Despite spillover, a shared pathogen promotes native plant persistence in a cheatgrass-invaded grassland

Area of Science:

• Ecology
• Plant Pathology
• Invasive Species Management

Background:

• Pathogen spillover dynamics and their impact on host community diversity remain unclear.
• In western US grasslands, black fingers of death (Pyrenophora semeniperda) spills over from invasive cheatgrass (Bromus tectorum) to native squirreltail (Elymus elymoides).

Purpose of the Study:

• To test theoretical predictions of how spillover pathogens affect host coexistence using field data.
• To quantify the net effect of pathogen spillover on cheatgrass and squirreltail populations.

Main Methods:

• Parameterizing a population growth model with field data from Skull Valley, Utah.
• Analyzing species-specific transmission rates, susceptibility, and pathogen tolerance.

Main Results:

• The pathogen weakly benefits squirreltail and weakly costs cheatgrass, despite cheatgrass being the more tolerant reservoir host.
• Squirreltail's lower susceptibility and buffered population growth mitigate seed loss impacts.
• The shared pathogen promotes native grass persistence by reducing competition from invasive cheatgrass.

Conclusions:

• Pathogen spillover can counterintuitively benefit native species by suppressing invasive competitors.
• Reservoir hosts may not benefit from shared pathogens and can incur greater costs than non-reservoir hosts.
• Quantitative models are essential for understanding pathogen spillover consequences by assessing species-specific differences.