What does it mean for a species to be rare?

Several summers ago, my wife and I visited the Bruce Peninsula in Ontario, Canada in search of wildflowers.

We were well rewarded by fields of prairie smoke flowers and forest carpets of white trillium, and then saw something quite unexpected. Along the road to a campground in the Bruce Peninsula National Park, our gaze locked onto large, abundant clumps of striking yellow flowers along the roadside — lady slipper orchids

This same species is rare and endangered here in Massachusetts, and I had once seen a few at one of their 12 reported spots in the commonwealth, but what on earth was going on to turn this marvelous, exotic plant into essentially a roadside weed?

I should have been less surprised. The Bruce Peninsula is a prime destination for orchid enthusiasts, with 40 species and an annual festival. Apparently, cutting roads through the dense spruce forest creates light gaps and wet ditches that mimic the disturbances in the local sandy, calcareous and boggy soils caused by wind-throw and storm damage that usually provide new sites for orchid colonization.

But beyond the elegant beauty and startling proliferation, this sight brought to mind a long-standing discussion in ecology — what does it mean for a plant species to be rare? In fact, on any given patch of ground, most species are in low abundance. Even when the number of species is high, most of the biomass is typically found in a few dominant species.

The total population of a rare plant or animal may have just a few individuals, or be restricted to a narrow geographic range, or both. Some rare species occur sparsely over a broad area. Sometimes plants are numerous, like the yellow orchids in Ontario, but they are crowded into a tiny area, perhaps a single county or peninsula. The very rarest plants have both a narrow range and few individuals.

Time and distance also play a part in rarity, as ecologists Daniel Simberloff and E.O. Wilson discussed in their theory of island biogeography in the late 1960s. Small populations in restricted areas (“islands”) are at risk of local extinction from such perils as severe weather events, biological invaders and human disturbance.

If there are other populations of an endangered plant or animal nearby, a site of local extinction may be recolonized, the speed of which depends on distance, the species’ mobility and the size of the island. Isolated patches of unusual soils, such as limestone outcrops or bogs, can act as islands.

Dispersal is far and wide in some plants, short and chancy in others. So species abundance is variable in space and time even in the absence of human activity, and plants like yellow orchids, though very rare in one locale, can become abundant as conditions change.

But what of the ecological importance and function of rare species? If there are very few individuals, might their impact on the local ecology be small and hence the local loss of that species not so disruptive? In some cases, we can identify a key role for a species low in total biomass or population size.

For example, through intensive sampling of plants and soils over many years at the Hubbard Brook Experimental Forest in New Hampshire, researchers found that perennial spring ephemerals, like the trout lily, perform a crucial ecosystem function. In spring, snow melt causes rapid flushing of calcium, an important plant nutrient in low supply in the acid soils of the White Mountains, into streams.

The trout lily is one of only a few plant species actively growing and taking up calcium at this critical time, thus storing it for release and use by other plants later in the season as its leaves die back.

In animal systems, top carnivores provide another example: They are often low in number and biomass but high in impact, and their local extinction has severe consequences. The wolves and cats that were once major factors in the control of deer herds in New England are gone, and deer populations explode, resulting in damage to forest seedlings and home landscapes and increasing the risk of tick-borne disease.

But in most cases we just don’t know how rare species fit into the web of life in a particular community. No matter how much research we do, how much we incorporate traditional knowledge, how closely we watch, for the most part we must admit that we live and act in profound ignorance about the detailed lives of the plants and animals around us.

We like to think we know — we are humans, after all. But there is just so much to know, so many interactions to account for. The 1973 Endangered Species Act, which seeks to conserve and protect endangered and threatened species and their habitats, is an acknowledgement of our limited knowledge, and a truly significant implementation of the precautionary principle: First, do no harm

We don’t really know, and perhaps can’t know the full ecological value of the snail darter, the desert pupfish, the piping plover, the lady slipper orchid. But it is imperative that we use what knowledge we do have to make thoughtful, careful choices about how we use our forests, fields, streams and oceans.

So scrounging the landscape for rare species sightings continues to be an important “green” activity. I love hunting for (but never picking) rare ferns and orchids. Not only does it allow us the very human pleasure of discovery and delight, but it also aids the effort to “save the pieces” for future generations.

Lawrence J. Winship is emeritus professor of botany at Hampshire College and a former board member of the Hitchcock Center for the Environment. Earth Matters has been a project of the Hitchcock Center for the Environment for 13 years. Amid the pandemic, the Hitchcock Center adapted its programming and has a sliding-scale fee structure for families facing financial challenges. To help the Hitchcock Center during this difficult time, consider a donation here.

Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 845 West St., Amherst, appears every other week in the Daily Hampshire Gazette. For more information, call 413-256-6006, or write to us.