July 17, 2012
Rodents Do Their Part To Help Save Tropical Trees
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redOrbit Staff & Wire Reports - Your Universe Online
There's no honor among thieves when it comes to rodent robbers – which turns out to be a good thing for tropical trees that depend on animals to spread their seeds, according to recent research.
Big seeds produced by many tropical trees were probably once ingested and then defecated whole by huge mammals called gomphotheres that dispersed the seeds over large distances. But gomphotheres were likely hunted to extinction more than 10,000 years ago. So why aren't large-seeded plants also extinct?
A new Smithsonian report in the early online edition of Proceedings of the National Academy of Sciences during the week of July 16 suggests that rodents may have taken over the seed dispersal role of gomphotheres. Studies also suggest that thieving rodents helped the black palm tree survive by taking over the seed-spreading role of the mighty mastodon and other extinct elephant-like creatures that are thought to have eaten these large seeds.
"The question is how this tree managed to survive for 10,000 years if its seed dispersers are extinct," says Roland Kays, a zoologist with North Carolina State University and the North Carolina Museum of Natural Sciences. "There's always been this mystery of how does this tree survive, and now we have a possible answer for it."
By attaching tiny radio transmitters to more than 400 seeds, Patrick Jansen, scientist at the Smithsonian Tropical Research Institute and Wageningen University, and his colleagues found that 85 percent of the seeds were buried in caches by agoutis, common, domestic cat-sized rodents in tropical lowlands.
Agoutis carry seeds around in their mouths and bury them for times when food sources are scarce. Radio tracking revealed a surprising finding: when the rodents dig up the seeds, they usually do not eat them, but instead move them to a new site and bury them, often many times. One seed in the study was moved 36 times, traveling a total distance of 749 meters and ending up 280 meters from its starting point. It was ultimately retrieved and eaten by an agouti 209 days after first dispersed.
"We knew that these rodents would bury the seeds but we had no idea that there would be this constant digging up of the seed, moving it and burying it, over and over again," said Kays. "As rodents steal the same seed many, many times, it adds up to a long-distance movement of the seed that one animal by itself could have never done."
Researchers used remote cameras to catch the animals digging up cached seeds. They discovered that frequent seed movement was primarily caused by animals stealing seeds from one another. Ultimately, 35 percent of the seeds ended up more than 100 meters from their origin. "Agoutis moved seeds at a scale that none of us had ever imagined," said Jansen.
"Previously, researchers had observed seeds being moved and buried up to five times, but in this system it seems that this re-caching behavior was on steroids," said Ben Hirsch, who ran the fieldwork as a post-doctoral fellow at the Smithsonian Tropical Research Institute in Panama. "By radio-tagging the seeds, we were able to track them as they were moved by agoutis, to find out if they were taken up into trees by squirrels, and to discover seeds inside spiny rat burrows. This resolution allowed us to gain a much better understanding of how each rodent species affects seed dispersal and survival."
"When you think about global climate change and habitats shifting, for a forest to move into new areas, trees need to have their seeds moved into new areas. This opens up a route to study how animals can help trees adjust to climate change through seed dispersal."