Sunday, January 02, 2011
Invasive species are all around us
Invasive species are all around us; they are ubiquitous. Invasive species as defined by Executive Order 13112 as a species "… that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health." Invasive species do not just happen, however; they are helped by human activities. Among these activities are human developmental disturbances, the chronic 'plowing' of natural areas, and the now global nature of the market place. The regular year after year pressures of human activity on the balance of existing ecological systems undermines the resiliency of the ecosystems and ultimately earth's biome. This is understandable as we extract resources from natural systems in order to develop our anthropic ecosystems to support human well-being. Moreover the pathways that our goods travel provide platforms for other species to hitch a ride from ecosystem to ecosystem throughout the world and across the planet.
An invasive species needs several things to happen in order for it to become established within a new ecological system. Among these is a pathway or a mechanism by which it can be transported from one ecosystem to another. And it needs this platform or vector to provide the pathway over time, that is more than once, so that multiple introductions can take place. For the most part, one introduction does not create an invasion event, though the gypsy moth introduction serves as a reminder that it is possible to do great harm through one well-meaning action. The very act of multiple introductions is a disturbance regime in the ecosystem that begins to alter the impacted ecological system. In addition the multiple introductions make it possible to overcome random events that might prevent its establishment.
The above mentioned random events are part of the resiliency of a complex system. An introduced seed may land on a rock and not germinate or in water where it drowns. An insect may be devoured by a bird or run into a windshield before it can lay its eggs or find a mate in order to reproduce. The initial invader may be stepped on or washed away before it can set down roots or start a family. And if it is the lone invader, it may never find that all important significant other. Multiple introductions of novel species increase the odds that two ecosystem immigrants may find each other and begin the process of establishment and eventual naturalization.
Examples of multiple introductions of plant species are easy to find and include kudzu, (Pueraria montana var. lobata), buffelgrass (Pennisetum ciliare), silver carp (Hypophthalmichthys molitrix), callery pears (Pyrus calleryana). Harder to demonstrate but almost certainly the result of many introductions over time are insects such as the emerald ash borer (Agrilus planipennis) or the brown marmorated stink bug, (Halyomorpha halys). The Asian lady bug (Harmonia axyridis) was repeatedly introduced as a biological control agent. Kudzu was intentionally planted for erosion control and as a forage crop in great numbers in the 1930s while the silver carp was stocked for recreational fishing. It is important to notice that not every invasive species was intentionally introduced. Many if not most were the result of accidental introduction or simply unnoticed hitchhiking.
For those who value the gardens, which they call natural areas, the constant influx of disease, insects, weeds and destructive animals from different ecosystems, leave no choice but to weed out the undesirable everyday, and to restrict the importation or movement of non indigenous possibly harmful species that may reduce or alter the 'garden' (natural area). This is what a farmer does everyday. He does not say that is alright to do nothing about the invasion of his 'garden" or fields. Rather he fences out the unwanted, weeds out the harmful plants, and fights the insects and diseases each and every day. The farmer may ask the government to protect the lands and his work from invasion from illegal alien species that have been shown to reduce his harvests. The farmer, moreover, is not alone. The ornamental landscape gardener does the same thing. Both decide which species should stay and which should not be allowed. The natural area manager does the same triage simply dealing with a larger palette of species and a corresponding greater complexity of interactions and relationships.
Invasive species have travelled with mankind ever since it first it settled down to a life style based upon farming. And in farming humanity undertook to plow the land and to tame nature which is to say to disturb the landscape annually. In doing so, mankind's actions favored certain species that were predisposed to find advantage in the constant disturbance of the environment. As agriculture allowed for the establishment of cities and dense populations of people, so it allowed for companion species to coevolve with the farm based cultures of man. And as man spread across the globe so these companion species came with him. The historic companions we call pests and weeds; the new companion species that are taking advantage of human disturbance and trade are called invasive species.