Common garden comparisons of native and introduced plant populations: latitudinal clines can obscure evolutionary inferences

Abstract

Common garden studies are increasingly used to identify differences in phenotypic traits between native and introduced genotypes, often ignoring sources of among-population variation within each range. We re-analyzed data from 32 common garden studies of 28 plant species that tested for rapid evolution associated with biological invasion. Our goals were: (i) to identify patterns of phenotypic trait variation among populations within native and introduced ranges, and (ii) to explore the consequences of this variation for how differences between the ranges are interpreted. We combined life history and physiologic traits into a single principal component (PCALL) and also compared subsets of traits related to size, reproduction, and defense (PCSIZE, PCREP, and PCDEF, respectively). On average, introduced populations exhibited increased growth and reproduction compared to native conspecifics when latitude was not included in statistical models. However, significant correlations between PC-scores and latitude were detected in both the native and introduced ranges, indicating population differentiation along latitudinal gradients. When latitude was explicitly incorporated into statistical models as a covariate, it reduced the magnitude and reversed the direction of the effect for PCALL and PCSIZE. These results indicate that unrecognized geographic clines in phenotypic traits can confound inferences about the causes of evolutionary change in invasive plants.

Keywords: EICA; biogeographical comparisons; clinal variation; common garden; evolution of invasive species; introduced plants; latitudinal gradients.

Figure 1

Simulated data for a standardized (i.e. mean = 0) phenotypic trait (e.g. biomass, seed set) to demonstrate the effect of latitude on inferred differences between native (open symbols) and introduced (closed symbols) populations. Circles represent population means; triangles denote average differences between ranges (i.e. native versus introduced), with 95% confidence intervals. (A) When no latitudinal clines are present there is no significant difference (P = 0.856) between the native and introduced ranges. (B) A parallel cline results in a significant difference (P < 0.0001) between ranges when latitude is not included in the statistical model; this difference is nonsignificant (P = 0.557) if latitude is included as a covariate. Seventy-five simulated population means from each range were drawn from a normal z-distribution (mean = 0, SD = 1), and populations represent an even sampling every 0.2° of latitude, from 40–55° in the native range and 35–50° in the introduced range. The slope of the gradient is −0.2 units per degree of latitude (B).

Figure 2

Estimated differences between native and introduced plant populations (i.e. range effect size, or β_R) from a statistical model including data from 32 common garden studies of 28 species from 13 flowering plant families. Positive effect sizes indicate larger values in introduced populations relative to native ones, and are estimated for principal components of all measured traits (PC_ALL), or separate measurements of plant size (PC_SIZE), reproduction (PC_REP), and defense (PC_DEF). We estimated effect sizes by excluding (open circles) or including (closed circles) latitude and range*latitude effects. Approximate standard error bars are shown along with results of significance tests, based on restricted maximum likelihood. Effect sizes that are significantly different from zero (P < 0.05) are indicated with an asterisk.

Figure 3

Population means of PC_REP (first principal component of reproductive traits) of native (open) and introduced (closed) plant populations plotted against latitude, showing the confounding effects of latitude and range. A single garden from each of four species from Appendix C is shown.