Flockhart D T Tyler, Pichancourt Jean-Baptiste, Norris D Ryan, Martin Tara G
Department of Integrative Biology, University of Guelph, Guelph, ON, N1G2W1, Canada.
Climate Adaptation Flagship, CSIRO, Ecosystem Sciences, GPO 2583, Brisbane, QLD, 4001, Australia.
J Anim Ecol. 2015 Jan;84(1):155-65. doi: 10.1111/1365-2656.12253. Epub 2014 Jun 25.
Threats to migratory animals can occur at multiple periods of the annual cycle that are separated by thousands of kilometres and span international borders. Populations of the iconic monarch butterfly (Danaus plexippus) of eastern North America have declined over the last 21 years. Three hypotheses have been posed to explain the decline: habitat loss on the overwintering grounds in Mexico, habitat loss on the breeding grounds in the United States and Canada, and extreme weather events. Our objectives were to assess population viability, determine which life stage, season and geographical region are contributing the most to population dynamics and test the three hypotheses that explain the observed population decline. We developed a spatially structured, stochastic and density-dependent periodic projection matrix model that integrates patterns of migratory connectivity and demographic vital rates across the annual cycle. We used perturbation analysis to determine the sensitivity of population abundance to changes in vital rate among life stages, seasons and geographical regions. Next, we compared the singular effects of each threat to the full model where all factors operate concurrently. Finally, we generated predictions to assess the risk of host plant loss as a result of genetically modified crops on current and future monarch butterfly population size and extinction probability. Our year-round population model predicted population declines of 14% and a quasi-extinction probability (<1000 individuals) >5% within a century. Monarch abundance was more than four times more sensitive to perturbations of vital rates on the breeding grounds than on the wintering grounds. Simulations that considered only forest loss or climate change in Mexico predicted higher population sizes compared to milkweed declines on the breeding grounds. Our model predictions also suggest that mitigating the negative effects of genetically modified crops results in higher population size and lower extinction risk. Recent population declines stem from reduction in milkweed host plants in the United States that arise from increasing adoption of genetically modified crops and land-use change, not from climate change or degradation of forest habitats in Mexico. Therefore, reducing the negative effects of host plant loss on the breeding grounds is the top conservation priority to slow or halt future population declines of monarch butterflies in North America.
对迁徙动物的威胁可能发生在年度周期的多个阶段，这些阶段相隔数千公里且跨越国界。在过去21年里，北美东部标志性的黑脉金斑蝶（Danaus plexippus）种群数量有所下降。人们提出了三种假说来解释这种下降：墨西哥越冬地的栖息地丧失、美国和加拿大繁殖地的栖息地丧失以及极端天气事件。我们的目标是评估种群生存力，确定哪个生命阶段、季节和地理区域对种群动态贡献最大，并检验解释观察到的种群下降的三种假说。我们开发了一个空间结构化、随机且依赖密度的周期性投影矩阵模型，该模型整合了年度周期中迁徙连通性模式和人口统计学生命率。我们使用扰动分析来确定种群丰度对生命阶段、季节和地理区域内生命率变化的敏感性。接下来，我们将每种威胁的单一效应与所有因素同时作用的完整模型进行了比较。最后，我们进行预测，以评估转基因作物导致寄主植物丧失对当前和未来黑脉金斑蝶种群规模及灭绝概率的风险。我们的全年种群模型预测，一个世纪内种群数量将下降14%，准灭绝概率（<1000只个体）>5%。黑脉金斑蝶的丰度对繁殖地生命率扰动的敏感度比对越冬地的敏感度高出四倍多。与繁殖地马利筋减少相比，仅考虑墨西哥森林丧失或气候变化的模拟预测种群规模更高。我们的模型预测还表明，减轻转基因作物的负面影响会导致种群规模更大且灭绝风险更低。近期种群数量下降源于美国马利筋寄主植物的减少，这是由于转基因作物的采用增加和土地利用变化所致，而非气候变化或墨西哥森林栖息地的退化。因此，减少寄主植物丧失对繁殖地的负面影响是减缓或阻止北美黑脉金斑蝶未来种群数量下降的首要保护重点。
