Strange worlds lie secluded within everyday surroundings we know well, yet they often remain overlooked and unappreciated.
Evergreen or deciduous, a tree can be wounded when wind tears off a branch, or woodpeckers and insects cut through bark to expose sapwood, and dead wood is excavated. Bacteria and fungi, invading opportunists, digest cellulose and lignin until a superficial injury grows deep as it penetrates the living trunk.
We don’t like these rotting holes in valued trees, yet that’s where some organisms find the habitat they require, and complex communities of small plants and animals develop in distinct populations and unique assemblages. Water collecting in a tree cavity provides favorable conditions for a vast array of tiny aquatic creatures.
The best tree holes from a naturalist’s point of view are these wet ones, often resulting from a limb having been ripped off, thereby creating an instant cavity in which rainwater is trapped in the summer, snow in winter. Because evaporation is retarded in such a protected, shaded place, its microclimate remains moist, even soaked, maintaining a reliable temperature throughout the seasons.
As tree holes age, they enlarge enough to put your fist in, perhaps an entire arm. Even the smallest flooded cavity is technically a miniature pond, complete with detritus-laden bottom and a “shoreline” of simple green plants (algae, liverworts, and mosses) growing around the perimeter. When I point out a New England tree hole, companions are reluctant to probe it with a finger, although nothing dangerous lives within. In the tropics, however, I have second thoughts myself, for venomous centipedes and other biting, stinging creatures may be residents of shadowy recesses.
Each little pool supports organisms adapted to life in or around the dark water where they feed upon decaying wood, reproduce, capture and eat one another. How do they reach such an isolated place? Some crawl over the bark seeking a sheltered haven. Others arrive flying on exploratory missions, ending up in a retreat suited to their needs. Tree holes attract birds—a nuthatch, perhaps, as it probes cavities for food. Birds invariably bear hitchhikers in the form of eggs, spores, seeds, and active microorganisms riding on feet and feathers, ready to be left off upon arrival.
As the wooden sides of a tree hole soften with decay, they are penetrated beneath the water’s surface by roundworms, rotifers, tardigrades, protozoans, mites, and fly and beetle larvae. Above the water level, ants explore the damp, pulpy mass that is home to myriad other mites, larval insects, centipedes, and millipedes, as well as slow-moving snails and slugs.
A tree hole is an active incubator, a culture vessel in which flourishing life develops, an entire little universe with its own complex food web interactions between organisms. Like any habitat outside, its status changes with time as stages in its population succeed one another. Some species are adapted to a new young hole, others to a mature one, still others to one that is drying out. A tree hole is a reflection in miniature of major ecosystems in the larger world beyond.
To learn about a tree hole’s hidden lives, dip out water and sediment with a small container, or suck up bottom detritus with a large basting pipette, and place your collection in a shallow container. A magnifying glass reveals a zoo of small creatures moving through decaying wood fibers and accumulated sediment. The most obvious inhabitants may be mosquito larvae—wrigglers—although seldom the common sort found in puddles out in the open. One species of mosquito breeds exclusively in tree holes and is known, logically if not imaginatively, as the tree hole mosquito.
To explore tree holes and their inhabitants beyond the reach of a simple magnifier, use a microscope—and be prepared for astonishing scenes as creatures dart across your field of view. The smaller they are, the more diverse, the more abundant, and the greater their activity.
Biologists study tree hole ecosystems most actively in tropical rain forests, yet here in Vermont and New Hampshire, equivalent microhabitats are commonplace—only the players are different. Admittedly many cavity inhabitants also live beneath forest leaf litter, in swamps, and inside recumbent rotting logs, but in these places they exist in different proportions to one another. With its little “pond” and vegetated humus “shoreline,” a tree hole is a unique, discontinuous extension of well-populated habitats common to a saturated, boggy forest floor.
Look closely. Poke a finger into the cavity, feel its warmth and richness. Search for its inhabitants. This protected and hidden world flourishes year after year, shielded from summer heat, unlikely to freeze in winter. For small lives, it’s a good place to be when conditions outside are harsh. They assemble here, seeking livelihood and a degree of security in a very unsure world.