Imagine that one morning you walked into your kitchen for breakfast and discovered that all your food was unfamiliar.  In the refrigerator, in the cupboards, everywhere you looked, you found food that looked similar to what you usually ate, but that tasted and smelled awful!  You drove to the store only to find that all your familiar brands were gone, replaced by strange, unappetizing, inedible products. Same story at the local fast food places and restaurants.  There was nothing for you to eat anywhere. You’d feel you had awakened in an alien land. You’d wonder how you could survive.

Even though humans are omnivores, with a much broader diet than other species, there are just some things nature hasn’t equipped us to eat.  We can’t eat holly berries or belladonna. We can’t eat milkweed or skunk cabbage. Some animals and insects can consume them easily. But we didn’t evolve with the ability to digest some foods or to resist the toxins in others.  For us, they aren’t food.

Bees and Butterflies – Now imagine that you’re a bee, out on your rounds, looking for food.  You’re looking for just the right kind of flower. But what if every time you land on a flower you find it has no food you can consume?  Or you’re a butterfly looking for a leaf on which to deposit your eggs. You need to find a leaf that will provide nourishment for your caterpillar offspring.  But what if all you find are flowers and leaves that are completely inedible. For you, as a bee or a caterpillar, you might as well have found yourself on a plastic plant.  Once again you’d feel you’re in an alien land.

To an insect, a species of plant that can’t sustain it might as well be an alien.  In fact, that’s what horticulturists call plants that are not native to a particular environment – alien species.  They didn’t evolve with the local insects, so insects have developed no way to use them for nourishment or to resist the toxins that may be present.

Birds – Have you ever walked through your yard in late summer and noticed that some leaves are riddled with holes, while others remain shiny and perfect?  Chances are the ravaged leaves are natives while the untouched ones are aliens. The native plants have been chewed on by insects. The alien plants, like plastic plants, survive with little damage.  

If native insects can’t feed on alien plants, then the plants will stay pretty, and insect populations will decline.  Sound good? Not really. What’s icky to us humans is delicious food for birds and other animals. Reduce the food supply, and birds and other animals will find it hard to survive.  

Loss of habitat – homes and food sources – has caused dramatic reductions in animal species.  It is estimated that we’ve experienced an almost 50% reduction in bird populations within the last 50 years.  Conservation ecologists believe that one quarter of all bird species will be extinct within the next 100 years.   

Alien plants aren’t really part of the environment.  If they don’t provide sustenance for native insects, those insects may be unable to survive.  And if those insects disappear, what will happen to the birds and other animals that survive by consuming them?

Biodiversity – We hear a lot of talk about the necessity of biodiversity.  For good reason. One big advantage to biodiversity is that a diverse environment is more resistant to attack.  If we had only elm and ash trees, we’d be in deep trouble because of Dutch elm disease and the emerald ash borer.  All our trees would be doomed. But because we also have oaks and hickories and maples and many other species, one invader is unlikely to kill all trees.  And the presence of those other species means that one invader has a more difficult time finding susceptible trees among all the trees in the woodlands. What is true for trees is also true for other plants.  

There’s an even more important reason for biodiversity.  As we learned in grammar school, life on planet Earth is a complex web of interactions and dependencies.  We need it to sustain the interdependent balance of life on earth that has developed over tens of thousands of years.  Like the building blocks in a child’s toy tower, biodiversity is essential to the stability of our ecosystem. Removing species from our ecosystem, by our intentional or unintentional extermination, is like removing blocks from the tower.  We risk imbalance. In the worst case, we risk collapse.

But if diversity is such a good thing, what’s wrong with planting non-natives?  Doesn’t that increase diversity? Yes, it does. Temporarily. Then two things happen.  First, without natural predators, the alien species multiply freely, crowding out the natives.  Second, since the native animals can’t use the aliens for food or shelter, the native animals begin to die out.  The net long-term effect is a reduction in diversity of both plant and animal life.

Smothered by Aliens – Have you ever seen a landscape of trees and bushes covered with vines?  Those vines are probably alien plants that were brought into the country intentionally for decorative value, or because they grew rapidly and were “pest free.”  Or the vines may have been brought into the country inadvertently, as seeds in containers of other plants. Those plants, such as kudzu, multiflora rose and oriental bittersweet, are now considered noxious invasives that threaten to smother our native landscapes.   

Inedible Aliens – Why can’t our native insects and other animals eat alien plants?  Sometimes the reason is that the alien plants have toxins that native animals have not developed the ability to tolerate.  Sometimes it’s the texture of the alien plants, or the difficulty of digestion, that causes a problem for native animals. Most insects are specialists, in that they eat only one or a few types of plants.  Most native plants are inedible for many native insects, so it’s no surprise that alien plants are even more of a challenge.

Birds and Alien Plants – Do birds have a problem with alien plants?  Yes, sometimes directly, and sometimes indirectly.  Many adult birds survive on the berries plants produce.  Some birds are able to eat the seeds and berries of alien plants.  But the birds then distribute the seeds of the alien plants, helping those plants to proliferate and dominate the environment.  Other birds are unable to tolerate alien berries and seeds, and disappear from the environment.

Baby birds have another challenge.  They can’t eat berries or seeds. They eat caterpillars.  Since many native caterpillars are unable to consume alien leaves, the birds must find native plants bearing caterpillars.  The more alien plants around, the fewer native plants. Fewer native plants mean fewer baby birds, which means fewer adult birds.  It’s the web of life.

What’s a Native? – Plants, animals and micro-organisms that have evolved together in an area adapt to the soil, moisture and weather conditions, and depend on one another for food or habitat or propagation.  If a plant is transported to a different area, it may confront an unfamiliar environment with unfamiliar species. It may be unable to thrive, or it may thrive but contribute little to its environment as food or habitat.  In the worst case, it may become wildly successful, uncontrollably dominating the landscape. Buckthorn and garlic mustard are among the many local examples of this latter problem.

A plant moved to an unfamiliar environment will, if it survives, take up space, water and soil nutrients that would otherwise be available to native plants.  But it may not serve as food or habitat for other species in the area. A native species interacts with its environment – taking and giving – while an alien species may become a free rider, using the resources of its surroundings while contributing little.

Can an alien species ultimately become a native species, interacting with its environment in the same way that long-time natives interact?  The unfortunate answer is that the slow pace of evolution means that such adaptation probably would take tens of thousands of years or more.

Alien Bugs – Besides taking up water and nutrients that would otherwise nourish native plants, and besides aggressively multiplying to dominate a landscape, alien plants pose an additional threat.  They can be transporters of alien insects and diseases against which native plants may have few defenses. The American chestnut tree was once a major part of the landscape, producing more nuts than oaks and hickories combined.  Animals from squirrels to deer to bear thrived on the nuts. Songbirds fed on the caterpillars that lived in chestnut trees. The American chestnut inspired Longfellow’s poem “Under the spreading chestnut tree, the village smithy stands…” and the well-known song “Chestnuts roasting on an open fire…”  But now the American chestnut is gone, a victim of disease brought to the U.S. with imported ornamental chestnut trees.

Containers of alien plants may also contain unwanted seeds – seeds of other alien plants.

Alien plants have been introduced accidentally, and intentionally.  Plants that nurseries promote as “pest free” are frequently aliens. “Pest free” means, of course, that native insects won’t eat the leaves, which may allow the plants to propagate more freely than natives.  It may also mean that the plants won’t feed the caterpillars that turn into butterflies or that feed the local bird populations. Fewer caterpillars mean fewer butterflies and fewer birds.

Balance of Nature – A healthy environment is one in balance – in equilibrium – in which no one member dominates another.  A garden in which all leaves are perfect and untouched is not contributing to the balance of nature. It provides no food source for caterpillars, which means no butterflies or birds.

We often think of pests as those that consumer our plantings.  And we work hard to keep those pests in check. But in an environment in which the natural enemies of those pests can thrive, they’ll keep the pests in check without the need for our intervention.  

Want butterflies?  You’ll need plants that provide both nectar for the adult butterflies and plants on which butterflies will lay their eggs and that provide food for the larvae.  Only a few plants do both. Milkweed, Coneflower and Black-eyed Susan? Yes. Butterfly bush?  No.

A store that sells only one product will have a big problem if the product is recalled or the supplier is unable to deliver it.  Stores that sell multiple products are more likely to survive shortages of one product. In the same way, a property that has only one type of vegetation – a large expanse of turf grass, for example – will have a problem if the grass is threatened by disease or drought.  Variety is the spice of life. It turns out to be the essence of life.

Source:  Bringing Nature Home, by Douglas W. Tallamy.  Dr.Tallamy’s book is an excellent resource, widely available, with beautiful photos and many valuable recommendations.