Ground Squirrels and Levee Vegetation

Vegetation removal activities of levee maintenance cause changes in the plant composition and structure on the levees. These activities also have a major impact on ground squirrels, the most important animal pest on levees.

Ground squirrels can be significant pests on levees due to the extensive network of underground burrows these animals create. Their channeling may weaken levee structure during floods and increase the likelihood of a levee break. The population biology and behavior of rodents make them good candidates for control through habitat modification (Davis 1972), and ground squirrels may be susceptible to this strategy.

The relationship of ground squirrel population density to the degree of soil disturbance has long been noted. For example, Linsdale (1946) observed that ground squirrel numbers rose and fell with the extent of overgrazing on pasture lands. High squirrel populations are characteristically associated wih barren ground, outcrops, or elevated areas (Owings and Borchert 1975), and nearby food sources. Lack of vegetation permits high visibility for the squirrels, which aids in social communication and predator detection (Owings 1977). With the construction of levees and the traditional maintenance practices associated with them, people have inadvertently created prime ground squirrel habitat which is lacking only an enriched food source, often supplied by adjacent agriculture.

Figure 3 shows the results of a detailed study on a section of levee which demonstrated the relationship of ground squirrel density to vegetation and other environmental features. The number of burrows is indicative of the population size of squirrels and is a direct indication of the damage to levees caused by the squirrels.

Squirrel burrows on a 3.2-km. (2-mi.) stretch of levee encompassing various environments were counted in the late summer of 1980, after the levee had been burned. The squirrels' particular attraction to areas of barren ground was demonstrated by the high burrow densities observed in the area where overgrazed pasture extended up onto the side of the levee. Burrow numbers on the levee dropped where the fence line ended.

A second strong determinant of ground squirrel distribution is availability of food sources. Exceptionally high burrow densities were present near the walnut orchard. The area immediately adjacent to the walnut orchard, which had 583 holes per km. (932 per mi.), had been fumigated earlier in the season, so counts may actually have been depressed there. Areas within the squirrel foraging range of the walnuts on both sides of the levee also had very high burrow counts. (It was impossible to count burrows in the area of stone revetment on the water side opposite the orchard, although squirrel densities were clearly high there. The high counts in areas near the orchard showed the enhanced effect on squirrel densities produced by both disturbed slopes and nearby food.)

Figure 3.
Distribution of ground squirrels on 3.2 km. (2 mi.) of levee at East Yolo Bypass, Yolo County, California,
August, 1980. Counts are in number of burrows per levee mile. Before burning, yellow star thistle
(Centaurea  solstitialis ) and wild oat (Avena  fatua ) comprised the dominant vegetation on much of
the otherwise undistinguished levee slope. The linear distance along the levee is not drawn to scale.

Areas lacking both nearby walnuts and the disturbed surface of the overgrazed pasture had lower burrow counts. In these areas, a weaker propensity of squirrels to burrow on land- versus water-side slopes may have been demonstrated. That is, there is a propensity to burrow on landside slopes, but that propensity is less pronounced in areas which are away from orchards and overgrazed pastures.

Vegetation on levee slopes during the summer and fall may be particularly important in habitat management of ground squirrels. During this period, young, first-year animals are actively dispersing, seeking new area in which to settle (Dobson 1979). Slopes covered with vegetation at this time of the year will be much less attractive to migrating squirrels than those freshly denuded by annual vegetation removal practices.

The traditional approach of annually burning levee slopes followed by dragging to obliterate burrow openings and smooth the levee surface improves the quality of ground squirrel habitat. In contrast, a program of deliberately maintaining certain vegetation on levee slopes may tend to discourage squirrel colonization. The tactic of revegetating levee slopes with appropriate plant species should be integrated into an active squirrel control program if permanent population reduction is to occur.