Experts say deforestation threatens the spectacular annual migration of millions of monarch butterflies from Canada and the US to forests north-west of Mexico City. Words and pictures: Kate Joynes-Burgess.
Conservation has suffered because the Monarch Butterfly Bio-Reserve crosses two states. Now a bilateral commission has been set up with $1.4m (£1m) to create sustainable tourism and fight illegal logging.
Macheros sanctuary, in Mexico state, is one area that will benefit. Local officials say they have been overlooked because neighbouring Michoacan state branded itself "the monarch butterfly state".
Jose Luis Juarez has been a guide at Macheros for a decade and fears for his livelihood if the butterfly migration is damaged. He says everyone relies on tourism, adding: "There is no other employment."
The bio-reserve only provides an income between November and March, when monarchs can be observed. Out of season, Macheros's residents eke out a living in construction, farming and fishing.
To support the community year-round, workers are encouraged to provide more services for tourists. Some are serving up locally-caught trout to hungry hikers or making handicrafts from pine needles.
Some tourists take the lengthy trail on horseback, which Prof Lincoln Brower - who has studied the monarchs since 1954 - says could be harmful. The horses create dust which can suffocate the butterflies.
Woodland on the Mexico state side of the reserve is well preserved, but tree cover along the western trail, which passes through Michoacan, is visibly thinner, which Prof Brower blames on logging.
The bio-reserve's status as a Unesco World Heritage Site, granted in 2008, could be lost unless deforestation is checked, says Mexico's representative to Unesco, Homero Aridjis.
Our guide holds the body of a female monarch he says died from cold. Hail struck during my visit, leaving a carpet of orange corpses. In 2002, a winter storm wiped out 80% of the overwintering population.
As the day warms up, monarchs take to the skies. Depending on numbers, the sound of their wings varies from a gentle rain to a downpour. Visitors watch their step to avoid crushing the butterflies.
In season, branches can snap from the weight of butterflies. Around 21 March they set off to breed and die in the southern US and the next generation flies to the Great Lakes, laying eggs en route.