Are rare species less able than common species to persist through environmental change? A meta-analysis of congeneric comparisons

Jennifer Boyd, Department of Biology, Geology, and Environmental Science, University of Tennessee at Chattanooga, Jessica Brzyski, Department of Biology, Seton Hill University, Jill T. Anderson, Department of Genetics, University of Georgia, Carol J. Baskauf, Department of Biology, Austin Peay State University and Jennifer Cruse-Sanders, Department of Plant Biology, University of Georgia; State Botanical Garden of Georgia, University of Georgia

Background/Question/Methods
The question of why some species are rare while others are common has important implications for ecological theory and conservation. To persist in a dynamic natural world, species must successfully respond to the environmental changes they experience through migration, adaptation, and/or acclimation. We aimed to advance understanding of species rarity with a comprehensive meta-analysis of empirical studies comparing fitness measures, functional traits, and/or genetic diversity of rare and common congeneric plant species. Our approach focused on characteristics of species that could influence their abilities to migrate, adapt, and/or acclimate in the face of environmental change and considered rarity types as defined by geographic distribution, habitat specificity, and local abundance. We extracted species-level means, variances, and sample sizes of continuous measures; numbers of events and sample sizes for binary measures; and genetic diversity parameters from studies as directly reported, mined from figures, obtained from associated datasets, and/or provided by authors. Fitness measures included survival and recruitment and reproductive success. Functional traits included those related to fecundity, growth, and physiology. Genetic parameters included percent polymorphism, number of alleles per locus, observed and expected heterozygosity, and inbreeding coefficient. Effect size of pairs of rare and common congeners were analyzed via phylogentically-controlled meta-analyses.
Results/Conclusions
We generated an extensive dataset from 156 studies representing >200 rare species from >150 genera. Most rare species were the focus of a single study, and most studies focused on a single rare species. Differences in fitness measures and functional traits between rare and common plant species were limited and dependent on rarity type. Specifically, rare species characterized by low local abundance exhibited lower fitness and smaller floral size than their common congeners. But differences were not detected when rare species were characterized by constrained geographic distribution or narrow habitat specificity. Analysis of genetic parameters supported the expectation that rare species have lower genetic diversity and higher levels of inbreeding than common species. Collectively, our findings of functional traits and genetic diversity suggest an effect of inbreeding depression in rare species characterized by low local abundance. Given the distinctions between rarity type in our findings and to motivate further research on the potential causes of rarity, we propose experimental approaches focused on specific types of rarity.