Image: Screenshot from USDA PLANTS database Lonicera maackii profile.
What factors cause exotic species to displace natives species?
Delving into this question, ecologists have suggested that both direct competition for resources and asymmetrical susceptibility to consumers may be important drivers of community change following an introduction. Exotic and native species may interact indirectly, displaying competitive effects due to a shared consumer (i.e. apparent competition). Of course, direct and indirect competitive forces are not mutually exclusive, and both may be important in a given system. For this reason, Orrock et al. (2015) Ecology designed a randomized factorial design experiment to simultaneously evaluate direct competitive and consumptive effects, and apparent competition between native plants and the invasive shrub Lonicera maackii.
Lonicera maackii is native to Asia and has been introduced throughout much of eastern North America. Depending on which U.S. state you’re in, L. maackii is unregulated or considered to be a noxious weed.
This table from Orrock et al. (2015) nicely describes some mechanistic hypotheses for how direct or indirect competition might occur between native plants and L. maackii, plus it proposes experimental tests for the mechanism along with its predicted outcome.
They set up an experiment to test these hypothesized mechanisms in 30 x 30 meter plots at the Busch Wildlife Conservation Area in Missouri, USA. Exclosures were 20 cm radius canisters that kept out deer and rodents, or just deer, depending on mesh sizes.
They found large direct effects of vegetation removal (which they hypothesize is due to competition for light), and from exclosures (both deer and rodents eat the native plants), and they found evidence for apparent competition by way of “food subsidies” from L. maackii berries. The main results are presented in Table 2. Interactions terms for (consumers) x (competitors) would be consistent with apparent competition, and only the one interaction term was significant, (fruit) x (consumer exclosure).
Interestingly, there was no significant effect of the consumers on the exotics! This asymmetry may be particularly important in allowing the spread of L. maackii.
This asymmetry is consistent with the novel weapons hypothesis, which reminded me of a seminar I saw last week by Eric Lind. In it he presented some work he did for this PLOS ONE publication, where they did not find consistent evidence for the novel weapons hypothesis. One thing they did find, however, was that herbivores tended to eat the more common plants (Fig 1 from Lind et al. (2010)). Percent cover was a good predictor of whether herbivores ate the plant, regardless of native status.
In Orrock et al. (2015), the top 5 most prevalent plant taxa, which accounted for >50% of all individual plants were Carex spp., Geum spp., Agaritina altissima, Sanicula spp., and Cardamine pensylvanica, all of which are native.