- Top of page
- Materials and Methods
- Supporting Information
Manipulative experiments designed to characterize how environmental factors such as temperature, light or resource availability affect evolution almost always focus on the direct effects of the manipulation. This is particularly true of studies conducted in controlled environments such as chemostats, mesocosms, growth chambers and glasshouses in which the environment is much simpler than the conditions most organisms experience in nature. Study of direct effects can be powerful for identifying selective agents and for elucidating the genetic basis of phenotypic variation. However, exclusive focus on direct effects may be misleading if the goal of the research is to predict evolutionary responses in natural environments. In this case, partitioning direct effects from indirect effects that result when the environment affects species interactions provides much greater insight into selective agents and the genetic constraints that influence evolutionary responses to environmental change.
Ecological indirect effects may be particularly important for understanding how environmental changes that affect the resource environment influence plant evolution. Changes in resource supply, including CO2, can alter the outcome of competitive interactions (Tilman, 1977; Brooker, 2006; Harpole & Tilman, 2006, 2007; Reich, 2009), and competition can be an important selective agent (Miller, 1995; Shaw et al., 1995; Dorn et al., 2000; Stanton et al., 2004). Therefore, many of the ecological and evolutionary effects of changes in resource availability that have occurred since the Industrial Revolution may have come about through indirect pathways mediated by competitors rather than through direct effects. For example, elevated atmospheric CO2 concentrations (eCO2) typically increase plant growth directly by increasing carbon supply, but because taxa differ in magnitude of CO2 response (reviewed in Poorter & Navas, 2003), some species may respond negatively to eCO2 because of increased competition from species that benefit most from eCO2 (reviewed in Brooker, 2006).
The increase in atmospheric CO2 concentration is one of the most globally extensive and rapid anthropogenic environmental changes, and many studies have demonstrated large effects of eCO2 on plant physiology, plant growth and reproduction, community structure and ecosystem functions (Bazzaz, 1990; Poorter & Navas, 2003; Niklaus & Körner, 2004; Reich et al., 2006; van Groenigen et al., 2011). The evolutionary consequences of eCO2 also have received attention. In contrast to the strong ecological effects of CO2, however, many evolutionary effects of CO2 are weak in magnitude (reviewed in Leakey & Lau, 2012). For example, many studies fail to detect genotype × eCO2 interactions (Zhang & Lechowicz, 1995; Luscher et al., 1998; Volk & Körner, 2001; Veteli et al., 2002; Wieneke et al., 2004), suggesting that genotypes may commonly respond in similar ways to rising CO2 concentrations and that predicted future eCO2 concentrations will not change either the rate of evolutionary change or which genotypes are favored by natural selection. Similarly, to our knowledge, no studies have documented adaptation to eCO2 even though experimental evolution approaches have detected genetic differences in plant traits between populations that have evolved under ambient vs elevated CO2 (Potvin & Tousignant, 1996) and even evidence for adaptation to subambient CO2 concentrations (Ward et al., 2000; Collins & Bell, 2004). Most tests of the evolutionary effects of eCO2, however, have been conducted in simplistic growing environments where indirect effects resulting from altered species interactions cannot contribute to evolutionary responses.
In previous studies, we detected little evidence that eCO2 alters patterns of selection or expected responses to selection in Arabidopsis thaliana populations grown in the absence of competition (Lau et al., 2007); however, other studies using similar approaches have found some evidence that eCO2 alters patterns of natural selection and/or heritabilities when individuals are grown in competitive environments (Bazzaz et al., 1995; Steinger et al., 2007; Lau et al., 2010). The differences across studies suggest that eCO2 may have minimal direct effects on evolutionary processes, but potentially, larger indirect effects that result from eCO2 altering interactions with other community members that are strong selective agents.
Here, we report on an experiment designed to characterize potential competitor-mediated indirect effects of increased atmospheric CO2 concentration on the predicted evolution of experimental Arabidopsis thaliana populations. We investigated how eCO2 affects interactions with competitors to alter: plant fitness; the expression of genetic variation (heritability); genetic trade-offs that can constrain or facilitate evolutionary responses; and natural selection on ecologically relevant plant traits. This study expands on previous work by our group that showed a surprising lack of direct effects of eCO2 on predicted A. thaliana evolution (Lau et al., 2007) and work that demonstrated that eCO2 alters the effects of competition on A. thaliana fitness and selection on A. thaliana traits (Lau et al., 2010). The rigorous quantitative genetics approach and large number of genotypes used in the current study, combined with experimental manipulation of both CO2 and competitive environment, directly investigates the potential for competitor-mediated indirect effects of eCO2 on all three components determining evolutionary responses: patterns of natural selection, heritabilities and genetic covariances.