Cite as:

Robert Fisher and (499 student). (In progress). Beaver activities and their effects on the ecosystem in the Santa Margarita River. San Diego State University Field Stations publication.

Abstract

Beavers are native to much of North America, but they have been present in the Santa Margarita River for less than 50 years. Beaver activities and their effects on the ecosystem in the Santa Margarita River is of concern because, as major ecosystem engineers, beavers have a great impact on the environment in which they live. As an introduced species, beavers may play a role in favoring the success of other introduced species. It is important to understand the effects of beavers in order to create a beaver management or control program.

The Santa Margarita River

General Characteristics of the River

The Santa Margartia River is normally sluggish and has many pools and marshy areas. The flow rate is generally about 2 ft3/sec; however, during the early part of the year, peak rates as high as 100 to 200 ft3/sec may occur. The pH ranges from8.0 to 8.5. The temperature flucuates from 4oC to 24oC. (unpublished paper, 1970's)

Native Fauna & Flora.

There are many species of fish native to the Santa Margarita river, including the green sunfish (Lepomis cyannelus), the mosquito fish (Gambusia), the red-eyed bass (Micropterus coosae), the brown bullhead (Ictalurus nebulosus), the golden Shiner (Notemiganus crysaleucas) and the rainbow trout (Salmo gairdnerii). The invertebrates living in the Santa Margarita include Crayfish, Gastropods (Physa), oligochaetes and various insects.

The vegetation of the Santa Margarita Ecological Reserve is largely comprised of coastal sage scrub and grassland. These plants are adapted to such environmental factors as summer drought, limited winter rains, shallow soils and fairly frequent fires. Some foreign species have begun to invade areas where fires have occurred. Riparian vegetation exists along the river banks and in the washes. Willow trees (Salix) are numerous along the river. Oaks occur sporadically in groves along the river. There are many introduced trees as well, such as Eucalyptus trees and fruit trees.

Beavers in the Santa Margarita River

Beavers are not native to the Santa Margarita River. They were introduced in sets of two pairs of beavers to each of three rivers in San Diego county in the 1940's by the Department of Fish and Game (Hensley 1946). The Santa Margarita river was not one of the three rivers into which beavers were introduced and the way in which beavers reached and colonized the Santa Margarita River is unclear. One possibility is that, during heavy winter storms, beavers living in other rivers may have been washed out to the ocean.

These beavers might have then swam into the Santa Margarita River.

The Effects of Beavers

The effects of beavers in the Santa Margarita area has not been well-studied, but there have been many studies performed in other areas. These studies highlight some of the specific concerns and problems of beavers in the Santa Margarita River.

Beavers are ecosystem engineers. They utilize many woody species in dam building as well as for feeding. Beaver dams impede the water flow and large pools often form behind them. Beaver tree-felling activity, their pools and their dams can greatly affect the surrounding ecosystem.

Some of the changes which occur when stream is converted into a beaver pond are summarized below, with emphasis on potential problems specific to the Santa Margarita River (based largely on Naiman et al, 1986 & 1988 and Hammerson 1994).

Beaver dams greatly affect water flow dynamics.

1) Decreased current velocity. Beaver dams lead to decreased current velocity by physically impeding the rate of water flow.

2) Increased trapping of sediment. The decreased current velocity leads to increased trapping of sediment (Naiman et al. 1986, 1988). This can, in the future, affect the kinds of plants which will grow in abandoned beaver ponds. Abandoned beaver ponds have been observed to grow into grasslands, greatly altering the distribution and abundance of organisms in the area (Neff 1957).

3) Storage of precipitation. The precipitation stored during the rainy season is gradually released throughout dry periods. This gradual release of water may be of concern in the Santa Margarita River where the native ecosystem is adapted to occasional dry spells during the summer. With beaver dams present, runoff may be

stabilized by the maintenance of a higher water table, which is drawn upon during periods of low precipitation (Hammerson 1994).

4) Elevation of the water table. This may result in flooding which can destroy wetland vegetation that had previously lived in the area.

5) Increased water depth. Beaver activity affects local vegetation, animal life and nutrient cycling

6) Changes in the distribution and abundance of various tree species.

In the spring and fall, beaver cut down trees and shrubs for feeding and dam-building. Willow trees (Salix), which are native in the Santa Margarita river area, are one of the species utilized by beavers. While willows are generally resistant to beaver tree-cutting activities (Slough and Sadlier 1977 in Hammerson 1994), other species, such as aspen do not fare so well (Moen et al. 1990). Other species of vegetation are also affected by both tree-felling and feeding activity.

7) Increased amounts of open canopy in forested areas.

This is caused by the killing of trees in the riparian zones, both directly by tree-felling and indirectly by flooding caused by beaver dams. (Naiman et al.1986)

8) Increased amount of organic matter. Wood debris from beaver activity and the subsequent soil erosion in beaver-affected areas cause an accumulation of organic material. This organic material may modify the biochemical composition of the waters, soils and

sediments in the surrounding areas (Lizarralde 1993).

9) Increased amounts of organic carbon, nitrogen and other nutrients. The increased amount of organic matter can affect carbon cycling. The rate of carbon turnover in beaver ponds is much slower than in a free-flowing stream (Naiman et al. 1986). The increase in the amount of organic material is favorable to methane-producing microorganisms, which leads to increased carbon output via methanogenesis (Naiman et al. 1986). The decomposition of this

increased amount of organic matter will lead to reduced oxygen levels in the spring and early summer (Naiman et al. 1988)

10) Pond invertebrates replace running-water taxa.

Naiman et al. (1988) found that when a stream is converted to beaver pond, the original host of stream invertebrates, including blackflies, Tanytarsini midges, scraping mayflies, and net-spinning caddisflies are replaced by pond invertebrates such as Tanypodinae and Chironomini midges, predaceous dragonflies, tubificid worms and filtering clams. (Naiman et al. 1988)

11) Beaver ponds may be favorable to the success of other introduced species. Beaver ponds may provide a refugia for introduced species which, without beaver ponds present, would probably be washed out of the river during the rainy season. This may give introduced species a foothold from which to invade the Santa Margarita River.

12) Beaver dams may negatively affect trout migration.

Rainbow trout are native to the Santa Margarita River. Beaver dams may pose an impediment to trout migration, although some trout have been observed to cross beaver dams (Gard 1961). At least two species of trout (Brown Trout and Brook Trout) are able to live primarily on pond invertebrates in beaver ponds. Rainbow Trout living in beaver ponds, however, still rely on stream invertebrates for food (Gard 1961). In a stream with several beaver ponds,

the Rainbow trout might be at a disadvantage when competing with Brown Trout and Brook Trout.

Conclusions

Beaver can have major impacts on their environment. Beaver activities can lead to many changes in water flow dynamics, species composition, nutrient cycling, carbon cycling and many other aspects of their surroundings. These effects may prove to be unfavorable in the rather delicate ecosystem of the Santa Margarita River in which beavers may negatively impact preservation or restoration efforts.

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