New Monarch Papers Cast Doubt on Summer Decline, Especially in the East

Monarch in migration in 2013 at Point Lookout, St. Mary's Co MD [photo by REB]

Monarch in migration in 2013 at Point Lookout, St. Mary’s Co MD [photo by REB]

I noted in an earlier post that a spate of new papers representing long-term citizen science monitoring of Eastern Monarch fall migration populations published last month failed to observe the precipitous decline in Monarchs reported from their Oyamel fir wintering grounds in Mexico or trumpeted in the media for the American plains.  Here are two of the studies, one based on the well-known Journey North data (18 years) and Michigan’s Peninsula Point Monarch Monitoring Project (15 years).

Local researcher Leslie Ries at Georgetown University is among the coauthors of a paper that similarly found high variation year to year in Eastern summer Monarch populations reported from NABA and other counts but no statistically significant trend, downward, precipitous, catastrophic or other.  The trend in Mexico is grim, the authors say, but appears disconnected from summer populations, suggesting that GMO crops and other factors on the summering grounds have had minimal impact on Monarch populations.  Whatever is going on is likely either going on in Mexico or on the southward migration (likely linked to their passage through Texas, some researchers theorize).

By contrast, the fall count at Long Point on Lake Erie shows high variability that, corrected for some factors, shows a gradual 3% year over year decline that the authors postulate is related to lower egg densities in milkweed habitat (i.e., Monarchs appear not to be laying more eggs in remaining habitat to compensate for habitat loss, a finding of the Oberhuaser lab at the University of Minnesota).  Another Oberhauser paper in the set suggests a predetermined egg-laying density that Monarchs observe regardless of milkweed abundance, that might be related either/both to predators/parasites and disease.

Taken together, these papers paint a rather confusing picture of Monarch abundance.  The only compelling evidence of a drastic decline in Monarch populations seems to be on the overwintering grounds, and it’s hard to reconcile this with historically quite stable populations on the summer grounds (anecdotal observations notwithstanding).  But one thing seems pretty certain — planting Monarch waystations across the Eastern US is unlikely to make much of a dent in populations here, which seem to have enough milkweed and then some for larval food supplies, especially as there is likely much more milkweed available in the East now from agriculture and road construction that ever existed historically prior to European colonization (when most of the East was forested).


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