Freddy Krueger of plants???????????????????????????????????????????????? So said the National Post on August 17, 2000.
Early last summer there was a day (there were many days, if the truth be told) when I was standing overwhelmed by the
noise music in the garden – the sound of bees, lots of them. We have purple loosestrife growing in areas of the garden and they were absolutely covered. Sometimes, if you stand really, really still in the garden, you hear it whisper to you. This time it whispered: “Thank you for the purple loosestrife. It makes up a bit for all that you have taken from us.”
I first remember hearing about purple loosestrife as an invasive species that crowds out native species back in the mid-80s. Yet in the 25 years since then, I don’t see monocultures of the plant. Yes, there are some areas where it grows densely but I also see other areas where there are other plants growing there as well.
So I started doing a bit of digging. It seems that purple loosestrife might be big business: searching a periodicals database for purple loosestrife in the title gave 44 hits across 10 journals with 14 of them being from a journal called Biological Control. I started coming across articles that questioned the popular view of purple loosestrife. Mark Sagoff argues in WHO IS THE INVADER? ALIEN SPECIES, PROPERTY RIGHTS, AND THE POLICE POWER that
[t]he belief that a non-native “generalist” or “weedy” species must “crowd out” native species adapted to particular environments, while a consequence of prevailing theory, has little empirical support.
Claude Lavoie poses the question Should we care about purple loosestrife? The history of an invasive plant in North America. One of the first studies, an exhaustive survey of the historical literature and herbarium specimens of purple loosestrife collected throughout the United States and Canada, was written by Ronald Stuckey in 1980. Despite the fact that his data source was herbarium specimens, he concluded: ‘‘As the history of its spread in North America reveals, L. salicaria has the ability to (…) eliminate other species in both natural and artificial wetland habitats’’.
In 1987, the United States Fish and Wildlife Service published an exhaustive review of the biology, the ecology, the spread and the control methods of purple loosestrife. It is from this point that ecologists and environmental managers began to consider purple loosestrife as a serious pest even though the long list of the impacts of the species on wetland flora and fauna that was presented was not supported by field or experimental data; the impacts were only suspected. Nonetheless, the report stated that
In the early 1990s, this report fired up a vitriolic campaign against the plant. And that, in turn, started a bit of a backlash. Much of the contrary work that was done was not peer reviewed although one study that examined peer reviewed articles concluded in 2001 that
‘‘despite two decades of accounts, the actual dynamics of L. salicaria and its capacity to displace other wetlands species remain in doubt.”
Lavoie’s study of 34 peer reviewed articles shows split conclusions depending on methodology. Manipulative studies where the environment is controlled showed some negative impacts (10 of 11) while field surveys (7) showed no negative impacts. Two studies used both methodologies with one one showing a reduction in abundance and richness in wetlands invaded by purple loosestrife and the other concluding that the competitive effect of purple loosestrife on a rare species was not stronger than that of other native plants.
It seems that the image of purple loosestrife depicted by scientific studies is far removed from that portrayed by newspapers. Purple loosestrife is certainly an invader and some native species likely suffer from an invasion, but stating that this plant has ‘large negative impacts’ on wetlands is probably exaggerated.
- The most commonly mentioned impact that purple loosestrife crowds out native plants and forms a monoculture is controversial and has not been observed in nature (with maybe one exception).
- There is certainly no evidence that purple loosestrife ‘kills wetlands’ or ‘creates biological deserts’, as it is repeatedly reported.
- There are no published studies (at least in peer-reviewed journals) demonstrating that purple loosestrife has an impact on waterfowl or fishes.
- All the other negative impacts associated with purple loosestrife in the press
have not been the object of a study, except for a possible impact on amphibians that has been tested (to date) only on two species, one reacting negatively.
For a biologist’s take on purple loosestrife, see The purple monster by Seabrooke Leckie. It’s a very interesting read, especially the parts why purple loosestrife is the target that it is.
Current methods of control include chemical – glyphosate aka Roundup, biological – introduction of loosestrife-eating weevils and beetles from Europe and Asia, and hand pulling. As for the use of Roundup on a wetland, what were they thinking???????? As for the introducing another species to control an introduced species regardless of whether they are natural loosestrife predators in Europe and Asia, one need only look at the cane toad in Australia to wonder about the sense of that as well.
So what’s the real buzz on purple loosestrife? “Thank you for the purple loosestrife. It makes up a bit for all that you have taken from us.” As we destroy more and more habitat at an increasing pace, pollinators are increasingly threatened. It’s time to re-frame our views about purple loosestrife.
Loosestrife research here.