You probably know the Island is under attack by invaders. They are not very conspicuous until one learns to look for them closely. For the most part, they are plants and are thus just seen as part of the background for most of us. Most of these plants have arrived here from exotic locations and have not brought with them the diseases, pests or predators that serve to keep their populations under control in their native habitats. Therefore they tend to multiply rapidly and usually out-compete native species to which, in many cases, we attach greater sentimental value.
As pointed out so clearly by Stephen Spongberg in our book Island Life, the characteristics of invasive plants that constitute a threat to our native habitats are several: “Germination, growth and maturation to flowering are rapid. The species . . . spread quickly either through efficient seed dispersal or by vegetative means, or both, and the onset of flowering is generally early in the season and continues for a prolonged period . . . Moreover, invasive species tend to have wide ecological tolerances that enable colonization of a spectrum of habitats. Most, additionally, tend to be perennials that are difficult to eradicate.”
Nearly everyone is familiar with the vine oriental bittersweet which can suffocate native junipers and ornamental trees and shrubs alike. Its orange fruits are often used as decorations and supply winter food for birds and mice which become agents of dispersal.
Autumn olive and Russian olive were once introduced by the state highway department as a windbreak and light barrier on dividers between highway lanes. Now both species have become common almost everywhere and have even spread to Noman’s Land where they are overtaking native shrubbery.
Purple loosestrife threatens to displace the vegetation in many shallow wetlands, crowding out native ferns, orchids, cranberries, rushes and other plants. Its purple spikes of flowers do not even seem to be particularly attractive to our native butterflies, but it spreads aggressively.
A plant that could easily become a serious pest in the future is garlic mustard, which is becoming more widespread on the Island. It is often a woodland undergrowth species that can be expected to flourish in areas where the tree canopy has been killed by the recent outbreaks of caterpillars. It will also grow happily in your yard, orchard or pasture edge. Be on the lookout for it.
You may ask, what can or should I do about these and 14 other such plants listed in Island Life? Total eradication seems an unrealistic objective. I note, however, that Fish and Wildlife Service personnel stationed on two of the smaller Hawaiian islands northwest of Kauai were able to completely eliminate all exotic plants over several years, simply by each person pulling them up for an hour each day. This shows that modest effort by individuals can have a significant effect. I applaud the programs of Sheriff’s Meadow Foundation and Vineyard Conservation Society which sponsor invasive plant removal events on their properties. The more people who participate, the better.
Other kinds of invasives should not be ignored either. Two species of aggressive exotic crabs have reached Island shores within the last 15 years, certainly to the detriment of our native species. A marine alga, codium fragile, has spread here from Europe and has done serious damage to native shellfish populations, particularly mussels and scallops. The alga attaches itself to the bivalve’s shells and eventually can suffocate it. The unfortunate effects caused by population explosions of winter moth, gypsy moth and Japanese beetle are well known.
Most of the nine avian invasives that have arrived here since 1950 came naturally as colonists, primarily from the south, during the cyclical climate warming pattern which has occurred in the last century. As far as we know, none of these birds has had a detrimental effect on the native species already here. The house (or English) sparrow and the European starling are exceptions. Both were introduced from Europe and are cavity-nesting species. The starling has had a particular impact on woodpeckers such as northern flickers and hairy woodpeckers by stealing their nest holes. The sparrow has had a very deleterious effect on our house wrens, eastern bluebirds and tree swallows by killing these birds in their nest boxes in order to appropriate the nest sites. If it is appropriate to eradicate bittersweet from preserves and refuges, it seems equally justifiable to eliminate starlings and house sparrows from places where native birds are supposed to receive sanctuary.
Allan Keith is a naturalist living in Chilmark and co-author with Stephen Spongberg of Island Life, A Catalogue of the Biodiversity in and around Martha’s Vineyard, available in most Island bookstores.