Given the rapid population decline and recent petition for listing of the monarch butterfly (Danaus plexippus L.) under the Endangered Species Act, an accurate estimate of the Eastern, migratory population size is needed. Because of difficulty in counting individual monarchs, the number of hectares occupied by monarchs in the overwintering area is commonly used as a proxy for population size, which is then multiplied by the density of individuals per hectare to estimate population size. There is, however, considerable variation in published estimates of overwintering density, ranging from 6.9-60.9 million ha-1. We develop a probability distribution for overwinter density of monarch butterflies from six published density estimates. The mean density among the mixture of the six published estimates was ∼27.9 million butterflies ha-1 (95% CI [2.4-80.7] million ha-1); the mixture distribution is approximately log-normal, and as such is better represented by the median (21.1 million butterflies ha-1). Based upon assumptions regarding the number of milkweed needed to support monarchs, the amount of milkweed (Asclepias spp.) lost (0.86 billion stems) in the northern US plus the amount of milkweed remaining (1.34 billion stems), we estimate >1.8 billion stems is needed to return monarchs to an average population size of 6 ha. Considerable uncertainty exists in this required amount of milkweed because of the considerable uncertainty occurring in overwinter density estimates. Nevertheless, the estimate is on the same order as other published estimates. The studies included in our synthesis differ substantially by year, location, method, and measures of precision. A better understanding of the factors influencing overwintering density across space and time would be valuable for increasing the precision of conservation recommendations.

译文

鉴于人口迅速减少,并且最近根据《濒危物种法》要求将帝王蝶 (Danaus plexippus L.) 列入名单,因此需要对东部迁徙种群数量进行准确估计。由于难以计算单个君主,因此通常将越冬地区君主所占的公顷数用作人口规模的替代指标,然后将其乘以每公顷的个人密度来估算人口规模。然而,在已公布的越冬密度估计值中存在相当大的变化,范围为690-6090万ha-1。我们从六个已发布的密度估算值中开发了帝王蝶越冬密度的概率分布。六个已公布的估计值的混合物中的平均密度为2790万蝴蝶ha-1 (95% CI [2.4-80.7] 百万ha-1); 混合物分布近似于对数正态分布,因此最好用中位数表示 (2110万蝴蝶ha-1)。根据关于支持君主所需马利筋数量的假设,美国北部损失的马利筋 (Asclepias spp。) 的数量 (8.6亿茎) 加上剩余的马利筋数量 (13.4亿茎),我们估计需要> 18亿个茎才能使君主恢复到6公顷的平均人口规模。由于越冬密度估计中存在相当大的不确定性,因此所需数量的马利筋存在相当大的不确定性。尽管如此,估计数与其他已发布的估计数的顺序相同。我们合成中的研究因年份,位置,方法和精密度而有很大差异。更好地了解影响时空越冬密度的因素对于提高保护建议的准确性将很有价值。

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