Jason P. Marshal

Power analysis of wolf-moose functional responses

Gray wolf (Canis lupus) predation is density dependent if the proportion of prey killed by wolves increases wit h prey density. The functional response is 1 component of wolf predation that has important effect on whether density-dependent predation occurs, and consequently whether wolves have the potential to regulate prey to a low-density equilibrium. Because the functional response alone may determine if predation is regulatory, researchers have been attempting to accurately describe the functional response for wolves. We used simulated datasets based on the typical variances found in wolf-moose (Alces alces) predation data to evaluate the sample size required to distinguish between 2 common forms of wol f functional response: Type II and Type III. We found that we required sample sizes considerably larger than those likely attainable for wolf-moose predation studies. This conclusion was true even for a strongly sigmoid functional response (C > 2 in the Michaelis-Mento equation). Our analysis suggests it would be better to devote more effort toward measuring total predation losses versus trying to determine the shape of the functional response.

Rainfall, El Nino, and dynamics of mule deer in the Sonoran Desert, California

We used long-term El Nino southern oscillation (ENSO), rainfall, and deer harvest records to investigate effects of ENSO and rainfall on mule deer (Odocoileus hemionus) population trends in the Sonoran Desert, southeastern California, USA. We found significant relationships between the southern oscillation index and rainfall (R 2 = 0.38, P≤0.001), and between rainfall and annual deer harvest (R 2 = 0.25, P ≤ 0.001). We also found that deer harvest (i.e., an index of deer abundance) in any year was related to accumulations of rainfall >5 years before that hunting season (R 2 = 0.34), whereas the change in harvest between years (i.e., an index of rate of population change) was most related to rainfall the year immediately prior to that hunting season (R 2 = 0.15). Fluctuations in deer populations in the deserts of California ultimately may be caused by ENSO events.

Factors Affecting Habitat Use and Distribution of Desert Mule Deer in an Arid Environment

Abstract We studied habitat use by desert mule deer (Odocoileus hemionus eremicus) in the Sonoran Desert to understand the relative importance of vegetation, terrain characteristics, human disturbances, and water sources in determining their distribution. We located 44 radiocollared female mule deer weekly over 5 years. In spring, when water was most scarce, deer were in areas with lower elevations, shallower slopes, and greater normalized difference vegetation index (NDVI) rates, when compared to random locations. Use of water developments (i.e., catchments) during spring was evident but their effect on deer distribution was small relative to other factors, and their importance varied by animal. More deer locations were recorded in areas of higher NDVI rate in spring, summer, and autumn, but they were also further from washes in autumn. The deer we monitored used lower elevations in spring and higher elevations during the rest of the year. The effect of slope was strong in all seasons. However, deer tended to select shallow slopes in spring and steep slopes during the rest of the year. Deer avoided roads in summer and autumn but were closer to them in winter. They avoided rivers or canals in summer but were closer to those features in autumn. Our results suggest that terrain characteristics in all seasons (slope and elevation), as well as forage quality in 3 of 4 seasons (as indexed by NDVI rate) were most important in determining distribution of deer. Concomitantly water catchments had a measurable, but minor, role during hot, dry conditions. We recommend investigators continue to evaluate the importance of water developments relative to other habitat factors, particularly via the use of multivariate studies, global positioning system technology, adaptive management, and temporary closure of water developments known to be used by radiocollared deer.

Rainfall, Temperature, and Forage Dynamics Affect Nutritional Quality of Desert Mule Deer Forage

Abstract Forage quality affects physiological condition, population dynamics, habitat use, and distribution of ungulates. We studied how rainfall, temperature, forage biomass, and forage growth are related to water content, crude protein (CP), and in vitro dry-matter digestibility (IVDMD) of some common forage species of desert mule deer (Odocoileus hemionus eremicus Mearns) in the Sonoran Desert, California. We established vegetation transects in desert washes to collect forage samples and to measure forage biomass, growth, rainfall, and temperature on a quarterly basis. Percent water and CP were positively associated with forage growth (P < 0.001) and with rainfall (P ≤ 0.025). There were positive relationships between IVDMD and forage growth (P < 0.001), forage biomass (P < 0.001), and the combination of temperature and rainfall (P < 0.001). These findings suggest that the highest quality landscapes for deer are those with rapidly growing forage where forage water, CP, and IVDMD are greatest. With the quantified relationships between rainfall, temperature, and forage characteristics presented here, the nutritional constituents for deer forage can be predicted.

SEASONAL FORAGE USE BY DESERT MULE DEER IN SOUTHEASTERN CALIFORNIA

Abstract We used microhistological analysis of fecal pellets to identify plant species in diets of desert mule deer (Odocoileus hemionus eremicus) in southeastern California and to investigate seasonal changes in use of forage classes. We identified 34 taxa of plants, 7 of which have not been reported previously in the diets of desert mule deer. Browse species were abundant in diets during all seasons, but were lowest in spring, when forb species were most commonly observed. Use of succulents generally was low, with highest use occurring in autumn (3 to 24%). Grasses composed ≤1% of the diet in all seasons. Our findings emphasize the diversity of plant species used by desert mule deer and, consequently, the importance of conserving habitats that provide for that diversity.

Scale-dependent selection of greenness by African elephants in the Kruger-private reserve transboundary region, South Africa

Foraging behaviour and habitat selection occur as hierarchical processes. Understanding the factors that govern foraging and habitat selection thus requires investigation of those processes over the scales at which they occur. We investigated patterns of habitat use by African elephants (Loxodonta africana) in relation to vegetation greenness to investigate the scale at which that landscape attribute was most closely related to distribution of elephant locations. We analysed Global Positioning System radio-collar locations for 15 individuals, using the Normalized Difference Vegetation Index as a representation of vegetation greenness in a Geographic Information Systems framework. We compared the importance of vegetation greenness at three spatial scales: the total home range, the seasonal home range and the 16-day home range. During the wet season, seasonal home ranges for both sexes were associated with intermediate greenness within the total home range; there was no evidence of selection based on greenness at finer scales. During the dry season, the strongest associations were within the 16-day home range: individual locations for males tended to be in areas of intermediate greenness, and those for females were in areas of intermediate and high greenness. Our findings suggest that the role of vegetation greenness varies with the scale of analysis, likely reflecting the hierarchical processes involved in habitat selection by elephants.

Mining activity and habitat use by mountain sheep (Ovis canadensis)

We studied mountain sheep in the vicinity of three high-wall limestone mines in San Bernardino County, CA, USA to evaluate factors that influenced habitat use and, specifically, to investigate the influence of mining activity on distribution of those specialized ungulates. We used aerial telemetry data to estimate a resource selection function by fitting a logistic regression model and then comparing environmental characteristics at observed sheep locations to those at random locations. Distribution of mountain sheep was most influenced by a fire in 1999 that resulted in an area they avoided. Mountain sheep used steeper slopes, areas of lower terrain roughness, higher elevations, and areas closer to escape terrain than were random points. In contrast, sheep avoided areas near roads (federal and state highways, local roads, and off-road vehicle trails) but used areas near hiking trails and a railway. Water sources had the smallest effect of the factors considered, with sheep being associated with areas further from water points than were random locations. The disturbed area associated with the mines had a moderate influence on distribution, with sheep being associated with areas closer to the mine than were random points. Mining activities can alter terrain features and vegetation structure or composition in a way that promotes occupancy by sheep if they create steep slopes and rugged terrain (escape terrain) or reduce vegetation density or height (i.e., improve visibility). Whether increased occupancy reflects a benefit depends on the demographic responses of those sheep to the resources and conditions available on mine sites; information about those responses remains lacking.

Gradients of Forage Biomass and Ungulate Use Near Wildlife Water Developments

Abstract The addition of wildlife water developments (i.e., catchments) to arid areas may concentrate foraging by desert ungulates and decrease forage availability near catchments. We looked for gradients in forage biomass and use by mule deer (Odocoileus hemionus) and bighorn sheep (Ovis canadensis) along dry riparian streams near catchments. We measured forage biomass and pellet group density in transects following vegetation along edges of desert washes near catchments and in control washes with catchments >3 km away. Ungulate use, as reflected by pellet group density, was greatest in washes with catchments in place >3 years. There was some evidence for a gradient in pellet density in spring (slope = −0.012, P = 0.088) and summer (slope = −0.013, P = 0.015), and for differences between catchment and control transects in all seasons (13–46 pellet groups/ha, P ≤ 0.077). We found no evidence of an effect of catchments on forage biomass in nearby washes: we detected no gradient in forage biomass nor an overall difference between washes with catchments and those without (P > 0.15). Desert ungulates used washes near catchments in our study area but had minimal effects on nearby vegetation.

Evidence for interspecific competition between feral ass Equus asinus and mountain sheep Ovis canadensis in a desert environment

Abstract Different ungulate species that co-occur over evolutionary time have been hypothesized to develop mechanisms to limit the degree to which they directly compete for the same limited resources (i.e. resource partitioning). In situations where an exotic ungulate has been recently introduced to a system, resource partitioning has not likely developed; this appears to be the situation between introduced feral ass Equus asinus and indigenous mountain sheep Ovis canadensis in the Sonoran Desert of southeastern California, USA. We analysed data from aerial surveys conducted during 1993–2003 to look for evidence of an effect of feral ass abundance on mountain sheep demography. After controlling for the influence of forage availability via rainfall, we found no evidence of relationship between an index of feral ass abundance and indices of reproduction or recruitment in mountain sheep (P≥0.360). However, we found strong evidence for an effect on indices of sheep abundance and population rate of change. There was an interactive effect of rainfall and feral ass, such that a negative relationship between abundances of mountain sheep and feral ass was strongest during the driest years (P=0.014). There was a negative relationship between rate of change for sheep populations and feral ass abundance (P=0.016), which was not affected by rainfall. These results indicated a competitive effect of feral ass on mountain sheep populations, but the mechanism of competition remains unclear. Further research should use manipulations in feral ass abundance to clarify interactions between these species.

Intrinsic and extrinsic sources of variation in the dynamics of large herbivore populations

We evaluated the relative importance of intrinsic (density-dependent) and extrinsic (density-independent) sour- ces of resource variability in the dynamics of bighorn sheep (Ovis canadensis Shaw, 1804) populations in southwestern USA. We tested the hypothesis that populations experiencing greater variation in forage availability are less likely to be at equilibrium density with forage supplies, and thus, would demonstrate weaker evidence of intraspecific competition. We used regression to relate population growth rate to rainfall (forage conditions) and to abundance (intraspecific competition) for 20 sheep populations, and we estimated the strength of the effects of density and rainfall for each population. Then we compared among populations to look for relationships between the strengths of rainfall and density effects and the variabil- ity in rainfall experienced by each population. Populations demonstrating a density effect were associated with environ- ments having lower variability than those that did not. Populations showing a rainfall effect were associated with environments having a variability higher than those that lacked a rainfall effect. These findings are consistent with the hy- pothesis that populations experiencing higher variation in forage resources are less likely to achieve an equilibrium density with forage supplies where intraspecific competition would be a large determinant to the dynamics of that population.