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Altered Size, Fecundity and Herbivore Defense in Introduced versus Native St. John's Wort (Hypericum perforatum): Evolution or Plasticity?

Saturday, April 13, 2002 - 11:00am - 11:50am
Keller 3-180
John Maron (University of Montana)
Joint work with Montserrat Vila (Centre de Recerca Ecol�gica i Aplicacions Forestals, Universitat Aut�noma de Barcelona, 08193 Bellaterra, Spain) and J.T. Arnason (Department of Biology, University of Ottawa, Box 450, Station A, Ottawa, Ontario, Canada).

My research explores two issues concerning the evolutionary consequences of plant invasions: first, do introduced plants evolve greater size or fecundity compared with native conspecifics? Second, do introduced plants gain or lose herbivore resistance depending on whether or not they have been exposed to biological control? To address these questions, we have initiated common gardens in Washington and Spain containing St. John's Wort (Hypericum perforatum) from three regions: the native range in Europe, the introduced range in central North America where populations have been long liberated from natural enemies, and the introduced range in western North America where populations have been exposed to over 55 years of successful biological control. For the last two years we have examined how size, fecundity and herbivore defense varies depending on region of plant origin. Across both gardens, introduced and native plants were not consistently different in size. In the Washington garden, introduced plants from western North America produced significantly more seed capsules than native plants from Europe. Central North American and European plants did not differ significantly in fecundity. In the Spain garden, in 2000 results were similar to those found in Washington. In 2001, pathogens killed many plants before they set seed in Spain. Not only were a greater fraction of individuals from western North America killed than plants from the other two regions, but those western North American plants that died produced significantly more seed capsules in 2000 than surviving western North American plants. Because these high fecundity western North American plants were eliminated, there were no differences in fecundity between introduced and native plants in year two. In both gardens, introduced plants from both regions of North America contained substantially lower levels of the defensive secondary compound hypericin than did native plants. On-going work is evaluating how this variation in plant defensive chemistry influences plant resistance to a variety of pathogens and a widespread biocontrol agent, Chyrsolina quadrigemina.