Study rejects widely-held belief explaining invasive plant success

Woolly bear caterpillar (Pyrrharctia isabella) Credit, IronChris.A recent study published in the Public Library of Science by Eric Lind and John Parker raises doubts about a commonly accepted hypothesis explaining the success of exotic plants in their non-native ranges.

Invasive species have long posed a threat to biotic systems, outcompeting native species and threatening the productivity of many agricultural and forestry crops.

The oft made hypothesis regarding the success of non-native species is that when these species leave their home range, they leave their coevolved predators behind as well, allowing them to proliferate without the natural curb on population growth.

A key question, however, is how these invasive species are able to avoid predation and parasitism by organisms in their introduced range.

This study sought to assess the validity of the novel weapons hypothesis, which argues that native predators are unable to cope with exotic plants’ biochemical defenses because they have simply not had enough time to adapt to do so.

Lind and Parker analyzed the chemical deterrence power of 19 invasive and 21 native plant species in the face of predation by a native generalist herbivore, the woolly bear caterpillar, P. isabella.

It was found that there was no significant pattern of deterrence by invasive or native chemical defenses, suggesting that the novel weapons hypothesis does not fully explain the success of invasive plants due to “evolutionary mismatches”.

The authors suggest that invasive and native species may be similar in their deterrence power and overall ecophysiological traits due to the ecological constraints of their shared environment.

Thus, it may be the case that biochemical novelty is not the determinant of invasion success, but instead that “fundamentally similar processes may promote the ecological success of both native and exotic species”.

A major strategy in the management of invasive species has been the introduction of exotic predators once found in the target species’ native ecosystem, thought to have evolved to effectively reduce the population growth of their prey.

However, if the concept of “evolutionary mismatches” between predator and prey lacks support, it may be time to rethink our weapons strategies against these exotic enemies.

--by Alexis Mychajliw

Lind, E., & Parker, J. (2010). Novel Weapons Testing: Are Invasive Plants More Chemically Defended than Native Plants? PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010429