Out of Place and Thriving: Exploring the Effects of Underground Natural Enemies on a Global Plant Invader

Introduced plants have followed in the footsteps of humans throughout the globe and are often accompanied by impacts on native ecosystems.

The factors that influence the spread of introduced plants may result from novel ecological interactions or evolutionary processes plants undergo outside their native range.

Soil microbes can act as natural enemies that keep populations of invasive species in check in their native regions, but also as beneficial symbionts in non-native regions that can fuel invasive spread.

In these studies, I tested how soil microbes influence the worldwide success of a highly invasive weed, Centaurea solstitialis (yellow starthistle).

I tested whether Centaurea from invasive populations grow more vigorously than from native or non-invasive populations and how soil microbes affect Centaurea performance throughout its global distribution.

To address these topics, I conducted greenhouse experiments using seeds and soils collected from populations spanning Centaurea's global distribution.

I found no support for the possibility of evolutionary changes that enhance Centaurea's competitiveness, but soil microbes had strong negative effects on plants from all regions.

There was a release from natural enemy pressure in some invasive populations, but patterns did not conform to the Enemy Release Hypothesis.

Rather, I observed global geographic mosaics of plant-soil microbe interactions that varied in their strength and influence on Centaurea invasions.

Negative feedbacks were particularly strong in soils from Argentina and California where Centaurea is spreading most rapidly.

I conclude that biogeographic mosaics of species interactions can be important processes that influence the global variability in performance of introduced plants.