Mortality of Monarch Butterflies (Danaus plexippus) at Two Highway Crossing “Hotspots” During Autumn Migration in Northeast Mexico

Abstract

The contribution to annual mortality of migrating monarch butterflies (Danaus plexippus) due to collisions with vehicles is poorly understood but likely significant. Recent estimates based on a study in Texas suggests that mortality during autumn migration may be of the order of 2 million per year or about 3% of the population. However, MaxEnt models used in that study are not well suited to quantifying mortality at hotspots where monarchs are concentrated by topography such as canyons when crossing highways. Potentially catastrophic mortality could occur at such sites if timing of migration and weather conditions conspire to force a large proportion of the migrating population across highways at low altitude. We investigated monarch mortality 15 October to 11 November, 2018 at two highway crossings in northeastern Mexico known for their frequent and extensive collisions (La Muralla and Santa Catarina). During a 15-19 day period of migration, we collected dead and injured monarchs along a series of 500 m roadside transects. We estimated a minimum total mortality during fall migration at just these sites of about 196,560 individuals. Monarchs exhibited a diurnal pattern of passage at Santa Catarina of peaks in late morning and late afternoon. Average vehicle speeds exceeded posted 60 km/h limits designed to protect monarchs, ranging from 75.1 to 99.6 km/h at La Muralla and 86.6 to 106.8 km/h at Santa Catarina. We recommend finer-scale documentation of migration pathways and an inventory of significant highway crossing hotspots for monarchs during fall migration in northeast Mexico. Mitigative measures could include better enforced vehicle speeds at least during the short period of migration, deflection structures to raise the height of crossing monarchs, and/or manipulation of habitat to lower the potential for monarchs descending to roost near key crossing points.

Origins of Six Species of Butterflies Migrating through Northeastern Mexico: New Insights from Stable Isotope (δ2H) Analyses and a Call for Documenting Butterfly Migrations

“…Following the authors of [29], we focused on a highway along the Sierra Madre Oriental in the state of Nuevo Leon (Highway MEX 40-D, landmark km 58, Saltillo-Monterrey; 25 • 39 18.54 N, 100 • 27 12.24 W). The highway dissects the natural habitat of the region and is adjacent to the high-density urban center of Monterrey.…”

“…Recently, Mora-Alvarez et al [29] documented a fall migration bottleneck for monarch butterflies in northeast Mexico associated with 2 major highway crossings and predicted the annual roadkill mortality of monarchs at that site to be at least 197,000 individuals. This high mortality results from the geographic concentration of the southward bound monarchs by the Sierra Madre Oriental mountains, which forces monarchs into narrow canyons intercepted by highways.…”

Enhancing road verges to aid pollinator conservation: A review

“…For example, bumblebees Bombus spp. can fly distances of several kilometres (Greenleaf et al, 2007), and migrating monarch butterflies readily cross roads, mostly at heights of >6 m (Mora Alvarez et al, 2019). However, most studies that we found focus on local-scale movement of pollinators, where pollinators with shorter flight ranges might be physically unable to cross roads, or otherwise where pollinators are more likely to be deterred from crossing roads because they are responding to local cues (e.g.…”

“…Pollinators that attempt to cross roads at low heights may be killed by collision with vehicles. Few studies provide information on road crossing height, though Mora Alvarez et al (2019) observed that most migrating monarch butterflies crossed highways at heights of >6 m, whilst other butterfly species have been observed exhibiting resourcesearching behaviour along roads (zig-zagging low to the ground) (Severns, 2008;Skórka et al, 2013), which puts them at high risk of being hit by road traffic. Most studies estimate mortality from vehiclepollinator collisions using counts of dead insects along roads, which are likely to underestimate (e.g.…”

A Method to Project Future Impacts From Threats and Conservation on the Probability of Extinction for North American Migratory Monarch (Danaus plexippus) Populations

“…The western population faces additional threats such as loss of overwintering or breeding habitat from climate-related fire and drought (Griffiths and Villablanca, 2015;Pelton et al, 2016). Furthermore, both populations face the threats of road kill mortality (Kantola et al, 2019;Mora Alvarez et al, 2019), diseases, parasitism, and predation (Altizer and de Roode, 2015;Oberhauser et al, 2015). Each of these threats contributes to the current and future resiliency of eastern and western monarch populations through population-specific responses that the SSA endeavors to make explicit.…”