A few years ago, I participated in a comprehensive conservation area plan for the lower Hudson Valley's 6,500-acre Great Swamp. The plan included a highly professional publication with many glossy pictures and a series of maps generated using Geographic Information System (GIS) technology. These maps included a data layer showing all the natural community types across this vast wetland complex, and it was here, for the first time, that I encountered what is certainly not native habitat in this landscape but now undeniably a fait accompli: a widely distributed invasive community type identified in the plan as a common reed/purple loosestrife wetland.
Did ecologists throw in the towel when they characterized this heavily invaded wetland by its dominant, invasive species cover? Has an invasive Phragmites australis/Lythrum salicaria plant community naturalized to such an extent here that it has effectively replaced the native wetland habitat that otherwise would have expressed itself at these sites across the swamp? Is the previous community type gone beyond hope of restoration, and what is the likely fate of the rest of the extraordinary calcareous wetlands complex that still supports a mosaic of rare and threatened species and natural community types?
For many natural areas land managers, the challenge posed by invasive plant and animal species, and by introduced forest pests and pathogens, is often the most severe threat facing the protected lands under their stewardship. Invasive species management strategies place heavy emphasis on early detection and rapid response, since there is an exponential growth curve over time in the expense and effort required to push back an invasion and a corresponding diminished chance of eradication or effective control.
So what should be the management response when in Montana, millions of acres of range land are made unpalatable for grazing because of spotted knapweed? What can be done when the entire Florida everglades are at grave risk of being smothered under a horrifically widespread expansion of Lygodium microphyllum (old world climbing fern), or when a calcareous sloping fen that happens to be bog turtle habitat becomes infested with purple loosestrife? Do we write off these ecosystems, habitats and species occurrences and accept a far simplified expression of biodiversity, a range land no longer suitable for grazing and therefore more vulnerable to more damaging uses like fossil fuel extraction, a forest where the herbaceous layer is microstegium and garlic mustard, the shrub layer is woody invasives, and nothing in the sub-canopy or canopy is able to regenerate?
All land management is about human values. Even doing nothing and letting nature take its course is a choice to let human impacts alter the landscape. Fossil fuel use, global dispersal of invasive plant and animal material, and the expansion of some native species that can outcompete others by profiting from the resources provided by your bird feeder and trashcan are all the result of human activity and affect both working lands and wilderness. If we value natural areas with the capacity to express a full compliment of native biodiversity, the sustainable use of federal range land, and the continued viability of threatened species where they still persist, then a response is needed to the impacts of invasive species on the things we value.
In some ways, invasive species management is a bit like throwing up walls around the monastery, keeping the barbarian hordes at bay until new technology makes it safe to come out from the refuge and share the ancient knowledge preserved in the cloister with the wider world. Keeping uninvaded areas free of invasion and holding the line in hope that effective biocontrol will emerge before it is too late is often the last, best hope with some virulent invasions. There are some apparent biocontrol success stories, notably the reduction in Melaleuca quinquenervia across Florida due to a combination of introduced predators, but introducing new species to combat undesirable ones is fraught with peril and there are many instances where the hoped for solution became a greater problem than the original invader.
Invasive species will always be with us, but we can make effective management decisions about the ones we have as well as preventing those on our doorstep. It probably helps the Great Swamp managers to know where the dense infestations of phragmites and loosestrife occur across the wetlands so that they can develop containment and prevention strategies to keep areas of conservation value uninvaded.
Those areas that are now common reed/purple loosestrife community types are not biologically sterile, as Erik Kiviat and Hudsonia have dutifully documented. Some marsh birds use phragmites for nesting habitat and cover, while beekeepers consider loosestrife an important nectar source. I personally would not trade a rich, graminoid fen and the biodiversity it supports for an invaded wetland with fewer and more common species, but it is vitally important to ask what outcomes natural areas management should achieve and what reclamation activity may be needed to complement invasive species control.
There are sites in New Jersey that support the federally threatened bog turtle and are heavily invaded by loosestrife. Eradication at these sites is highly difficult and the threat of damage to the rare reptile species limits what can be done to reduce loosestrife cover. The important question at these places again reflects values and management outcomes. The conservation value of greatest importance at the site is the bog turtle, not the invaded calcareous fen. The threat posed to bog turtle from loosestrife is the loss of basking habitat due to shading. Managers have elected to take brush cutters to the 6 ft high loosestrife stems before they set seed and mow them to less than 1 foot high, allowing light to penetrate to the areas used by bog turtles. If these sites are critical to maintain for the recovery of this rare species, then the solution is to manage the invasive species for the turtle's benefit.
Jennifer Foreman Orth's Invasivespecies Blog recently discussed the possible consequences of putting some of our heavily invaded areas to use to supply invasive Phragmites australis as a thatching material in an emerging North American market. She quite rightly asks what happens to the inflorescences and whether harvesting phragmites for this purpose could be a vector for its spread. I have done some research on this subject, and believe that both the leaves and seed heads are removed from the culm before the stems are bundled and sent to the thatcher. From a practical standpoint, wild stands of phragmites need to be mowed so that only this year's growth is harvested to prevent premature biological and erosive decay when used as thatch. This use is clearly not an invasive species control method , but just as I enjoy my annual Knotweed pie - reminiscent of rhubarb when picked young - so, too, I would like to see our naturalized invaders start to earn their keep where possible.
Another miserable wetland invader, reed canary grass, is apparently a major source of biomass for electrical and thermal energy production in Europe. Conceivably, agricultural areas that are dominated with this plant and adjacent to important wetland habitats might be harvested for biomass prior to setting seed and therefore only spread from the site rhizominously.
An effective invasive species management strategy needs to integrate multiple techniques, including but not limited to preventing the spread of invasive material to the uninvaded areas we value. For those areas where the invaders are here to stay, we can certainly try to contain their spread. We can hope for a viable biocontrol solution. And in some cases, we can even make lemonade out of these uninvited lemons by finding new values in degraded habitat.