Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?

Abstract

1. During the 2009-2010 overwintering season and following a 15-year downward trend, the total area in Mexico occupied by the eastern North American population of overwintering monarch butterflies reached an all-time low. Despite an increase, it remained low in 2010-2011.2. Although the data set is small, the decline in abundance is statistically significant using both linear and exponential regression models.3. Three factors appear to have contributed to reduce monarch abundance: degradation of the forest in the overwintering areas; the loss of breeding habitat in the United States due to the expansion of GM herbicide-resistant crops, with consequent loss of milkweed host plants, as well as continued land development; and severe weather.4. This decline calls into question the long-term survival of the monarchs' migratory phenomenon.Resú men. 1. Durante la temporada invernal 2009-2010, y siguiendo una tendencia a la baja de 15 an˜os, la superficie total ocupada por mariposas monarca en Me´xico, provenientes del este Ame´rica del Norte, llego´a su punto ma´s bajo. A pesar de su incremento, dicha superficie siguio´siendo baja en 2010-2011.2. Aunque que el conjunto de datos disponibles es au´n pequen˜o, esta disminucio´n de la abundancia de mariposas es estadı´sticamente significativa, tanto si se usan modelos de regresio´n lineales como exponenciales.3. Hay tres factores que parecen haber contribuido con esta tendencia de reduc-cio´n del nu´mero de mariposas: la degradacio´n de bosque en las a´reas de invernacio´n en Me´xico; la pe´rdida de ha´bitat de reproduccio´n en los Estados Unidos, debido a la expansio´n de cultivos gene´ticamente modificados resistentes a herbicidas, con la consiguiente pe´rdida de las plantas hospederas de algodoncillo, y por continuos cambios en el uso del suelo no favorables para ellas; y, las recientes condiciones cli-ma´ticas severas.4. Esta disminucio´n hace que nos cuestionemos sobre la posibilidad de supervivencia a largo plazo del feno´meno migratorio de las mariposas monarca.

Understanding a migratory species in a changing world: climatic effects and demographic declines in the western monarch revealed by four decades of intensive monitoring

“…These plants are ruderal in nature, and have experienced declines in recent years in some areas, potentially in association with increased herbicide use on agricultural lands (Hartzler 2010;Pleasants and Oberhauser 2012;Zalucki and Lammers 2010). For the overwintering monarchs in Mexico, severe weather and forest degradation are further stressors that compound habitat and host loss on breeding grounds in the USA (Brower et al 2012;Flockhart et al 2015). Despite apparent stressors and declines in monarchs at their overwintering grounds (Vidal and Rendon-Salinas 2014;Saenz-Romero et al 2012;Brower et al 2002), numbers have not declined at some of the fall stopover sites in the Eastern USA (e.g., Davis 2012) or summer breeding grounds (Ries et al 2015).…”

Assessing Role of Major Drivers in Recent Decline of Monarch Butterfly Population in North America

“…Neonicotinoid insecticides have been widely incorporated into commercial agriculture recently compared to the decline of eastern monarchs, therefore they may not be a primary driver of monarch declines, which have occurred since the 1990's (Brower et al, 2012). Moreover, by the time neonicotinoids were widely used (Elbert et al, 2008), milkweed plants were largely absent from agricultural fields (Pleasants and Oberhauser, 2013), thereby decreasing the potential for in-field neonicotinoid exposure of monarch larvae.…”

“…After 1999, growers began rapidly adopting genetically modified glyphosate resistant (GR) varieties of maize and soybeans enabling them to increase their usage of glyphosate as an herbicide (Brower et al, 2012). The timing of this change is suggested to be closely linked with the beginning of monarch declines.…”

“…Several interacting factors have been reported to be associated with the decline of the Eastern population of monarch butterflies in this region (Figure 1). A reduction in quality of overwintering habitat in Mexico, a sharp decrease of breeding habitat in the United States, periodic extreme weather conditions that reduce spring breeding populations (Brower et al, 2012), and modern agronomic practices including various pest management tactics have all been implicated. The Western monarch butterfly population migrates annually from its overwintering habitats along the California coast to its breeding and feeding ranges (summer habitats) in the western United States west of the Rocky Mountains.…”

Evaluation of Annual Modis PTC Data for Deforestation and Forest Degradation Analysis

“…In the last 20 years, the study area has experienced deforestation and forest degradation caused mainly by illegal logging (Honey-Rosés 2009, Navarrete et al 2011, Brower et al 2011, Vidal et al 2013). …”