Abstract
Both competitors and natural enemies can limit plant population growth. However, demographic comparisons of the effects of these interactions on introduced versus co‐occurring, related native species are uncommon. We asked: (1) does plant competition, insect herbivory, or their combination reduce population growth rate, log λ, of the Eurasian thistle Cirsium vulgare sufficiently to explain its limited invasiveness in western tallgrass prairie; and (2) how do the effects of these interactions compare to those for C. altissimum, its co‐occurring, synchronously‐flowering native congener? We developed integral projection models (IPMs) to estimate log λ for both species, using parameter estimates from field experiments. Our models predicted that the growth potential (growth rate at minimal competition and herbivory) for the introduced thistle (log λ = 3.5 (2.5, 4.6)) was twice as large as for the native thistle (log λ = 1.6 (0.4, 3.1)); however, a high level of competition and ambient insect herbivory together reduced log λ to similar values for both thistle species (C. vulgare: log λ = −1.3 (−2.4, −0.3) vs C. altissimum: log λ = −0.9 (−1.4, −0.3)). This suggests that the introduced thistle was more affected by competition and insect herbivory. For the introduced thistle, neither competition nor insect herbivory alone led to negative log λ. In contrast, for the native thistle, high competition alone also led to negative population growth (log λ = −0.8, percentile limits do not overlap with zero). Ambient herbivory alone prevented the spread for both thistle species (percentile limits include zero). Overall, the results show that interspecific competition followed by ambient levels of insect herbivory strongly constrained log λ for both thistles, limiting C. vulgare invasiveness and C. altissimum abundance. The outcome highlights the importance of synergy between the two biological interactions in limiting plant population growth. Improved understanding of mechanisms limiting log λ for weedy plants enhances our ability to predict when biotic resistance will contribute to invasive plant species management.
Highlights
Most plant introductions fail, and species that do establish typically remain at low densities (Rejmanek 1989, Williamson and Fitter 1996, Mack et al 2000)
Using integral projection models (IPMs), we addressed two specific questions: (1) does competition from the resident plant community, herbivory by native insects, or their interaction significantly reduce the population growth rate of the introduced C. vulgare and (2) how do the effects of these biotic interactions on C. vulgare compare to their effects on the co-occurring native C. altissimum? Given the limited occurrence of C. vulgare in our region (Andersen and Louda 2008), we expected that population growth rates of the introduced C. vulgare would be impacted more severely by resident competitors and native insect herbivores than the population growth rates of the related native thistle
Studies show that 96% of the native thistle-feeding arthropod fauna found on the native C. altissimum, including the most abundant thistle specialists, feed on C. vulgare (Takahashi 2006)
Summary
Species that do establish typically remain at low densities (Rejmanek 1989, Williamson and Fitter 1996, Mack et al 2000). This is surprising because introduced species generally leave their coevolved competitors and natural enemies behind. On the other hand, introduced species inevitably encounter novel competitors and natural enemies in their new habitats (Mitchell et al 2006). Some of these new antagonists may be pre-adapted, or evolve rapidly, to compete with or to consume the introduced plants (Maron et al 2004, Parker and Hay 2005). Elton’s ‘‘biotic resistance hypothesis’’ posits that native communities often limit the colonization, establishment, and spread of introduced species through intense antagonistic interactions (Crawley 1997, Keane and Crawley 2002)
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