Ant bridges connect shy tropical tree crowns

Internet and telephone connections are essential for effective communication and business success. New results from the Smithsonian Tropical Research Institute (STRI) in Panama show that connections between trees may be important for maintaining the rich diversity of tropical forests.

Below, the rainforest canopy looks like a giant, illuminated puzzle. A thin, bright outline of light isolates each tree from the others. Biologists call this tendency for each tree to stand alone “crown shrinkage.”

Rainforests are home to more than 40 percent of all the world’s terrestrial species. Toucans fly and monkeys jump in the gap between the trees. But to raccoons, raccoons and tiny ants, every tree is an island.

One of ecology’s long-standing rules of thumb is that large islands—those surrounded by water—have more species on them than small islands. This also applies to individual forest patches surrounded by agricultural fields: large patches of forest generally have more species than small patches.

“Nature is so variable, it’s sometimes frustrating to try to explain even what seems like an obvious pattern,” said STRI research associate Steve Yanoviak, who is also the Tom Wallace Chair in Conservation at the University of Louisville. “In this case, we were very pleased to see that something as complex as the diversity in the rainforest canopy could be explained simply by seeing trees as islands.”

Yanoviak spends as much time as he can climbing into the treetops, where he studies tropical insect communities, from mosquito and dragonfly larvae that breed in the tiny pools of water that form in the center of pineapple-like plants that grow on branches, to ants that slither back on the trunk of the tree when they are brushed from one end.

Yanoviak’s doctoral student, Ben (Max) Adams, compared the number of ants in more than 200 tree crowns representing more than 30 tree species. Adams and his assistants used a mixture of honey and meat (ham, tuna or chicken) as bait to trap 128 species of ants.

Despite their tendency to be shy, some trees remain very connected through a complex network of woody vines known as lianas. Stefan Schnitzer, STRI research associate and professor at Marquette University, has written papers showing that vines and lianas take over many tropical forests. He devised a terrifying experiment to find out what the impact of this entanglement might be, cutting all the lianas from the trees in one section of the forest at the Barro Colorado Nature Reserve and leaving the lianas in another.

Adams collected 92 species on trees with lianas compared to 58 on trees without lianas. He discovered that on average, from two to 20 different species of ants live in a single tree crown. Larger trees tended to host more ant species than smaller trees, but only when the trees were not connected to lianas. Trees connected to other trees with vines and lianas had more than 10 ant species, while unconnected trees had an average of eight ant species.

Liana trees often have more of the ant species in which individual foragers go out to forage alone, rather than foraging in groups as other ant species do. And ant species that build large colonial nests were equally common on trees with and without lianas.

Lianas can be thought of as telephone lines or the lines on a map that show the geographic connections between cities through the World Wide Web. Understanding the importance of these vital connections is especially important in a world where continuous timber harvesting and road construction are destroying them.