I just finished reading a very good article from The Guardian's Environment section. (The Guardian, IMO, does the best job of reporting on environmental issues of any major newspaper currently) The article described an emerging trend in some U.S. cities to make biodiversity and the the enhancement of urban wildlife a prominent concern in decisions surrounding the kinds of trees planted on city land, such as streets, parks and even schools. While it might seem like an obvious criteria, the fact is that over the past several decades most urban planners, and most private residents living in the city, have given little to no regard to such concerns when choosing what variety of trees to plant for ornamental purposes. In many cases, the primary factor has been a tree's aesthetic appeal.
There has been a remarkable level of disregard, when it comes to urban landscapes, to the interrelationship between tree species and urban wildlife. Consider this fact:
Of the Audubon Society's list of the 20 common birds in decline here in America, all of them have seen their populations decline by at least 50% just since 1970. Of 800 bird species, about 17% are in decline.
Much of that population decline is the result of habitat loss. The mistake we often make, however, is thinking of habitat loss strictly as a function of urban development, as in the Joni Mitchell song
They paved paradise, put up a parking lotWhile the physical loss of habitat cannot be discounted as a factor in the decline in wildlife populations, the composition of tree species in what habitat that is left also plays a huge role in the ability of birds and other wildlife to survive and thrive in the urban environment.
This point is driven home by taking the single example of one bird species many of you are probably familiar with, the Carolina Chickadee.
A single pair of Carolina Chickadees, according to Professor and Chair of the Department of Entomology and Wildlife Ecology at the University of Delaware Douglas Tallamy, must capture 6,000 to 9,000 caterpillars and take them back to their nest in order to raise a typical brood of 6 nestlings. Those caterpillars, in turn, depend upon specific host trees during their life cycles. And when it comes to trees, it should come as no surprise that all varieties are not created equal. An oak tree may support as many as 537 different caterpillar species. In contrast, the Ginkgo (Maidenhair) tree, which is native to China and widely used by cities and homeowners around the country as an ornamental specimen, hosts only 3 caterpillar species. That consideration carried far less weight in planting decisions than did its appearance and the fact that it can grow in even poor soil conditions with little care.
In thinking about urban biodiversity, it seems intuitively logical that native trees and shrubs would support more native wildlife than non-native species. But, it is more nuanced that that even. The Tulip tree is native to North America, and is another species that has been widely planted over the years as an urban ornamental tree. It grows fairly quickly, reaching a height of some 160 ft. Yet compared to the oak tree mentioned above, a Tulip tree supports only about 21 species of caterpillars.
The most basic requirements of wildlife, in the end, are the same as for humans. They need food and they need shelter. In that respect, trees matter. Landscaping matters. It is not the number of trees, or the fact that you landscape your home alone that matters. What you plant is often more important than the act of planting in and of itself. If we value biodiversitity...if we are serious about addressing the decline in bird populations and sustaining wildlife in our urban environments, we must move considerations of their needs for both food and shelter higher up on the list of criteria used in making such planting decisions. And this consciousness needs to become institutionalized at both the city level and, individually, as homeowners.
Another of the problems associated with using non-native landscaping in the urban environment is that, to paraphrase a popular civic a motto, "What happens in Vegas doesn't always stay in Vegas." Some non-natives, though certainly not all, refuse to stay put in the urban settings they are first introduced to. Think about English Ivy, or Kudzu, or even certain trees. As the Jeff Goldblum character in "Jurassic Park" warned early on in the film, "Nature finds a way." Mostly, it finds a way to reproduce and spread. As these non-native plants escape the confines of their urban gardens and landscapes, they spread into more "natural" ecosystems, often displacing native vegetation and forest species to the detriment of wildlife by reducing vital food sources and breeding habitat.
There is an intrinsic irony with respect to non-native plants that is parochial in nature. A home owner might think that he/she is actually increasing the biodiversity of their immediate environment by planting a non-native species on their lot. After all...this is a plant that doesn't ordinarily grow here, so if I plant it, I have made the environment, by definition, more "diverse." That's not the case, however. Lacking natural competitors or pathogens, many of these non-natives often have the ability to crowd out native species and alter the environment. From a global perspective, the rapid spread of non-native species is having the effect of actually decreasing biodiversity, and is leading us inexorably towards a global homogenization of plant and wildlife diversity. I have seen it referred to as a global march towards the McPlanet...where no matter where you travel, one encounters the same plants and animals.
Even within a large urban environment, however, there is also an unmistakable socio-economic component to biodiversity. The more affluent a neighborhood, it turns out, the more diversity exists. Upon reflection, that's hardly surprising. More affluent neighborhoods are characterized by more discretionary income that is used for landscaping. If you were a bird or an urban mammal, where would you rather reside and rear your young?
One of the more interesting web based developments that has arisen is the marriage between two distinct databases in order to analyze the links between urban tree species and biodiversity. From Yale Environment 360:
A new study in the journal Landscape and Urban Planning also looks at better ways of understanding urban wildlife and habitat in combination. The study uses birds as bio-indicators for other wildlife types because they are easier to count than shy, often nocturnal, mammals, and because they are more broadly familiar to the public.One can only hope that urban planners and city agencies involved in environmental decisions regarding street trees and open spaces will make use of such information in the future. On a more individual level, what can be done to educate homeowners about how their landscaping choices impact biodiversity? That is, perhaps, a more vexing question. People like pretty, eye catching specimens. And they really, really like easy. Are native trees any harder to cultivate than non-natives? Not really, in most cases. Sure, there are some non-natives that are more tolerant of salt, if you live in the snow belt, or which accomodate more neglect and inattention.
The study proposes a marriage of i-Tree and eBird, two current methods for keeping track of the natural world. Designed by the U.S. Forest Service, i-Tree is software used by organizations around the world to record data on urban tree cover, from single trees to entire forests. Its counterpart, eBird, from Cornell Lab of Ornithology, is a checklist system enabling thousands of birders around the world to log their observations into a central database. The combination of the two enables researchers to assess not just which trees characterize a neighborhood, but how good they are as bird habitat, and which birds are using them.
But if you are like me, you will come to the conclusion that the trade off is not really worth it. Perhaps you enjoy birds, and even put out feeders to try to attract them. And perhaps, like me, you have been disappointed by the variety of birds that come to your feeder...even discontinued feeding them for that reason. These birds...indeed, all urban wildlife, require more than simply a handout in the form of birdseed. They need an ecosystem. They need native food. They need insects and caterpillars, which in turn depend upon specific host plants. Maybe you plant flowers, hoping to attract butterflies...and have also been disappointed at the low turnout. Butterflies need more than just nectar from blossoms. They need host plants during their larval stage...and often times those host plants aren't the most attractive plants for landscaping.
As more and more cities spray roadsides to control weeds, many of those host plants are disappearing. Learn what butterflies are native to your area and google them. Learn what plants they require over their entire life cycle. You may not want to plant wild fennel, for example, in your front yard...but perhaps you have an "out of sight, out of mind" strip on the side where it would provide beneficial habitat without marring the aesthetics of your landscaping vision.
But if we, as individuals, and as urban citizens, value wildlife and hope to stem the loss of biodiversity here, we must take into consideration what we plant and why we plant it. Forget that Ginkgo tree, or that Norfolk Island Pine. or that Amur Maple. Instead of planting an evergreen shrub like juniper, consider a fruiting shrub. See a vacant lot in your neighborhood? How about seeding it with an inexpensive can of wildflower seeds? Call your local city's department in charge of tree planting/environmental services, and urge them to consider native trees that support wildlife instead of choosing the most "neglect tolerant" tree out there.
We can do this.