This is a great time of year for nature observation in Massachusetts. Almost every day something new is happening: a new plant is flowering, some new insect has emerged or I’ve spotted a new bird, recently returned from its winter away. So these days, when I’m out and about, I try to keep my eyes pealed to spot these “happenings”. 

A few days ago, while poking around the woods in my backyard where I was checking on the blooming mayflower and the new wild indigo shoots, I noticed a number of just-starting-to-unfurl bracken fern fiddleheads. And then I noticed that every single one of them was covered in tiny black ants. What could these developing ferns be providing that is so appealing to the ants? 

The answer is foliar nectaries. Foliar nectaries are specialized plant structures that are not flowers but still produce sugar. These are often referred to as extrafloral nectaries in flowering plants, but since ferns never produce flowers, “foliar” (i.e., related to the leaves or foliage) is more appropriate. In this case, the bracken fern foliar nectaries secrete sugars and amino acids that set the stage for a mutualistic relationship between the fern and the ants. The ants benefit from a valuable high-energy food source. In return, fern benefits by being protected from insect herbivores by the ants. 

Although it’s a nice story, some scientists had questioned whether there’s actually a protective benefit conferred to the fern. To answer this, researchers in California designed an experiment  where they observed the densities of sawfly larvae (a bracken fern herbivore) on ferns with ants and on ferns where ants had been excluded. They found that sawfly larvae abundance was significantly higher on ferns where ants had been excluded (i.e., where the protection had been removed). They also found that in addition to deterring actual herbivory, ants can also deter insects, such as sawflies, from laying their eggs on the ferns in the first place, further adding to the benefit the ants provide to the plant. 

In most cases, nectar secretion is highest in young bracken ferns during the early stages of growth, which further explains why the ants were so numerous a few days ago. This ant-fern mutualism is a good example of John Muir’s quote: “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” What interesting connections have you noticed lately?