A new perspective on trait differences between native and invasive exotic plants: comment.

Abstract

In a recent article in Ecology, Leffler et al. (2014) presented a potentially new perspective on the importance of trait differences between native and invasive exotic plants in explaining invasions in local native communities. The new perspective brought forward is that, if trait differences between invasive and native species are likely to be important in explaining exotic plant invasion, the differences must be larger than those observed between native species in the new community. A meta-analysis of previous studies searching for trait differences was presented, with the general finding that the magnitudes of trait differences between invasive and native species tend not to differ from those observed between native species only. Leffler et al. (2014) interpret this result as evidence that trait differences are highly context dependent, and that mechanisms other than trait differences are likely to be more important in most cases of invasion. We acknowledge that there is no universal explanation of successful exotic invasion into native communities. Moreover, we do not believe that invasive plant species always have trait values that differ substantially from the traits present in the native community, or that trait differences are important for invasion in all cases. However, we cannot agree with the criterion stipulated by Leffler et al. (2014), namely that a trait difference between invasive and native species can only be important to invasion success if it is greater than the differences among natives. Leffler et al. (2014) do not explain the logic behind the criterion, but a flaw of the criterion is that it will discount cases when a successfully invading species has intermediate trait values that are not represented by native species. Leffler et al. (2014) seem to focus on trait differences as representing niche differences among species. Consider the scenarios of niche differences among native and exotic invasive species in Fig. 1. If a trait is related to the niche space occupied by native species in the community and the invader, for a trait difference to be important in invasion success under the criterion of Leffler et al. (2014), only the scenario in Fig. 1a would qualify. Here, the invader occupies a niche at the extreme of the niche space, compared to native species. The average niche-related trait difference between the invasive species and the natives will be greater than the average difference among natives. However, consider Fig. 1b. Here, the invader occupies a vacant niche that is intermediate between the native species (Stachowicz and Tilman 2005), and the invader would have an intermediate, niche-related trait value not represented by the native community. However, the average trait difference between the invader and native species in Fig. 1b will be smaller than the difference among native species, and under the criterion proposed, the native-invasive trait difference would be considered unimportant. Thus, the criterion proposed by Leffler et al. (2014) cannot distinguish between cases where trait values may lie between those of native species but are still distinct and cases where they are very similar to native species. Exotic species may not only invade a community by having different niche-related traits compared to native species. Some of the traits considered in the metaanalysis of Leffler et al. (2014), e.g., biomass, are arguably traits related to fitness. Such fitness-related traits also do not have to be more different between invasive and native species than among natives, for them to be important for invasions. All that is required is for the trait difference to be large enough for invasive species to have greater fitness than the native species (Fig. 1c). If this occurs and there is niche overlap between the invasive species and a native species, then the invasive species should displace the native species (MacDougall et al. 2009). The trait difference between invasive and native species should always be greater than the average native-native difference only when the trait is related to niche space and the invader is occupying a vacant niche at the extremes of the niche space available to the whole community. Thus, cases that meet the Leffler et al. (2014) criterion could be viewed as representing only one of three possible scenarios where differences in traits between native and invasive species are potentially important, and the only scenario where native-native differences are relevant. The challenge is to understand which of the many traits we can measure are actually related to fitness and niches of invasive and native species, and then to identify whether fitness or niche differences (or perhaps even both) have led to invasion. Manuscript received 15 July 2014; revised 7 August 2014; accepted 10 September 2014. Corresponding Editor: D. C. Laughlin. 1 Ecology Lab, Department of Biology, University of Konstanz, Universitaetsstrassse 10, Konstanz D78457 Germany. 2 E-mail: wayne.dawson@uni-konstanz.de