In the simplest terms, an ecological niche is analogous to the basic “ice-breaker” question asked at social gatherings: “So, what do you do for a living?” Of course, outside of a few indigenous cultures, what humans do for a living is a far stretch from what our ecological niche is as a species. What most other species do, however, is a direct result of their structural, physiological, and behavioral adaptations, which in turn are inextricably linked to their habitat. Downy woodpeckers eat insects. But because they are adapted to excavate prey from under bark or inside dead or dying trees, they are found in forested habitats, not open grasslands.
Theoretical ecologists like to contemplate the differences between a species’ fundamental niche (a theoretical ideal) and its realized niche (actual, real-world conditions). In any given habitat, a species that is free from interference from other species is capable of occupying the full range of environmental conditions for which it is adapted – its fundamental niche. Imagine if the downy woodpecker was the only bird in a given forest stand. It could forage and nest wherever it wanted without worrying about competition for space and food. Nature is complex, however, and rarely, if ever, are competitors completely absent from natural communities. In New England, downy woodpeckers commonly share their territories with 3 or 4 (and sometimes as many as 5 or 6) other woodpecker species. In addition, they may compete with several other less closely related species for both food and breeding space. Just how these competitors allocate space and resources within an area (niche partitioning) determines what portion of the habitat each species actually occupies – its realized niche. For example, the diminutive downy woodpecker reduces competition with its larger and more aggressive relative, the hairy woodpecker, by focusing its foraging efforts on smaller branches and weed stems where hairys, with their larger bills and feet, cannot forage efficiently.
Species that have broad ecological niches are considered generalists, while those that have extremely narrow niches are specialists. Often, species with broad niches are widely distributed geographically while specialists have limited ranges. Consider two different hawks – the red-tailed hawk, a generalist that eats a wide variety of readily available food found in a wide variety of habitats stretching from Mexico to Alaska – and the snail kite, a specialist that feeds only on apple snails and whose range in the United States is limited to a few places in southern Florida. Narrow or broad, species seem to instinctively recognize their particular niche. In an experiment during the 1940s, two ornithologists removed 542 birds of 42 species from a 40-acre, spruce-fir forest. By the next morning, a bird of the same species as the original occupant had set up housekeeping in every territory that had been vacated the night before.
Remarkably, birds of closely related species may occupy separate niches within a single tree. In an elegant experiment conducted in the 1950s, R. H. MacArthur found that four species of warbler he studied in Maine spruce forests always had distinct niches even though the physical space they occupied for feeding and nesting often overlapped. MacArthur found that magnolia warblers foraged for insects on the lower trunk and nested within 15 feet of the ground; black-throated green warblers preyed on foliage-eating insects close to the trunk and nested in crotches of lower branches; blackburnian warblers foraged high in the treetops, caught insects on the wing, and nested on a high branch well out from the trunk; while Cape May warblers ate sap or insects gleaned from the bark and nested near the top of a tree.
Niche partitioning, as this is called, may even occur within the same species, and may be based upon gender. Male red-eyed vireos, for instance, glean insects from the upper canopy of deciduous forests, while females forage in the lower canopy closer to the ground. Although they eat similar food, primarily caterpillars, each sex utilizes different areas of the same forest tract.
Ecological relationships and processes are often difficult to tease apart due to the infinite number of variables that need to be considered. Whenever ecologists examine a species carefully, differences in niches are discovered, suggesting that any given niche is occupied by only one species. Or, said another way, each species helps define its own niche. How’s that for job security?