Someone asked me why I keep calling the Bradford pears awful, when they are so beautiful, and come so early in the spring.
Well, they’re horribly invasive. That’s easily seen along the highway, where you see them miles from any place anyone could have planted them.
They’re invasive because they out compete the native trees. Why is that so bad? Well, the problem is that the insects can’t eat them. Plants throw up a chemical defense against the insects that want to eat them. If you look at a chemical analysis of a plant, there’s what they need for photosynthesis, and then there’s a poison or two, like arsenic.
In other words, invasive species have no predators. Obviously, that’s a huge competitive advantage over native species, which do have predators. Native plants and native insects have co-evolved for thousands, maybe millions of years, and they have reached a classic predator-prey balance. The insects can eat them, but usually not to the extent that it kills the plant.
The implications of the invasive species replacing the native species are vast. If the insects can’t eat, they die. If the birds can’t eat, they die, too. That helps explain why we have lost such a high percentage of our birds – something like 29% in the past 50 years. The crowding out of native species by invasive species is an important part of habitat loss, which is acknowledged to be a major cause of the decline of virtually every North American species. This is not just a theoretical problem. Where we visit in northern Indiana, it’s easy to see that the Japanese honeysuckle is simply taking over, crowding out everything else. And it’s starting to happen here.
The biggest problem is that this threatens the very foundation of the food chain that we all depend on to live. Biologists have varying estimates of how close we are to food chain collapse, but there are few who don’t acknowledge it as a concern.
A huge problem is that the nursery industry promotes many of the species they sell as “pest resistant.” That sounds good, until you realize that they’re basically talking about starving the insects and birds. For instance, Home Depot and many others are selling plants treated with Imidacloprid. That’s the stuff that’s banned in Europe because it’s killing off the bees. So, someone goes to Home Depot and buys milkweed, because it is known to attract butterflies. It does. But after it attracts the butterflies, it kills their larvae. That’s not exactly what most people who buy milkweed are trying to achieve. Last I heard, Home Depot is unconcerned with the problem, saying that it’s a legal product, and they are going to go on selling it.
In short, this is why conservation organizations like the Audubon Society are urging people to plant native species.
A good overview of the subject can be found in Douglas Tallamy’s Bringing Nature Home: How You Can Sustain Wildlife With Native Plants.