Darwin’s naturalization conundrum: Exotic-native phylogenetic relationships and local scale invasion success
Understanding and predicting invasion success of novel species is a fundamental goal of invasion biology and offers insight on mechanisms determining community assembly. We examined invasion success of non-native species using Darwin’s Naturalization Conundrum framework. Darwin predicted that novel species that were both not related to and closely related to the native community would be most successful invading new landscapes. Here, we examine the phylogenetic relatedness of exotic to co-occurring native plant species in an intermountain bunchgrass prairie and adjacent canyon lands in northeast Oregon. We used standard line-point intersect surveys to identify and estimate percent cover of all native and exotic plant species occurring in 1 hectare plots located across the Zumwalt Prairie Preserve. We constructed a phylogenetic tree for the 163 taxa identified and used standard phylogenetic distance indices to compare phylogenetic community structure with exotic cover for each 1-ha plot. In addition, we modified standard phylogenetic metrics to calculate the phylogenetic relationships of exotic to co-occurring native species for the whole community and different functional groups.
We found that the percent cover of the exotic community increased along a gradient of increasing phylogenetic relatedness of the native community. There was also a positive relationship between exotic cover and the relatedness of the exotic to nearest co-occurring native species. This relationship was particularly strong for the perennial community. The canyon habitat supported a significantly different community comprised of native shrubs and exotic forbs. Therefore, the canyons had a net relatedness index that was significantly lower than expected by chance, indicating the exotic species were from different higher taxonomic clades than the native community. Similarly, annual exotic grasses were less closely related to co-occurring native [perennial] grasses than were perennial exotic grasses. Together, our study suggests that exotic species with novel life-history strategies or growth form traits are more successful invaders, supporting Darwin’s Naturalization Hypothesis. This is the first community-wide phylogenetic test of Darwin’s Naturalization hypothesis for local plant communities and serves as a case study of how to incorporate phylogenetic analyses into invasion biology and restoration projects.