In exchange for this Aphaenogaster ants spreading their seeds attach bloodworm a nice ‘handle’ that gives the ants a reward and a way to carry the seed to their nest.
Alex Wild
By Elizabeth Pennisi
Trilliums, carrots, violets – many spring wildflowers in eastern North America bloom through ants. The small six-legged gardeners have collaborated with these plants as well as about 11,000 others to spread their seed. The plants ‘pay’ for the service by attaching a calorie-laden appendage to each seed, just as fleshy fruits reward birds and mammals that sow seeds or throw them out. But there is more to the ant-seed relationship than that exchange, researchers reported last week at the annual meeting of the Ecological Society of America, which was held online.
Far more than just transporting the seeds, the ants are active gardeners, preferring some seeds over others and possibly keeping their loads safe from disease. “It’s becoming clear that this is not a simple two-way interaction,” said Douglas Levey, an ecologist at the National Science Foundation.
The importance of this partnership also comes into focus. In forests disturbed by human activity, where ants can be scarce, seeds may not find their way to fertile soil, and ecosystems may suffer. “If ants are lost, the real chances are we will lose plants, as are the other species that depend on ants and plants,” says Judith Bronstein, an evolutionary ecologist at the University of Arizona.
Many ants eat seeds, but in abandoned forests in Europe and North America, Australian arid forests, and South African shrubs called fynbos, a few dozen anti-species save the seeds in favor of something better. Certain plants attach a food source called an elaiosoom to their seed coats, which serves as lunch for the young of the ants and gives ants a grip on seeds that may be larger than their head. Until now, researchers have assumed that ants simply carry the seeds to their nests, feed the eliozome to their hatchery, and deposit the seed either outside or inside at the colony ‘garbage dump’, providing a fertile environment for sprouting. But Charles Kwit, an ecologist at the University of Tennessee, Knoxville, thought that ant seeds could help with more than just transportation.
The common seed-spreading ants in the genus Aphaenogaster, like others, secrete antimicrobial chemicals to clean themselves and other ants. Kwit wondered how these disinfectants could affect the microbial communities of the seeds – and their health. He and his student Chloe Lash have teamed up with Melissa Cregger of Oak Ridge National Laboratory to isolate and track DNA from microbes on the seed coats of three common ant-dependent plants: wild ginger, blood root, and twin leaves. To begin with, the seed of each species had a complex and unique microbiome – its community of bacteria and fungi. But after an ant treated a seed, the microbiome shrank and became more similar to that of other seeds of different species, Lash reported at the meeting – apparently because of the antimicrobial treatment. Wild ginger and twin leaf also have fewer plant pathogens. The microbiome changes, says Levey, “can affect postdispersal seed predation, dormancy, seed viability, timing of germination, and the health of the resulting seedlings.”
Kwit’s lab has also found that when it comes to seed, ants have preferences that can affect the success of the plants. In both the field and the lab, his student Chelsea Miller presented ants with seeds of different trillium species and found the ants were quick to pick up seeds of some species while others left rats, Miller told the meeting. “So having less preference really has consequences,” Bronstein says.
To find out how ants make their choice, Miller and Susan Whitehead at the Virginia Polytechnic Institute and State University (Virginia Tech) used mass spectroscopy and other techniques to analyze the chemical makeup of elaiosomes. They found that ant seeds pick based on the specific combination and concentrations of oleic acid and other compounds made by the plant, 20 of which are unique to trilliums. The taste of ants can affect plant species ‘, says Kirsten Prior, an ecologist at Binghamton University:’ Widespread trillium species [are] the preference by seed-spreading ants compared to rare trillium species. ”
Human activities can also affect ant-seed partnerships. Many researchers assumed that ants survive survival as deforestation and move quickly back into disturbed areas. But Katie Stuble, an ecologist at the Holden Arboretum in Kirtland, Ohio, found otherwise. “History of land use has affected communities,” she said at the meeting. Its arboretum covers 1416 hectares, much of it has been cleared of agricultural land at various times in the past century before the trees grew again.
Even in areas that were cleared decades ago, their team found higher concentrations of invading earthworms and lower concentrations of seed-spreading ants than in forests that were never cleared. Earthworms break down fallen leaves and organic waste, leaving possibly too little cover for ants. “This suggests that there are major effects of past land use that are likely to run deeper than we previously thought,” says Stuble. This influence could explain why secondary forests lack dense undergrowth, and why plants that rely on ants to spread their seed are poor there.
At the meeting, Prior and her student Carmela Buono reported that a 20-page survey in northeastern North America showed a similar trend. Compared to never cleared forests, secondary forests had fewer Aphaenogaster ants, which spread up to 70% seeds in a deciduous forest, Buono said. The secondary forests had less leaf litter and less decaying logs for ants to colonize. They also had more invasive snails, which compete with ants by eating the elaiosomes – and leaving seeds, instead of spreading them. The loss of seed-bearing ants “has major implications for forest communities and restoration,” says Prior. “In order to restore underlying plant communities, we might also need to think about restoring this important type of interaction.” For example, it can help to ensure that there are plenty of degrading logs and leaf litter for the ants to thrive in.
Bronstein notes that in the past, ecologists deciphered the role of ants as gardeners through painful observations. Now, she says, “there are exciting testable hypotheses, well-designed experiments, serious phytochemical analyzes, and supposed statistical approaches,” such as genome sequencing and fine-scale chemical analyzes.
Melissa Burt, an ecologist at Virginia Tech, hopes these studies bring ants new respect. “A lot of people I talk to about ants only know them as pests that take over their kitchens, but a lot of ants have important functions in ecosystems,” she says. “Seed propagation is just one of those.”
