title: Experimental reduction in interaction intensity strongly affects biotic selection.
abstract: The link between biotic interaction intensity and strength of selection is of fundamental interest for understanding biotically driven diversification and predicting the consequences of environmental change. The strength of selection resulting from biotic interactions is determined by the strength of the interaction and by the covariance between fitness and the trait under selection. When the relationship between trait and absolute fitness is constant, selection strength should be a direct function of mean population interaction intensity. To test this prediction, we excluded pollinators for intervals of different length to induce five levels of pollination intensity within a single plant population. Pollen limitation (PL) increased from 0 to 0.77 across treatments, accompanied by a fivefold increase in the opportunity for selection. Trait-fitness covariance declined with PL for number of flowers, but varied little for other traits. Pollinator-mediated selection on plant height, corolla size, and spur length increased by 91%, 34%, and 330%, respectively, in the most severely pollen-limited treatment compared to open-pollinated plants. The results indicate that realized biotic selection can be predicted from mean population interaction intensity when variation in trait-fitness covariance is limited, and that declines in pollination intensity will strongly increase selection on traits involved in the interaction.
authors.affiliation: Plant Ecology and Evolution, Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18 D, SE-752 36, Uppsala, Sweden.
authors.forename: Nina
authors.initials: N
authors.lastname: Sletvold
authors.affiliation: Plant Ecology and Evolution, Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18 D, SE-752 36, Uppsala, Sweden.
authors.forename: Jon
authors.initials: J
authors.lastname: Ågren
mesh_terms: D000818:Animals; D017753:Ecosystem; D007915:Lepidoptera / Q000502:physiology*; D029595:Orchidaceae / Q000235:genetics* / Q000502:physiology*; D011058:Pollen / Q000502:physiology*; D054817:Pollination; D012641:Selection, Genetic
keywords: 
  Gymnadenia conopsea
  ; floral evolution; natural selection; opportunity for selection; plant-animal interactions; pollen limitation; pollinator-mediated selection; selection intensity; trait-fitness covariance