A multi-scale evaluation of pack stock effects on subalpine meadow plant communities in the Sierra Nevada.

We evaluated the influence of pack stock (i.e., horse and mule) use on meadow plant communities in Sequoia and Yosemite National Parks in the Sierra Nevada of California. Meadows were sampled to account for inherent variability across multiple scales by: 1) controlling for among-meadow variability by using remotely sensed hydro-climatic and geospatial data to pair stock use meadows with similar non-stock (reference) sites, 2) accounting for within-meadow variation in the local hydrology using in-situ soil moisture readings, and 3) incorporating variation in stock use intensity by sampling across the entire available gradient of pack stock use. Increased cover of bare ground was detected only within "dry" meadow areas at the two most heavily used pack stock meadows (maximum animals per night per hectare). There was no difference in plant community composition for any level of soil moisture or pack stock use. Increased local-scale spatial variability in plant community composition (species dispersion) was detected in "wet" meadow areas at the two most heavily used meadows. These results suggest that at the meadow scale, plant communities are generally resistant to the contemporary levels of recreational pack stock use. However, finer-scale within-meadow responses such as increased bare ground or spatial variability in the plant community can be a function of local-scale hydrological conditions. Wilderness managers can improve monitoring of disturbance in Sierra Nevada meadows by adopting multiple plant community indices while simultaneously considering local moisture regimes.

Transporting juvenile salmonids around dams impairs adult migration.

Mitigation and ecosystem-restoration efforts may have unintended consequences on both target and nontarget populations. Important effects can be displaced in space and time, making them difficult to detect without monitoring at appropriate scales. Here, we examined the effects of a mitigation program for juvenile salmonids on subsequent adult migration behaviors and survival. Juvenile chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss) were collected and uniquely tagged with passive integrated transponder (PIT) tags at Lower Granite Dam (Washington State, USA) on the Snake River and were then either transported downstream in barges in an effort to reduce out-migration mortality or returned to the river as a control group. Returning adults were collected and radio-tagged at Bonneville Dam (Washington-Oregon, USA) on the Columbia River 1-3 years later and then monitored during approximately 460 km of their homing migrations. The proportion of adults successfully homing was significantly lower, and unaccounted loss and permanent straying into non-natal rivers was higher, for barged fish of both species. On average, barged fish homed to Lower Granite Dam at rates about 10% lower than for in-river migrants. Barged fish were also 1.7-3.4 times more likely than in-river fish to fall back downstream past dams as adults, a behavior strongly associated with lower survival. These results suggest that juvenile transport impaired adult orientation or homing abilities, perhaps by disrupting sequential imprinting processes during juvenile out-migration. While juvenile transportation has clear short-term juvenile-survival benefits, the delayed effects that manifest in adult stages illustrate the need to assess mitigation success throughout the life cycle of target organisms, i.e., the use of fitness-based measures. In the case of Snake River salmonids listed under the Endangered Species Act, the increased straying and potential associated genetic and demographic effects may represent significant risks to successful recovery for both target and nontarget populations.