Facilitation in plant communities: Driver of evolutionary adaptation?

Background/Question/Methods

Once seen as anomaly, facilitation is now fully accepted as important determinant of the structure and functioning of various natural communities. Over the last two decades, facilitation got increasingly embedded in ecological theory, with the stress-gradient-hypothesis as most prominent concept.  In contrast, the evolutionary aspects of facilitation in plant communities have received much less attention: the importance of facilitative interactions as an evolutionary cause of change has been overlooked for a long time. Theoretical studies suggest that facilitation can lead to evolutionary adaptations in interacting plants, but empirical evidence is thus far limiting. The few available studies indicate that facilitation can be a mechanism to maintain the genetic diversity within populations or conserve mal-adapted lineages, acting as an ‘ecological time machine’.  Further evidence comes from a handful of field experiments that indicate that facilitation can help the colonization of mal-adapted ecotypes in stressful environments.  What we now need are empirical studies that focus on the impacts of facilitation on evolutionary adaptation of interacting species, in terms of phenotypic variation and genotypic population structure. We set-up two different experiments to investigate the impacts of facilitation on the phenotypic variation and genotypic population structure of protégé species.

Results/Conclusions

Results of a field experiment performed in grazed salt marshes show evidence for local adaptation of the protégé species Elytrigia atherica, and that facilitation by the nurse plant Juncus maritimus can act as a selective force for particular ecotypes of E. atherica. Our lab study indicates that facilitation can favour particular genotypes of the protégé species Brachypodium distachyon: the diploid type is less drought resistance than the polyploidy type and is therefore more dependent on the presence of a facilitating plant. Ongoing lab studies with mixed population of diploid and polyploid B. distachyon under low and high drought stress, with and without a facilitator, are being followed for several generations to determine to what degree the phenotypic variation and genotypic structure of the B. distachyon populations are changing over time. These studies indicate the potential of facilitation as driver of evolutionary adaptations in protégé species.