Thursday, June 26, 2014

Monarchs and the Changing of the Season

I’ve been thinking about those events that mark the changing of the season from spring to summer here in the northern hemisphere.  I was prompted initially (in a decidedly negative fashion on my part as will be evident) by physicist Adam Frank’s oral essay titled Seeking the Solstice:  Kick Off Your Summer of Cosmic Sunsets that aired on National Public Radio’s All Things Considered on June 19, 2014.  In it, Frank, who teaches at the University of Rochester and blogs for NPR, drew attention to the summer solstice which was to be reached in two days on the 21st.  He suggested that, to explore the “cosmic link between the solstice and the Earth’s seasons,” listeners should pick a place to watch the sun set over the course of the summer, somewhere with a fixed object on the horizon, and then observe how the spot at which the sun sets moves southward, relative to that fixed horizon point.  He noted that, had his listeners been tracking the sunset from the beginning of the year, they would have seen the sun appear to move north until, as we neared, reached, and passed the summer solstice, that northward movement would have slowed, ceased, and eventually reversed.  He suggested watching sunsets with your children and . . .
after a few weeks and you've watched the sunset's tack like sailboat through interplanetary winds, then you can be their science hero. Then you can recount to them the grand links between sunset, sunrise and seasons.
Wait.  Has he ever done anything with children?  Does he not know that one of the first, loudest, and ultimately most persistent question out of their mouths will be, “Why?  Why does this happen?”

Sadly, Frank’s little ad for families doing science together never even hints at an explanation of why the point of the sunset moves over time, never once mentions the tilt of the Earth’s axis which does explain the phenomenon, why we see what we see.

I was then struck by how, for me, the signal of the season change in this part of the year could never be the result of careful charting of the perceived movement of the sun (or sunset) or measurement of steadily lengthening or decreasing daylight.  The signal of when a corner has been turned, when a seasonal change has occurred, bringing a new perspective to the present year, is more visceral, less intellectual, less scientific (?) than that.  This year, I knew we had entered my summer when I became aware that the buds of a cluster of beebalm (Monarda didyma) in a corner of the garden had exploded with blossoms, blossoms like fireworks sending scarlet tracks arcing into the air:

and that the green pompoms of buds on the gangly common milkweed (Asclepias syriaca) had opened into whitish pink flowers exuding a wonderful, arresting fragrance:

For me, these are moments that mark the change from spring to summer, not the end of sun’s march to the north.  Admittedly, my signals of the onset of summer are not the same every year.  Sometimes it’s when I'm told that a summer cottage, with its rusted hot water heater and aged roof, has become my destination.

This year, in addition to the flowers that sparked my sense of a seasonal change, there was another event – related to the opening of these flowers – and one that brought a bit of hope that a migratory miracle, a phenomenon that, in many ways, seems fated to slip into extinction, may not yet be doomed.  While out with the dog early on the 22nd of June, the first full day of summer, I came upon a stand of milkweed and, there, amid a ball of flowers, a monarch butterfly sipped nectar.  It was a fresh, female of the species (Danaus plexippus).

A member of the second generation born of the first generation which was spawned by that long-lived generation that overwintered in Mexico?  I would like to think so, but perhaps not, given that Maryland lies well outside the primary spring and summer breeding areas in what Chip Taylor, director of Monarch Watch at the University of Kansas, calls the “milkweed/monarch corridor.”  The generations of monarch that are conceived and born in this corridor are the primary ones participating in the monarch’s annual migratory phenomenon – the journey from their overwintering sites in Mexico to as far north as Canada and then back again.

This past winter saw the wooded area occupied by the overwintering monarchs in Mexico sink to the smallest recorded since the winter of 1994-95.  Rather than attempt the impossible (counting individual monarchs), a proxy measure is used – the area (in hectares) occupied by the overwintering monarchs.  It was 0.67 hectares (1.7 acres) in 2013-14, down from 7.81 hectares (19.3 acres) in 1994-95.  The peak recorded in this time period was in 1996-97 – 20.97 hectares (51.8 acres).  (Chip Taylor, Monarch Butterfly Recovery Plan.)  Chip Taylor attributes the downward trajectory in recent years to three primary factors:
1) the widespread adoption of herbicide tolerant corn and soybean varieties by North American farmers which has had the effect of eliminating milkweeds (the host plants for monarchs) within the crop fields; 2) the ethanol mandate passed by Congress in 2007 that increased the price of corn and soybeans which in turn led to the conversion of grasslands to crops thus [to the] elimination of the milkweeds that occurred in these areas, and 3) three consecutive years during which the reproductive success of the summer breeding population was limited by unfavorable weather conditions.  (Monarch Butterfly Recovery Plan.)
Taylor's Recovery Plan is a hopeful document, offering some concrete steps that can be taken to help mitigate those factors we can influence.  The President's recent Executive Memorandum, Presidential Memorandum -- Creating a Federal Strategy to Promote the Health of Honey Bees and Other Pollinators (The White House, June 20, 2014), suggests there will be some much-needed attention in high places to the fate of the monarch (sadly, not included in the memorandum's title but lumped in with "Other Pollinators").

On the Annenberg Foundation’s student-centered website Journey North, the actual movement of the monarchs north from Mexico to their spring and summer breeding grounds can be tracked.  The dots on the map are color-coded to reflect the sightings of first monarchs in discrete time periods from the late winter through to summer.  Pressing the “animated map” button at the bottom of the map does just that, bringing the data to life as we watch the timing and extent of the spread northward.

These data gathered in this website by citizen scientists do not tell us anything about numbers of butterflies.  To know whether numbers have rebounded at all, or continued their spiral downward, we’ll have to wait until the hectares in Mexico covered by the overwintering monarchs can be counted during the winter.  Those specific monarchs are the super-generation, the one that makes the long haul in the fall to Mexico, spends the winter there, and then moves north to give birth to a new generation in the spring and to one of the most beautiful and fragile signals of season change of all.

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