Why are invasive species so bad? One answer is that they negatively impact ecosystems; they bring an imbalance which at the very least causes stress and may under extreme conditions begin to destroy integral relationships and interaction necessary to a functioning ecosystem. The introduction of kudzu or Lythrum may result in a monoculture or biological desert. The complex web of food chains between trophies levels is eliminated and the system simplified. The ecosystem resources ability to provide services are correspondingly reduced to, for example, a possible regulating service such as erosion control on the part of kudzu or an informing service (aesthetics) on the part of Lythrum . The complex web of weak ecological interactions is reduced.
Sustainability in a certain sense is the process of maintaining the homeostatic relationships of an ecosystem. Invasive species reduce the efficiency of an ecosystem by replacing a web of weak interactions with a few strong interactions. Invasive species interfere with the feedback mechanism of an ecological system. An addition of a top predator such as mute swans, feral swine or cats can quickly bring an imbalance to the system. The feedback mechanisms which would normally tend the system towards equilibrium no longer function and the system begins to fragment.
The premise that this is a bad thing, the imbalance and degradation of the ecosystem is based upon an assumption that the ecosystem services as a resource have a value and a cost. A person who assumes that the resources are infinitely available would take strong issue with the idea that his or her use of these services should be restricted or should cost. The introduction of an invasive species, therefore, is an extranality to which the person is entitled by right. This sets up the dichotomy between market preference and public value. Our economic system and assumptions is based upon this infinite resource premise; it is hard to borrow money if you own a bog without a plan to drain it and pave it. A bog has “no” value for it generates no measurable return (ecotourism aside).
This in a world of infinite ecosystem services (regulating, provisioning, and providing) which are considered free, the introduction of Hedera helix, English ivy, in Maryland is not a problem but rather a solution to a short term landscape need accordingly a use of a high level ecosystem service, that of informing. The possible negative impacts to lower level ecosystem services are not easily measurable in a market preference sense and thus are not taken into consideration.
A sustainable landscape would “value” the complete palette of services attempting to support complex interactions and relationships. At the very least it would try to do no harm. So we wobble between our values system and our market choices a dynamic political tension with no linear solutions and great probability of unintended consequences no matter which side we take. Do nothing is not an option.