Predicting invasion in grassland ecosystems: is exotic dominance the real embarrassment of richness?

Invasions have increased the size of regional species pools, but are typically assumed to reduce native diversity. Invasions have increased the size of regional species pools, but are typically assumed to reduce native diversity. However, global-scale tests of this assumption have been elusive because of the focus on exotic species richness, rather than relative abundance. This is problematic because low invader richness can indicate invasion resistance by the native community or, alternatively, dominance by a single exotic species. Here, we used a globally replicated study to quantify relationships between exotic richness and abundance in grass-dominated ecosystems in 13 countries on six continents, ranging from salt marshes to alpine tundra. We tested effects of human land use, native community diversity, herbivore pressure, and nutrient limitation on exotic plant dominance. Despite its widespread use, exotic richness was a poor proxy for exotic dominance at low exotic richness, because sites that contained few exotic species ranged from relatively pristine (low exotic richness and cover) to almost completely exotic-dominated ones (low exotic richness but high exotic cover). Both exotic cover and richness were predicted by native plant diversity (native grass richness) and land use (distance to cultivation). Although climate was important for predicting both exotic cover and richness, climatic factors predicting cover (precipitation variability) differed from those predicting richness (maximum temperature and mean temperature in the wettest quarter). Herbivory and nutrient limitation did not predict exotic richness or cover. Exotic dominance was greatest in areas with low native grass richness at the site- or regional-scale. Although this could reflect native grass displacement, a lack of biotic resistance is a more likely explanation, given that grasses comprise the most aggressive invaders. These findings underscore the need to move beyond richness as a surrogate for the extent of invasion, because this metric confounds monodominance with invasion resistance. Monitoring species' relative abundance will more rapidly advance our understanding of invasions.Fil: Seabloom, Eric W.. University of Minnesota; Estados UnidosFil: Borer, Elizabeth T.. University of Minnesota; Estados UnidosFil: Buckley, Yvonne. The University Of Queensland; AustraliaFil: Cleland, Elsa E.. University of California, San Diego; Estados UnidosFil: Kendi, Davies. State University of Colorado Boulder; Estados UnidosFil: Firn, Jennifer. Queensland University of Technology. Brisbane; AustraliaFil: Hartpole, Stanley W.. Iowa State University; Estados UnidosFil: Hautier, Yann. University of Zurich; SuizaFil: Lind, Eric. Universidad de Minnesota; Estados UnidosFil: MacDougall, Andrew. University of Guelph; CanadáFil: Orrock, John L.. University of Wisconsin; Estados UnidosFil: Prober, Suzanne M.. CSIRO Ecosystem Sciences; AustraliaFil: Adler, Peter. Utah State University; Estados UnidosFil: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Anderson, T. Michael. Wake Forest University; Estados UnidosFil: Bakker, Jonathan D.. University of Washington; Estados UnidosFil: Biederman, Lori A.. Iowa State University; Estados UnidosFil: Blumenthal, Dana. Rangeland Resources Research Unit; Estados UnidosFil: Browns, Cynthia S.. State University of Colorado - Fort Collins; Estados UnidosFil: Brudvig, Lars A.. Michigan State University; Estados UnidosFil: Caldeira, Maria. Technical University of Lisbon; PortugalFil: Chu, Chengjin. Lanzhou University; ChinaFil: Crawley, Michel. Imperial College London; Reino UnidoFil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Damschen, Ellen I.. University of Wisconsin; Estados UnidosFil: D'antonio, Carla. University of California; Estados UnidosFil: Decrappeo, Nicole. Geological Survey Forest and Rangeland Ecosystem Science Center; Estados UnidosFil: Dickman, Chris. University of Sydney; AustraliaFil: Du, Guozhen. Lanzhou University; ChinaFil: Fay, Philip. Grassland Soil and Water Research Lab; Estados UnidosFil: Frater, Paul. Iowa State University; Estados UnidosFil: Gruner, Daniel S.. University of Queensland; AustraliaFil: Hagenah, Nicole. University of KwaZulu-Natal; SudáfricaFil: Hector, Andrew. University of Zurich; SuizaFil: Helm, Avellina. University Of Tartu.; EstoniaFil: Hillebrand, Helmut. Carl-von-Ossietzky University; AlemaniaFil: Hofmockel, Kirsten. Iowa State University; Estados UnidosFil: Humphries, Hope. State University of Colorado - Fort Collins; Estados UnidosFil: Iribarne, Oscar Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Jin, Virginia L.. Agroecosystem Management Research Unit; Estados UnidosFil: Kay, Adam. University of St. Thomas; Estados UnidosFil: Kirkman, Kevin. University of KwaZulu-Natal; EstoniaFil: Klein, Julia. State University of Colorado - Fort Collins; Estados UnidosFil: Knops, Johannes M. H.. University Of Nebraska; Estados UnidosFil: La Pierre, Kimberly. Yale University; Estados UnidosFil: Ladwig, Laura M.. University Of New Mexico; Estados UnidosFil: Lambrinos, John G.. Oregon State University; Estados UnidosFil: Leakey, Andrew. University of Illinois at Urbana-Champaign; Estados UnidosFil: Li, Qi. Northwest Institute of Plateau Biology, Chinese Academy of Sciences; ChinaFil: Li, Wei. Southwest Forestry University; ChinaFil: McCulley, Rebecca. University Of Kentucky; Estados UnidosFil: Melbourne, Brett. State University of Colorado Boulder; Estados UnidosFil: Mitchell, Charles. University of North Carolina; Estados UnidosFil: Moore, Joslin. Australian Research Centre for Urban Ecology; AustraliaFil: Morgan, John. La Trobe University; AustraliaFil: Mortensen, Brent. Iowa State University; Estados UnidosFil: O'Halloran, Lydia. Oregon State University; Estados UnidosFil: Pärtel, Meelis. University Of Tartu.; EstoniaFil: Pascual, Jesus Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; ArgentinaFil: Pyke, David A.. University of California; Estados UnidosFil: Risch, Anita. Swiss Federal Institute For Forest, Snow And Landscape Research; SuizaFil: Salguero Goméz, Roberto. The University Of Queensland; AustraliaFil: Sankaran, Mahesh. National Centre for Biological Sciences; IndiaFil: Schuetz, Martin. Swiss Federal Institute For Forest, Snow And Landscape Research; SuizaFil: Simonsen, Anna. University of Toronto; CanadáFil: Smith, Melinda. State University of Colorado - Fort Collins; Estados UnidosFil: Stevens, Carly. Lancaster Environment Center; CanadáFil: Sullivan, Lauren. Iowa State University; Estados UnidosFil: Wardle, Glenda M.. University of Sydney; AustraliaFil: Wolkovich, Elizabeth M.. University of British Columbia; CanadáFil: Wragg, Peter D.. The University Of Queensland; AustraliaFil: Wright, Justin. University of Duke; Estados UnidosFil: Yang, Louie. University of California; Estados Unido