TY - JOUR
T1 - Density estimates of monarch butterflies overwintering in central Mexico
AU - Thogmartin, Wayne E.
AU - Diffendorfer, Jay E.
AU - López-Hoffman, Laura
AU - Oberhauser, Karen
AU - Pleasants, John
AU - Semmens, Brice X.
AU - Semmens, Darius
AU - Taylor, Orley R.
AU - Wiederholt, Ruscena
N1 - Publisher Copyright:
© 2017 Thogmartin et al.
PY - 2017
Y1 - 2017
N2 - 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.
AB - 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.
KW - Danaus plexxipus
KW - Density estimation
KW - Mixture distribution
KW - Monarch butterfly
KW - Uncertainty modeling
UR - http://www.scopus.com/inward/record.url?scp=85018650703&partnerID=8YFLogxK
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U2 - 10.7717/peerj.3221
DO - 10.7717/peerj.3221
M3 - Article
C2 - 28462031
AN - SCOPUS:85018650703
SN - 2167-8359
VL - 2017
JO - PeerJ
JF - PeerJ
IS - 4
M1 - e3221
ER -