Influence of insects and fungal pathogens on individual and population parameters of <Emphasis Type="Italic">Cirsium arvense</Emphasis> in its native and introduced ranges |
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Authors: | Michael G Cripps Graeme W Bourdôt David J Saville Hariet L Hinz Simon V Fowler Grant R Edwards |
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Institution: | 1.Bio-Protection Research Centre,Lincoln University,Lincoln,New Zealand;2.AgResearch Ltd, Lincoln,Christchurch,New Zealand;3.Saville Statistical Consulting Ltd,Lincoln,New Zealand;4.CABI Europe-Switzerland,Delémont,Switzerland;5.Landcare Research,Lincoln,New Zealand;6.Agriculture and Life Sciences Faculty,Lincoln University,Lincoln,New Zealand |
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Abstract: | Introduced weeds are hypothesized to be invasive in their exotic ranges due to release from natural enemies. Cirsium arvense (Californian, Canada, or creeping thistle) is a weed of Eurasian origin that was inadvertently introduced to New Zealand
(NZ), where it is presently one of the worst invasive weeds. We tested the ‘enemy release hypothesis’ (ERH) by establishing
natural enemy exclusion plots in both the native (Europe) and introduced (NZ) ranges of C. arvense. We followed the development and fate of individually labelled shoots and recorded recruitment of new shoots into the population
over two years. Natural enemy exclusion had minimal impact on shoot height and relative growth rate in either range. However,
natural enemies did have a significant effect on shoot population growth and development in the native range, supporting the
ERH. In year one, exclusion of insect herbivores increased mean population growth by 2.1–3.6 shoots m−2, and in year two exclusion of pathogens increased mean population growth by 2.7–4.1 shoots m−2. Exclusion of insect herbivores in the native range also increased the probability of shoots developing from the budding
to the reproductive growth stage by 4.0× in the first year, and 13.4× in the second year; but exclusion of pathogens had no
effect on shoot development in either year. In accordance with the ERH, exclusion of insect herbivores and pathogens did not
benefit shoot development or population growth in the introduced range. In either range, we found no evidence for an additive
benefit of dual exclusion of insects and pathogens, and in no case was there an interaction between insect and pathogen exclusion.
This study further demonstrates the value of conducting manipulative experiments in the native and introduced ranges of an
invasive plant to elucidate invasion mechanisms. |
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