The role of phenotypic plasticity for species coexistence

Background/Question/Methods

Stable coexistence between competing species is promoted when niche differences counteract the destabilizing effect of fitness differences. Niche differences have been mostly related to differences in traits such as root depth phenology or resource acquisition. However, trait variation in response to competition (i.e. trait plasticity) can also promote coexistence, although its importance remains little explored. Trait plasticity can increase observed niche differences or equalize fitness differences, reducing the extinction probability of an inferior competitor (i.e. lower fitness).  Further, trait plasticity can allow the occurrence of rapid adaptive evolution, selecting trait mean that maximize niche differences on the new competition scenarios. We test whether plastic responses of Lasthenia californica, an early phenology Californian annual species, in response to competition to three European annual invasive species promotes stable coexistence in serpentine grasslands. These three invasive species follow a phenological gradient along the season (Bromus madritensis, early phenology; Centaurea melitensis, middle phenology; and Lactuca serriola, late phenology). We hypothesize that the relative importance for coexistence of trait plasticity will be higher under an overlapping scenario in phenology between species (i.e. absence of niche differences). We measure variation in height and variation in phenology as estimators of variation in fitness and niche differences respectively, and then, we used a parameterized annual plant population model to project the competition outcomes.

 Results/Conclusions

Lasthenia californica exhibited high trait plasticity in response to the three exotic competitors. Plastic responses in height and phenology primarily reduced fitness inequality and they did not increased niche differences. Thus, contrary to our expectations, they were not strong enough to promote stable coexistence in the absence of niche differences (i.e. early phenology). However, they did enhance stable coexistence between species with different phenology. Our results suggest that phenotypic plasticity in response to competition can be an important mechanism driving species coexistence if the fitness inequality in absence of niche differences is not too high.