range may also influence whether a niche shift occurs. Based on these considerations, we propose conditions under which niche shifts are most likely. Such understanding is important for predicting and mitigating current and future anthropogenic impacts on species ranges. he spatial distribution of a species is determined by all those factors that constrain positive population growth. One such factor – reflected in the species’ fundamental niche – is the inability of populations to adapt to abiotic conditions beyond the species’ boundary. A second – reflected in the realized niche – is the species’ exclusion from some parts of its fundamental niche through negative interactions with other organisms. A third is the inability of the species to disperse to all potentially suitable areas (the intersection of regions A, B and M, Fig. 1B) (Pulliam 2000, Soberón 2007, Svenning et al. 2008). In seeking to understand the relative importance of these factors, we can learn much from the many natural experiments produced by the reshuffling of the Earth’s biota by human activities (Holt and Keitt 2005, Parmesan et al. 2005). For example, if a species spreads rapidly when introduced to a new region, it is clear that it was previously limited by its capacity to disperse; and if the species occupies the same niche in the introduced and native ranges (Peterson 2003, huiller et al. 2005), we can conclude that similar factors limit its distribution in both regions. hus, alien species provide excellent model systems for understanding both niche dynamics and the factors influencing species ranges (Levin 2003b, Holt et al. 2005, Sax et al. 2007, Sexton et al. 2009). In this paper, we explore how theories about the range margins of species, especially plants, can help in understand- ing niche dynamics of alien species, and vice versa. We begin by attempting to reconcile evidence for rapid evolution in introduced populations with the observation that many spe- cies occupy the same niche in their native and introduced regions. We then consider whether the various hypotheses that have been proposed to explain range margins (Table 1, Fig. 1) (Hoffmann and Blows 1994, Gaston 2003) are more consistent with niche conservatism (stasis) or shifts. We review evidence for niche shifts in introduced populations, and based upon this analysis propose circumstances under which niche shifts are most likely to occur. Finally, we sug- gest potentially fruitful avenues for future research. Reconciling rapid evolution and niche dynamics of alien species At a geographic scale, the niche of a species is defined by the range of environmental conditions within which it can maintain positive population growth rates (Grinnell 1917, Hutchinson 1957). However, populations within species may be genetically adapted to local conditions, implying that the niches of these populations are different (Mau-Crimmins et al. 2006). Indeed, individuals of one ecotype recipro- cally transplanted to the habitat of another may be unable to reproduce, in which case local adaptation is not merely a “fine-tuning” of the genotype to its environment but is crucial for its persistence (Potvin 1986). Alien populations provide many examples of how rapidly such genetic differen- tiation can arise. For example, genetically determined clines in plant traits along latitudinal gradients have been observed in introduced populations of Echinochloa crus-galli (Roy et al. 2000), Hypericum perforatum (Maron et al. 2004), Impatiens glandulifera (Kollmann and Bañuelos 2004), Lythrum salicaria (Montague et al. 2008), Solidago species (Weber Oikos 119: 1377–1386, 2010 doi: 10.1111/j.1600-0706.2009.17977.x © 2010 he Authors. Journal compilation © 2010 Oikos Subject Editor: Eric Seablom. Accepted 20 April 2010