Monarch butterflies appear headed for a perhaps unprecedented population crash, according to scientists and monarch watchers who have been keeping tabs on the species in their main summer home in Eastern and Central North America.
There had been hope that on their journey north from their overwintering zone in Mexico, the insects' numbers would build through the generations, but there's no indication that happened.
Only a small number of monarchs did make it to Canada this summer to propagate the generation that has now begun its southern migration to Mexico, and early indications are that the past year's record lows will be followed by even lower numbers this fall.
Elizabeth Howard, the director and founder of Journey North, a citizen-scientist effort that tracks the migrations of monarchs and other species, says one indicator for the robustness of the monarchs is the number of roosts they form in late August and September, something Journey North monitors throughout the migration periods.
"During migration, monarchs form overnight roosts in places like Point Pelee or Long Point [in southern Ontario], where the monarchs are congregating before crossing the Great Lakes, places where people generally see huge overnight clusters of monarchs gathering."
Howard told CBC News that at this time in 2011, Journey North had already received 55 reports of roosts, followed by just 25 in 2012. This year, only 17 reports of roosts came in.
"This is really a proxy for peak migration because this is where people see really large numbers of monarchs and we're just not getting the reports, it's looking pretty bad," she says.
Amazing migration
The monarch butterflies that are now flying south are the fourth generation of those that left the few hectares in central Mexico where millions of monarchs spend the winter.
Monarch butterflies overwintering in Mexico descend on a small creek near El Rosario, Dec. 12, 2012. The monarchs began their journey north in March. (WWF)
Beginning in March, after spending the winter literally hanging out in the Oyamel forest's coniferous fir trees, those long-distance fliers head north in search of milkweed plants on which to lay their eggs.
Last winter's annual survey of monarch numbers, which actually measures the area occupied by monarch colonies, found the numbers were at their lowest since measurements began 20 years ago.
While monarch numbers have fluctuated dramatically over the years, for the past 16 years the trend has been downward. Last year's unfavourable climatic conditions — a hot spring followed by the drought in the U.S. Midwest — had monarch watchers hoping for a recovery in 2013.
"This year it was again too warm in March in Texas and too cold in April to June. In the northeast it was not only cold it was raining," Chip Taylor, the head of the citizen-scientist group Monarch Watch told CBC News. Taylor, who founded Monarch Watch in 1992, is also an insect ecologist at the University of Kansas.
"What we're seeing is the result of two years in which the reproduction of monarchs has been limited because of the physical conditions, weather conditions," he says.
"We've also seen a decline in monarchs because of loss of habitat. So those two things together mean monarch numbers are much lower than they've ever been in the past."
Sharon Burkhard, a Monarch Watch citizen-scientist in Tottenham, 70 kilometres north of Toronto, says she has seen just one monarch this year and that wasn't until the second week of September, an absence she describes as "very scary."
"Based on what I saw this year, I'm very concerned they're not going to bounce back that well, and my fear is I'm going to see them extinct within my lifetime," Burkhard said.
New understanding of monarch migration
A study published last month, by University of Guelph biologist Ryan Norris and other scientists, describes "for the first time" the migratory patterns of monarchs during the entire breeding season in Eastern North America.
They found that the overwintered butterflies came from all over the breeding zone. The next few short-lived generations showed a northern progression until August, when the journey south begins.
They found that most of the monarchs that make it to Canada were born in the Corn Belt of the U.S. Midwest, followed by another Canadian-born generation.
Their major finding, Norris told CBC News, is that "the Corn Belt really acts as a central hub for the expansion later on in the breeding season."
In this "key region" for monarchs, from June onwards, the population explodes around the Corn Belt, radiating outwards. After June, the monarchs not only move north but east and northwest as well.
Norris says it's not surprising that monarch numbers in Ontario are near zero this year. "You're going to see the largest fluctuations in the northern part of the breeding range because when the population's low, you're going to get really low numbers."
Egg-laying monarchs gain a reproductive advantage from fewer competitors, so Norris's theory is that when there are really low numbers, the monarchs don't need to come as far north to be free from competition. That's also why, "in years of low numbers we don't see them up here until really late."
Norris says that although we don't have the full story yet, understanding the migration patterns is one of the steps towards figuring out how to help conserve the species.
At tipping point?
So what does all this portend for monarchs? Could they indeed become an endangered species?
Both Howard and Taylor point out that, as insects, monarchs have the capacity to bounce back. "The really good thing about monarch butterflies is they have a very robust reproductive capacity and that means if conditions are favourable they can recover very fast," Taylor notes.
He predicts they will recover. "The conditions can't be like this forever and they will turn around, we will see favourable conditions for reproduction sometime in the future. The question is when."
Elizabeth Howard of Journey North says their migration records at this point make her quite concerned that monarch butterflies will again be at record low numbers this winter in Mexico. Howard poses in the monarchs' overwintering area. (Mary Housier/Courtesy Elizabeth Howard)
Howard is less certain. "I wouldn't want to say that all they need is the right climate conditions because they also have disease and parasite effects as well."
For next year at least, Howard says, "our migration records make me at this point quite concerned that we won't see more than the record low."
She doesn't foresee extinction, but does worry that the monarch numbers may be at a tipping point.
For Norris, "It's looking like a long-term decline."
He expects "there will likely be a bit of bounce-back but I think we're looking at numbers staying low," not going back up to where they were, "not unless there's some really major conservation action done."
Loss of habitat
The three scientists all stress the loss of habitat for monarchs needs to be reversed.
Taylor says that "in the Midwest, we're seeing a tremendous loss of habitat due to the type of agriculture that been adopted here, Roundup-ready corn and soybeans, which has taken the milkweeds out of those row crops, and we're seeing overzealous management of roadside marshes, excessive use of herbicides here and there."
Monarch Watch and Journey North are trying to promote monarch habitat restoration by planting milkweeds, which is the main food source for monarch larvae.
But Taylor says "the difficulty is that the loss of habitat is so great that the effort that we're making at this point needs to be ramped up probably a thousand-fold to get the kind of impact that we want."
Although it fluctuates from year to year, the monarch butterfly population overwintering in Mexico seems to be on a downward trend. (WWWF-Telcel-CONANP)
Anda sedang membaca artikel tentang
Monarch butterfly numbers drop to new lows
Dengan url
http://teknounikmenarik.blogspot.com/2013/09/monarch-butterfly-numbers-drop-to-new.html
Anda boleh menyebar luaskannya atau mengcopy paste-nya
Monarch butterfly numbers drop to new lows
namun jangan lupa untuk meletakkan link
Monarch butterfly numbers drop to new lows
sebagai sumbernya
0 komentar:
Posting Komentar