Symbiosis – How Woodpeckers and Fungi Collaborate with Each Other

      Associations between organisms are widely found in nature. Symbiosis – from Greek meaning “living together” -- is a term referring to any close relationship between two distinct organisms rather both benefit (mutualism), only one species benefits while the other has a neutral income (commensalism), or one species has advantage or harm the other through the relationship (parasitism). However, symbiotic associations are far more complex than these; they often include several other interactions with more organisms and the environment, and the way that many of them work is still unknown.

    One intriguing symbiotic interaction poorly understood is the potential mutualism between cavity excavators and fungi. It is hypothesized that excavating species – such as woodpeckers – may benefit from fungi because they soften wood. Thus, woodpeckers can open holes in trees more easily. At the same time, fungi may gain advantage of facilitated dispersion, because excavators would carry them to other trees.

    This theory was experimentally tested for the very first time in a field survey in the Marine Corps Base Camp Lejeune (MCBCL), on the central coast of North Carolina with an endangered woodpecker species: the Red-cocked Woodpecker (Picoides borealis) which population has been monitored in the site since 1986.

Image: Red-cocked Woodpecker (Picoides borealis). Photo by Shailesh Pinto, Macaulay Library gallery. ML293097971 Red-cockaded Woodpecker Macaulay Library.

   

 Red-cocked Woodpecker is an endangered species endemic to North Caroline. Unlike other woodpeckers which normally excavate dead wood, Red-cocked woodpeckers gouge living pine trees through the sapwood into the heartwood – a process that can take from one year to decades! Obviously, this demanding work is worth it: Red-cocked nests are much more durable compared to those of other woodpeckers. This dedicated carpenter is a social species and lives in family groups reusing the cavity for many years. Once the family leaves the tree, many other species utilize the cavities.

    Because Red-cocked Woodpeckers are endangered and provide services for many other organisms, understanding their interactions with other organisms can help protect this species and the communities that rely on their cavities.

    To verify the symbiotic hypothesis, researchers first swabbed 11 woodpeckers. You know that covid test you had to take? Yeah, almost like that, but for woodpecker’s luck no nasal sampling was needed: only wings, feet and around their beak. Next, researchers collected samples from active woodpecker’s cavities, 36 complete and 42 which had only been started. After that, they selected 60 non-excavated trees similar to the ones woodpeckers would prefer and drilled a hole in each of them mimicking Red-cocked cavities. Half of those were closed impeding woodpeckers to access, and the other half was opened. They collected samples from all of those at the start of the study and after a two-years period.

    They found out that woodpeckers indeed carried fungi communities, and those were remarkably similar to the ones found in their natural complete cavities. Cavity starts had some similarity too, and in the few artificial drills woodpeckers ended up using there were some of those fungi. Non-excavated trees and inaccessible drills, on the other hand, had quite different fungi community composition.

Figures: see dots with colors for Red-cocked Woodpecker (RCW) complete cavities (red), cavity starts (yellow), non-excavated trees (blue), woodpeckers (black), inaccessible drills (light blue), and accessible drills (purple). The two axes are a function of fungi communities. To simplify, observe how the values overlap in the graphics. (Jusino et al., 2016).

    These results offered great support for the “bird facilitation hypothesis” that states that fungi gain dispersal advantage when associated with birds. You could think of it as if Red-cocked Woodpeckers give a “ride” to fungi species to disperse across trees and have them as "cavity roomies” in their nests. Many of those woodpecker-friendly fungi were wood decay agent species (a.k.a. wood decomposers), which suggests that woodpeckers benefit from fungi interactions in a mutualistic association, but the authors emphasize that more research with experiments to test it is needed.

    Nevertheless, this study proves that Red-cocked Woodpeckers and fungi have a symbiotic association; whether it is mutualism or commensalism, however, is an inquiry for another future research.

References:

Jusino, Michelle A., Daniel L. Lindner, Mark T. Banik, Kevin R. Rose, and Jeffrey R. Walters. “Experimental Evidence of a Symbiosis between Red-Cockaded Woodpeckers and Fungi.” Proceedings of the Royal Society B: Biological Sciences 283, no. 1827 (March 30, 2016): 20160106. https://doi.org/10.1098/rspb.2016.0106.