The purple monster

It’s late August, and one of the most maligned flowers to be introduced to North America is blooming in wetlands and damp areas across the continent. The plant is Purple Loosestrife, Lythrum salicaria, and coinciding with the start of its blooming period I’ve been seeing posts on the colourful purple flowers popping up on many of the blogs I read. The general sentiment toward the plant is acknowledgment of its pretty purple blooms but antagonism towards its presence.

There have been big promotional campaigns toward its eradication. I remember as a kid my sisters and I joined my father pulling up some plants that had seeded themselves at the edges of our little swamp and the pond across the road (actually, what stands out most clearly about that afternoon was walking down my neighbour’s driveway to say hello to the woman, who was also pulling loosestrife, while swinging a just-pulled loosestrife plant, with a big clod of dirt at its base, by my side like a walking stick, and her alarmed reaction admonishing me as it would spread seeds). The website PurpleLoosestrife.org has the subheading, “A Beautiful Killer”. Talk about melodramatic! But it is the promoted notion of the plant.

But why loosestrife, over all the other widespread and incredibly invasive plants? The wildflowers in our roadsides and meadows are probably more than half introduced species (if my own observations are any indication), and yet no one’s crying for any of their metaphorical heads. Why has there been no campaigns to control Butter-and-eggs, or Viper’s Bugloss, for instance? There’s no shortage of invasive species; Invasive.org lists 694 exotic plant and 228 non-native insect species on their website as well as 43 other organisms. What makes loosestrife such a target? One reason might simply be its visibility, the bright purple flowers that really stand out in a wetland.

Interestingly, I’ve read a number of articles that have accused the anglers and hunters organizations as being the primary proponents of loosestrife eradication. Do a Google search for “purple loosestrife control” and the #2 site to come up is the page for OFAH (Ontario Federation of Anglers and Hunters) and #3 is PurpleLoosestrife.org, which, when you look closely, is hosted by Ducks Unlimited Canada. The latter is the #5 site returned by the search, and many of the others on the first page are government or conservation authority sites (I’ll refrain from painting all of these with the same brush, but will state that within the couple of authorities that I’ve had insider insight into, the majority of employees are themselves either hunters or anglers). Why would these organizations take such an interest in Purple Loosestrife? (Conversely, compare a search for Eurasian Watermilfoil, another terribly invasive aquatic species, and only one of the top ten – OFAH – is a hunting/fishing organization.) Only they know for sure, but the argument I’ve seen presented is that loosestrife creates denser habitat that’s harder for anglers and hunters to navigate through, and reduces their productivity on outings. (To be fair, that could all be a bunch of bunk presented by someone with anti-hunting sentiments; that’s the downside of the internet, it’s harder to tell fact from fiction.)

Of course, that’s not the story that’s given for controlling the spread of the species. Whether it was through anglers associations, governments, or other media lines, the general picture presented to the public is one of a ruthless invader, moving into and decimating wetlands (as illustrated by the PurpleLoosestrife.org subheader). We are told that if left unchecked it will completely take over wetlands at the expense of native species, forming near monocultures of a plant that’s non-native and therefore practically unusable by our native fauna. We’re lead to believe that if we don’t do something right away, that sprig of purple flowers you see at the edge of the cattail patch will, within a few years, have wiped the whole stand off the map.

Okay, maybe it’s my turn to lean towards the melodramatic, but it’s not that far from the truth. A lot of research has gone in to loosestrife control, and no fewer than four non-native loosestrife predators (two beetles and two weevils) have been approved for introduction across North America as a biocontrol method. OFAH states that these insects have been established at over 300 release sites across Ontario alone. They also state that these insects have no other known food items, and so their population will be limited by the availability of loosestrife, with the plant and insect eventually establishing a relatively stable relationship at low numbers. (This may be true. Of course, there’s also this saying – most famously stated by Dr. Ian Malcolm in Jurassic Park – that nature will find a way.)

Of course, it all begs the question: Is all this hoopla really warranted? Is it really as bad as they say? In university, a chapter of one of my courses, Conservation Biology, examined the danger of accepting unproven hypotheses – that is, making assumptions without doing the background research. The example the professor presented for this section of the class? Purple Loosestrife. I don’t remember a lot of what I learned in university, but those lectures really stuck with me. Over the years and through several moves, however, my course notes got misplaced, so when I decided I wanted to do a post on Purple Loosestrife, I emailed my course prof and asked for some of the material pertaining to that segment of his course. He generously obliged with six papers he happened to have on hand on his computer.

The papers are all dated within the last 10 years. They examine a number of common (and prior to that, unproven) assumptions regarding the plants, and actually carry out manipulative experiments (meaning that they manipulate the different variables involved in order to pinpoint the truth of the situation) or quantification studies (where they take samples from plots representing a range of loosestrife conditions, from absent to monoculture) to prove or disprove these assumptions.

The results show a number of things. The first and perhaps most important is that loosestrife, like so many introduced species, requires disturbed habitat to become established. Loosestrife seedlings would only take root and prosper in plots that were experimentally cleared of reed canary grass to simulate herbivore foraging disturbance; 53% of seeds sowed in such plots became established, while 0% became established in non-disturbed plots. A second study showed that in undisturbed habitats, loosestrife was unable to invade and establish itself, as seedlings can’t compete with established, mature native vegetation. However, once a disturbance creates a situation where the native vegetation is removed, allowing the seedlings a window of opportunity to get established, the mature loosestrife plants are able to prevent the young native plants from growing in again. This is where the problem really lies.

This information says to me that the wetlands where loosestrife is found are perhaps not the healthiest to begin with, if they are experiencing so much disturbance as to allow the invader to spread and turn into a monoculture. It could be a natural disturbance – say, severe harvesting of cattail roots by muskrats – but I rather suspect non-natural disturbances such as boats are also at least partially to blame. Perhaps it’s just coincidence, but in our shallow lake the corridors where the motorboats rip back and forth from one end of the lake to the other lack much vegetation on the lake bottom, while areas that don’t get much motorized traffic grow thick and weedy. But these are just my hypotheses, unproven by any sort of scientific research, of course!

As for the claim that loosestrife reduces local biodiversity, it simply isn’t true. One study showed that plant diversity in invaded habitats was actually higher than it was in stands without the loosestrife (and no, it’s not simply because the loosestrife is the additional species). They found that species richness also increased with increasing abundance of loosestrife. They pointed out that this raised the question as to whether loosestrife naturally preferred to invade more diverse stands, or if some factor of the loosestrife’s presence created that diversity; they don’t have an answer, but do point out that previous studies didn’t seem to show a preference for invading stands of either higher or lower diversity.

A second study of the same thing, but in a different location, showed no significant difference between plots with and without loosestrife, and no relationship between species diversity and loosestrife abundance. They did, however, observe that several species were significantly more likely to be found in plots containing purple loosestrife, but no species were significantly more likely to occur in plots without it (again, whether this is due to characteristics of the stand that make it appeal more to both loosestrife and the other species, or if it’s a direct result of the loosestrife’s presence, is the subject for future studies).

Another study looked at aquatic invertebrate diversity between plots with and without loosestrife, and found no significant differences, although they did note that the invertebrates in loosestrife stands tended to be a tad smaller than those found in plots without loosestrife. Whether this is due to dietary deficiencies, some characteristic of the habitat allowing larger bugs to get picked off leaving just small ones, or a local selective pressure toward overall smaller bug sizes, is for a future research project.

I’m pretty sure that, aside from our afternoon plucking plants that one year when I was young, the wetlands by my parents’ have had no control measures, no removal crews, no beetle releases. They also have no muskrats, no people, and no boats. And, they have very little Purple Loosestrife. Even fifteen years later there are still just the odd few plants here and there, mostly along the edge of the road; far from the monotypic stands broadly predicted among all the doom and gloom surrounding the species. Here at our new home, we have a couple plants that are blooming, standing singly at distances from each other, adding a bright splash of colour but hardly threatening the ecosystem.

I think the truth is somewhere in between. I suspect that there are instances, highly disturbed areas that suffer heavy herbivore and/or human activity, that will require human intervention to keep the opportunistic loosestrife from moving in to the compromised wetland. However, I don’t think, based on the research to date, the classifications the plant has received (“noxious weed”, “beautiful killer”, etc) are necessarily warranted. If we humans hadn’t muddled around with the wetlands in the first place, I strongly doubt that loosestrife would be as widespread and abundant as it is today; the low abundance of the plant in relatively untouched areas is evidence of this. Something to ponder next time you see these purple flowers growing at your local patch.