Modern Wildlife Management

Rural System's

Modern Wild Faunal Resource System Management

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Forest Edge as a Cant

Edges between fields and forests are said to be good for wildlife. Maybe. Forest edges need to be viewed as "cants." A cant is a log with its four faces sawed off. It is a rough wooden beam.. Here's why a new view can be useful to people interested in forests and wildlife management.

Ever since the early '30s when the first textbook on wildlife management was written, wildlife managers have discussed edges, places, for example, between fields and forests. It seems that where two cover types come together, there are more animals than in either area alone. This is easily greeted with... "duh!" Maybe 2 plus 2 does equals 4!? Others say that there is an extra, not just an additive effect of two cover types coming together. At the edges, there are more species and greater numbers than in nearby areas. When this occurred, wildlife workers called this edge effect. They were emphasizing game birds such as quail, grouse, and turkeys that are abundant in or near edges.

The increased abundance does not always occur, but some people do not understand this and throw "edge effect" around as if they believed that to produce or create an edge is always a good thing for all animals. Others have claimed that forest edges are bad because crows, cowbirds, and other nest destroyers hit birds nesting there harder than elsewhere. Maybe edges have a bad effect.

We now know that edges are too complicated to allow simple slogans to be useful. Some species benefit from them; others avoid them; some use them, but they are not superior areas for them; some may be harmed by their presence.

Sketch of an edge between a forest and fallow field. The edge volume is V or Volume 1 + Volume 2
and Volume 1 = Height-1 x Width-1 x Length
and Volume 2 = Height-2 x Width-2 x Length

Strangely, defining an edge is pretty difficult. It is a line on a map, but even that has some width. How wide is it? I think it has one width (Width 1) into the forest and another one into an opening or field (Width 2). Grouse and turkeys nest more abundantly within 50 feet of edges than elsewhere in the forest. Wind and drying conditions are only evident a little ways into the forest. Shade effects and soil moisture drains are evident for only a few feet out into most fields or openings. Edge that is relevant to animals has width. Insect counts follow evident gradients into and out of forests. Edge relevant to animals is more than a line. It is a line with width.

It also has length (Figure A). The least length of edge for any opening occurs when that hole in the forest is made as a circle. The more long and thin the area of the hole, the more will be the edge length. There has to be some minimum width of an opening or else "edge" makes no sense (imagine an acre opening that is only one foot wide!)

Forest openings have greatest value as insect producers for birds or as mouse producers for birds of prey. Openings are as important as the hole is for a doughnut to have meaning - they produce the special conditions in the forest needed for many species of forest wild animals.

In addition to length and width, edges have two heights, the height of both types of plants at the contact points or "contiguity." A four-foot grassy area (Height 2 in Figure 1) adjacent to a 50-foot pine forest (Height 1) is an example of the two heights needed to calculate the edge volume. The edge is clearly a volume - like a cant. It is a volume with widths, length, and heights. Length of edge has been the measure in the past to seek out correlations with wild animals. "Edge effect," always positive for game animals, was claimed, usually for edge length. There were correlations found, but they were poor. Perhaps adequate measures were not made. Perhaps edge is not universally good for all animals. Perhaps we need to be more inclusive and study edge volumes for each species along with the ages of each half of the volume. I think so. Edge volumes are pretty dynamic things. What may be a firm relationship for animals today may disappear within a year or two.

In the future, abundance of each species of wildlife may be studied relative to the edge volume that has just been described, that is, to the cant volume. When we do this and stop over-generalizing, realize how edge volume changes (as height and width-effects change as the forest grows and matures), then we can see some progress in understanding what goes on at the edge. Edge is not a fixed measure like the line on a map but it is a changing volume. Long narrow volumes may be equivalent to short, wide ones. With better analyses of past studies and a few new studies, then we may be better able to explain past populations, understand the present number of species and their abundance, and confidently include edges in our predictions of and prescriptions for the many profitable new yields from the forest. Edge is a changing volume that affects each species differently. With the cant idea, we can improve wildlife management in the forest.