Apparent competition in canopy trees determined by pathogen transmission rather than susceptibility

Area of Science:

• Ecology
• Forest Pathology
• Invasive Species Biology

Background:

• Epidemiological theory posits that asymmetries in transmission, susceptibility, and mortality drive pathogen dynamics.
• Apparent competition occurs when a highly infectious host reduces populations of less infectious or susceptible hosts via a shared pathogen.

Purpose of the Study:

• To investigate if the invasive pathogen Phytophthora ramorum causes apparent competition among canopy trees.
• To determine if transmission differences among tree species drive sudden oak death severity in California coast redwood forests.

Main Methods:

• Path analysis was employed to assess the impact of inoculum-producing species on host mortality rates.
• The study examined the roles of transmission, susceptibility, and mortality in driving disease dynamics.

Main Results:

• Phytophthora ramorum causes apparent competition among canopy trees, with transmission differences driving disease severity.
• California bay laurel (Umbellularia californica), a prolific inoculum producer, significantly impacted tanoak (Lithocarpus densiflorus) mortality.
• Lack of negative impacts on bay laurel may lead to increased its density, creating positive feedback for the pathogen.

Conclusions:

• Invasive, generalist pathogens can rapidly alter forest canopy composition.
• Differences in pathogen transmission are more critical than host susceptibility in shaping apparent competition patterns.