Three million years ago the Panama Isthmus rose from the sea to join North and South America for the first time since the days of Pangea. The result was the Great American Biotic Interchange (GABI), where animals moved from one continent to another, affecting the species that inhabit the north, and utterly transforming the south's ecology. Today, deforestation could unleash something similar on a much smaller scale, but a depleted band of jaguars serve like the Night's Watch, a crumbling defense against invasion.
In the course of GABI, North America initially got a consignment of ground sloths and terror birds, among many other species that did not make it to our own time, and the ancestors of opossums and porcupines. Southward migrants included cougars, tapirs, and an abundance of rodents to name just a few of the arrivals that displaced much of the pre-existing South American fauna.
For most of the time since its formation, however, the isthmus has been covered with thick rainforest, providing a barrier to species that need more open habitat or fear its occupants. A study in the Journal of Mammalogy has shown that since humans deforested much of the region, previously blocked creatures have been edging closer to crossing.
Dr Roland Kays of the Smithsonian Tropical Research Institute has been examining the spread of coyotes and crab-eating foxes. Rather than true foxes, crab-eaters are members of the dog family whose ancestors came south during GABI or a preceding over-water migration isolating them from other canids.
"We knew the coyotes were moving south and the foxes north, but we didn't know how far they'd gotten, or what would happen when they met up," Kays said in a statement. Through camera traps and local reports, Kays and co-authors found the two members of the canid family now overlap in areas of Panama dominated by agriculture. However, the coyotes in particular are giving a wide berth to jaguar-inhabited forests, which has so far prevented them from reaching South America.
However, co-author Dr Ricardo Moreno added: "If the population of jaguars decreases and deforestation increases in Darien, surely the coyote will soon enter South America."
We don't know what will happen if either coyote or fox does make it properly into the other's domain. However, invasive generalist predators have often proven devastating to ecosystems once they gain a foothold, exemplified in the sad story of Australia's smaller mammals, overwhelmed by cats and true foxes.
There are always many reasons to protect remnant tropical rainforest. The study shows the Panamanian forests have one more; maintaining a green wall from which these watchers can guard an entire continent against northern invaders.