The invaders lurk out there, along highways, in meadows, forests and rivers, masquerading as pretty flowers or attractive autumn foliage. They have exotic names, like Flowering Rush, Purple Loose-strife, Japanese Honeysuckle and Water Chestnut.

For 20 years, Leslie Mehrhoff, curator of the University's George Safford Torrey Herbarium, has been closing in on this invasion of alien plant species, whose abundance and spread are wreaking havoc and threatening many of the native flora in New England's habitats with extinction.

His tools are boots, knapsacks, good hats and a yen to traverse the lesser traveled rivers and streams, highways and byways of New England in search of the invading plant specimens.

"I love field work," says Mehrhoff, who often uses his lunch hour to trudge through fields near the Storrs campus, hunting for plants and seeds. "It's basic to my research."

Mehrhoff, who specializes in plant ecology and plant geography, has been documenting the spread of non-native invasives around New England for 25 years.

Along with a small staff of students, he tends diligently to a collection of dried pressed plant specimens of state- or regionally-i mperiled species stored in the herbarium, which is the official repository for the State Geological and Natural History Survey.

Standing amidst the stacks that house more than 120,000 specimens, Mehrhoff notes that "some just see this as storage space." But instead, he explains, its function is like that of a library. "Every single sheet in here represents a single plant's

distribution at a certain vignette of time. What that does is provide an historical record, a composite record of the distribution of the species in space and time."

Scientists believe the current global rate of plant extinction is unprecedented. Since the 1800s, the United States has lost close to 200 plant species. Today, 5,000 plant species are considered vulnerable to extinction, and more than 600 are on the federal threatened and endangered species list. These plants are disappearing because humans are destroying their habitats.

Native plants are an integral part of all ecosystems, says Mehrhoff. Each species belongs to a carefully balanced system that supports other species. In a healthy ecosystem, plant and animal species interact to keep the system working properly. Birds feed on berries from shrubs, and seeds from the berries are deposited by the bird, thus dispersing the shrubs. Earthworms churn up the soil, aerating it and improving plant growth; the plants drop their leaves, providing the earthworms with organic matter for food.

The intricacy of ecosystems - each with thousands of species of plants, animals, insects, and bacteria - boggles the mind, he says. There is no way to know beforehand what the loss of even one species will mean to an ecosystem and, subsequently, to species such as humans who rely on that ecosystem.

Additionally, Mehrhoff notes, communities of native plants hold in reserve an irreplaceable gene pool that scientists can draw upon in their search for new medicines or improved agricultural crops. Of all medicinal drugs, 40 to 50 percent originate in wild plants, but only 2 percent of the world's plant species have been chemically analyzed.

Although the movement was slow to start, each year more people become concerned about the problem of invasive species. It was the focus last month of an Executive Order from President Clinton directing federal agencies to develop a detailed invasive species management plan within 18 months.

Mehrhoff estimates that of the approximately 3,000 vascular plants that comprise New England's flora, about a thousand are non-native plant species, 200 of which can be considered invasive.

"Perhaps three dozen are documented as seriously invasive and another three or four dozen either have recently been reported in the region or have the potential to become invasive, based on their actions elsewhere," he says. "This means less than 3 percent of the vascular plant species in the region are both non-native and invasive. Yet these 3 percent have the potential to seriously damage our native plant communities."

Mehrhoff, who is currently working on a book with the working title "Non-Native Invasive Flora of Connecticut," describes five key biological traits that characterize invasive plants species: each plant produces large quantities of seeds; they have very effective dispersal mechanisms; they germinate quickly and are readily established; they grow rapidly; and they are very effective competitors.

"Once established in a plant community, the non-indigenous invasives take over and dominate the vegetation," says Mehrhoff. "Ecological principles and common sense suggest that these 'hostile take-overs' must be at the expense of other plants."

One simple solution is to identify the problem plants. Mehrhoff describes his book as a "preemptive effort" to educate the public on what the invading plants look like.

"Consider the damage done by the purple loosestrife, a beautiful, seemingly harmless flower one might be pleased to find in a meadow. This species is now found in 36 states, including Connecticut, and costs $45 million to manage," he says.

"I want to aim my book at land managers, whether they be those tending to their back yards or nature preserves," he continues. "The easiest thing we can do is establish an early detection system and yank the plants up before they get established."

David Bauman