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June 1999
The day I learned to see structure in habitat, it was as though I was seeing for the first time. I had been invited into the house where previously I was catching only glimpses of life through the windows.
Vertical structure is described as layers in a natural community. These seven layers, or levels, are underground, ground, herbaceous, shrub, midstory, canopy and supracanopy. Each natural community has a different vertical structure. In some, such as a meadow, the canopy is absent. In other places, the shrub layer or midstory may be missing because a dense canopy limits the sunlight reaching the forest floor. Wildlife species have made adaptations to satisfy their requirements for food and cover and, accordingly, use of forest layers differs among species.
The underground layer provides shelter as with burrows and tunnels for red fox, coyote, groundhogs and snakes. The northern redbelly snake, the gardeners' friend because it eats mostly slugs, may use an anthill for hibernation. The underground is the location of food sources, such as the Japanese beetle grubs relished by moles and earthworms, the treat of the robin redbreast.
The ground layer provides nesting sites for some bird species, such as the white-throated sparrow and the dark-eyed junco. The smooth green snake is usually found on the ground but has been observed in shrubs and on vines. Food sources on the ground level include grass seeds and acorns, insects and invertebrates in the leaf litter, and salamanders and mice.
The herbaceous vegetation layer is made up of forbs, grasses and woody seedlings. Herbaceous plants provide cover for some bird species nesting on the ground, and those nest sites would not be selected unless there was adequate cover. These plants are forage foods for many species, such as deer, moose, woodchucks, snowshoe hare and bear.
The shrub layer includes shrubs and seedlings, usually less than fifteen feet high. Many bird species use thickets for nesting sites or to hide from predators. The ruby-throated hummingbird, gray catbird, brown thrasher and the northern cardinal all nest in the shrub layer. Fruit bearing shrubs attract a variety of wildlife to this layer and buds are browse food.
The midstory includes deciduous or coniferous tree saplings and small-growing trees. Many are shade tolerant, like the hemlock and beech, which grow beneath the canopy. Openings in the canopy allow less shade tolerant species to grow, increasing the habitat's vertical structure. Alternate leaf dogwood and witch hazel are examples of native small-growing trees that may be part of the midstory.
The canopy is made up of the crowns of trees and is often described by its closure. Simply put, when you are standing in the forest and look up from horizon to horizon, how much of the area is filled with treetops and how much sky can you see? If you see mostly sky, this is characteristic of a clearcut or non-forested habitat such as a meadow with scattered trees present. If you see mostly treetops and very little sky, this is considered closed canopy forest. Large acreage of closed canopy forest is essential habitat for forest interior-dwelling birds, such as the scarlet tanager, the ovenbird, and the American redstart as well as many other species of warblers and vireos.
Exposed perches add to the structure of habitat. High perches, in the supracanopy level, are dead or live perches on trees above the surrounding forest canopy. Pasture trees are high perches above the herbaceous level. These high perches are frequently used as hunting perches. Shrubs, fence posts and dead tree stubs less than ten feet high are low perches used by some bird species for hunting, hawking for insects, and for announcing territory.
The belted kingfisher is an example of a species that uses several habitat levels. For its nest site, it makes a burrow three to six feet deep in a bank of sandy clay or gravel, within a mile of water. Their preferred feeding habitat is the shallow borders of bodies of water. Here they must have perches at shrub level or midstory to sight prey in clear water and to use as feeding sites. Without perches, they cannot take full advantage of the habitat's resources.
Knowledge
of habitat layers allows me to focus my observations.
In my yard, in the neighborhood or walking in the woods, with eyes to see
I have many wildlife adventures.
By Nancy Coverstone (ncstone@umext.maine.edu), University of Maine Extension Educator in Androscoggin and Sagadahoc Counties
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