What is an invasive species?

In the last post, That Blog asked what a native plant is. Asking what an invasive plant is might seem redundant – if it’s not native, it must be invasive, right?

Actually, no. If it’s not native, it must be non-native. But not all non-native plants are invasive. An invasive plant is one that meets a special definition:

An invasive plant is one which tends to spread, and which causes harm to the local ecosystem.

A non-native plant that stays put in a garden, or that escapes into the wild but causes no discernible disruption to the existing plant community, is not considered to be invasive.

Where do invasive species come from? Some – like Kentucky bluegrass – arrive by accident, as stowaways in other shipments. But half or more of the invasive species spreading across the United States today were deliberately introduced by the nursery trade. People planted these ornamental exotics in their yards, and the plants then cheerfully multiplied beyond the property lines.

In recent years, some states have tried to deal with this problem by banning the sale of invasive species. Some of these measures have passed; others have not. Even when they do pass, though, they tend to ban plants which have already been sold in abundance, and which have already spread into natural areas. Once a species has invaded an area, it is usually very difficult to eradicate. Banning the continued sale of such species is unlikely to make a dent in the problem.

An approach that might be more effective is to make every property owner responsible for eliminating invasives from their own land: if we each take care of our own piece of the Earth, together we might be able to accomplish something. Municipalities have used this approach for a long time, in the form of banning so-called “noxious weeds”. But these weed bans are often not enforced, and, more importantly, they tend to take aim at plants that are unpopular, rather than focusing on plants that are truly harmful. As just one example, these rules often forbid milkweed, the family of native flowers that are crucial to the survival of the monarch butterfly.

Before we pause to reflect on That Blog’s third anniversary, let’s take a look at one more definition worth knowing: a naturalized plant is not one that has settled harmoniously into its new home. Rather, it is a non-native species that is capable of surviving and reproducing without human help; in other words, a species poised to become invasive.

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What is an invasive species?

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