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Ecological Studies at the Woodworth Study Area

Effects of Silv-Ex on Terrestrial Wildlife

Nimish B. Vyas*, James W. Spann, and Elwood F. Hill
National Biological Service
Patuxent Environmental Science Center
11510 American Holly Drive
Laurel, Maryland 20708


The U.S. Bureau of Land Management (BLM) and the U.S. Forest Service are the primary agencies responsible for the control of wildfires on public lands in the United States. These fires may be anthropogenic or of natural origin and may strike forests, rangelands or grasslands. Wildfires may occur in habitats that support endangered species. Resource agencies must decide whether fires should be allowed to burn and potentially destroy the habitat or whether they should be controlled using chemicals that may possibly be toxic to endangered species or impact critical habitats. Three National Biological Service Science Centers are collaborating to develop environmentally sound wildfire management strategies with regards to the use of chemicals. The Patuxent Environmental Science Center is studying the impacts of the wildfire control chemicals on terrestrial vertebrates and invertebrates. The Northern Prairie Science Center and the Midwest Science Center are responsible for determining the impacts of the chemicals on terrestrial vegetation and aquatic species, respectively.

Foams are one class of wildfire control chemicals. Foams exclude oxygen from burning fuels and allow for a slower release and a more efficient use of water (1). Silv-Ex® (Ansul Corporation; use of brandnames does not constitute endorsement by the U.S Government) is a commonly used wildfire control foam and was therefore recommended for laboratory and field testing by the BLM and the National Interagency Fire Center, Boise, Idaho. Acute and subacute toxicity of Silv-Ex® is reported to be low for white-footed mice (Peromyscus leucopus), Northern bobwhites (Colinus virginianus), American kestrels (Falco sparverius), and red-winged blackbirds (Agelaius phoeniceus). Acute oral limit tests for single-dose 24-h median lethal dosages (LD50) were greater than 2,000 mg Silv-Ex®/kg body weight. Subacute dietary limit tests demonstrated that five-day median lethal concentrations (LC50) were greater than 5,000 mg Silv-Ex®/kg body weight. Silv-Ex® was selected for field testing based on the results of the laboratory aquatic toxicity tests (2) and the paucity of information on its impact on mammals and birds. This work was funded by the National Interagency Fire Center, Boise, Idaho.

The objectives of our study were to, 1) determine the population-level effects of Silv-Ex® on small mammals, 2) determine the reproductive success of birds exposed to Silv-Ex®, 3) determine the effects of Silv-Ex® on the abundance and diversity of insects, and 4) determine residue levels of Silv-Ex® on vegetation.

Small mammals were selected for primary focus in our study since they are not highly mobile and were expected to be exposed to the chemical within the treated area. Birds, however, likely foraged outside the study site. Further, the density of small mammals was expected to be greater within the study area than birds. Eggs and nestlings of birds nesting in the study sites were to be monitored because they may be exposed to the chemical via direct contact or ingestion.


Our study was conducted at the Woodworth Study Area (WSA), North Dakota. Two treatments were selected, control and 0.3% Silv-Ex®. The 0.3% Silv-Ex treatment is the most common Silv-Ex® concentration used for the control of grassland fires (Wiesse, pers. comm.).

To assess the response of small mammal and insect populations, 12 1-acre (0.4 ha) plots (6 controls and 6 treatments of 0.3% Silv-Ex®) were sampled pre- and post-application for approximately 3 months (May - August, 1993). Estimates of small mammal abundance and survival were conducted using live mark-recapture methodology. Experimental design followed the combined closed and open population models (3). One hundred Sherman live traps were arranged in a 10 X 10 matrix on the control and Silv-Ex® plots. Small mammals were individually marked with Monel metal fingerling tags or Avid pits and immediately released at the capture site. Data on body weight and reproductive condition were recorded at the initial capture and at all subsequent recaptures. Each plot was sampled immediately prior to treatment (pre-treatment sampling) to determine species diversity and abundance. Plots were sampled three times following treatment (post-treatment sampling). A total of 1,200 small mammal live traps (100 traps per plot) were checked every day for 5 consecutive days at 2 week intervals. Estimates of survival rate and population size were determined and the treatment effect was evaluated with a t-test.

The reproductive success of birds nesting in and around the study site was to be monitored for hatching success and nestling survival. However, due to inclement weather, the initiation of breeding by birds was delayed until well after chemical application. Therefore, no data were collected on the impact of Silv-Ex® on avian reproduction.

The effects of Silv-Ex® on insect abundance and diversity was to be measured using standard sweep nets. Methods involved sweeping the nets along ten transects across each plot. Because of excessive rain and cool temperatures, sweep netting in plots resulted in collection of only a few individuals. Therefore, we sampled ants from mounds located in the study plots. Two mounds (one on a control plot and one on a treated plot) were sampled once pre-treatment, and twice post-treatment. The two mounds were similar in that they measured approximately 40 cm high and their flat tops were 20 cm in diameter. Adhesive packaging tape (2.54 cm width) the length of the diameter of the mound was place across the flat top of the mound (adhesive side down). The tape and ants were quickly placed in a zip-lock bag and refrigerated prior to counting. Sampling per mound was conducted in triplicates. Treatment effect was assessed with a t-test.

Vegetation samples were collected for analysis of residue levels of Silv-Ex®. Forty grams per sample were collected from three randomly selected locations in each plot. Samples were frozen and shipped to Patuxent for analysis.

Results and Conclusions

Small mammal trapping data indicated the meadow vole, Microtus pennsylvanicus was the most common mammal species. Other species trapped included the thirteen-lined ground squirrel (Citellus tridecemlineatus) and Peromyscus spp. Only the meadow vole was abundant enough for statistical analysis. No treatment effect was found on the survival rate (P=0.3490) and the population size (P=0.9938) for the meadow vole. We also were unable to identify a treatment effect on ants (P>0.05).

The WSA was subject to extremely high amounts of precipitation during summer 1993. The area also experienced cooler temperatures than average. We believe this weather was responsible for our small sample size of mammals and insects. Only about 30% of the traps were successful during each trapping period. This was considerably lower than the trapping success (70-80%) we obtained in a similar experiment in Nevada. The small sample size may have masked the treatment effect. Further, we believe that excessive rain events may have diluted the level of Silv-Ex® in our treatment plots and transported it away from the treatment area.

1.  Schlobohm, P. and Rochna R. (1988) An evaluation of foam as a fire suppressant 
         is available.  National Wildlife Coordination Group.  Foam Applications 
         Wildlife and Urban Fire Management 1,6-7.

2.  Poulton, B. C. (1996) Effects of two fire suppressant foams on benthic invertebrates 
         colonizing artificial substrates in portable limnocorrals.  Proc. N.D. Acad. Sci. 50.

3.  Pollock, K. H., Nichols, J. D., Brownie, C. and Hines, J. E. (1990) Statistical 
         inference for capture-recapture experiments.  Wildl. Monog. 107.

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