Biotic resistance and plant phenology: insights from coexistence theory and field experiments

Background/Question/Methods

Coexistence theory provides a valuable framework for understanding how biotic resistance influences the population dynamics of introduced species, and for predicting biotic resistance from the traits of invaders and resident competitors.  It suggests that some species differences, termed stabilizing niche differences, favour the establishment of invaders but also limit their impact on the resident community.  Other species differences, termed average fitness differences, favour either successful biotic resistance or the eventual dominance by invaders, with the outcome depending on which taxa have greater competitive fitness.  The influence of trait differences on invasion outcomes should therefore depend on the ability of traits to predict the stabilizing niche differences and average fitness differences between invaders and resident taxa.

We explored this hypothesis by examining how plant phenology determines biotic resistance and invasion success in a California annual plant community.  Specifically, we quantified how the seasonal timing of growth relates to the niche differences and average fitness differences that collectively determine invasion outcomes. To do so, we first defined these two determinants of competitive outcomes from models that describe community dynamics in annual plant systems.  We then field parameterized and validated model predictions in experimentally assembled communities including invaders that displayed earlier, coincident, or later phenology than their native competitors.

Results/Conclusions

We found that phenology differences between the exotic and native taxa predicted both stabilizing niche differences and average fitness differences.  However, phenological differences were more strongly related to average fitness differences, and this overwhelmed the influence of phenology on niche differentiation.  It caused later native communities to more successfully repel earlier exotic invaders, and later invaders to outcompete earlier native residents. Our results suggest that disentangling how species differences drive biotic resistance and invasion success benefits greatly from coupling population dynamic models, trait information, and field experiments.