Chasing the ghost of competition past: Interactions between immigration history and environmental gradients influence modern plant community assembly

Background/Question/Methods

Until recently, direct evidence of historical contingency in community assembly has been limited to laboratory manipulations and field experiments over short time scales. Advances in the resolution of molecular dating of phylogenetic relationships have made it possible to reconstruct immigration order over evolutionary time scales. Isolated island ecosystems, with low immigration rates and high levels of endemism, provide a unique opportunity to leverage this information to study the role of immigration and radiation histories in shaping communities. In the Murchison Mountains of New Zealand’s South Island, the community dominance of monophyletic alpine plant species was recently shown to increase with radiation age. In this study, we have combined data on the ages of 17 monophyletic New Zealand alpine plant genera with species abundance data collected in 262 5 x 5 m plots across an elevation range of 780 – 1620 m and a precipitation gradient of 2500 – 5000 mm yr^-1. While the strength of priority effects has been previously shown to vary across environmental gradients, little is known about how this process interacts with radiation history during community assembly. Using Bayesian methods, we investigated interactions between radiation age and abiotic conditions to explain modern community structure.

Results/Conclusions

Two measures of community dominance by genera, cover dominance (% cover of the focal genus / % cover non-focal genera) and richness dominance (# focal congeneric species / # non-focal species), were found to be positively correlated with radiation age when analyzed at the plot level. The magnitude of these effects varied spatially across environmental gradients, with significant direct effects of elevation and precipitation on both dominance measures and a negative interaction between elevation and radiation age for cover dominance, but not richness dominance. The negative interaction between radiation age and elevation suggests that increasing abiotic stress, associated with increasing elevation, diminishes the importance of ecological interactions, and thus priority effects. These results suggest that immigration order and priority effects, on a time scale of millions of years, may influence modern community structure, but that these effects are mediated by current, local conditions.