The Evolutionary Ecology of Facilitation: A Case Study Using Potentilla diversifolia

Abstract

Facilitation occurs when the presence of one or more neighbours ameliorates abiotic conditions for a focal individual, leading to an increase in its absolute fitness. Facilitation is strong and widespread among sessile organisms such as plants. Nevertheless, the linkage between facilitation and patterns at the individual, population and species-level have only recently become subjects of investigation. My research sought to understand the role of facilitation as an agent of natural selection, and what, if any, consequences the amelioration of abiotic conditions by neighbours has for evolutionary processes such as local adaptation. Plant ecology and evolution along elevation gradients are well understood and form an excellent basis upon which to examine the ecological and evolutionary consequences of facilitation. Therefore, my studies considered Potentilla diversifolia, an alpine herbaceous perennial found in North America. A neighbour-removal experiment demonstrated that P. diversifolia experiences strongest facilitation at lower elevations within its range where the presence of neighbours reduces chronic heat stress. This association conflicts with conclusions of previous findings that the likelihood and strength of facilitation among alpine plants increases with elevation. A phenotypic selection experiment examined how the presence of neighbours affects the strength and direction of selection on stem length, leaf number, leaflet length and trichome density at different elevations. Neighbourhood significantly affected individual plant performance, but only stem length experienced directional selection, and this could not be clearly attributed to the effect of facilitation. Initial results from an ongoing reciprocal transplant experiment revealed that P. diversifolia is locally adapted to conditions at different elevations, and that the presence of neighbours appears to weaken the magnitude of local adaptation, especially at lower elevations where facilitation is the dominant interaction. Finally, an individual-based simulation model demonstrated that stable range limits could develop even where facilitation is the dominant interaction. The research presented here highlights the importance of considering species traits, and not just an individual’s location along an environmental gradient, when trying to understand the ecological and evolutionary consequences of facilitation. The results demonstrate that strong individual-level facilitation may not translate into significant consequences at the population- or species-level.