Herbicide-tolerant GM crops a major factor in the decline

A new study (abstract below) has found that the monarch butterfly population in the US has “a substantial probability of quasi-extinction, from 11–57% over 20 years”.

The study repeats the well-documented fact that a major factor in the decline of the monarch is the adoption of herbicide-tolerant GM corn and soybeans. The herbicide spraying has killed off the monarch larvae’s only food, the milkweed plant.

Given the findings of the new study, it seems premature to give credence to a suggestion made in one article that the monarch population may be “on the rebound”.

As the article also states, while the population of monarchs who made their annual trip to Mexico this past December increased three-and-a-half times from the previous year, the totals are still far below those from two decades ago. And the rise may be a temporary blip caused by favourable weather conditions.  

The article quotes Tierra Curry, a senior scientist at the Center for Biological Diversity, as saying, “The increase is certainly great news, but the bottom line is that monarchs must reach a much larger population size to be resilient to ever-increasing threats.”

Indeed, the study below says, “An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction.”
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Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)

Brice X. Semmens, Darius J. Semmens, Wayne E. Thogmartin, Ruscena Wiederholt, Laura López-Hoffman, Jay E. Diffendorfer, John M. Pleasants, Karen S. Oberhauser & Orley R. Taylor
Scientific Reports 6, Article number: 23265 (2016)
doi:10.1038/srep23265
Published online: 21 March 2016
http://www.nature.com/articles/srep23265

Abstract

The Eastern, migratory population of monarch butterflies (Danaus plexippus), an iconic North American insect, has declined by ~80% over the last decade. The monarch’s multi-generational migration between overwintering grounds in central Mexico and the summer breeding grounds in the northern U.S. and southern Canada is celebrated in all three countries and creates shared management responsibilities across North America. Here we present a novel Bayesian multivariate auto-regressive state-space model to assess quasi-extinction risk and aid in the establishment of a target population size for monarch conservation planning. We find that, given a range of plausible quasi-extinction thresholds, the population has a substantial probability of quasi-extinction, from 11–57% over 20 years, although uncertainty in these estimates is large. Exceptionally high population stochasticity, declining numbers, and a small current population size act in concert to drive this risk. An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction across all thresholds considered. Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability.