Exotics and Disturbance at Home and Abroad

JOSÉ HIERRO

Ascona, Suiza

Workshop; Colonization versus invasion: do the same traits matter? Towards a joint perspective in research on range expansion of native and biological invasion of non-indigenous plants; 2007

Disturbance is one of the most important factors promoting exotic plant invasion.  If disturbance per se is sufficient to explain exotic success, then “invasion” abroad should not differ from “colonization” at home.  Comparisons of the effects of disturbance on organisms in their native and introduced ranges are crucial to elucidate whether this is the case; however, such comparisons have not been conducted.  I investigated the effects of disturbance on the success of the Eurasian native Centaurea solstitialis (Asteraceae) in two invaded regions, California and central Argentina, and one native region, southern Turkey, by conducting field experiments consisting of simulating three different types of disturbance and adding locally collected C. solstitialis seeds.  I also conducted preliminary tests for differences among C. solstitialis genotypes in these three regions and the effects of local soil microbes on plant performance in greenhouse experiments.  Disturbance increased C. solstitialis abundance and performance far more in nonnative ranges than in the native range.  Centaurea solstitialis biomass and fecundity were similar among populations from all regions grown under common conditions.  Eurasian soil microbes suppressed growth of C. solstitialis plants, while Californian and Argentinean soil biota did not.  These results question the assumption that disturbance per se is sufficient to explain the remarkable dominance of some exotic invasives.  If these findings are general, then the common and powerful effects of disturbance must act in concert with other factors, allowing certain species to attain community dominance only where they occur as exotics.  Of the factors investigated up to now, escape from soil pathogens may contribute to this pattern.  In my current work, I am further investigating colonization versus invasion patterns by conducting additional biogeographical field experiments where disturbance, competition from local ruderals, insects, and fungi are manipulated.