Immature Monarch Survival: Effects of Site Characteristics, Density, and Time

Nail, Kelly R.; Stenoien, Carl; Oberhauser, Karen S.

doi:10.1093/aesa/sav047

Cited by 74 publications

(111 citation statements)

References 33 publications

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“…Assuming 28.5 milkweed stems are needed to produce a single monarch (Nail, Stenoien & Oberhauser, 2015), 127 million monarchs equaled ∼3.62 billion stems (127 million monarchs × 28.5 milkweed/monarch).…”

Section: Resultsmentioning

confidence: 99%

Density estimates of monarch butterflies overwintering in central Mexico

Thogmartin

Diffendorfer

López‐Hoffman

et al. 2017

PeerJ

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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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Section: Resultsmentioning

confidence: 99%

Density estimates of monarch butterflies overwintering in central Mexico

Thogmartin

Diffendorfer

López‐Hoffman

et al. 2017

PeerJ

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“…Our model selection analysis indicates that over the years, populations of adult monarchs consistently build up during the summer, beginning in the first (southern USA) breeding generation. There is some evidence that the density of monarch eggs has declined since 2007 (Stenoien et al ), and that larval survival has also declined (Nail et al ); these results, however, are inconsistent with the trends in summer adult counts and initial number of fall migrants (Fig. ).…”

Section: Discussionmentioning

confidence: 98%

Linking the continental migratory cycle of the monarch butterfly to understand its population decline

Inamine

Ellner

Springer³

et al. 2016

Oikos

135

160

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Threats to several of the world's great animal migrations necessitate a research agenda focused on identifying drivers of their population dynamics. The monarch butterfly is an iconic species whose continental migratory population in eastern North America has been declining precipitously. Recent analyses have linked the monarch decline to reduced abundance of milkweed host plants in the USA caused by increased use of genetically modified herbicide‐resistant crops. To identify the most sensitive stages in the monarch's annual multi‐generational migration, and to test the milkweed limitation hypothesis, we analyzed 22 years of citizen science records from four monitoring programs across North America. We analyzed the relationships between butterfly population indices at successive stages of the annual migratory cycle to assess demographic connections and to address the roles of migrant population size versus temporal trends that reflect changes in habitat or resource quality. We find a sharp annual population decline in the first breeding generation in the southern USA, driven by the progressively smaller numbers of spring migrants from the overwintering grounds in Mexico. Monarch populations then build regionally during the summer generations. Contrary to the milkweed limitation hypothesis, we did not find statistically significant temporal trends in stage‐to‐stage population relationships in the mid‐western or northeastern USA. In contrast, there are statistically significant negative temporal trends at the overwintering grounds in Mexico, suggesting that monarch success during the fall migration and re‐establishment strongly contributes to the butterfly decline. Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch's population decline. Conservation efforts therefore require additional focus on the later phases in the monarch's annual migratory cycle. We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs.

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“…Our result that local summer abundance of monarchs has declined is consistent with those of other studies. Nail et al () found a decline in survival rates of immature monarchs from 1997 to 2014, Stenoien et al () detected a decline in the density of eggs per plant from 2006 to 2014, and Ries et al () indicated a possible (but not significant) decrease from 2011–2014 in end‐of‐summer counts conducted by the North American Butterfly Association. Furthermore, Pleasants and Oberhauser () and Pleasants () showed that if milkweed loss within agriculture fields in the upper Midwest is taken into account, monarch recruitment declined from 1999–2012, consistent with the decline observed in the overwintering population.…”

Section: Discussionmentioning

confidence: 99%

Local and cross‐seasonal associations of climate and land use with abundance of monarch butterflies Danaus plexippus

et al. 2017

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Quantifying how climate and land use factors drive population dynamics at regional scales is complex because it depends on the extent of spatial and temporal synchrony among local populations, and the integration of population processes throughout a species’ annual cycle. We modeled weekly, site‐specific summer abundance (1994–2013) of monarch butterflies Danaus plexippus at sites across Illinois, USA to assess relative associations of monarch abundance with climate and land use variables during the winter, spring, and summer stages of their annual cycle. We developed negative binomial regression models to estimate monarch abundance during recruitment in Illinois as a function of local climate, site‐specific crop cover, and county‐level herbicide (glyphosate) application. We also incorporated cross‐seasonal covariates, including annual abundance of wintering monarchs in Mexico and climate conditions during spring migration and breeding in Texas, USA. We provide the first empirical evidence of a negative association between county‐level glyphosate application and local abundance of adult monarchs, particularly in areas of concentrated agriculture. However, this association was only evident during the initial years of the adoption of herbicide‐resistant crops (1994–2003). We also found that wetter and, to a lesser degree, cooler springs in Texas were associated with higher summer abundances in Illinois, as were relatively cool local summer temperatures in Illinois. Site‐specific abundance of monarchs averaged approximately one fewer per site from 2004–2013 than during the previous decade, suggesting a recent decline in local abundance of monarch butterflies on their summer breeding grounds in Illinois. Our results demonstrate that seasonal climate and land use are associated with trends in adult monarch abundance, and our approach highlights the value of considering fine‐resolution temporal fluctuations in population‐level responses to environmental conditions when inferring the dynamics of migratory species.

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Immature Monarch Survival: Effects of Site Characteristics, Density, and Time

Cited by 74 publications

References 33 publications

Density estimates of monarch butterflies overwintering in central Mexico

Density estimates of monarch butterflies overwintering in central Mexico

Linking the continental migratory cycle of the monarch butterfly to understand its population decline

Local and cross‐seasonal associations of climate and land use with abundance of monarch butterflies Danaus plexippus

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