Giving Ecological Purpose to Your Landscape: Part 1
By Douglas W. Tallamy
As the human footprint continues to expand at the expense of the natural capital that sustains us, there is a growing need and increasing demand for residential, corporate, urban, and suburban landscapes that generate natural resources rather than destroy them. At our current population levels, a culture that segregates humans from nature is not a sustainable option and by whittling away at functional ecosystems, such a culture has led to a reduction in the earth’s ability to produce essential renewable resources (aka ecosystem services) by more than 60% (2005 Millennium Ecosystem Assessment). To believe there will always be sufficient oxygen, clean air and water, carbon sequestration, pollinators, and the biodiversity that produces these resources, regardless of how we treat local landscapes – or to suggest that technology can effectively replace them – is folly in its most misguided form.
Fortunately, we already have the knowledge required to integrate human habitats with the natural world. Indeed, the concept itself is ironic because humans are products of the natural world – one of millions of life forms that natural systems sustain every day – and we have never been even partially independent of earth’s bounty. What types of landscapes are capable of sustaining humans and nature simultaneously? Ones that feature plants that interact with the species around them. Such plants are the key; every ecosystem service required by humans (and most other animals as well) is created either directly or indirectly by plants. We have degraded ecosystem function by removing plants from local ecosystems, or by assuming that all plants function equally well in every environment. It follows that we can quickly repair the damage we have inflicted on the typical built landscape simply by putting the right plants back. And who better to lead the way in this most noble endeavor than gardeners who know and love plants.
Nature equals specialized relationships
A pattern is emerging from conservation efforts around the world: if you want to save a particular species, you have to save the specialized relationships that support that species. If, for example, you want to save the resplendent quetzal (a gorgeous but endangered bird in Central America), you have to restore populations of wild avocado trees, because the fruits of that species are an essential component of the quetzal diet. If you want to save jaguars, you need to protect large populations of palm species that make small palm nuts (as opposed to coconuts). Why these palms? Because palm nuts sustain peccaries, the wild pigs that are jaguar prey. If you want great green macaws in the future, you need to restore populations of wild almond trees because they are the only trees those birds will nest in. Such specialized relationships are so common in the tropics that they are the rule rather than the exception.
What surprises many people, however, is that specialized relationships, particularly involving food webs, are also the rule in the temperate zone, and we cannot create living landscapes if we exclude them. If you want your may apples to spread by seed, you need a population of box turtles. May apple seeds germinate best after passing through the gut of a box turtle that has eaten the may apple fruit. If you want pileated woodpeckers in your neighborhood, you need trees that harbor large colonies of carpenter ants, because carpenter ants are what these birds feed their young. If you want your Phlox divaricata to produce viable seed, you need the plants that support the larval development of day-flying sphinx moths (such as the snowberry clearwing moth pictured above), for these moths are the primary pollinators of Phlox.
Even species that do not seem to depend on specialized relationships often do, especially during reproduction. The Carolina chickadee is an excellent example. As anyone with a bird feeder knows, chickadees are seed eaters during the fall, winter, and early spring. When it comes time to feed young, however, chickadees join the 96% of the terrestrial birds in North America that rear their young on insects. And not just any insect: chickadees feed their nestlings caterpillars. Chickadee parents could feed their young other insects, but the overwhelming majority of their prey during reproduction is caterpillars. And not just any caterpillar, but those that are not covered in hairs or spines. Because chickadees rear their young on caterpillars, there will be no chickadees where there are not enough caterpillars to bring a clutch of eggs to independence from parental care.
How many caterpillars is that? Carolina chickadees bring somewhere between 390 and 570 caterpillars to their nest each day, depending on the number of chicks in the nest (Brewer 1961). Parents feed nestlings in the nest for 16 to 18 days before the young fledge and then for 30 more days after fledging. If we focus only on the caterpillars required to reach fledging, it takes 6,240 to 10,260 caterpillars to fledge a single clutch of chickadees: an astounding number, even to those who study bird behavior. No one knows how many more caterpillars are required during the 30 days after fledging. What’s more, chickadees are tiny birds; a Carolina chickadee weighs 1/3 oz, the equivalent of four pennies. In comparison, a red-bellied woodpecker, which also rears its young on insect larvae, weighs eight times more than a chickadee. How many larvae are required to create a red-bellied woodpecker? How many insects are required to sustain an entire population of chickadees and woodpeckers . . . and titmice, and orioles, bluebirds, warblers, wood thrushes, catbirds, cardinals, buntings, flycatchers and all of the other birds that signal healthy temperate zone ecosystems? The numbers are mind-boggling. Consequences of specialization
Suggesting that designed landscapes should produce rather than destroy insects would have been ludicrous, if not heretical, in the past. After all, if plants are simply decorations, we would want them to be forever flawless and untouched by natural processes. In fact, if flawless plantings are really the goal, using silk or plastic plants seems like the more logical option. If our goal, however, is to create landscapes that contribute to, rather than detract from, local ecosystem function, then we must include “the little things that run the world” (E.O. Wilson). Decades of research have shown that insects are essential for pollination, nutrient recycling, pest control, and especially for feeding other animals. A world without insects is a world without biological diversity; and a world without biological diversity is – eventually – a world without humans. If insects were to disappear, humans would not last more than a few months. Seen in this light, waging war on insects where we live, work, farm, and play seems counterproductive at best.
How, then, can we design landscapes that support lots of insects but also stay in a balanced equilibrium with the natural enemies that control them? Before we answer this question, we have to consider the most important and abundant specialized relationship on the planet: the relationship between the insects that eat plants and the plants they eat. Read Part II »
Douglas W Tallemy is the author of Bringing Nature Home and co-author (with Rick Darke) of The Living Landscape. He is currently Professor and Chair of the Department of Entomology and Wildlife Ecology at the University of Delaware.
Excerpted from “Beyond the Rock Garden: Giving Ecological Purpose to Your Landscape” by Douglas W. Tallamy, which originally appeared in Rock Garden Quarterly, publication of the North American Rock Garden Society. Look for Part 2 in our May 2018 issue.