Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)

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.

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“…The dashed gray line is an upper-end quasi-extinction risk threshold (0.25 ha) described by Semmens et al (2016).…”

Section: Resultsmentioning

confidence: 99%

“…Semmens et al (2016) used a state-space formula for estimating a corrected time series of the areal estimate of the overwinter population size. This state-space formula enabled estimation of the underlying true state of the population corrected for observation noise.…”

Section: Methodsmentioning

confidence: 99%

Density estimates of monarch butterflies overwintering in central Mexico

Thogmartin

Diffendorfer

López‐Hoffman

et al. 2017

PeerJ

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“…We developed a simulation‐based power analysis to predict the trade‐offs between sampling breadth and intensity for the IMMP. When monitoring densities of milkweed, monarch eggs, or adult monarchs, we evaluated potential sampling designs for their statistical power to detect (a) a population trend of the magnitude observed for the monarch butterfly population in the eastern U.S. (−4% per year; Semmens et al, ), or (b) various magnitudes of differences among land‐use sectors. We expected that scenarios allowing successful detection of population trends would also allow detection of differences between sectors, as monitoring trends is generally more data‐intensive than detecting differences (Urquhart, ).…”

Section: Introductionmentioning

confidence: 99%

Balancing sampling intensity against spatial coverage for a community science monitoring programme

Weiser

Diffendorfer

Grundel

et al. 2019

Journal of Applied Ecology

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Community science is an increasingly integral part of biodiversity research and monitoring, often achieving broad spatial and temporal coverage but lower sampling intensity than studies conducted by professional scientists. When designing a community‐science monitoring programme, careful assessment of sampling designs that could be both feasible and successful at meeting programme goals is essential. Monarch butterflies (Danaus plexippus) are the focus of several successful community‐science projects in the U.S., but broader coverage is needed to monitor breeding areas and explain population declines observed in overwintering areas. The U.S. Monarch Conservation Science Partnership's Integrated Monarch Monitoring Program (IMMP) will representatively monitor monarchs and milkweed across North America. We performed a simulation‐based power analysis to predict trade‐offs between sampling breadth (number of sites and years) and sampling intensity (number of visits or subplots per site and year) for the IMMP. We evaluated whether each sampling design would produce sufficient statistical power to detect population trends and differences among land‐use sectors in densities of milkweed, monarch eggs, and adult monarchs. Sampling breadth had much stronger effects than sampling intensity on statistical power for all three monitoring targets. Depending on land‐use sector, monitoring 400–800 sites over 10–15 years would detect trends in densities of milkweed and adult monarchs, but no scenarios were successful for monarch eggs. Sampling 400–800 sites would also detect small (for adult monarchs) to large (for milkweed) differences among land‐use sectors in density of all three monitoring targets within the first 2–5 years. As more data become available from the IMMP, the sampling goals can be updated. Synthesis and applications. Careful sample design is an essential step in developing a successful monitoring programme. For monarchs and milkweed, we found that sampling breadth (number of sites and years) had a much stronger effect on statistical power than sampling intensity (number of visits or subsamples per site), suggesting field protocols could be tailored to maximize recruitment and retention of volunteers by minimizing the effort required to monitor each site. Many long‐term monitoring programmes might similarly benefit from evaluating the statistical trade‐offs between sampling breadth and intensity in their sampling designs.

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“…The lowest recordings of the monarch densities on the wintering ground are from 2013 to 2015 (Rendón-Salinas and Tavera-Alonso, 2015), and the probability that the fascinating migration of the eastern monarch will go extinct within 20 years is estimated to be 11-57% (Semmens et al, 2016). Alarmed by these perspectives, initiatives across borders (US, Mexico, Canada) have been started to conserve the monarch butterfly.…”

Section: Case Study 3 the Monarch Butterfly And Landscape Level Effementioning

confidence: 99%

Complex Outcomes from Insect and Weed Control with Transgenic Plants: Ecological Surprises?

Bøhn

Löveï

2017

Front. Environ. Sci.

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Agriculture is fundamental for human survival through food production and is performed in ecosystems that, while simplified, still operate along ecological principles and retain complexity. Agricultural plants are thus part of ecological systems, and interact in complex ways with the surrounding terrestrial, soil, and aquatic habitats. We discuss three case studies that demonstrate how agricultural solutions to pest and weed control, if they overlook important ecological and evolutionary factors, cause "surprises": (i) the fast emergence of resistance against the crop-inserted Bt-toxin in South Africa, (ii) the ecological changes generated by Bt-cotton landscapes in China, and (iii) the decline of the monarch butterfly, Danaus plexippus, in North America. The recognition that we work with complex systems is in itself important, as it should limit the belief in reductionist solutions. Agricultural practices lacking eco-evolutionary understanding result in "surprises" like resistance evolution both in weeds and pest insects, risking the reappearance of the "pesticide treadmill"-with increased use of toxic pesticides as the follow-up. We recommend prioritization of research that counteracts the tendencies of reductionist approaches. These may be beneficial on a short term, but with trade-off costs on a medium-to long-term. Such costs include loss of biodiversity, ecosystem services, long-term soil productivity, pollution, and reduced food quality.

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Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)

Cited by 170 publications

References 21 publications

Density estimates of monarch butterflies overwintering in central Mexico

Density estimates of monarch butterflies overwintering in central Mexico

Balancing sampling intensity against spatial coverage for a community science monitoring programme

Complex Outcomes from Insect and Weed Control with Transgenic Plants: Ecological Surprises?

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