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Invasions of plants

The term "invasive species" has recently come into popular use in preference to "weed." A weed usually means a plant that is unwanted in an agricultural area or a garden. However, an "invasive species" much more accurately describes the problems associated with a plant that has been brought in from other places and tends to aggressively invade areas, competing with native species, taking over habitats, even destroying ecosystems.

A clear example of this which recently came to the fore is that of cobbler's pegs, shepherd's needles or beggar's ticks (Bidens pilosa) on Majuro. When in blossom, this plant has rather attractive, although small, daisy-like white and yellow flowers. The young leaf-tips can even be used as a vegetable, and in some parts of the world, it serves as a poor substitute for spinach after it is boiled for a while. But when those pretty flowers mature, they form little dark-brown pompons of narrow seeds with tiny hooks on the end which readily attach to clothing, shoes, animals fur or just about anything they can get a hold of. This provides an easy means of spreading this species from one place to another... to another... and another.

So it is that from its native Central America, B. pilosa has expanded its range to much of the world, including the Marshall Islands. It does exceptionally well in freshly cleared areas as it likes lots of sunshine and doesn't require much water or even a lot of nutrients in the soil.

It's bad enough to have such a pestiferous species "sneak" into an area as B. pilosa tends to do, but the trouble is further aggravated once such invasive species are established. Then, after people are used to having them around, conditions can change, and these unwanted species can really take over.

B. pilosa has been in the Marshalls since at least the early 1980's when it was reported to have been already introduced into Enewetak. However, when a survey of Pacific Islands' weeds was published in 1997, it listed this species as a major weed on some Pacific Islands, yet not even as a minor weed in the Marshalls. Then Typhoon Paka hit and there was an extended drought. Around Majuro Atoll, other plants suffered during this time, but B. pilosa went to town - and out of town. Everywhere, from Rita to Laura, and all along the road between, even on the more isolated northern islets, in almost every field or clearing, there could be found very healthy looking B. pilosa. Even hardy native plants, such as the beach sunflower or "markubwebwe" (Wollanstonia biflora) was overrun by this aggressive import.

Once an invasive species like this becomes so well established on an atoll, it is virtually impossible to get rid of. The thin limestone soil makes the use of herbicides risky since these poisons can easily seep down into the valuable water lens. Often, though, the most that can be hoped for is to prevent further spreading. If people are aware of the potential problems associated with invasive species, they may be able to help prevent the spread of seeds into new areas. This is easier said than done with "hitchhiking" style seeds, like those of B. pilosa, which seem to jump out and snag onto clothing and then are indiscriminately tossed about afterwards.


References:
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