No Declines in Monarch Migration along East Coast for Past Two Decades

Monarchs in migration at Cape May NJ [courtesy Monarch Monitoring Project of New Jersey Audubon/Cape May Bird Observatory]

Monarchs in migration at Cape May NJ [courtesy Monarch Monitoring Project of New Jersey Audubon/Cape May Bird Observatory]

In a paper sure to cause a firestorm of controversy about the much-hyped population freefall of the Monarch butterfly, a paper now in press at the Annals of the Entomological Society of America will report when it comes out shortly that two decades of monitoring the fall migration of Monarchs through Cape May could find NO statistically significant changes in the numbers of migrants observed there.

Dick Walton, who heads up the Monarch Monitoring Project of the New Jersey Audubon’s Cape May Observatory, reports in the current The Peregrine Observer (NJA’s magazine) that a paper his team has in press at Annals will note that, if the Monarch population is serious jeopardy, you surely couldn’t tell it from the annual migration census data from Cape May.  While there have been some down years, there is not now and never has been a detectable downward trend in Monarch numbers migrating through the Cape, his team will conclude.

So, what to make of this in the face of the petition to declare the Monarch a protected species under the Endangered Species Act?

There are a couple of ways to interpret these data, which it should be noted are among the only long-term, rigorously collected figures on Monarch migration in North America.

One, it is theoretically possible that the Monarchs migrating through NJ in the fall don’t winter in Mexico and thus are not affected by the grim warnings about the diminishing size of the winter roosts.  That would presume that this Eastern population is replenished (presumably) by stock from the southeastern US.  EXCEPT that many Monarchs banded at Cape May have later been recovered in Mexico.  By contrast, it would appear that if there are terrible things going on with the Mexican roosts, they somehow have spared the New Jersey crowd.

Two, it is theoretically possible that something dreadful happens to the Monarchs between Cape May and Mexico in the fall.  EXCEPT that many Monarchs banded at Cape May have later been recovered in Mexico, and plenty of them replenish the spring migration here, enough to provide a continuing robust autumnal migration along the coast.

Three, it is possible that the current kerfuffle over the declines in Monarchs simple does not apply here in the East.  [A fourth possibility is that the data on on the Midwest Monarch declines are not supportable with available evidence, but that goes beyond the Walton paper]

I know this last won’t sit well with Monarch aficionados but it is my considered position.

While agricultural practices and GMO crop adoption in the Midwest may — may — have reduced the milkweed host plant populations to historically low levels (although this has not been demonstrated empirically to my knowledge. especially the comparison with pre-Columbian milkweed populations in the prairie states), and this de minimis availability of milkweed is causing population crashes among Monarchs in the Midwest, this is manifestly not the case here in East.  Milkweed stands go begging.  Acres and acres of Eastern milkweed support only a modest population of Monarchs, which lay eggs here, eclose, and hopscotch on north in the summer.  Monarchs don’t hang around here much in the mid-Atlantic, for example, during the summer — their genetic imperative is to make it as far north as they possibly can, perhaps to escape predation or disease pressures.  Or for some quality of northern milkweed we don’t yet understand.  Milkweed abundance just does not seem to be a factor in the East.  Much to apparent dismay of local butterfly gardeners, who wring their hands that their carefully tended Monarch Waystations have few or no takers despite the intense marketing of these gimmicky projects here in the Eastern US.

A more likely limiting factor for Eastern Monarchs is the quality of fall nectar sources, the most important of which is seaside goldenrod, and that’s a plant under severe downward pressure from coastal erosion and dune development.  But it appears that even that hasn’t reached a critical threshold yet, or there are sufficient alternative nectar sources to sustain the journey to Mexico.

What Walton and his colleagues have noticed, however, is that the dates of peak migration are occurring later in the season, possibly as a consequence of climate change.  Over time, this could have — although it hasn’t yet had, judging from the lack of declines in the fall migration — important ramifications if the migration falls out of “sync” with available nectar for the southward migration.

It bears repeating that these data ONLY reflect the Eastern migratory phenomenon, but it does make it pretty clear that any discussion about Eastern Monarchs or their migration pathway being in any peril CURRENTLY is seriously at odds with the science.  I’ll post the link to the paper here when it comes out.

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