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