Invasive species come in all varieties, warm-blooded and cold-blooded animals, all kinds of plants, even fungi. Some are notable but cause little problems – armadillos, some are notable and troublesome – feral hogs, and some you never see but are dangerous – infectious bacteria and viruses in some biting insects.
Whenever a species is introduced to a non-native environment there can be negative consequences, earthworms for example. The last ice age covered North America as far south as southern Illinois. Any native earthworms would have been wiped out. After the glaciers receded, the upper midwest evolved for thousands of years without earthworms. Much later, that locale was populated with earthworms by incidental introduction from colonial ships from Europe. Rock and dirt used as ballast and dumped onshore was the likely method of introduction.
Generally speaking, worms are good as they churn and aerate the soil. But there is a dark side. Portions of hardwood forests in Minnesota and boreal forests in Canada have been negatively impacted by invasive earthworms. Fallen leaves accumulate on the forest floor and create a rich organic layer called duff. This duff layer is the natural growing environment for native woodland wildflowers. It also provides habitat for ground-dwelling animals and helps prevent soil erosion – basically natural mulch. When earthworms are present however they eat this leaf litter. In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forest and even degrade fish habitat. Many tree seedlings, ferns, and wildflowers don’t survive this altered environment.
In the “unseeable” category of invasives are numerous fungi, smuts, rusts, mildew, mold, etc. Many of these are serious agricultural pests causing untold damage to food crops. The tropics where much fruit is grown for export are particularly susceptible. A new threat has recently come to the western hemisphere. Tropical plantations are subject to problems such as fungi because of the warm, moist climate and especially because plantations are generally monoculture. When only a single species of plant is grown in large concentration, the condition is likely to favor pests.
Guatemala alone exports over 3 billion pounds of bananas to the United States annually. All together we import over 8 billion pounds of bananas a year and this crop is at risk due to a fungus from southeast Asia. Although there are over a thousand varieties of bananas, 95 percent of all commercial bananas are essentially a clone of one – the Cavendish banana. It replaced a previous clone, Gros Michael or Big Mike. It succumbed to a fungus called fusarium wilt. A similar fate seems to await the Cavendish. In some parts of southeast Asia, a new variety of this wilt is reducing production there by forty percent per year. The wilt, a soil organism, just turned up in Colombia and could appear across central America.
Invasive species generally have an advantage as natural controls are absent. Monoculture acts to increase the distribution of the invader. Devastation of the banana crop in “northern triangle” of Central America will only serve to accelerate immigration to the United States as families seek some sort of income.
Dr. Bob Allen is Emeritus Professor of Chemistry at Arkansas Tech University.