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What it takes to eat wood

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While many organisms thrive solely feeding on the leaves, roots and fruits of trees, very few are capable of efficiently incorporating wood into their diets. In Northern Wisconsin, wood becomes an important resource that helps provide the needed energy for some over-wintering forest -dwellers. But, eating wood requires special structures and stomachs full of symbiotic partners to effectively break down the strong fibrous chains that give wood its strength.  

As my WXPR field notes colleague Scott Bowe has highlighted in prior episodes, wood is made up of functionally dead plant cells that form incredibly strong interlocking chains of a molecule called cellulose. At the chemical level, cellulose looks a lot like sugar. It is a carbohydrate chain of repeating structures with strong bonds between each link. Compared to sugar which has weak bonds between links, the bonds in cellulose are much harder to break.

Organisms readily turn sugars into energy. It takes only a bit of enzyme to help quickly break the bonds of sugar providing a ‘sugar rush’ of energy (an occurrence that was readily evident in my young boys over the holidays). In contrast, cellulose requires a lot of energy to break apart, such as a lightning strike or the high heat required to start a wood fire. Some fungi, bacteria and archaea produce enzymes that assist in breaking down cellulose but few animals have the ability.

Termites are one of the best (and most destructive) examples of wood eaters. We all know this household pest that plagues man made structures in woody regions throughout the world. Termites have special structures and relationships that make it possible to digest wood. Their hindgut is modified with a long, swollen cavity that contains no oxygen. This region contains an entire community of single-celled organisms that thrive in low or no-oxygen conditions. Trichonympha is one of the most abundant unicellular residents of the termite gut and can number in the thousands They are covered in flapping tiny hair-like structures and they compete with other swimming single-celled organisms in the termite gut.  Trichonymphs contain multiple tiny Bacterial and Archean microbes that help break down wood. In this multiscale symbiotic relationship, termites consume wood and feed and house all these organisms. In turn, the single-cell symbiotes break down the wood and share energy released from cellulose in a way that is beneficial for all.

Porcupine and beaver are also uniquely adapted and contain symbiotic microbes to digest wood. They have ever-growing teeth helpful for gnawing bark and, shooting off their lower intestine, both species have a large pocket called a caecum that is full of microbes. In the low oxygen environment of the caecum, microbes ferment wood particles, breaking them into simpler sugars that porcupines or beavers can absorb. Wood can make up a significant portion of winter diets for these animals, but does not provide sufficient amino acids, nutrients, and salts to sustain year-round health. Therefore, beavers shift to leaves, roots, and aquatic plants in spring and summer and porcupines consume buds, berries, and other vegetation to supplement their woody diets.

Very few animals are efficient enough at breaking down the cellulose of wood to make it energetically worth eating. Instead most unharvested wood ends up being consumed by fungi and bacteria that in turn replenish the nutrients and energy needed to keep our forests healthy.

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Gretchen Gerrish works for UW-Madison's Trout Lake Station through the Center for Limnology. She studies how evolutionary and ecological processes interact to allow natural systems to deal with change over time.