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Pathogens and Plant Invasion Ecology What do invasive plants have to do with us? What do invasive plants have to do with us? • Framework for study of pathogen invasions What do invasive plants have to do with us? • Framework for study of pathogen invasions • Pathogens’ role in plant invasions What do invasive plants have to do with us? • Framework for study of pathogen invasions • Pathogens’ role in plant invasions • Mechanism for introduction of pathogens Invasive species: The second biggest threat to biodiversity today. (Vitousek et al. 1997) Photo: Charles Webber, California Academy of Sciences Ecosystem Effects • Soil chemistry/nutrient cycling Ecosystem Effects • Soil chemistry/nutrient cycling • Fire frequency & intensity Ecosystem Effects • Soil chemistry/nutrient cycling • Fire frequency & intensity • Hydrology & sedimentation Ecosystem Effects • Soil chemistry/nutrient cycling • Fire frequency & intensity • Hydrology & sedimentation • Erosion Effects on Biota • Displacement of native species – E.g., ice plant and native shrubs Effects on Biota • Displacement of native species – E.g., ice plant and native shrubs • Hybridization with native species – E.g., Spartine alterniflora and S. foliosa Effects on Biota • Displacement of native species – E.g., ice plant and native shrubs • Hybridization with native species – E.g., Spartine alterniflora and S. foliosa • Promote (or introduce) non-native animals & microbes – E.g., Myrica faya in Hawaii What makes a species invasive? • “The ability to increase when rare.” (Crawley 1997) What makes a species invasive? • “The ability to increase when rare.” (Crawley 1997) • …Well, duh. So what makes a species likely to be a problem? A cautionary tale: Baker’s traits and USDA GMO deregulation • Keeler (1989) suggests using Baker’s traits as predictor of weed risk in GM crops • Williamson (1994) proves this method has little/no predictive value • As of 1997, APHIS continues to accept list as only evidence to discount ecological risk in petitions to deregulate crops Invasible Ecosystems • Disturbed areas • Ports of entry • Community composition/diversity Pathogens’ Role • Natural Enemies Hypothesis: plants leave enemies behind; are better competitors Pathogens’ Role • Natural Enemies Hypothesis: plants leave enemies behind; are better competitors • Biotic Resistance Hypothesis: native pathogens prevent new plants from establishing The Enemy Release Hypothesis Problems with Empirical Model • Beneficial associations: – Natives – Exotics • Biotic resistance Gilbert & Parker, UCSC Photos: I.M. Parker, UCSC California Coastal Clovers • 7 exotic, 9 native • Beneficial association with fungal endophytes in 2 most invasive • Leaf spot (Lesptosphaerulina, Pseudopeziza, Phoma) most severe on exotics (BR) • Leaf necrosis (Stemphylium) slightly more severe on natives (NE) • Damping-off most severe in natives (NE) Introduced plants can bring their pathogens • Chestnut Blight • Dutch Elm Disease • Phytophthora infestans