Posted by Bert Cregg
Reading through Linda’s recent article in Arboriculture and Urban Forestry “Nonnative, Noninvasive Woody Species Can Enhance Urban Landscape Biodiversity” (Arb. & UF 41:173-186) reminded me of some thoughts I had while I sat through a talk by Doug Tallamy at the New England GROWS conference back in February. As most GP blog readers are likely aware, Tallamy is the author of Bringing Nature Home. The basic premise of the book is that native insects only feed on native plants and therefore the use of exotic landscape plants will cause ecological food webs to collapse and the end of the world as we know it.
Looking at things critically, there was certainly plenty to nitpick in the talk. For example, to illustrate that insects feed more on native than non-native genera, Tallamy contrasted Quercus, a genus that includes hundreds of species distributed across the northern hemisphere, to Ginkgo, a monospecific genus (heck, a monospecific order) with a limited native range in China. Tallamy also compared the food value for birds of seeds of five native shrubs with five Asian exotics. The North American plants all had higher nutrition, which leads to one inescapable conclusion: If you’re a bird-lover in Asia you need to plant more exotics from North America.
But, as I said, those items are nitpicking. In an apparent effort to appease critics, Tallamy addresses the question of whether it’s ever OK to plant exotics. He says yes, but think of them as “statues” – nice to look at but devoid of any ecological value – and then put up a slide of a landscape with a bunch of statues photoshopped in. All analogies break down eventually but the ‘exotics as statues’ model fell apart before the slide hit ever the screen. First off, native insects, birds, and other wildlife do utilize nonnative plants and I’ll refer you to Linda’s review and her 100+ citations on that point. But just as importantly, nonnatives can also provide many of the same environmental benefits as native plants. Statues, on the other hand, don’t cool buildings, sequester carbon, reduce stormwater run-off, and create vertical structure and cover for wildlife – OK, statues provide perches for pigeons, but that’s about it. Landscape plants, and trees in particular, can provide a myriad of ecological services – but they can only provide these functions if they are alive to do so. Despite the intuitive appeal of, “Native plants are adapted to our local environment”, in many urban, and even suburban, environments this is simply not true due to urban heat island effects, top-soil removal, compaction and other site disturbances.
In his talk, as in his book, Tallamy demonizes the typical suburban landscape showing a photo of an expansive lawn with a lone Bradford pear tree. OK. Is the problem here the pear? If we changed the pear to, say, a lone shagbark hickory would anything change dramatically? No, but this does bring me to a point of agreement with Tallamy. The fundamental problem we face in our landscapes is lack of diversity; not only species diversity but also structural diversity and age class diversity. If we can build diversity and generate ecosystem services with natives; great, go for it. But we need to understand that our list of suitable native trees is shrinking (as an aside, ignore Tallamy’s recommendation to plant green or white ash if you live in eastern North America). Vilifying, or even banning, nonnatives will ultimately reduce diversity in urban and community forests and their ability provide ecosystems services, not enhance it.