Thriving on the bare minimum

Bembix americana

Few things look less auspicious than a patch of bare sand. With no plants or animal visible to the casual observer, it’s easy to assume that bare ground scores an ecological zero. But in nature, nothing is useless, and barren soil is a crucial element in the lives of a surprising number of species.

How a patch of bare ground became empty in the first place has a big effect on what will use it, and over what time frame the spot will become inhabited. An empty patch in our front yard, for example, must surely have resulted from a spill of something toxic, maybe oil or antifreeze, in an area once used as a driveway: the spot remained utterly barren and compacted for years, and only now, after years of rain have leached away whatever was dumped here, has a crust of lichen and moss finally gotten established.

Elsewhere, a sufficiently hot wildfire can burn off all the organic material, leaving empty mineral soil behind. Or repeated disturbance can produce a similar effect: think of a heavily worn footpath, or the tire ruts on a regularly traveled dirt road. When the fire is past, or if the pattern of disturbance is interrupted, these spots embark on a trajectory of regrowth. Simple plants like mosses colonize the site by means of airborne spores, or lichens (amazing amalgams of algae and fungi) may appear. Over time, the organic remains of these colonizers form the beginnings of soil; other plants arrive, adding more organic matter and sending down roots through sand grains to loosen the substrate. Wait long enough and you’ll have a forest.

But before all that happens, the very barrenness of the spot is a resource that wildlife can use. The punctured tiger-beetle, for example, Cicindela punctulata, spends most of its time hunting on the emptiest, flattest, and most compacted ground it can find. A visual hunter that relies on speed to run down its prey, this half-inch-long beetle eats ants and other smaller insects that venture across the bare soil. With no vegetation to obstruct its view, the tiger-beetle readily spots a target, and like a tiny, six-legged cheetah, it sprints so quickly after its victim that the eye can barely follow it. For this insect, bare ground furnishes the ideal place to hunt.

Many bee and wasp species use open ground not for hunting but for building their nests. Kicking sand through their legs like a terrier digging up daffodil bulbs, these ground-nesting insects move impressive amounts of soil as they dig the tunnels where they will lay their eggs. These nest burrows may be surprisingly long and elaborate, descending inches or even feet beneath the surface and sometimes branching into networks of side-tunnels and chambers. Each chamber will hold one or more eggs, and it will be stocked by the adult insect with either pollen or a paralyzed prey item for the young wasp or bee to feed on as it grows.

It’s not clear why such bees and wasps prefer bare ground. It may be as simple as making it easier to relocate the nest when they return from a foraging trip. Or perhaps bare ground signals a site where you can burrow with no interference from roots. But whatever the reason, the soil must be bare, and moreover, each species of burrowing wasp or bee has a strict preference for the type of soil it will nest in: sand or clay, coarse or fine, wet or dry, loose or compacted. So maintaining populations of all these beneficial insects requires not just the odd patch of bare ground, but many such patches, in different places and with different characteristics.

For some of the species that use bare ground, it seems like a very limited expanse will suffice. Some insects will dig their burrows in the space between clumps of grass or weeds. For other ground-nesters, it seems like a large bare area is required, or at least an area with many small bare spots in a small area. For instance, a bee called Bembix americana (it has no common name, though it is a common and widespread bee) prefers to nest in loose colonies of dozens or even hundreds of individual burrows. In an abandoned construction site near my home, such a colony covers an area of bare soil about 30 feet in diameter.

You probably already have some insects in your yard using snippets of bare ground. I’ve actually encouraged some bare spots to get larger and stay bare, and the result has been rewarding: new insects turn up, ones with interesting lives, beneficial habits, and, in some cases, exquisitely beautiful appearance. In nature, no niche goes unoccupied for long: even nothing is something, for the right creature.