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| Monarch caterpillar and milkweed flower |
published in the Pacifica
Tribune February 12, 2020
What’s Natural
The Monarch Abundance
Roller-Coaster- part 2
Indeed, some Monarch populations have declined in recent
decades. However, the species as a whole is not endangered. Monarchs experience booms
and busts as do many insects. So, we still need to determine if recent
population declines are part of natural cycles or due to human disturbance. Counter-intuitively, humans purposefully and
unwittingly have both increased and decreased monarch populations.
Eighteen thousand years ago, most of the breeding habitat
for North America’s eastern monarch populations was covered in ice sheets and
permafrost. Unfortunately the monarchs main food plants, Common
Milkweed and Showy
Milkweed, are frost intolerant. Today
those milkweeds die back each autumn, forcing monarchs to migrate south.
Habitat south of the ice sheet was covered with dense forests, which also limited
the milkweed species that require warm open habitat, and disturbed ground. As
the earth warmed and ice retreated, milkweed migrated northward colonizing
glacially disturbed landscapes. Likewise, monarch populations expanded.
However, some scientists
suggest the monarch’s awe-inspiring abundance really boomed during the past
200 years after European colonists began extensively logging America’s dense
southern and eastern forests. Logging created more open fields and pastures, more
farms and roadways; habitat milkweeds still favor today. More milkweeds, more
monarchs.
In addition, gardeners adored showy milkweed flowers, so began
planting milkweed across the globe. Again, the monarchs followed. Suddenly
monarchs expanded out of North America and across the globe. Around 1850
monarchs reached Hawaii likely as stowaways on trading ships, then spread to
several Pacific Islands. With optimally warm climates monarch populations
boomed, feeding on introduced milkweeds and closely related native species. But
monarchs often decimated their food plants causing island monarch populations to
bust.
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| Monarch butterfly's global expansion |
By the turn of the century monarchs were found in Australia,
New Zealand, Indonesia, the Philippines and southeast Asia. They also spread
across the Atlantic to the Azores, Canary Islands, Spain, Portugal and Morocco.
In many regions, monarch
populations are now stable. Where warm temperatures permitted milkweeds to
grow all year, monarchs no longer migrated. Having
successfully colonized much of the suitable regions of the world, insect
experts don’t fear monarch extinction. However, concern remains for the USA’s
eastern population that winters
in Mexico.
The wintering population in Mexico was first surveyed in
1993. By 1997, the population boomed, tripling its abundance. But then winter populations
worrisomely declined. Paradoxically, surveys of monarchs in their midwestern
breeding habitats found no evidence of declining populations. But such
surveys were done in “natural” habitat, not agricultural fields. It now appears
the rise and fall of milkweed in agricultural fields drove the booms and busts
of 20th century monarchs.
Having successfully colonized roadway ditches and any open
disturbed landscapes, milkweed species began invading
the open fertilized ground between rows of crops. Monarch populations
boomed, while the farmers’ crops suffered. Studies estimated milkweed
competition reduced harvests of wheat
and sorghum by 20% and most states declared milkweed a noxious plant. But when farmers
tried to eradicate milkweed by mowing, they only stimulated its underground
roots promoting a greater infestation. Likewise, for herbicides that only
eliminated stems and leaves. Tilling the fields only fragmented milkweed roots,
again causing milkweed to multiply. The growing
battle to eliminate milkweed started the monarch’s mid 20th
century decline. With the 1970s discovery that the herbicide glyphosate killed
the whole plant, the loss of milkweeds in the monarch’s human-made breeding
grounds accelerated.
Still, there was room for optimism. Monarchs continue
to breed throughout their traditional habitats. More efficient agriculture
allowed more land to revert to “natural” states. Furthermore, the federal Conservation Reserve
Program (CRP) was successfully compensating farmers to take environmentally
sensitive land out of crop production. The good news was the majority of
Midwest monarch breeding habitat was found on lands enrolled in the CRP. But fossil fuel fears reversed
all that promise. The 2005 Energy Policy Act and the 2007 Energy
Independence and Security Act instituted subsidies
and quotas that rewarded biofuel production.
As a
result, U.S. corn harvests for ethanol rose from 6% in 2000 to 43% percent in
2012. The monarch’s remaining “natural” breeding habitat was increasingly
eroded as corn acreage increased by 17 million acres since 2006. Lured by
lucrative biofuel subsidies, farmers increasingly abandoned the CRP program. Soon
thirty percent of the CRP’s sensitive lands converted back to growing corn and
soybeans for biofuels.
Perhaps
the loss of milkweed in agricultural lands, will only reduce monarch
populations to their “natural’ levels of the 19th century, before modern
agriculture opened the land for more milkweeds. Monarchs may become less
common, but not endangered. For monarch lovers, our best safeguard is to halt
the spread of biofuels and plant more milkweed in our gardens.
