So what exactly is an invasive species? There is no textbook answer, or at least not one that everyone agrees on. The most common professional definition is a species not indigenous to the area that does ecological and/or economic damage when it becomes established. It is not merely any species that cause a nuisance, as many native species (such as water lilies or coontail) can do reach nuisance densities and are quite native to New England. It is not a species that invades but maintains a low density or even goes unnoticed, not impairing any use of the lake. And there are such species to be sure. When professionals talk about aquatic invasive species, they are usually referring to plants or animals that arrive at a lake and become a dominant component of the aquatic community, negatively impacting other species or uses of the lake. Several non-indigenous species of Myriophyllum, the watermilfoil genus, qualify, but there are native milfoils as well, some of which are even on various state endangered species lists! The zebra mussel is a great example, not well established in many New England lakes, but causing both economic and ecological harm when it invades. The list of invasive species for each state varies a bit, but there are a few dozen species that just about everyone agrees we would be better off without.
What is it about invasive species that make them objectionable? For the most part, species termed invasive displace other species by some competitive advantage or lack of predators, and become abundant enough to influence lake features that affect lake uses. Dense plant growths can include native species, but among the worst conditions are associated with Eurasian or variable leaf watermifoil, fanwort, and hydrilla, all species that came to New England in the last century and have not been well integrated into aquatic communities. It is possible that at some point balance will be achieved, and some people or even agencies make the argument that we don’t need to act; if we wait them out, the invaders will become part of functioning aquatic systems. This might be true in some cases, but given the track record, it does not seem responsible to wait that long to test the theory.
Invasive animal species, like the zebra mussel or spiny water flea, alter the flow of energy in a lake and affect the aquatic food web. Aggressive snakehead fish similarly impact the food web, but from the top down via predation. Just how much damage is done can vary greatly depending on the condition of the infested lake. Lakes with very healthy native plant communities that cover the bottom in the zone where light is adequate tend to resist colonization by invasive plants, although it is reasonable to expect eventual dominance by the invader. Lakes with no hard substrate or very little calcium in the water column are not likely to support dense populations of zebra mussels, but one can expect that all native freshwater clam shells will be colonized and those species are likely to be eliminated. There can even be some upside, as with clearer water from the filtering effect of zebra mussels, but this tends to favor buoyant cyanobacteria, so ultimately zebra mussels may promote objectionable algae blooms.
Invasive species are analogous to infectious diseases. Not every disease will kill you, but none are considered pleasant or desirable. Living with a disease is highly personal experience, but acting as a vector for that disease is irresponsible. Having an invasive species in a lake and deciding not to act to control it may be a valid position in some circumstances, but the potential impacts on other lakes in the area should be considered in making management decisions. This is a complicated area of judicial, regulatory and scientific interaction, and blanket statements that universally apply are hard to come by.
The cost of impacts vs. the cost of control is also a difficult topic. Actually putting dollar figures on impacts is not always easy, and even accurate estimation of control costs can be challenging. Ideally, eradication is the goal, but that may not be feasible in all cases. There is a whole school of thought on invasion ecology that considers the potential for control along a timeline of species establishment and impact. Often invasive species are not noticed or managed until they have reached the point on the curve where eradication is very expensive. Clearly prevention is the most cost-effective approach to invasive species management, and rapid response is the clear second choice for action, but both of these are given way less attention than they deserve in our monitoring and regulatory systems at the state level. Maintenance and restoration are the more expensive alternatives that apply once an invader has become established, and the cost can indeed by staggering. Millions of dollars are spent annually in New England alone to manage invasive species, rarely with eradication as a result or even a goal.
There is a very real need to enable citizens with an interest in lakes to recognize invasive species and to empower groups to take early action. In Massachusetts, the process and timeline for getting a permit for a rapid response program are the same as for addressing a longstanding infestation or addressing nuisance native species. We have no mandate to control invasive species, but we have laws and regulations that protect many species; if the control of an invasive species conflicts even a little with protection of an endangered species, the chances of getting a permit to control the invader are very slim. A more holistic approach is needed, but until we reach that stage of enlightenment, it is important for lake monitors to recognize invasive species and bring them to the attention of appropriate state agencies.