Figure 1. Box-type, parallel bar barrier protecting hooded inlet overflow pipe.

Figure 2. Box-type, parallel bar barrier protecting riser inlet overflow pipe at Bass Pond.

Figure 3. A perforated intake pipe that carries water from near the bottom of the pond into the box-type, parallel bar barrier. The pond was very low when the photo was taken; the pipe is normally underwater.

Beaver damage concerns more landowners than damage caused by any other wildlife species in Oklahoma. No pesticides, toxicants, or fumigants are registered with the Environmental Protection Agency for beaver control. Trapping and shooting are the most effective lethal control techniques. Conibear traps are probably the most effective beaver traps. Beavers are primarily nocturnal so effective shooting generally must occur at night. Landowners and the general public cannot legally use conibear traps or shoot beavers at night without attending a special course and obtaining a Nuisance Beaver Control Permit from the Oklahoma Department of Wildlife Conservation.

All beavers from an impoundment or a relatively small watershed can be temporarily eliminated with lethal control techniques, but beavers usually move back into the area. To be effective, lethal control methods should be combined with frequent periodic monitoring of beaver activity. One of the disadvantages of lethal control techniques is new beaver damage generally occurs before landowners realize beavers have returned.

Although beavers can negatively impact timber, ponds and drainages, they also have beneficial attributes. Beavers are natural components of local natural ecosystems. Their impoundments and cutting can add diversity and enhance habitats for many other species. Beavers can help suppress black willow, which tends to be a weedy pest around some ponds. We do not dislike beavers — we dislike their damage. If possible, we prefer to control beaver damage while coexisting with beavers. Nonlethal control techniques allow this.

We estimate we expended 40 to 80 hours annually for beaver damage control at the NF Pasture Demonstration Farm during 1981-86. This labor involved trapping beavers, shooting beavers, removing beaver-felled trees from fences, unplugging beaver-plugged overflow pipes, removing beaver dams from earthen spillways, monitoring beaver activity, and associated travel.

In 1986, we began investigating means to decrease our annual labor commitment to beaver damage control. We began creating relatively permanent beaver damage prevention measures in an attempt to save money and labor while more effectively controlling damage and allowing beaver populations to exist. We selected seven ponds and a drainage ditch at the Pasture Demonstration Farm to receive beaver exclusion devices.

Exclusion Device Tests:
Overflow Pipe Protection
We constructed five box-type, parallel bar barriers over the inlets of overflow pipes on ponds where we experienced problems with beaver plugging the pipes (Figures 1 and 2). We constructed parallel bar barriers with 1-inch gaps between rods to prevent beaver damage because they are relatively maintenance free and restrict fish movement through overflow pipes.

During the nine years these barriers have been present, beavers restricted water flow through only one of the five overflow pipes protected with these barriers. Beavers built a dam around three sides of one barrier. After removing the beaver dam, we installed a perforated intake pipe on the bottom of the pond in front of the barrier (Figure 3). We plugged the lower end of the pipe with wood. This pipe was anchored to the bottom of the pond and inserted into the front of the barrier. Beavers did not attempt to construct a dam around the outside of the barrier probably because they could not raise the water level with the perforated intake pipe carrying water into the barrier.

Preventing Beaver Damage: Tree Protection
We protected 317 trees with 268 exclosures (Figures 4 and 5) around seven ponds and drainage. We constructed some beaver exclosures around trees each year from 1986 through 1996. We placed beaver exclosures around trees that were located from zero up to 168 feet from the water's edge, with an average distance of 35 feet. We used five types of wire for beaver exclosures. Exclosure heights varied from 28 to 32 inches, and diameters ranged from 13 to 106 inches, with an average diameter of 43 inches. Tops of the exclosures were usually separated at least 12 inches from tree trunks, allowing trees room to grow. We drove an average of five steel rods along most exclosures to provide support and to anchor exclosures. We used more rods to provide extra support on large exclosures, exclosures on uneven ground, and exclosures exposed to cattle.

Figure 4. A 2-inch by 4-inch woven wire beaver exclosure protecting a pecan tree at 2 NE Pond.

Figure 5. Rock placed around a 2-inch by 4-inch woven wire beaver exclosure protecting a bald cypress tree at water's edge on Dalrymple Pond.

We examined trees and shrubs more than 4 feet high for beaver damage during July and August 1997. At each site, we examined trees and shrubs out to the distance of the beaver exclosure farthest from the water's edge. At one pond where all exclosures were located at water's edge, we examined trees and shrubs within 24 feet of the water's edge. We examined a total of 2,079 trees and shrubs outside the exclosures.

All exclosures adequately prevented beaver damage, and when fenced from cattle, were still functional in 1997. Rubbing and trampling by cattle usually destroyed exclosures built with welded wire. We replaced welded wire exclosures with 2-inch x 4-inch woven wire (Figures 4 and 5) wherever cattle destroyed them.

Eight of 317 protected trees were gnawed but not killed by beavers during the 11-year period that exclosures were in use. All eight trees gnawed by beaver were bald cypress planted at the water's edge. Beavers cut some lower limbs of two trees outside of the exclosures, but did not harm the main stems. Beaver cut some roots of four trees outside the exclosures by trenching along the exclosures, but did not harm the stems. Beavers gained access inside two exclosures by trenching under them, where they cut the main stems. We saw no beaver damage on protected trees away from the water's edge.

Beaver cutting the roots and main stems on five of the eight damaged bald cypress trees occurred several years ago during the beginning of the project. We recognized that beaver used the wet soil at water's edge to facilitate their trenching activity. To prevent additional damage, we placed two to three rows of 6- to 12-inch rock around the bases of exclosures that were located at the water's edge (Figure 5). Fifty-four exclosures received such rock. No additional trenching under the wire exclosure occurred after placement of the rock.

At the eight sites with exclosures, visible beaver damage to unprotected trees and shrubs varied from 4 to 60 percent, with an average of 39 percent. These measurements of beaver damage probably underestimated the actual damage. Some beaver damage occurred every year of the project, so most of it occurred prior to the damage evaluation in 1997 and was difficult to find.

Using 1997 prices, material costs to build the wire exclosures in this project ranged from $1.40 to $43.68. For the average exclosure used in this project (new 2-inch x 4-inch woven wire, diameter of 43 inches, and 5 rods), 1997 material costs were $8.93. Construction of a typical exclosure generally required 30 to 60 minutes of labor including travel, accumulation of materials and tools, cutting of materials, and installation. Exclosures on steeply sloping ground and those requiring rock generally required more labor.

Preventing Beaver Damage: Conclusions
Exclosures provide an effective means to prevent beavers from plugging pond overflow pipes and cutting individual trees. Overall, beaver gnawing was seen on only 3 percent of the trees protected with exclosures, while at least 39 percent of the trees and shrubs outside exclosures were damaged. Most of the damage to trees with exclosures could have been prevented by placing two to three rows of rock around exclosures located at water's edge.

Properly constructed, exclosures should last 20 to 40 years with minimal maintenance. These exclosures require significant initial investments of time and money, but in many situations should provide better protection and lower cost over the long-term than trapping, shooting, and damage cleanup.