Facilitation and adaptation: How do positive-neighbour interactions affect local adaptation to harsh abiotic conditions?

Background/Question/Methods Facilitation among plants is predicted to occur in environments characterized by high abiotic stress, where the presence of neighbours is thought to ameliorate environmental conditions (e.g. wind, temperature).  Recent studies confirm this prediction. However, despite advances in our understanding of the ecological effects of facilitation, its evolutionary consequences remain unexplored.  How does facilitation affect adaptation to harsh abiotic conditions?  If facilitation ameliorates abiotic conditions it could potentially weaken selection for abiotic tolerance in harsh environments, decreasing local adaptation.  Alternatively, by increasing population size facilitation might increase the strength of selection.  We constructed a 2-D cellular automata model, for a plant species exhibiting heritable variation for a continuous trait (z) that affected susceptibility to mortality due to abiotic conditions.  Abiotic conditions varied from mild (low mortality) to harsh (high mortality) along a linear gradient. Interactions with neighbours had either a negative (i.e. competitive) or positive (i.e. facilitative) impact on individual survival, the sign of which depended on an individual’s position along the abiotic gradient and the strength of which depended on the number of neighbouring plants.  Competition occurred where abiotic conditions were mild, while facilitation occurred where abiotic conditions were harsh.   

Results/Conclusions Results indicated that the presence of neighbours where abiotic conditions are harsh can weaken abiotic selection leading to an increase in the number of individuals with trait values adapted to positions on the abiotic gradient where growing conditions are less stressful. Facilitation thus slows the rate of adaptation to harsh abiotic conditions.  The range limit of the species (threshold level of abiotic harshness beyond which the species could not persist) reflected a balance between facilitation allowing individuals to survive where they otherwise could not, and selection against individuals poorly-adapted to harsh abiotic conditions.  The results suggest that variation in the strength of selection due to facilitation may have broader implications as a novel mechanism to explain why the rate of adaptation appears to slow at species range margins.