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What makes invasive species so dangerous?

Just over a year ago, the largest known chestnut tree in North America was found in Western Maine. About 150 years ago, giant chestnut trees were family common in New England, where they were a favorite tree to plant in cities. Chestnuts were largely wiped out in the US after an invasive fungus swept the country in the early 1900s. This story – of invasive species driving plants and animals to the brink of extinction – is becoming more common.


Young Japanese knotweed – a rapidly spreading invasive plant – near Orono

What makes a species invasive? Invasive species are non-native, which means they evolved in areas far from their current range. Most non-native species in Maine come from northern Europe and mountainous parts of East Asia (particularly Japan), where the climate is similar to that of New England. Non-native species may arrive by human efforts (like shipping and the exotic plant/animal trade), but others may arrive simply by chance. An invasive species differs from a non-native species in its impact on new ecosystems: invasive species are likely to drive native species to near-extinction.

There are two main modes invasive species follow: they either outcompete native species for resources (like eating so fast and so much at Thanksgiving that there is no food left for your in-laws), or they eat native species so effectively that they drive the native species extinct (like eating so much turkey that turkeys go extinct).

Invasive species may outcompete native species by having fewer natural enemies. For every plant, there are usually several insects that graze almost exclusively on said plant (like monarch butterflies and milkweed). For every animal, there are usually parasites or diseases that infect only that animal (like moose and winter ticks). It is unlikely for a newly arriving invasive species to carry its natural enemies with it. So while native species lose a good portion of their energy or substance to their natural enemies (or invest in costly defenses), invasive species are able to grow and spread unburdened, thereby outcompeting native species.

Invasive species can also cause significant damage when their new prey lack specialized defenses. Tree diseases are one major example of this. There are hundreds of native insects and fungi that feed on trees without killing them. Over time, trees have evolved specialized defenses that strongly limit how much of the tree any pest can eat. These defenses include thicker bark, toxic chemicals that make eating large quantities of leaves poisonous to pests, and even communication with helpful defenders (yes, some plants release hormones when they are being eaten by insects; these hormones attract wasps that eat the insects). However, many of these defenses are specific to the pest they are used against, so they may be completely ineffective against invasive species. This is why chestnut blight or hemlock wooly adelgid can quickly decimate forests that have withstood hundreds of native pests for centuries.

So what happens in the long run when an invasive species sets up shop? Over long time frames, some of the problematic aspects of invasive species may be sorted out: native species may over time evolve or learn to feed on invasive species. Resistance to invasive diseases can evolve – scientists have found resistant strains of some trees plagued by invasive diseases (e.g. beech, chestnut). Scientists have even used selective breeding to speed up this process, increasing the chance that our grandchildren may once again see chestnut in the wild. Pollen records in lake bottoms show that some trees in North America nearly vanished at times over the last 12,000 years, likely due to invasive species outbreaks. These tree species have since recovered, likely due to slow evolution of resistance to the invasive species.

So, invasive species in Maine are likely to have high impacts on our ecosystem, though evolution and animal learning may dampen these effects over the long term. Unfortunately, by “long-term,” I mean taking much longer than a human life-span. Our economies (and enjoyment of the woods) function on much shorter time scales. So what can you do to help protect our lands over the short term? If you are a landowner (or have a favorite park or wild land), consider taking training on invasive species identification, and working with your local cooperative extension or farm association. If you want to get really creative, you can even eat invasive species.

If you have a question about ecology in Maine or see some cool science you want to share, please get in touch via email, Twitter, or Facebook!

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