Esther S. Rubin

Interactions between Feral Horses and Desert Bighorn Sheep at Water

Abstract We studied sympatric populations of native bighorn sheep (Ovis canadensis) and feral horses (Equus caballus) to quantify their spatial and temporal overlap and to determine whether horses interfered with use of water by bighorn sheep. We observed no evidence of direct competition, but our field experiment, which involved placing desert-acclimated domestic horses near watering sites used by bighorn sheep, demonstrated that bighorn sheep avoided sites with horses nearby. The presence of domestic horses near a watering site preferred by bighorn sheep resulted in a 76% reduction in the number of groups of bighorn sheep coming to water at that location and a concomitant increase in the number of bighorn sheep watering at other sites. An experimental approach to studying competition between large mammals has been problematic and to our knowledge this study constitutes the 1st manipulative field experiment to test for competitive interactions between feral horses and native ungulates.

Modeling demographic processes in an endangered population of bighorn sheep

We developed a demographic simulation model to explore how population vital rates, initial size, and the addition of animals influenced the viability of an endangered population of bighorn sheep (Ovis canadensis) consisting of 8 subpopulations. Perturbation analyses indicated that quasi-extinction risk was more sensitive to changes in adult female survival than to changes in reproduction or survival of young animals. This pattern was similar in 8 subpopulations that had different initial sizes, survival rates, and recruitment rates. Subpopulation viability was related to the initial number of females and to adult female survival, but not reproduction. Management actions that increase adult survival may be most effective when implemented in the largest subpopulations, whereas actions involving the addition of animals may be most effective if implemented in subpopulations with high survival rates. Subpopulation augmentation in yearly increments was more effective at reducing quasi-extinction risk than was adding the same total number of animals at the beginning of the simulation. The level of augmentation needed to substantially reduce quasi-extinction risk exceeded reported levels of female movement among populations or subpopulations. This finding led us to speculate that rescue effects, which are uncommon events for bighorn sheep, may be too rare or of inadequate magnitude under current conditions to effectively reverse bighorn sheep population declines.

Impacts of feral horses on a desert environment

BackgroundFree-ranging horses (Equus caballus) in North America are considered to be feral animals since they are descendents of non-native domestic horses introduced to the continent. We conducted a study in a southern California desert to understand how feral horse movements and horse feces impacted this arid ecosystem. We evaluated five parameters susceptible to horse trampling: soil strength, vegetation cover, percent of nonnative vegetation, plant species diversity, and macroinvertebrate abundance. We also tested whether or not plant cover and species diversity were affected by the presence of horse feces.ResultsHorse trailing resulted in reduced vegetation cover, compacted soils, and in cases of intermediate intensity disturbance, increased plant species diversity. The presence of horse feces did not affect plant cover, but it did increase native plant diversity.ConclusionAdverse impacts, such as soil compaction and increased erosion potential, were limited to established horse trails. In contrast, increased native plant diversity near trails and feces could be viewed as positive outcomes. Extensive trailing can result in a surprisingly large impact area: we estimate that < 30 horses used > 25 km2 of trails in our study area.

Flawed model has serious conservation implications: Response to Turner et al.

Turner et al. (2004) developed a habitat selection model for a population of desert bighorn sheep (Ovis canadensis) in the Peninsular Ranges of southern California that is listed as a threatened and endangered population by the state of California and the federal government, respectively. We are concerned that the recent publication of an article by Turner et al. (2004) could be detrimental to the management and recovery of bighorn sheep in the Peninsular Ranges because it lends credibility to a flawed analysis of bighorn sheep habitatuse patterns. The model attempts to extrapolate conclusions from a limited subset of bighorn sheep data that is not representative of the study area and was not gathered in a manner conducive to the analysis methods used by the authors. The authors classified habitat pixels as “active” or “inactive” based on the presence-absence of bighorn sheep observations without considering monitoring intensity. Turner et al. (2004) also failed to consider the implications of basing their model almost entirely on a bighorn sheep subpopulation known to have atypical habitat selection patterns. This subpopulation in the northwestern Santa Rosa Mountains frequently used food and water sources within hillside urban areas. Because the Turner et al. (2004) model was developed using data primarily from this atypical subpopulation, the model has low external validity and is unlikely to accurately predict habitat selection by other bighorn sheep subpopulations in the Peninsular Ranges. Furthermore, with the NW sub-population used in model development now excluded from urban areas, the Turner et al. (2004) model is unlikely to accurately predict habitat selection patterns of even this sub-population. We suggest the Turner et al. (2004) model is at best only applicable to this subpopulation between the years 1994–1998.Abstract Turner et al. (2004) developed a habitat selection model for a population of desert bighorn sheep (Ovis canadensis) in the Peninsular Ranges of southern California that is listed as a threatened and endangered population by the state of California and the federal government, respectively. We are concerned that the recent publication of an article by Turner et al. (2004) could be detrimental to the management and recovery of bighorn sheep in the Peninsular Ranges because it lends credibility to a flawed analysis of bighorn sheep habitat-use patterns. The model attempts to extrapolate conclusions from a limited subset of bighorn sheep data that is not representative of the study area and was not gathered in a manner conducive to the analysis methods used by the authors. The authors classified habitat pixels as “active” or “inactive” based on the presence–absence of bighorn sheep observations without considering monitoring intensity. Turner et al. (2004) also failed to consider the implications of basing their model almost entirely on a bighorn sheep subpopulation known to have atypical habitat selection patterns. This subpopulation in the northwestern Santa Rosa Mountains frequently used food and water sources within hillside urban areas. Because the Turner et al. (2004) model was developed using data primarily from this atypical subpopulation, the model has low external validity and is unlikely to accurately predict habitat selection by other bighorn sheep subpopulations in the Peninsular Ranges. Furthermore, with the NW subpopulation used in model development now excluded from urban areas, the Turner et al. (2004) model is unlikely to accurately predict habitat selection patterns of even this subpopulation. We suggest the Turner et al. (2004) model is at best only applicable to this subpopulation between the years 1994–1998.

RESPONSE OF BIGHORN SHEEP (OVIS CANADENSIS) TO FERAL HONEY BEES (APIS MELLIFERA) AT WATER

Abstract We used remote cinematography to evaluate the interactions between feral honey bees (Apis mellifera) and desert bighorn sheep (Ovis canadensis) at an artificial water source (guzzler) in Anza-Borrego Desert State Park in southern California from July through September 1995. Honey bees, determined by molecular analysis to be non-Africanized (i.e., not A. m. scutellata), were present at the guzzler collecting water from dawn to dusk whenever bighorn sheep were videotaped. Bighorn sheep exhibited behavioral responses (violent head shaking, rapid withdrawal from the water source, and temporary refusal to drink) to honey bees during 62% (66 of 107) of their visits to the guzzler. Bighorn sheep spent significantly (P < 0.001) more time at the guzzler when their visits were interrupted by honey bees than when they were not interrupted. We concluded that honey bees altered the behavior of bighorn sheep and that honey bees and bighorn sheep were competing for water resources at the guzzler.