ALBERT

Notes: Bighorn sheep (Ovis canadensis) once were ubiquitous in large areas, including lands now contained in the national parks of the Intermountain and Colorado Plateau regions of the United States. Due to catastrophic declines in the late 1800s and early 1900s, most extant populations now occur as small, isolated groups with a highly fragmented distribution. Three different subspecies of bighorn sheep were extirpated from 14 of 18 areas that are now managed by the National Park Service. We describe an eight-year plan to restore bighorn sheep to currently suitable historic habitats in the national parks within a six-state intermountain region of the western United States, 1991–1998. A seven-point program was recommended for each park unit that included: (1) survey the existing populations; (2) conduct a GIS-based habitat assessment to identify suitable habitat; (3) convene scientific advisory panels to review results of habitat assessment; (4) convene interagency panels to discuss metapopulation management and to plan the restoration(s); (5) draft interagency restoration and management plans; (6) conduct translocation(s); and (7) monitor the populations. We evaluated 38,781 km2 of area; 32% (12,329 km2) was potential habitat for bighorn sheep. The scientific advisory panels and the GIS modeling recommended bighorn restoration on 73 sites within these areas. By 1996, 36 of these sites (2,647 km2 or 22% of the entire suitable area) were inhabited by bighorn sheep. By 1999, the translocated animals increased 25%, and restoration efforts will continue in many of the remaining sites.

Notes: Habitat analysis is an important component of animal population restoration. We tested a habitat evaluation procedure for Rocky Mountain bighorn sheep (Ovis canadensis canadensis) using a geographic information system (GIS). We applied the model to eight bighorn sheep translocation sites in Colorado and compared the model's habitat suitability assessments with translocation results. The model considered the habitat unsuitable for four failed translocations, and did not recognize suitable habitat for four successful translocations. We performed a sensitivity analysis to determine how individual parameters affected model suitability assessments. To improve the model's ability to distinguish between suitable and unsuitable habitat, we relaxed the suitability criteria for four parameters: barriers created by dense vegetation, barriers created by fences, buffer zones on human use areas, and horizontal visibility. The refined model can be useful to bighorn sheep restoration efforts by facilitating evaluation of large areas of potential habitat, but numerical thresholds for required amounts of suitable habitat must consider the scale of data used for analysis.

Notes: Using the example of a reintroduced bighorn sheep population in Badlands National Park, South Dakota we demonstrate the usefulness of neutrality tests and demographic data for detecting a severe genetic bottleneck (Ne 〈 10). From demographic data the effective population size of the founding population at Badlands was estimated to be six, and a heterozygosity excess test revealed evidence of a severe population bottleneck. We discuss the criteria for intervention when there is evidence of a severe bottleneck, and propose methods of mitigating the potentially deleterious long-term consequences of such bottlenecks. These issues are presented in the context of bighorn sheep reintroductions, but the issues are also of general importance to restoration efforts involving other large vertebrates.

Notes: By 1950, bighorn sheep were extirpated from large areas of their range. Most extant populations of bighorn sheep (Ovis canadensis) in the Intermountain West consist of 〈100 individuals occurring in a fragmented distribution across the landscape. Dispersal and successful colonizations of unoccupied habitat patches has been rarely reported, and, in particular, translocated populations have been characterized by limited population growth and limited dispersal rates. Restoration of the species is greatly assisted by dispersal and successful colonization of new patches within a metapopulation structure versus the existing scenario of negligible dispersal and fragmented, small populations. We investigated the correlates for the rate of colonizations of 79 suitable, but unoccupied, patches by 31 translocated populations of bighorn sheep released into nearby patches of habitat. Population growth rates of bighorn sheep in the release patches were correlated to Ne of the founder group, and early contact with a second released population in a nearby release patch (logistic regression, p = 0.08). Largest population size of all extant released populations in 1994 was correlated to potential Ne of the founder group, the number of different source populations represented in the founder, and early contact with a second released population (p = 0.016). Dispersal rates were 100% higher in rams than ewes (p = 0.001). Successful colonizations of unoccupied patches (n = 24 of 79 were colonized) were associated with rapid growth rates in the released population, years since release, larger area of suitable habitat in the release patch, larger population sizes, and a seasonal migratory tendency in the released population (p = 0.05). Fewer water barriers, more open vegetation and more rugged, broken terrain in the intervening habitat were also associated with colonizations (p = 〈0.05). We concluded that high dispersal rates and rapid reoccupation of large areas could occur if bighorn sheep are placed in large patches of habitat with few barriers to movements to other patches and with no domestic sheep present. Many restorations in the past that did not meet these criteria may have contributed to an insular population structure of bighorn sheep with limited observations of dispersal.

Notes: We examined the size, species, location (x and y coordinates), and microsite inhabited by colonizing trees and shrubs in five abandoned pastures in southern Costa Rica. All woody stems greater than 1 m tall in the pastures were measured and mapped, from the overhanging forest edge to 50 m into the abandoned pasture. Species composition of colonists differed substantially among pastures: Croton draco (Euphorbiaceae) dominated one site, two species of Miconia (Melastomataceae) another site, and Verbesina tapentiensis (Asteraceae) a third site. Site 4 had the highest cover of rotting logs (11%), and a four-fold greater density of colonizing woody plants than the site with the next highest colonist density. For all species pooled, and for several individual taxa, density was positively correlated across sites with abundance of log microsites.Four of the six most common woody species in site 4 occurred on logs significantly more often than expected had they been randomly distributed relative to logs. Site 5 had less abundance of logs, but the common Miconia species was again significantly more likely to be found on log microsites. These results strongly suggest that rotting wood microsites facilitate establishment of bird-dispersed pioneer trees, which in turn could foster regrowth of other forest species.

Notes: Sources of forest regeneration (soil seed bank, seed rain) and barriers to seedling establishment were examined in a recently abandoned pasture in eastern Puerto Rico. Few woody species were found in the soil seed bank or in the seed rain. The number of seeds and species in the seed rain and soil seed bank declined with distance from the adjacent secondary forest. Nine species naturally dispersed and colonized plots during the study, with the wind-dispersed tree Tabebuia heterophylla being the predominant colonizer (91% of all seedlings). Barriers to seedling establishment were determined using a blocked field experiment with eleven woody species representative of a variety of life histories. Each species was planted under the pasture vegetation (control) or in areas where all vegetation was removed (removal). Germination was enhanced for four species in the control treatment, five species were not affected, and two species did not germinate under either treatment. Survival to 6 months was higher in the removal treatment for two species. Seedling biomass was greater in the removal treatment at 12 months for one species. Seed mass was a good predictor of germination success and final shoot biomass, but not survival. This study demonstrates that seeding recently abandoned pastures with a mix of known pioneer species may accelerate the rate of secondary succession, but some species will have to be planted in later successional stages in order to overcome strong barriers to establishment.

Notes: Forest recovery in degraded pastures is often highly variable, possibly due to variation in the availability of adequate recruitment sites. In an actively grazed pasture in northeastern Costa Rica, this study examines how recruitment of woody species in patches of the fern Nephrolepsis sp. and near logs compares with recruitment in grassy areas. Fern patches and logs had five and eight times higher densities of woody recruits, respectively, as grassy areas. They also had more than twice the species richness and growth as grassy areas. Grass apparently presents a competitive barrier against invading woody recruits, and also attracts cattle that may trample and/or consume recruits. Both logs and patches of fern appeared to provide superior conditions for establishment and growth of woody species, and they did not attract cattle. This study suggests that fern patches and logs can be managed to accelerate forest succession.