Ants play a vital role in the health of forested ecosystems as seed dispersers, decomposers of wood and leaves, aerators of soil via their nesting and tunneling, recyclers of nutrients, and more. As the composition of local forests changes from the impact of an increasingly warmer and wetter climate, Maggie Anderson wondered how those changes would affect ants and the roles they play in the forest.
A graduate student at the University of Minnesota who studies insect ecology, Anderson sampled ants within a meter of more than 400 trees – white pines, hemlocks, red maples, and red oaks – at Harvard Forest in Petersham, Massachusetts, in 2018. She collected a total of 473 ants of 11 species.
Among her findings was a greater diversity of ants and a greater number of functional roles played by ants around the deciduous trees compared to those found around the coniferous trees. In particular, deciduous trees were associated with more ant species that serve as soil movers and wood decomposers.
“The reason for these differences likely lies in the ability of deciduous oaks and maples to annually shed their large, nutrient-rich leaves to the forest floor,” Anderson wrote in the journal Northeastern Naturalist. “Accumulating leaf litter under trees provides a dry and sheltered nesting habitat for many ants, especially leaf-litter specialists.”
According to Anderson, the abundance of ants around deciduous trees increases the cycling of nutrients in soils by facilitating the faster decomposition of leaf litter. As the speed of decomposition increases, nutrients such as nitrogen and phosphorous are cycled more rapidly.
The succession of New England forests from coniferous-dominated to deciduous-dominated tree species, as scientists indicate is already happening, could have long-lasting impacts on ant communities, she said, as well as on the trees and other plants in the forest.
Eighty percent of the ants Anderson collected were of just two species, both of which are primarily known as seed dispersers, and they were found in greater abundance around the deciduous trees. Plants whose seeds are dispersed by ants make up about half of the herbaceous understory in most New England forests, so Anderson’s results suggest that as forests become increasingly dominated by deciduous trees, the increased prevalence of seed-dispersing ants could cause understory plants such as trillium to become more widespread.
Her data also suggests that as New England forests shift over time to become dominated by deciduous trees, a subsequent shift in ant communities may also occur. “We expect to see a change in those ant species and functional groups that are better adapted to forest soil habitats derived from deciduous leaf litter as forest succession in New England progresses from coniferous-dominated to deciduous-dominated mixed forest types,” she wrote.
“The trees are having a cascading effect on these small ants,” she added. “And that’s something not everyone is thinking about in land management. Most foresters think about soil health and quality, but it’s also important that they consider how this association between trees and ants and soil may actually change the forest in the future.”