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

Authors: SEABLOOM, ERIC - University Of Minnesota; BORER, ELIZABETH - University Of Minnesota; BUCKLEY, YVONNE - University Of Queensland; CLELAND, ELSA - University Of California; DAVIES, KENDI - University Of Colorado; FIRN, JENNIFER - Queensland University Of Technology; HARPOLE, STANLEY - Iowa State University; HAUTIER, YANN - University Of Zurich; LIND, ERIC - University Of Minnesota; MACDOUGALL, ANDREW - University Of Guelph; ORROCK, JOHN - University Of Wisconsin; PROBER, SUZANNE - Commonwealth Scientific And Industrial Research Organisation (CSIRO); ADLER, PETER - Utah State University; ALBERTI, JUAN - National University Of Mar Del Plata; ANDERSON, T. - Wake Forest University; BAKKER, JONATHAN - University Of Washington; BIEDERMAN, LORI - Iowa State University; Blumenthal, Dana; BROWN, CYNTHIA - Colorado State University; BRUDVIG, LARS - Michigan State University; CALDEIRA, MARIA - University Of Lisbon; CHU, CHENGJIN - Lanzhou University; CRAWLEY, MICHAEL - Imperial College; DALEO, PEDRO - National University Of Mar Del Plata; DAMSCHEN, ELLEN - University Of Wisconsin; D'ANTONIO, CARLA - University Of California; DECRAPPEO, NICOLE - Us Geological Survey (USGS); DICKMAN, CHRIS - University Of Sydney; DU, GUOZHEN - Lanzhou University; Fay, Philip; FRATER, PAUL - Iowa State University; GRUNER, DANIEL - University Of Maryland; HAGENAH, NICOLE - University Of Kwazulu-Natal; HECTOR, ANDREW - University Of Zurich; HELM, AVELIINA - University Of Tartu; HILLEBRAND, HELMUT - Carl von Ossietzky University Of Oldenburg; HOFMOCKEL, KIRSTEN - Iowa State University; HUMPHRIES, HOPE - University Of Colorado; IRIBARNE, OSCAR - National University Of Mar Del Plata; Jin, Virginia; KAY, ADAM - University Of St Thomas; KIRKMAN, KEVIN - University Of Kwazulu-Natal; KLEIN, JULIA - Colorado State University; KNOPS, JOHANNES - University Of Nebraska; LA PIERRE, KIMBERLY - Yale University; LADWIG, LAURA - New Mexico State University; LAMBRINOS, JOHN - Oregon State University; LEAKEY, ANDREW - University Of Illinois; LI, QI - Chinese Academy Of Sciences; LI, WEI - Chinese Academy Of Sciences; MCCULLEY, REBECCA - University Of Kentucky; MELBOURNE, BRETT - University Of Colorado; MITCHELL, CHARLES - University Of North Carolina; MOORE, JOSLIN - University Of Melbourne; MORGAN, JOHN - La Trobe University; MORTENSEN, BRENT - Iowa State University; O'HALLORAN, LYDIA - Oregon State University; PARTEL, MEELIS - University Of Tartu; PASCUAL, JESUS - National University Of Mar Del Plata; PYKE, DAVID - Us Geological Survey (USGS); RISCH, ANITA - Swiss Federal Institute; SALGUERO-GOMEZ, ROBERTO - University Of Queensland; SANKARAN, MAHESH - Gujarat Agricultural University; SCHUETZ, MARTIN - Swiss Federal Institute; SIMONSEN, ANNA - University Of Toronto; SMITH, MELINDA - Yale University; STEVENS, CARLY - Lancaster University; SULLIVAN, LAUREN - Iowa State University; WARDLE, GLENDA - University Of Sydney; WOLKOVICH, ELIZABETH - University Of San Diego; WRAGG, PETER - University Of Minnesota; WRIGHT, JUSTIN - Duke University; YANG, LOUIE - University Of California

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2013
Publication Date: 10/16/2013
Citation: Seabloom, E., Borer, E.T., Buckley, Y., Cleland, E., Davies, K., Firn, J., Harpole, S., Hautier, Y., Lind, E., Macdougall, A., Orrock, J.L., Prober, S.M., Adler, P., Alberti, J., Anderson, T.M., Bakker, J.D., Biederman, L.A., Blumenthal, D.M., Brown, C.S., Brudvig, L.A., Caldeira, M., Chu, C., Crawley, M.J., Daleo, P., Damschen, E.I., D'Antonio, C.M., Decrappeo, N.M., Dickman, C.R., Du, G., Fay, P.A., Frater, P., Gruner, D.S., Hagenah, N., Hector, A., Helm, A., Hillebrand, H., Hofmockel, K.S., Humphries, H.C., Iribarne, O., Jin, V.L., Kay, A., Kirkman, K.P., Klein, J.A., Knops, J.M., La Pierre, K.J., Ladwig, L.M., Lambrinos, J.G., Leakey, A.D., Li, Q., Li, W., McCulley, R., Melbourne, B., Mitchell, C.E., Moore, J.L., Morgan, J., Mortensen, B., O'Halloran, L.R., Partel, M., Pascual, J., Pyke, D.A., Risch, A.C., Salguero-Gomez, R., Sankaran, M., Schuetz, M., Simonsen, A., Smith, M., Stevens, C., Sullivan, L., Wardle, G.M., Wolkovich, E.M., Wragg, P.D., Wright, J., Yang, L. 2013. Predicting invasion in grassland ecosystems: Is exotic dominance the real embarrassment of richness? Global Change Biology. 19:3677-3687.

Interpretive Summary: Introduced exotic plant species comprise 20-80% of some plant communities, and can cause large losses in the abilities of ecosystems to provide crucial goods and services. However, not all systems become dominated by exotic species, and the reasons for their uneven distributions remain unclear. This study measured exotic plant species numbers (richness) and abundance (cover) in 54 grasslands from nine countries on four continents. We compared exotic species abundance with several factors that could influence abundance, including climate, and native species productivity and diversity. The study shows that exotic abundance is most strongly associated with geographic regions, and that species abundance was a better predictor than richness of exotic species impacts on ecosystems. Future studies of exotic species impacts should focus on their degree of dominance of the plant communities rather than on numbers of species.

Technical Abstract: For two centuries there has been a perception that while exotic species are dominant in many areas, others remain largely unaffected. This unquantified observation suggests a fundamental ecological question: why do exotics dominate some locations and not others? While invasions are clearly important globally, there have been no globally-replicated studies that measure exotic dominance. Rather, there has been a focus on exotic richness, because of the lack of standardized data on exotic abundance. We measured the richness and cover of exotic plant species in 54 grass-dominated ecosystems in nine countries on four continents, ranging from salt marshes to alpine tundra. We found that exotic richness was a poor proxy for exotic dominance at low levels of exotic richness, because exotic richness cannot resolve sites that had a few unimportant invaders (low exotic richness and cover) and those that were dominated by a few highly abundant exotics (low exotic richness and high exotic cover). We also quantified a bimodal pattern of invasion, with nearly all sites either dominated by exotic species or largely uninvaded, and few had an even mix of natives and exotics. Exotic dominance varied mostly among regions (subcontinents) and ecosystems with little variation among sites and blocks nested with ecosystem types. Exotic species were most dominant in areas that had low native grass richness at the site or regional scale suggesting a role for biotic resistance, as grasses comprise the most aggressive invaders. This work underscores the need to move beyond richness as a measure of exotic impact.