Early snow melts caused by climate change could deliver a mighty one-two punch to the delicate Mormon fritillary butterfly, shrinking the insect’s favored nectar supply, killing caterpillars and causing a steep population decline, a new study in the journal Ecology Letters finds.

In all, the researchers report, the combined effects of two consecutive years of early snow melt explained as much as 84 percent of the variation seen in the fritillary’s population growth rate between 1980 and 2005.

“It is very unusual for research to uncover such a simple mechanism that can explain almost all of the variation in growth rate of an insect population,” one of the study’s co-authors, David W. Inouye, a biology professor at the University of Maryland, said in a statement.

The deadly power of an early snow melt has to do with the particulars of the butterfly’s life cycle. During the first summer of a female fritillary butterfly’s life, she lays eggs and dies. Caterpillars from these eggs spend the winter without food and become adult butterflies the following summer.

The adult butterflies eat nectar, mostly from the aspen fleabane, which blooms from mid-late July to mid-late August — the peak of the fritillary flight season. The number of eggs laid has been found to depend upon how much nectar the fritillary consumes, according to laboratory experiments cited in the journal paper.

By analyzing decades of data on fritillary populations, aspen fleabane blooms and snow melt dates in the Colorado Rocky Mountains, scientists discovered that one early snow melt can throw these phases out of whack. Working out of the Rocky Mountain Biological Laboratory, Dr. Inouye and the the study’s other author, Carol L. Boggs, found that two consecutive years of early snow melt and frosts at the beginning of the growing season make matters worse for the fritillary.

That is because early season frosts can kill both young caterpillars and aspen fleabane buds. As snow begins to melt, signaling winter’s end, the caterpillars and fleabanes enter a development stage in which they are poorly equipped to survive frost. If flower buds are killed off early in the season, that means less nectar will be on tap for each butterfly come summer, which in turn leads to fewer eggs. So the adults are eating less and laying fewer eggs, while the next generation is being struck down by frost.

Dr. Boggs and Dr. Inouye considered more than 30 years of records on the first day of bare ground, and the number of aspen fleabane flowers blooming in seven wildflower plots in the Elk Mountains of Colorado. They analyzed the annual population density of male fritillaries during nine pairs of sequential years between 1980 and 2005.

Estimated population densities during that time spanned two orders of magnitude, ranging from just 28 males per hectare (2.47 acres) in 1985, apparently due to a severe snowstorm in late June, to nearly 4,500 males per hectare in 1980. More often, the researchers found fewer than 2,000 fritillary males per hectare.

The road is not getting any easier for the Mormon fritillary, said Dr. Boggs, a professor of biology at Stanford.

“We already can predict that this coming summer will be a difficult one for the butterflies,” she said in a statement, “because the very low snowpack in the mountains this winter makes it likely that there will be significant frost damage.”

(Source: www.nytimes.com )