Centre for Ecology and Hydrology, Hill of Brathens, AB31 4 BW, Banchory, UK.
Department of Biology, University College London, Gower Street, London, UK.
Oecologia. 2001 Dec;129(4):561-570. doi: 10.1007/s004420100770. Epub 2001 Aug 2.
Guanacos (Lama guanicoe) are the only wild ungulate species widely distributed across the Patagonian steppe and have undergone a precipitous population decline since the introduction of domestic sheep (Ovis aries) 100 years ago. There has been speculation that sheep ranching may have played a major role in guanaco population decline after monopolising the most productive land because of competition for forage plants. Our aim in this work was to estimate guanaco abundance and account for its variation across nine different sites, two seasons and two years. We conducted over 100 ground surveys of both guanacos and sheep and estimated the availability of the preferred plant species both animal species might select in their diet. We found that (1) sheep densities were up to 23 times higher than guanaco densities in sympatric conditions, (2) at a protected site without sheep, guanaco density was one order of magnitude higher than at the rest of the sites, (3) across nine different sites, sheep densities alone accounted for around 60% of the variation in guanaco abundance, (4) guanaco densities were negatively related to both total plant cover and availability of the preferred plant species in their diet, which were both positively associated with sheep density, and (5) within-site changes in guanaco densities between seasons and years were negatively related to changes in sheep densities. Our results are consistent with predictions on interspecific competition for food resources, although we cannot rule out possible effects of other human-related activities influencing guanaco abundance. We conclude that sheep compete with guanacos for forage in arid Patagonia.
原驼（Lama guanicoe）是唯一广泛分布于巴塔哥尼亚草原的野生有蹄类动物，自 100 年前引入绵羊（Ovis aries）以来，其数量急剧下降。有人推测，由于争夺草料植物，绵羊养殖在垄断了最肥沃的土地后，可能在原驼种群减少中发挥了主要作用。我们的目的是估算原驼的数量，并解释其在九个不同地点、两个季节和两年内的变化。我们对原驼和绵羊进行了 100 多次地面调查，估算了这两种动物可能在其饮食中选择的首选植物物种的可利用性。我们发现：（1）在共存条件下，绵羊的密度比原驼高 23 倍；（2）在没有绵羊的保护区，原驼的密度比其他地点高一个数量级；（3）在九个不同的地点，绵羊的密度 alone 解释了原驼丰度变化的 60%左右；（4）原驼的密度与总植物覆盖度以及其饮食中首选植物物种的可利用性呈负相关，而这些因素与绵羊密度呈正相关；（5）在同一地点，季节和年份之间原驼密度的变化与绵羊密度的变化呈负相关。我们的结果与关于食物资源的种间竞争的预测一致，尽管我们不能排除其他可能影响原驼数量的人为活动的影响。我们得出结论，绵羊和原驼在干旱的巴塔哥尼亚争夺草料。
