A Series by New Scientists
On a beautiful spring day I find myself yet again balanced on a rickety stepladder on the side of a dirt road in Costa Rica, waving a tree trimmer above my head and antagonizing a colony of tree-dwelling ants. As I explain to the locals who pass by, I am trying to collect the ants in plastic vials, not (as appearances would suggest) my hair. To me, my behavior is an entirely rational extension of my newfound interest in the biological impacts of climate change. However, I suspect that the average passerby on the road thinks I am at least a little bit crazy.
The path that leads me to the ants started a few months earlier, in a class I was taking on tropical ecology. I had learned about how the migration of birds to higher elevations could be linked to global warming. Species in the tropics typically are adapted to a very specific set of temperature and moisture conditions. In mountainous regions this means that if you travel along a slope you stay in the same ecosystem, but if you go uphill or downhill you quickly enter a new ecosystem with entirely different communities of plants and animals. Global warming is predicted to force each of these communities to move upslope, following their preferred climate conditions.
What’s the big deal? After all, scientists have only shown a few species of birds moving further upslope—not entire communities. It turns out that every species in the forest, from plants to insects to reptiles to birds, is likely moving upslope. This migration is hard to document, however, because nobody knows where most species live now, let alone where they lived 50 years ago. Records of the colorful birds of Monteverde have been kept for decades by visiting birders, but few people come to the region to observe the less charismatic critters. We simply don’t know if most species live at the same elevations today as they did a century ago, or if they have been forced upslope from lower elevations.
So I developed a research project to look for evidence that climate change was affecting more than just birds around our research station. I combed through the literature and found that in the 1980s the ecologist John Longino had worked locally on the symbiotic relationship between four species of Aztec ants and the two species of cecropia trees that they inhabit. The ants live in the hollow limbs and defend the trees by swarming and biting herbivores (and, I learned quickly, scientists with tree trimmers). Most importantly for me, Longino’s study documented the historical range of each of these species. Both the ants and the trees they live in are restricted to narrow elevation bands with distinct high-, middle-, and low-elevation species. After I found this gem of a study, I set off to see if the ranges of these species had shifted after 25 years.
This is how I come to be balanced on the ladder by the side of a road with biting ants in my hair. Each day I search for more trees with ants, log their elevation, then take the ants back to the lab to identify them under a microscope. A month in, I have surveyed over 100 trees and collected enough data to show that two of the four species of ants have moved 100 meters upslope since the 1980s. The other two types of ants and both kinds of tree have expanded their ranges upslope, but their lower limit remained.
My simple study isn’t perfect. I hoped to discover how much my ants had moved upslope prior to the 1980s, but there appears to be no way to know.
This shift to higher elevations doesn’t seem to be much of a problem for my ants. They still have plenty of habitat to fill and are quite abundant. However, if this shift to higher elevations is happening for all the species in the region, the consequences for the highest-elevation species could be dire. The cool cloud forest communities on the ridgetops have nowhere upslope to migrate and will be pushed off the mountaintops by species adapted to warmer and drier conditions. If ranges can shift upslope by as much as 100 meters in 25 years, then we should expect mountaintop extinctions to continue to accelerate.
Although the narrow climatic tolerances of tropical plants and animals make them particularly vulnerable to mountaintop extinction, the same challenge occurs globally. I now live in southern Arizona and when I hike the mountains, I think about the slow, uphill migration of the colorful wildflowers. Each new generation lives just a little higher than its parents. With the relentless pace of climate change, some day this familiar colorful carpet may be pushed off the mountaintops altogether, like my ants in Costa Rica. It won’t happen all at once, and perhaps only the keen will mourn the loss.
Header photo of a cecropia tree overlooking Costa Rica’s San Luis Valley by Garrison Loope.