title: A comprehensive test of evolutionarily increased competitive ability in a highly invasive plant species.
abstract: OBJECTIVE
  A common hypothesis to explain plants' invasive success is that release from natural enemies in the introduced range selects for reduced allocation to resistance traits and a subsequent increase in resources available for growth and competitive ability (evolution of increased competitive ability, EICA). However, studies that have investigated this hypothesis have been incomplete as they either did not test for all aspects of competitive ability or did not select appropriate competitors.
  
  
  METHODS
  Here, the prediction of increased competitive ability was examined with the invasive plant Lythrum salicaria (purple loosestrife) in a set of common-garden experiments that addressed these aspects by carefully distinguishing between competitive effect and response of invasive and native plants, and by using both intraspecific and interspecific competition settings with a highly vigorous neighbour, Urtica dioica (stinging nettle), which occurs in both ranges.
  
  
  RESULTS
  While the intraspecific competition results showed no differences in competitive effect or response between native and invasive plants, the interspecific competition experiment revealed greater competitive response and effect of invasive plants in both biomass and seed production.
  
  
  CONCLUSIONS
  The use of both intra- and interspecific competition experiments in this study revealed opposing results. While the first experiment refutes the EICA hypothesis, the second shows strong support for it, suggesting evolutionarily increased competitive ability in invasive populations of L. salicaria. It is suggested that the use of naturally co-occurring heterospecifics, rather than conspecifics, may provide a better evaluation of the possible evolutionary shift towards greater competitive ability.
authors.affiliation: Plant Ecology Group, University of Tübingen, Auf der Morgenstelle 5, 72076 Tübingen, Germany.
authors.forename: Srijana
authors.initials: S
authors.lastname: Joshi
authors.affiliation: Plant Ecology Group, University of Tübingen, Auf der Morgenstelle 5, 72076 Tübingen, Germany michal.gruntman@bot.uni-tuebingen.de.
authors.forename: Michal
authors.initials: M
authors.lastname: Gruntman
authors.affiliation: Plant Ecology Group, University of Tübingen, Auf der Morgenstelle 5, 72076 Tübingen, Germany.
authors.forename: Mark
authors.initials: M
authors.lastname: Bilton
authors.affiliation: Mitrani Department of Desert Ecology, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, 84990, Israel.
authors.forename: Merav
authors.initials: M
authors.lastname: Seifan
authors.affiliation: Plant Ecology Group, University of Tübingen, Auf der Morgenstelle 5, 72076 Tübingen, Germany.
authors.forename: Katja
authors.initials: K
authors.lastname: Tielbörger
mesh_terms: D005075:Biological Evolution; D058865:Introduced Species; D016014:Linear Models; D031561:Lythrum / Q000254:growth & development / Q000502:physiology*; D013045:Species Specificity
keywords: Competitive effect; EICA; Lythrum salicaria; Urtica dioica; competitive response; evolution of increased competitive ability; interspecific competition; intraspecific competition; invasive species; purple loosestrife; stinging nettle