The ant species Philidris nagasau and epiphytic plants in the Squamellaria genus have a complicated relationship. Together they form an obligate farming mutualism in which ant and plant require each other to survive. Ants fertilize the plants, disperse their seeds, and provide defense against herbivores. In turn, the plants produce food rich in sugars and amino acids.
Between 2014 and 2019, Guillaume Chomicki and colleagues studied what they call “nonhuman farmers” in rainforests on two Fijian islands to figure out the evolutionary tradeoffs that exist in this arrangement. They reported in PNAS on January 21 that “food rewards were 7.5-fold higher in plants cultivated in full sun than in plants cultivated in full shade.” Plants in full sun in turn received greater protection against herbivores. The researchers write in their report that they expected to find that nitrogen fertilization by the ants, which aids in photosynthesis, would also peak in sun-cultivated plants. But they observed the opposite: these plants contained the lowest amounts of nitrogen in their tissues.
The scientists argue that epiphytic plants sacrifice nitrogen input because farming by ants still results in nitrogen levels roughly three times higher than in non-farmed plants.
Over millions of years, ants have evolved to optimize crop yield by choosing specific light conditions. “While ants cannot simultaneously maximize all services to their epiphyte crops,” the paper concludes, “our work supports the idea that they buffer environmental variation by selecting the environment where their crop is most productive.”
G. Chomiki et al., “Tradeoffs in the evolution of plant farming by ants,” PNAS, doi:10.1073/pnas.1919611117, 2020.
Amy Schleunes is an intern at The Scientist. Email her at email@example.com.