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Biological Assessment for the Preferred Alternative

VI. Existing Environment and Environmental Effects

Species Dropped From Further Analysis

The following species were on the latest species list (reference 1-1-00-SP-1652) issued by the USFWS for this project. However, they were dropped from further analysis, with consultation with USFWS, because they do not occur in the analysis area, as described below, or they will not be directly, indirectly, or cumulatively affected by the proposed activities. Anadromous fish species occurring within the planning area, in consultation with NMFS, will be addressed through a separate biological assessment.

Vernal Pool Tadpole Shrimp

Geographic Distribution

Lepidurus packardi is endemic to the Central Valley of California. It occurs from the vicinity of Bella Vista, Shasta County in the north to Tulare County in the south, in the Sierra Nevada and Cascade mountain foothills below 800 ft, and as far west as Solano and Alameda counties. It is distributed sporadically within this range where suitable habitat exists. All known sites are below an altitude of about 800 ft. Collection localities attributed to L. packardi in Lassen, Modoc, Plumas, Shasta and Siskiyou counties are for a different, undescribed, but closely related species (Rogers 2000). According to McGriff (2000), there are no records for L. packardi on Forest Service lands in the California Natural Diversity Database (CNDDB). Based on this evidence, it does not appear likely that this species occurs on Forest Service lands and this species will not be given further analysis.

Biology

Lepidurus packardi is hermaphroditic and only capable of self-fertilization. Clutch size ranges from 8-61 eggs and is positively correlated with body size. Gravid individuals occur in late fall and early spring. Eggs must undergo a period of dehydration to induce hatching. According to laboratory studies conducted by Ahl (1991), eggs in standing water hatch within 3 weeks. Lanway (1974) observed the appearance of hatchlings within 2-3 days after ponding under natural conditions, and 10-13 days after inundation and incubation at 10°C.

Vernal pool tadpole shrimp reach sexual maturity in 6-7 weeks depending on water temperature and food availability (Ahl 1991). Individuals having a carapace length of 10-12 mm reproduce as long as standing water of suitable quality exists. As the vernal pool dries, tadpole shrimp produce small, resistant, spherical bodies called cysts that allow them to remain dormant until wet conditions return. Cysts may remain viable for many decades and Ahl (1991) observed that most hatch after 4 days of inundation when incubated at 10°C.

Vernal pool tadpole shrimp are eaten by aquatic insects and small mammals, and may provide a major food source for birds. Dispersal is accomplished by a variety of means. Passive dispersal may occur by wind, and active dispersal agents are waterfowl, shore birds, wading birds and mammals, presumably including humans. Cysts may be dispersed in mud adhering to feathers, fur or feet, or in feces.

Food Habits

Lepidurus packardi is an opportunistic omnivore and consumes plankton, fine organic particles (detritus) and benthic organisms, including fairy shrimp and aquatic insects such as predaceous diving beetle larvae of the family Dytiscidae. They can also be cannibalistic. It has been observed feeding on the blades of, especially Glyceria declinata, an aquatic weed introduced from southern Europe.

Habitats

Lepidurus packardi occurs sporadically in a variety of natural ephemeral habitats including vernal pools and swales. It may also occur where water collects seasonally in artificially created habitats like stock ponds, reservoirs, ditches, pits created by backhoes and ruts created by vehicular traffic. Intermediate levels of disturbance appear to enhance habitat suitability. Ponds with extensive vegetative growth can become unsuitable for this species because decomposition of dead vegetation may deplete available oxygen and acidify the water. Tolerances to variation in water chemistry are largely unknown, but water temperatures less than about 10°C appear to be most favorable.

Threats

Impacts from grazing are probably the single most important threat to vernal pool tadpole shrimp and the vernal pools inhabited by them. According to Rogers (2000), grazing by livestock may have either a positive or negative influence on vernal pool habitat integrity and suitability for tadpole shrimp. Grazing may enhance habitat suitability by reducing the density of vegetation adjacent to vernal pools by consumption or trampling which avoids the loss of water quality that might otherwise result if the vegetation were to decompose in the vernal pool. Grazing may also fragment the canopy of herbaceous plants adjacent to vernal pools and promote plant diversity by preventing competitive dominates from displacing other plant species. Heavy grazing is usually detrimental because tadpole shrimp may be directly trampled or die if turbidity becomes excessive. Livestock may also defecate into the water resulting in lowered water quality as this organic material decomposes depleting dissolved oxygen and increasing acidity.

Other threats to vernal pool tadpole shrimp and their habitat include recreation, infestation by noxious weeds, modification of natural hydrology, water chemistry or water temperature, and increased sedimentation.

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USDA Forest Service · Pacific Southwest Region

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