Keystone Role of Beavers in a Restored Wetland (Ohio)

The Shaker Trace Wetlands is a restored wetland area in southwestern Ohio (Klein 1992). Several years ago, a small number of American beavers (Castor canadensis) took up residence in an ephemeral lake in the wetlands. These beavers cut down willows (Salix spp.)—thus allowing more light to reach other vegetation—built a lodge, and dug an extensive system of canals (Figure 1). There is no stream flowing into the lake, so no dam. In some places, this lake can be over 1 m deep during the spring and early summer, but by late summer it may dry up completely owing to normal dry conditions. During the driest periods the canals dug by beavers provided an important reservoir of water for amphibians, fingernail clams, snails, and other aquatic organisms (Figure 1). During canal excavation, the beavers churned the soil and exposed seeds from deeper in the soil seed bank. In addition to these ecological benefits, the beavers provided wildlife viewing opportunities for visitors to the wetlands. After a couple of years in this ephemeral lake, the beavers abandoned their lodge and moved on, probably to a river or pond less likely to dry up.

Within a couple of years of the beavers’ departure, the extensive growth of willows and other woody vegetation made mechanical removal with a Hydro-Axe machine necessary (Conover and Klein 2010). This operation opened extensive areas to more light, and the large tires of the Hydro-Axe machine left deep tracks in some areas. Churning the soil exposed seeds from deeper in the soil seed bank. The Hydro-Axe operation served to increase plant diversity in the wetlands and provided more habitats for herbaceous species. The tire track depressions hold water during the summer drought period longer than surrounding areas, giving larval amphibians more time to develop into adults. Snails, fingernail clams, and other aquatic species also benefit from water held in these depressions. In other words, the Hydro-Axe operation provided services at a cost of about $5,000 for 20 ha—services that had been provided for free by the beaver colony before they moved on.

Figure 1.

Beavers as ecological engineers can provide beneficial ecological services. A canal (above) dug in an ephemeral lake in Shaker Trace Wetlands in Ohio leads to the beaver lodge. A wildlife viewing shelter can be seen in the background. The canal retained water longer into the drought period, allowing tadpoles to survive (below) when other areas were already dry.

Beavers were extirpated from Ohio by 1830 (Chapman 1949) but have been making a comeback during the last few decades. In some parts of the country, beavers have been reintroduced and have provided rapid improvements in hydrology, riparian vegetation, and wildlife habitat (e.g., Albert and Trimble 2000). In restored ephemeral wetlands similar to the Shaker Trace Wetlands, beavers can play a keystone role by cutting down woody vegetation and digging canals that hold water for longer periods into the dry season, and by churning the soil, exposing seeds buried deeper in the soil seed bank. Beavers could be encouraged to remain in such wetlands by scooping out some deeper pools. This would benefit aquatic organisms by holding more water longer into the drought period, and it might enable beavers to remain in the wetlands. Other parts of the wetlands could still be allowed to dry up during drought periods, discouraging the establishment of fish that could prey on amphibian larvae. Desirable larger trees in the wetland area can be protected from beavers by wrapping aluminum, chicken wire, or steel screen around the trunks up to a height of 80 cm (Albert and Trimble 2000).