How borders are drawn and enforced has far-reaching consequences, whether we live on either side of them or halfway across the world.

In the winter months of 2015-2016, monarch butterflies had their best migration in years, arriving in record numbers to the Central Mexico forests where they hibernate. Unfortunately, those forests had a really bad year.

Severe storms toppled many of the oyamel fir trees where millions of the iconic orange-winged insects rest after their long trip from Canada. More than 70 hectares of forest were damaged, the biggest loss since the 2009-2010 winter, according to data (link in Spanish) released Tuesday (Aug. 23) by the World Wildlife Fund (WWF). In more bad news, experts expect the butterflies’ overwintering grounds to get hit by this sort of extreme weather more frequently in the future due to climate change.

*This chart shows winter seasons, starting in 2009-2010 and ending in 2015-2016.

The lost habitat is a depressing setback for monarch lovers. There are many along the butterfly’s international route who have been fighting for the monarch’s survival, including the presidents of the US and Mexico. The two countries, along with Canada, have been trying hard to bring back migrating populations to healthier levels after an 80% decline over the last decade.

President Barack Obama last year created a “flyway” along an interstate highway, from Duluth, Minnesota down to the the Texas-Mexico border, to help facilitate monarch migration. The plan involves lining the highway with milkweed, which has been depleted by herbicides used in industrial farming. That flower appears to be the only food picky monarch caterpillars will eat.

In Mexico, the government is working with environmental groups to replace damaged trees, and to get local communities to protect them instead of harvesting them for timber. It’s working, as the chart below shows. (Illegal logging, once the overwhelming reason for forest degradation, accounted for 15% of the lost hectares in the 2015-2016 winter season.)

*This chart shows winter seasons, starting in 2005-2016 and ending in 2015-2016.

Last winter, total area occupied by the butterflies more than tripled since 2015, growing to a level not seen in a few years.

But stopping climate change is a lot harder than restoring milkweed or oyamel firs. Last winter storms killed 7% of the the 84 million butterflies that made it to Mexico, according to the country’s attorney general for environmental protection. Heavy rain and wind knock butterflies off trees. Low temperatures freeze them to death.

It’s hard to imagine something more tragic than a carpet of dead butterflies, but a forest littered with dead trees might be even worse. The forest is an essential stop for the insects, protecting them from heat and cold and allowing them to recover for the journey back home. Without it, the monarchs, the only butterflies in the world to make such a long two-way trip, might stop migrating altogether. There’s a 16% to 62% chance of that happening over 20 years, a study published in Nature earlier this year predicts.

That risk was calculated using past climate data, says Chip Taylor, who contributed to the paper. Climate models suggest the weather will be more extreme in the future. Just in the past 14 years the monarch’s reserve in Mexico has been hit by three major storms the likes of which had never been recorded before, he adds. This winter’s 60 storm-ravaged hectares are but a sliver of the reserve’s core of around 13,500 hectares, but at that pace, the forest could shrink considerably in a few years.

It’s all the more reason to plant more milkweed. “The way you increase your probability of survival is by having a robust population,” says Taylor, who directs Monarch Watch, a conservation non-profit. If you want to help, he’ll give you the plants for free (save the shipping cost).