“When you look at the 25-year trend, it seems quite dire,” [Anurag] Agrawal, a Cornell University professor of ecology and evolutionary biology and the author of Monarchs and Milkweed, says. “There has been a steep, persistent decline in the number of monarchs that overwinter in Mexico every year. But no one is arguing that the monarch butterflies are threatened or endangered as a species. What we are arguing is that the eastern North American migration, in which hundreds of millions of monarchs travel several thousand kilometers every year, is in serious trouble and may be lost.”
While a host of human factors are contributing to the stark decline in the number of monarchs overwintering in Mexico each year – some 20 million are thought to be killed by vehicles alone – the conventional wisdom for years has been that widespread herbicide use throughout great swaths of the United States has decimated milkweed, which is the only source of food for monarch caterpillars.
But simply pointing to herbicide use as the butterfly-killing bogeyman is oversimplifying a complex issue, Agrawal warns.
Agrawal thinks that the narrative is once again starting to change as scientists and non-scientists alike come to realize the true complexity of the problem. When a species has a life cycle as complicated as that of the monarch butterfly, which has four generations every year and faces a long and treacherous migration, it’s very difficult to put a relative importance to the various dangers it faces.
Read full, original article: No shortage of dangers’ and no easy answers for the monarch butterfly