Being a rainforest, the Amazon isn’t supposed to burn out of control, unlike California’s drier landscape, which is built to burn and burn explosively. Yet here we are, watching swaths of the Amazon go up in flames. And we can easily nail down the cause: humans. Deforestation is what’s driving these blazes, and there is some horrifying science behind that.

Since the 1970s, 20 percent of the Amazon has been deforested, totaling about twice the area of California. But deforestation isn’t an organized shrinking of the rainforest, paring it down from the edges in. Humans carve out farmlands, sometimes leaving a neat edge where the forest meets the fields, or even creating islands of forest surrounded by crops or grazing fields for cattle. Indeed, agriculture is far and away the primary driver of deforestation in Brazil.

You might think that well, things could be worse, at least these can operate like actual islands—self-contained spots of green in a sea of agriculture. And you wouldn't be alone.

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“People used to think an island was just a miniature version of the Amazon,” says University of Florida ecologist Emilio Bruna, who studies the area. “So if you had an area of the Amazon that had a jaguar, and you had an area that's 10 times bigger, you'd have X more jaguars because of their territory size.”

But it turns out that when you isolate a fragment of rainforest, things get a lot more complicated. Like on a real island, animals on an Amazon island can’t flee to other islands—they’re trapped. Even birds, which are more than capable of leaving, have evolved to live in the dense rainforest, and may have become behaviorally averse to crossing open areas where predators might lurk.

Meanwhile, the edges are closing in on the species of a rainforest island. Deep in the Amazon, it’s dark and cooler and humid, with the multitudinous trees dominating the climate. At the edge of a rainforest island, though, humidity plummets and temperatures skyrocket. Decomposition rates fall as fungi retreat, causing dead foliage to pile up as tinder, and new kinds of trees proliferate at the edges. “All the big ones start to get stressed out hydrologically and die, and then they keel over and open up a giant hole in the canopy,” says Bruna. “Pioneer” species get in there, which have lower-density wood and therefore sequester less carbon. “You change the composition of the Amazon and get lots of local extinctions from that spot.”

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You don’t even need to create a rainforest island to get these so-called edge effects. Gouge a highway through the Amazon, and that opening begins to change the forest around it. More roads then split off from that one highway, tearing through the Amazon like branches of a lightning bolt. All it takes is one cigarette thrown out a truck window to ignite the dry vegetation at the edges of the forest.