The last couple installments of Conservation Corner have looked at invasive species, rather at length and to the point where my editor is begging me to get off this topic. With this installment, I’d like to round off the topic with a discussion of the methods used to eliminate or control invasive species, including their pros and cons.
Obviously, the same kinds of invasive control methods cannot be applied to all species across the board. What works well for terrestrial organisms may fail entirely in an aquatic environment – and in some cases, can actually increase the problem. Methods that work on plants may fail for animals, and so on.
Controlling aquatic organisms brings one large advantage over that of a terrestrial environment. For the most part, aquatic organisms can’t exactly get away – they’re trapped in whatever body of water they’re in. The exception comes when the containing body is large enough to negate this advantage, such as with very large lakes (e.g., North America’s Great Lakes), or in the ocean. In these cases, control can be much more difficult.
To make matters easy, let’s discuss methods that work for plants first, and then move onto animals (ignoring the other 99.9 percent of life for now).
The most obvious method of control for any type of invasive organism is manual removal. In many regions of the country, kudzu or air potato (or other plant) round-ups are scheduled, where volunteers simply comb an area removing all of an invasive plant.
Manual removal actually works really well for aquatic plants, but unfortunately has certain practical problems, most notably that you have to be underwater to remove the plant. However, divers have been employed in some lakes to remove aquatic plants. Unfortunately, dive teams are quite expensive, and the method is at best a control method. Complete elimination of an offending plant is unlikely. Worse, divers may serve to spread the plant by fragmenting it – broken pieces of aquatic plants can resettle elsewhere. However, unlike other methods of plant control, this method has the distinct advantage of being extremely selective. Divers can remove only one species of plant, leaving everything else alone.
For surface growing plants (e.g., water lettuce or hyacinth, salvinia), specially designed skimming boats are sometimes deployed – they kind of resemble the oil skimmers we all saw during the Gulf Oil Disaster last summer, only smaller. They’re great for opening up large areas of lakes, but tend to miss areas that aren’t boat friendly.
A similar boat is often deployed for submerged vegetation, with what amounts to giant lawnmower blades. The boat simply cruises across a lake, mowing up all aquatic vegetation and pulling it into a large bed. This removes only the top of the plant, and leaves shallow areas infested, requiring frequent “mowing.” This, really, is admitting that the war is lost. It allows recreational use of the body of water again, but leaves the invading plant. Worse, the mower blades greatly fragment the plants, resulting in spread. (Ironically, this, like all forms of manual removal, has a “silver lining.” The plants that are removed carry huge amounts of nutrients: removing the plants “cleans” the lake.)
For smaller patches of plants, heavy mats may be laid down. These simply smother the entire patch of plants – native and invasive alike. Whatever is underneath receives no light, and dies.
Lastly, there’s the “scorched earth” method. In this case, powerful herbicides are applied to a body of water, killing off all aquatic plants. This method is completely non-selective. All the nutrients biosequestered by the aquatic plants are released as they rot, which can turn a healthy, though weed chocked, body of water into a stinking cesspool.
Manual removal works well for animals, too. In the case of fishing, manual removal often involves encouraging anglers to remove fish. In the past, bounties have actually been put out on certain species (although this has mostly been used for terrestrial organisms). Alternatively, agencies often simply encourage anglers by offering good recipes and creating hype about how good a particular invasive species tastes. This method is being heavily employed along the Atlantic Coast to help control the invasive lionfish. Similarly, you can find information on cooking oscar cichlids, carp, and even plecostomus, all of which are invasive in various regions.
Many groups sponsor a “Catch and Never Release” program for invasive species, where anglers are encouraged to destroy invasives, even if they’re not interested in keeping them. It’s actually technically illegal to release an exotic species into U.S. waters, meaning that if you happen to catch one, technically, putting it back is a crime. The only downside of this is that sometimes native species take a hit, too. For example, the exotic northern snakehead bears a resemblance to our native bowfin – and many bowfin are killed as a result. While fish collecting in many areas, I’ve littered a shore with captured invasive jewel cichlids, Gambusia, and other invasives – much to the delight of whatever birds are nearby.
As with aquatic plants, poisons are often employed. The most common of these is rotenone. The process of poisoning an entire body of water is given euphemism of “reclamation.” The poison is applied, all the fish in the lake are killed, and it’s restocked with natives. There are a number of disadvantages to this, most notably that it doesn’t work on flowing water. Secondly, many invasive species are able to tolerate the poison. Very often, state agencies simply stock game fish (bass, trout, etc.), ignoring the native bait fishes. Lastly, the number of dead fish can significantly impact the nutrient levels of the lake.
Finally, one method that is often employed for both plants and animals is one that I call the “Old Lady and the Spider” method. In this method, a control organism is released, usually from the native range of the invading species. Ideally, this control organism begins to eat the invading species. The control organism will never completely eradicate the invading organism, but will hopefully reduce its numbers to a less, well, invasive level. In an ideal world, the control organism will eat only the invading organism. For the most part, this is true, but there are significant cases where it simply is not (especially when the control organism is not that selective). For example, carp are often used to control aquatic vegetation – and they soon cause as much problems as the aquatic vegetation did.
The most important method of control, however, is prevention. Nonnative organisms should never be released into the wild for any reason. Care should be taken to prevent the spread of organisms already present in the wild: many invasive aquatic plants have been spread by boats, fishing tackle, etc. Aquatic plants should never simply be flushed or thrown away, but instead dried in the oven until crumbly, preventing their spread. Most importantly, fish or plants placed in ponds should be chosen carefully – there are so many beautiful native fishes, invertebrates (e.g., snails) and plants that I simply don’t understand why people choose species that can cause so many problems. Ponds should be carefully planned to avoid releases during high water, and of course, no nonnative organisms should ever be stocked in a naturally occurring pond.