Journal of Plant Ecology Advance Access originally published online on May 22, 2009
Journal of Plant Ecology 2009 2(2):77-86; doi:10.1093/jpe/rtp008
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Enhanced fitness due to higher fecundity, increased defence against a specialist and tolerance towards a generalist herbivore in an invasive annual plant
Institute of Environmental Sciences, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
* Correspondence address. Institute of Environmental Sciences, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. Tel: +41-44-635-4747, Fax: +41-44-635-5711; E-mail: joshi{at}uwinst.uzh.ch
Aims: The superior performance of many non-indigenous species in a new range can be attributed to different factors such as pre-adaptation to environmental conditions in new areas or to factors inherent to displacement mechanisms such as loss of co-evolved pathogens and herbivores that increase the speed of evolutionary change towards a shift in allocation from defence to growth and reproduction. To assess the importance of the different mechanisms governing the success of Conyza canadensis, a globally successful invader, we simultaneously tested several recent hypotheses potentially explaining the factors leading to biological invasion.
Methods: We tested (i) whether plants from the non-native range showed a higher fitness than plants from the native North American range, (ii) whether they differed in resistance against an invasive generalist herbivore, the slug Arion lusitanicus and against a recently established specialist aphid herbivore, Uroleucon erigeronense and (iii) experimentally assessed whether C. canadensis releases allelopathic chemicals that have harmful effects on competing species in the non-native range. We compared populations along a similar latitudinal gradient both in the native North American and invasive European range and analysed patterns of adaptive clinal variation in biomass production.
Important Findings: The invasion success of C. canadensis in Europe cannot be attributed to a single trait, but to a combination of factors. Invasive plants benefited from increased growth and above all, increased reproduction (a key trait in an annual plant) and were less attacked by a co-migrated specialist enemy. The observed loss of defence against generalist slugs did not translate into a decreased fitness as invasive C. canadensis plants showed a high re-growth potential. In contrast to earlier in vitro studies, we detected no allelopathic effects on the competing flora in the non-native range. The latitudinal cline in vegetative biomass production in the non-native range observed in our common garden study indicates a high adaptive potential. However, only further genetic studies will provide conclusive evidence whether the differentiation in the non-native range is caused by pre-adaptation and sorting-out processes of putatively repeatedly introduced populations of this composite, long-distance disperser with highly volatile seeds or evolved de novo as a rapid response to new selection pressures in the non-native range.
Keywords: allelopathy • biological invasions • Conyza canadensis • EICA hypothesis • generalist/specialist herbivory • latitudinal clines • biogeographic comparison • common garden
1 Present address: Institute of Biochemistry and Biology, Biodiversity Research/Botany, University of Potsdam, Maulbeerallee 1, 14469 Potsdam, Germany.