The role of climatic niche shifts and study type for phenotypic differentiation – A meta-analysis on native and invasive plant performances in common environment studies

Background/Question/Methods

The multitude of common environment experiments that aimed at testing for differences in native and invasive populations have often shown contradictory results either providing evidence for adaptive evolution but also rejecting this hypothesis. The present study aims at explaining some of this inconsistency by accounting for additional sources of covariation which might be partly brought about by differences in climatic niches of species between native and invasive ranges.

We conducted a meta-analysis on 37 common environment experiments with 27 invasive species and calculated standardized range effect size (invasive vs. native populations) for variables of growth and size. Then we addressed the effect of study type (greenhouse vs. common garden) and common garden location (native vs. invaded range) on range effect sizes. We compiled distribution data on the basis of presence/absence for all species. Climatic niche overlap between native and invaded ranges was calculated using Schoener’s D metric and tested for niche equivalency and niche similarity. To combine niche shift data and the results of the meta-analysis, a nested mixed model was applied to test for covarying effects of the amount of niche shift on range effect sizes in variables of plant size.

Results/Conclusions

Positive range effect sizes confirmed previous findings of increased growth of invasive populations, while the location of the common garden in either the native or the invasive range had no significant effect. Study type turned out to be of particular importance, as the superiority of invasive populations in aboveground biomass and individual plant height was only detectable in common garden but not in greenhouse situations. In climatic niche analyses, eleven species could be identified as niche shift indicator species, whereas five species indicated niche conservatism and displayed climate match across ranges. The magnitude of climatic niche shift alone had no significant effect on range effect sizes in variables of plant size. However, we found a significant interaction between measures of climatic niche shift and study area on range effect sizes in the common garden studies, thus indicating different effects of climatic niche differences depending on the location of the study area being in the invasive or in the native range.

In summary, changes of climatic niches are likely to be connected to invasion processes and the interactive role in these evolutionary processes should be investigated in further research preferably on those species clearly identified as climatic niche shift indicators.