Phenology as a niche mechanism of community resistance to invasion

Background/Question/Methods

Exotic species can successfully be established in a native community depending on either a fitness advantage or niche differences from resident species. While niches differences promote coexistence between an invader and the resident community only a high asymmetry on species fitness allows the invader to become dominant or allows the residents to repel the invasion. The invasion outcome can be determined by phenology. Phenology has been traditionally viewed as a key trait promoting niche differences but little emphasis has been done to quantify its importance for community dynamics and hence for biological invasions. We hypothesize that an overlap in phenology between species (i.e. absence of niche differences) will promote competitive exclusion by limiting similarity whereas different phenology will promote coexistence. We focused our experiment in annual species of serpentine grasslands in California. These grasslands have been heavily invaded in the past by many European species. We selected three native communities and three invasive species with contrasted phenology: Early (flowering early-middle spring), Middle (flowering middle spring-early summer) and Late (flowering early summer-late summer). In a factorial experiment, we estimated niche and fitness differences for each invasive-native community combination and then, we used a parameterized annual plant population model to project the invasion outcomes.

 Results/Conclusions  

Invasive species and native communities with similar phenology limited themselves more than species with different phenology. Thus, invasive and native species with different phenology were projected to coexist at long-term. In the absence of niche differences, invasive species became eventually dominant and led some native species to extinction in the early and middle phenology communities due to higher fitness differences on average.  Interestingly, native species in the late phenology community could resist the invasion because showed similar fitness as invasive species. This result was due to a fitness increase of native species and not to a fitness reduction in invasive species. Since all species germinated at the same time, this result highlights that the extent of the phenology can be viewed as a cumulative effect through time determining the fitness output. Here we suggest that the higher is the phenology extent the higher will be the fitness output.  We conclude that phenology is a key mechanism which primarily promotes niche differences and in a lesser extend fitness differences. Thus, it is important for community dynamics and species assemblage under biological invasions scenarios.