Phenotypic plasticity (PP), or the capacity of a given genotype to render different phenotypic values for a given trait under different environmental conditions, is a basic concept in genetics and evolutionary biology that has attracted the attention of ecologists for many years (Bradshaw 1965; Bradshaw 2006). The current interest in plasticity results from an urgency to predict species responses to global change (Potvin & Tousignant 1996; Rehfeldt et al. 2001) and from the emerging ideas on the importance of plasticity for understanding trait-mediated species interactions (Callaway et al. 2003). Distribution shifts triggered by climate change are projected using correlational bioclimate envelope models (see Discussion by Hampe 2004), which can overestimate species losses because key aspects such as plasticity are ignored (Thuiller et al. 2005). Analogously, basic models and empirical approaches to community dynamics assume that they are governed by the densities of the interacting species, without considering trait changes that can alter the per capita effect of the reacting species on other species (trait–mediated interactions; Werner & Peacor 2003). Ecological communities are replete with trait–mediated interactions arising from trait plasticity that are often as strong or stronger than density effects (Callaway et al. 2003; Werner & Peacor 2003). Thus, the quantification of phenotypic plasticity becomes essential not only for investigators exploring species responses to the environment but also to those aimed at modelling both the effects of global change on species distribution and the outcome of species interactions in community dynamics.
The concept of plasticity is being widely used in an expanding number of disciplines (see Fuller 2003; DeWitt & Scheiner 2004), with an exponential increase of publications in recent decades (Scheiner & DeWitt 2004). For instance, phenotypic plasticity has frequently been reported as the primary mechanism enabling exotics to colonize environmentally diverse areas, a topic explored for more than three decades (e.g. Marshall & Jain 1968) and attracting increasingly more recent attention (Williams et al. 1995; Sexton et al. 2002; Niinemets et al. 2003; Parker et al. 2003; Peperkorn et al. 2005; Sharma et al. 2005). However, even though the literature on phenotypic plasticity is extensive, it fails to provide a clear consensus on the adaptive and evolutionary meaning of plasticity (Via et al. 1995; DeWitt & Scheiner 2004). There is agreement on the notion that the degree of phenotypic change across environments differs among species and traits, and that the amount of phenotypic change observed depends on the type of environments considered (Pemac & Tucic 1998; Valladares et al. 2002a,b, 2005a; West-Eberhard 2003; Bradshaw 2006). However, there is disagreement regarding its quantification and on the way that natural selection influences reaction norms (trait vs. environment plots; Pigliucci 2001). While those dealing with plasticity accept the working hypothesis that plasticity functions as a way of adapting to variable environments, evolutionary biologists assess plasticity in terms of genetic variation and fitness consequences, plant ecophysiologists translate it in terms of stress tolerance and carbon gain, and developmental biologists in terms of mechanisms by which the environment affects trait development (Dudley 2004). Plasticity sensu stricto has been typically focused on developmental aspects using known genetic lines (e.g. Cheplick 2003; Van Kleunen & Fischer 2003), while plasticity sensu lato has been focused on the responses of different species and populations in their ecological context (e.g. Callaway et al. 2003; Valladares et al. 2005b). The fields of ecology and development are now rapidly developing new insights into plant evolution with plasticity emerging as a key to the understanding of plant development in an ecological context (Farnsworth 2004; Sultan 2005). Ecological development, or ‘eco-devo’, aims to bridge the gap between the study of developmental mechanisms and the study of ecological and evolutionary diversity ((Ackerly & Sultan 2006), the major ‘new frontier’ in biology (Kafatos & Eisner 2004). Plasticity has become a central focus of this ecological and evolutionary research, bringing new insights into understanding phenotypic variation that shapes ecological interactions and selective change ((Ackerly & Sultan 2006).
Research in plasticity has expanded from its initial focus on abiotic factors, such as irradiance or water, to that of biotic factors such as competitors, predators or pollinators (Schlichting 2002; Sultan 2004). A crucial step in ecological approaches to phenotypic plasticity is the quantitative estimation of the phenotypic change induced by the environment, which is of particular relevance in comparative studies of different species and populations (Valladares et al. 2000a, 2005a; Balaguer et al. 2001). This estimation must be simple, particularly in ecological studies dealing with an ample number of species and traits (e.g. Navas & Garnier 2002; Gratani et al. 2003; Castro-Díez et al. 2006). In fact, research goals requiring a simplified estimation of plasticity have given rise to a plethora of plasticity indices (e.g. Cheplick 1995; Valladares et al. 2000a,b; Richardson et al. 2001). Selection of the quantitative estimator of plasticity has an important bearing on both the way plasticity is assessed and the ecological and evolutionary implications that can be extracted. By condensing experimental data, indices can facilitate the presentation and interpretation of complex results, and the use of the same index by different investigators facilitates comparisons of different studies (Weigelt & Jolliffe 2003). However, indices can be flawed and misapplied in different ways, and indices built from similar primary measures can be defined differently, complicating comparisons between studies and the meta-analyses of published data.
In the present study, we first review different approaches undertaken to quantify phenotypic plasticity with special attention to the most common indices used in comparative studies. Secondly, we conducted an experimental case study of plastic responses of woody seedlings to light, to evaluate the degree of coincidence of the various indices in ranking genotypes according to their plasticities. We then introduce a new approach to quantify phenotypic plasticity based on the phenotypic distances between individuals of a given species exposed to different environments, which is summarized in a relative distance plasticity index (RDPI). RDPI is applied to the study case, and the other indices are regressed against it to determine its utility and consistency. A rescaling of RDPI that includes the environmental range giving rise to each phenotype is also introduced and discussed. Finally, we assess the appropriateness of RDPI and the other indices according to the objectives in each kind of study.