Strolling the pastoral pathways of the Polly Hill Arboretum in West Tisbury, you would never suspect that among the shimmering medley of goldenrod and milkweed and beneath the hypnotic drone of sunbathing insects, a silent battle is being waged. It is the yearly struggle between invasive and native plant species for the natural heritage of the Island and for the very future of biodiversity in the region. It is a battle that Doug Tallamy desperately hopes can be won by the home team.

Professor Tallamy, chairman of entomology and wildlife ecology at the University of Delaware, will speak tomorrow night at 7:30 p.m. at Polly Hill about his new book, Bringing Nature Home: How Native Plants Sustain Wildlife in our Gardens, and again on Thursday in a talk titled Vineyard Oak Appreciation, from 10 to 11:30 a.m., about the ubiquitous tree and its particular importance to local ecology.

His message is simple and logical: native ecosystems depend on native plants for their survival.

Plants convert sunlight to energy, insects eat plants, and birds and other animals eat insects. In fact, 96 per cent of terrestrial birds rear their young on insects, and with good reason, as he points out in his new book: “Pound for pound, most insect species contain more protein than beef.” Take away the plants that insects depend on, and the insects will disappear. Take away insects, and the rest of the food web will collapse. We do this, he argues, when we populate our gardens and landscape our properties with plants that, although perhaps satisfying some aesthetic purpose, are practically useless from an ecological perspective and only serve to impoverish local biodiversity.

“Most people landscape just because that’s what their neighbors are doing,” he said in a telephone interview from his home in Pennsylvania over the weekend. “They don’t know anything about plants, they don’t care whether it’s native or nonnative. They’re simply doing what they think you’re supposed to do, without understanding what the consequences are.”

As more land is turned over to development and rich native habitats are replaced with sterile, nitrogen and herbicide-intensive lawns, the consequence is ecological collapse, he said.

He has found an apt setting for his talk. The arboretum has launched an aggressive campaign to promote local plants, collecting and cataloguing seed from a variety of locations on the Island, mapping local plant populations with GPS software, creating an insect and plant-host relationship database and selling specimens of native species.

The plants available for purchase read like postcards from around the Island: beetlebungs from Beetlebung Corner (not unimportantly the diet of the spicebush swallowtail butterfly), asters from Chilmark, and butterfly weed, an orange-flowering milkweed that dots Island grasslands, sustaining the local population of monarch butterflies.

“It’s extraordinarily hard to propagate in horticulture and normal nurseries just don’t sell it,” said arboretum director Tim Boland. Without butterfly weed, there would very simply be no monarch butterflies on the Vineyard.

To understand why certain insects can thrive only in the presence of certain native plants is to understand a creature’s evolutionary history within an ecosystem.

After all, plants are just as selfish with the energy they capture from the sun as the insects who seek to appropriate it. Mr. Tallamy explains: “Plants want to use their energy for their own growth and reproduction and that’s why they protect their leaves from being eaten. They do that by putting secondary metabolic compounds — nasty chemicals — into their leaves that make them distasteful and sometimes downright toxic. So that means that the animals that are going to eat these plants need to adapt to these defenses. They need to be able to circumvent them, to break them down to store and excrete them, and then they can eat the plant without being poisoned.”

The only way for a creature to develop the necessary defenses against such toxins is to be exposed to the plant over a long time scale — a nearly impossible task with nonnative species, Mr. Tallamy said.

Using an example familiar to Islanders, he said: “The monarch is a milkweed specialist; it only eats milkweed because it has, over time, come up with the adaptations to break down cardiac glycosides, one of its toxins. The good part about that is that it allows the monarch to eat a plant that almost nothing else can eat. The bad part is that it restricts the monarch to eating milkweed. When it developed the ability to eat milkweed it lost the ability, in the monarch’s case, to eat everything else, and that’s the downside of specializing.”

He said 90 per cent of insects are such specialists, depending often on only a single plant for survival.

At the arboretum and around the Island, native plant species such as milkweed are often elbowed out by foreign invaders such as bittersweet and autumn olive (“just a horrible plant,” Mr. Boland said), brought to the Island as ornamentals from China. They were also used as part of a USDA program in the 1950s that promoted aggressive plants for erosion control.

One invasive species on the Island, often erroneously believed to be native, is the black locust tree, a comely tree prized by fence-builders for its hardiness and unmatched resistance to rot, but which nonetheless fixes nitrogen in its root system and invites more aggressive species to the neighborhood. The black locust offers its own unique challenges, Mr. Boland said: “Polly Hill herself loved it and everyone on the Island loves it and it smells great, so that’s a management dilemma!”

Other offenders from abroad include foreign grasses such as European fescues, also known as cool-season grasses, that were imported for grazing and served an agricultural purpose but have since become pesky invaders, robbing native warm-season grasses of nutrients by germinating earlier in the season and monopolizing soil resources. To wipe out the cool-season agricultural grasses the arboretum has conducted a series of prescribed burns of their fields in early spring, clearing the way for the local and less aggressive bluestem grasses that take off in the summer.

But Mr. Tallamy said there are more prosaic steps the average gardener can take to turn the tide against invasives. The solution is as simple as gardening with native species. When Mr. Tallamy and his wife moved into their southeastern Pennsylvania home in 2000, they noticed that a full 35 per cent of their backyard had been annexed by exotic species, including bittersweet and autumn olive, the same ones that afflict the average Vineyard landscape. He quickly set about to reclaim the local ecosystem. “It really encourages me,” he said. “It’s actually working. Every year we see new species come back to this area. So I’m optimistic, first of all because it’s easy to do and it’s something we all can do.”

He also believes the Vineyard has a built-in advantage over the mainland in this effort.

“People who live on Islands understand there are limits,” he said. “You understand there’s only this much land, whereas people who live on the mainland always think that nature’s happy some place else. On your Island you can see exactly what you’ve taken and what you’ve left.”


Doug Tallamy delivers the annual David H. Smith Memorial Lecture tomorrow at 7:30 p.m. at the Polly Hill Arboretum in West Tisbury. Admission is $10; $5 for arboretum members.