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Boyce, P. N., & McLoughlin, P. D. (2021). Ecological Interactions Involving Feral Horses and Predators: Review with Implications for Biodiversity Conservation. Jour. Wild. Mgmt., n/a(n/a).
Abstract: ABSTRACT For many ecosystems, feral horses are increasingly becoming an important if not dominant component of ungulate biomass and hence influence on community dynamics. Yet we still know little of how horses contribute to key ecological interactions including predator-prey and indirect competitive relationships at a community level. Notably, feral species like horses can exhibit life-history traits that differ from that of native (mainly artiodactyl) herbivore competitors. Artificial selection for traits like increased, early, or extended reproduction that have yet to be reversed by natural selection, coupled with naturally selected differences in anatomy and behavior, in addition to unique management objectives for horses compared to other species, means that the dynamics of feral horse populations are not likely to align with what might be expected of other large herbivores. Unexpected population dynamics and inherent biological asymmetries between native ungulates and feral horses may therefore influence the former via direct competition for shared resources and through enemy-mediated interactions like apparent competition. In several localities feral horses now co-exist with multiple native prey species, some of which are in decline or are species at risk. Compounding risks to native species from direct or indirect competitive exclusion by horses is the unique nature and socio-political context of feral horse management, which tends towards allowing horse populations to be limited largely by natural, density-dependent factors. We summarize the inherent asymmetries between feral horse biology and that of other ungulate prey species with consequences for conservation, focusing on predator-prey and emerging indirect interactions in multi-prey systems, and highlight future directions to address key knowledge gaps in our understanding of how feral horses may now be contributing to the (re)structuring of food webs. Observations of patterns of rapid growth and decline, and associated skews in sex ratios of feral horse populations, indicate a heightened potential for indirect interactions among large ungulate prey species, where there is a prevalence of feral horses as preferred prey, particularly where native prey are declining. In places like western North America, we expect predator-prey interactions involving feral horses to become an increasingly important factor in the conservation of wildlife. This applies not only to economically or culturally important game species but also at-risk species, both predators (e.g., wolves [Canis lupus], grizzly bears [Ursus arctos]) and prey (e.g., woodland caribou [Rangifer tarandus caribou]), necessitating an ecological understanding of the role of horses in natural environments that goes beyond that of population control. ? 2021 The Wildlife Society.
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van Duyne, C., Ras, E., de Vos, A. E. W., de Boer, W. F., Henkens, R. J. H. G., & Usukhjargal, D. (2009). Wolf Predation Among Reintroduced Przewalski Horses in Hustai National Park, Mongolia. Journal of Wildlife Management, 73(6), 836–843.
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Dalmau, A., Ferret, A., Chacon, G., & Manteca, X. (2007). Seasonal Changes in Fecal Cortisol Metabolites in Pyrenean Chamois. J Wildl Manag, 71(1), 190–194.
Abstract: We studied seasonal changes in fecal cortisol metabolites (FCM), which have been widely used as indicators of stress, in a population of Pyrenean chamois (Rupicapra pyrenaica pyrenaica) in the Cadí Range of northeastern Spain. We collected fecal samples from 2001 to 2003 in 3 particular locations with different altitudes and male or female presence, and we analyzed them for FCM and fecal nitrogen as an indicator of diet quality. We observed a clear seasonal pattern, with the highest FCM in winter, and we obtained correlations between FCM and monthly mean minimum temperatures and fecal nitrogen. We observed no effects of tourism presence, trophy hunting, or rut season on FCM. Analysis of cortisol metabolites in feces can be a good measure of winter stress in Pyrenean chamois.
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Baldi, R., Pelliza-Sbriller, A., Elston, D., & Albon, S. (2004). High Potential For Competition Between Guanacos And Sheep In Patagonia. Journal of Wildlife Management, 68(4), 924–938.
Abstract: Guanacos (Lama guanicoe) are the largest native Artiodactyl in South America and the most widely distributed. In arid Patagonia, densities are low and negatively related to domestic sheep numbers in space and time consistent with interspecific competition theory. Although guanacos and domestic sheep have been described as intermediate feeders sharing food resources, no studies have been conducted to compare their diets in sympatric conditions and explore whether the potential exists for direct interspecific competition. We assessed the diet of both species across 9 different sites and 2 seasons by microhistological analysis of fecal samples. We found that (1) guanacos and sheep are generalist herbivores feeding on a wide range of plant species; (2) both are intermediate feeders able to include both monocotyledoneous and dicotyledoneous plants in their diet; (3) both are able to change their diets seasonally; and (4) food niche overlap is high, particularly in summer when food resources are more scarce than in spring. We conclude that the potential for competition between guanacos and sheep is high and could have played a major role in the demise of guanacos. Consequently, current management practices focused on maximizing sheep numbers are not compatible with the recovery of guanaco populations.
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McKnight, T. L. (1958). The Feral Burro in the United States: Distribution and Problems. The Journal of Wildlife Management, 22(2), 163–179.
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Goodloe, R. B. (2000). Population characteristics of feral horses on Cumberland Island, Georgia and their management implications. J Wildl Manag, 64(1), 114–121.
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Kirkpatrick, J. F., Kasman, L. H., Lasley,, B. L., & Turner, J. W. J. (1988). Pregnancy Determination in Uncaptured Feral Horses. J Wildl Manag, 52(2), 305–308.
Abstract: The urinary excretion of estrone sulfate ($\text{E}{1}\text{S}$) by 25 free-roaming feral horses (Equus caballus) was measured by radioimmunoassay applied to extracts of urine-soaked soil. Twelve of 15 mares having $\text{E}{1}\text{S}$ concentrations >1.0 mg/mg creatinine (x = 2.64 +- 1.02 [SD]) produced foals. All 10 mares with $\text{E}{1}\text{S}$ concentrations <1.0 mg/mg creatinine (x = 0.44 +- 0.26) did not foal. Extracting urine from soil and measuring $\text{E}{1}\text{S}$ and creatinine can be used to determine pregnancy in free-roaming feral horses without the stress of capture or immobilization.
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Eberhardt, L. L., Majorowicz, A. K., & Wilcox, J. A. (1982). Apparent Rates of Increase for Two Feral Horse Herds. The Journal of Wildlife Management, 46(2), 367–374.
Abstract: Rates of increase for 2 Oregon feral horse (Equus caballus) herds were estimated from direct aerial counts to be about 20% per year. These rates can be achieved only if survival rates are high, and reproduction exceeds that normally expected from horses. A population dynamics model suggests adult survival to be the key parameter in determining rates of increase, and there is some direct evidence of high adult survival rates. Management implications are discussed.
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