Butterfly Mortality and Salvage Logging

By: Lincoln P. Brower, Ernest H. Williams, Pablo Jaramillo-Lopez, David R. Kust, Daniel A. Slayback, and M. Isabel Ramirez

from the March 2016 Storm in the Monarch Butterfly Biosphere Reserve in Mexico



A severe late spring storm in central Mexico in March 2016 that struck the Monarch Butterfly Biosphere Reserve was unique because it was accompanied by high-velocity winds that eliminated the normal thermal protection provided by the Oyamel fir forest. Temperatures throughout the forest merged with the colder open-area ambient temperatures. The storm was in effect a rain and snow storm sandwiched within a powerful and sustained wind storm, followed by lethal freezing that killed 31–38% of the butterflies in the Sierra Chincua and Cerro Pelón overwintering colonies. Several lines of evidence point to greater than 40% mortality of monarchs in the Rosario colony. Our estimates are a five-fold increase over the 7.4% mortality reported by the press. Tens of thousands of trees were blown down and subjected to extensive salvage logging in the Core Zone of the Reserve. This loss of canopy cover will diminish the normal microclimatic protection provided by the intact forest. The unexpected effects of this storm may take place at greater frequency in an era of changing climate.

Research summarized in Williams and Brower (2015) has shown that the intact Oyamel forest ecosystem provides microclimatic protection of the overwintering monarch butterflies (Danaus plexippus L.) by acting as a blanket that holds heat beneath the forest canopy, as an umbrella that reduces wetting of the butterflies clustering on the tree boughs, and as a heat source from the tree trunks radiating warmth that protects the butterflies clustering on the trunks from freezing. Although several past winter storms have struck the overwintering area (Calvert et al. 1983, Marriott 1996, Taylor et al. 2000, Brower et al. 2004, Taylor 2004, Brower et al. 2009), the March 2016 storm was exceptionally severe, with intense and continuous wind in combination with rain, snow, and lethally cold temperatures. In this paper, we describe this storm and how the severe wind obliterated the microclimatic protection by homogenizing sub-freezing temperatures throughout the Oyamel fir forest ecosystem and the effect this had upon the butterflies. We also provide the first quantitative estimate of butterfly mortality caused by the storm, and we describe and address potential effects of the authorization to permit the salvage logging of tens of thousands of trees blown down by the storm.

Historical Decline in the Numbers of Overwintering Butterflies

Over the past 24 years, the numbers of monarch butterflies overwintering in Mexico, as measured by the combined area of forest occupied by all known colonies, declined by 90%, from a high of 18.2 hectares during the 1996–1997 overwintering season to a low of 0.67 hectares in 2013–2014 (Brower et al. 2012, Vidal and Rendón-Salinas 2014, Rendón-Salinas and Tavera-Alonso 2014). In the 2014–2015 season, the numbers increased slightly to 1.13 ha, but that was still the second lowest number in the historical record (Rendón-Salinas et al. 2016). The severe decline resulted in widespread press coverage (Partlow 2014, Stevenson 2014, Wade 2014, Wines 2014) expressing concern that the migration and overwintering phenomenon might be on the verge of extinction, as had been predicted by Brower and Malcolm (1991). The dwindling population was formally addressed on 26 August 2014 in a petition to the U.S. Fish and Wildlife Service to designate the monarch butterfly migration as a threatened phenomenon (Center for Biological Diversity et. al. 2014). Following that, a multidisciplinary study concluded that the eastern North American population of the butterfly is at substantial risk of extinction (Semmens et al. 2016). Modeling the overwintering numbers over a 22-year period (1993–2014), the authors pointed out that the historical fluctuations and the precipitously small recent number of hectares occupied by the butterflies have resulted in the eastern North American population approaching what they termed “a quasi-extinct population,” defined as a population with so few individuals remaining that recovery becomes impossible. Their model predicted that if the overwintering area declined to between 0.01–0.25 hectares, a quasi-extinction probability over 20 years would reach 11–57%. The study concluded that reducing the probability of extinction by 50% over the next two decades will depend on increasing the size of the breeding population in the U.S. and Canada, in turn leading to an increase in the area occupied in Mexico by up to at least 6 ha, with a goal of doing so by 2020.

The 2015–2016 overwintering season heralded moderately good news: on 26 February 2016, World Wildlife Fund–Mexico together with two Mexican government agencies, SEMARNAT (The Secretary of the Environment and Natural Resources) and CONANP (The National Commission of Protected Natural Areas), reported that the area measured during the second half of December 2015 had increased from 1.13 ha in December 2014 to 4.01 ha (Rendón-Salinas et al. 2016). The press (Burnett 2016) responded to the positive news, quoting Daniel Ashe (Secretary, U.S. Fish and Wildlife Service): “Our task now is to continue building on that success” (see also Associated Press 2016a). Williams (2016) immediately responded to the optimism by urging caution in interpreting the increase, warning that it was for one year in a general 20-year downward trend, and that “the challenges facing monarchs remain.” Two weeks later (7–11 March 2016), a severe and unusually late winter storm struck the overwintering area and, with speculation on the likely mortality, was reported as a “heartbreaking development” (Cave 2016).

Here, we describe the 7–11 March 2016 storm and analyze weather data recorded on the Sierra Chincua that indicate that the intensity and seeming uniqueness of the storm had heretofore unseen effects on the microclimatic protection that the Oyamel fir forest ecosystem normally provides the overwintering butterflies (Williams and Brower 2015). Based on our weather data and the known freezing resistance physiology of monarchs, we estimate the mortality that the storm might have caused. We then present field censuses of mortality in the Pelón and Chincua overwintering colonies and estimate that mortality was higher in the Rosario colony. Finally, we address the official decision to allow extensive post-storm salvage logging and the likely negative impact it will have on the microclimate of the Reserve.

Qualitative Description of the 7–11 March 2016 Storm

Personal and press reports (Rangel 2016, Romero 2016) stated that the four-day storm in Angangueo (the town located near the center of the Reserve at 2,630 m elevation) began with winds early in the morning of 7 March that built to a maximum in the afternoon of 9 March and ended early in the morning of 11 March. Rain commenced on the afternoon of 8 March, continued for at least 16 hours through the afternoon of 9 March, and changed to snow at higher elevations (Romero 2016; Zubieta, pers. comm.). Severe west-to-east winds caused widespread damage to rooftops, power lines, power transformers, and trees. Sleet and 4–5 cm hail fell on Angangueo, and up to 10 cm of sleet fell below the entrance to the Rosario overwintering colony area. At higher elevations, snow was widespread, including on the Sierra Chincua (Figs. 1, 2), the Sierra Campanario, Cerro Pelón, and the San Andrés massif to the west of the Reserve (Rangel 2016; Romero 2016; Zubieta, pers. comm.; pers. observation).

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Published: Oxford Academic



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