Giving Ecological Purpose to Your Landscape - Part II
By Douglas W. Tallamy:
Excerpted from Giving Ecological Purpose to Your Landscape Read Part I »
It is not actually the number of species in an ecosystem that controls ecosystem function, but rather the number of interactions among species. This explains why invasive plants are decreasing rather than increasing ecosystem function around the world. Non-native plants are just meeting the plants and animals in the “novel” ecosystem for the first time in evolutionary history, which means they have not had the time to develop the adaptations required to interact with many of the other species in that ecosystem.
Which plants should we use?
There are huge differences among plant genera in their ability to make caterpillars and thus support other creatures. Oaks (Quercus) in the Mid-Atlantic states, for example, serve as host plants for 557 species of caterpillars, tulip poplars (Liriodendron) only feed 21 species, and yellowwood (Cladrastis) is not used by any caterpillars at all. These are order-of-magnitude differences among plant genera that are all native to eastern North America. Second, a mere 5% of the native plant genera in any North American ecosystem support 73-75% of the caterpillar species.
We don’t yet understand why some plant genera are responsible for so much of the life around us, while most pass on minimal energy, and some none at all, to local wildlife. But we do not need to understand the basis of the relationship to use it effectively in landscape design. Wherever we are in the U.S., we can create plantings that sustain birds, reptiles, amphibians, and mammals by generating tens of thousands of insects. Landscape designers and architects, land managers, restoration biologists, and above all home gardeners can learn which native plant genera contain core species at the National Wildlife Federation website under “Native Plant Finder” at www.nwf.org/NativePlantFinder. Enter your zip code, and a list of plant genera found in your county, ranked from most to least productive, will appear.
Creating trophic balance
Because our past goal in constructing built landscapes has been to create beauty using plants rather than ecological integrity, a primary concern has been the aesthetic appeal of the plants themselves. A perfect specimen unmarred by insect damage has been the ideal. As we have seen, though, a perfect plant is one that has not interacted with other species in our landscapes, and a landscape full of perfect plants is an ecologically barren space devoid of animal life.
When species interact over long periods of time, a balance among plants, herbivores, and natural enemies (predators, parasites, parasitoids, and diseases) emerges that typically keeps any one species from eliminating the others. This is the ideal that we should strive for in our built landscapes. If we use native plants that support dozens of species of insect herbivores, we will create a food resource for hundreds of species of the natural enemies of those insects, so they too will become residents in the landscape and will keep insect populations below the aesthetic injury level. The spiders, assassin bugs, damsel bugs, ladybird beetles, lacewings, predatory stink bugs, digger wasps, parasitic Hymenopterans, bluebirds, tree swallows, cardinals, hummingbirds, catbirds, and many other insectivores all kill tens of thousands of insect herbivores before plants suffer noticeable damage. But natural enemies will not be in our landscapes if there is not enough food to support them. Fortunately, we have some wiggle room here, for studies have shown that people do not even notice insect damage until about 10% of the leaves have been eaten. Most plants are viewed at a distance; even the oak tree that supports hundreds of caterpillars looks untouched from 20 feet away.
Using more plants
Today our built landscapes are dominated by turf grass. We have favored large lawns bearing few plants for two reasons. First, we prefer savanna-like landscapes, presumably because we feel safer in such environments. Second, large, flawless lawns have been a status symbol of the rich for centuries. Such landscapes may have met our physical and social needs when we were hunter-gathers, but they are an environmental disaster in today’s world of 7.5 billion people. We now have over 40 million acres of lawn in the U.S. and we are adding 500 square miles more lawn each year.
Raising the bar
To achieve a sustainable relationship with the earth, we must raise the bar for what we ask of our built landscapes. In the past we have asked that they be attractive, well-tended spaces. We have achieved this in grand style. But our need for ecosystem services is now so great that we can no longer rely on the remaining degraded and fragmented “natural” spaces to produce enough. We must now design beautiful landscapes that also support complex food webs, which in turn support the biodiversity that runs our ecosystems. We need landscapes that sequester carbon: lawn sequesters 27 times less carbon than a meadow. We also need landscapes that clean and manage water. A lawn-dominated landscape impedes infiltration, creates disastrous storm water runoff, and adds nutrient and pesticide pollutants to aquatic ecosystems. Finally, we need to design landscapes that support diverse pollinator populations. Pollinators across the US are in steep decline due in large part to the loss of nesting sites and seasonally abundant forage. Manicured lawns provide neither resource. Pollinators, including the 4000 species of native bees that did all of the pollination in North America before the introduction of the honeybee, are not optional. They pollinate 80% of all plants and 90% of all flowering plants. If we were to lose pollinators, we would lose 80-90% of all plants, including 1/3 of our crop species.
Homegrown National Park
We have it in our power to create a new national park of sorts simply by redesigning the landscapes in which we live, work, and play. If we were to replace half of the area now in lawn with 3-dimensional plantings of powerful native plant communities, we could create over 20 million acres of spaces that generate, rather than destroy, ecosystem services. Our “Homegrown National Park” will be enormous – bigger than all of the major national parks combined – and it will provide us with many of the benefits we derive from visiting our official national parks. Just 15 minutes in the solitude of a well-planted garden can lower blood pressure, reduce stress (cortisol), improve attention span, raise immune responses, and provide unlimited entertainment as we observe the life around us. Our new plantings will fill the gaps between fragmented natural areas, creating biological corridors that reconnect them. If habitat fragments are reconnected, they will support populations that are large enough to withstand normal fluctuations without disappearing. To be sure, this is an optimistic view of our future but it is also a feasible one and will yield enormous ecological payoffs both for humans and our fellow earthlings.
A living landscape rich in productive native plants can also include less powerful plants and even plants that are strictly decorative, leaving plenty of room for the perennials and annuals we love so much.
Douglas Tallamy is Professor & Chair of the Dept. of Entomology & Wildlife Ecology at University of Delaware