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While wood is made up of the same carbohydrates called cellulose, wood is held together by a complex molecule called lignin. This makes wood tough and hard to digest as part of the cellulose remains undigested. Some bacteria however have found the right enzyme to break it down and termites have such gut bacteria. That makes them (along with some beetles) the only animals capable of eating wood. Now what if large animals got this ability?

Now try and picture what a wood eating animal would look like. Large stout jaws with long teeth to bite and chew wood. Thick skin to protect them from woodchips. Multiple stomachs to process and ferment the plant matter. This calorie intake would make them very large and powerful, capable of unrooting trees and fending off predators like hippos do with their terrifying bites. Their size would make them energy efficient and very slow creatures due to their metabolism slowing down thanks to the square-cube-law. Their young would be incapable of chewing wood right away so the parents would take care of them for years until they are developed enough. This might lead to herding behaviors like elephants or hippos but they might also be solitary. Birds would be useful to get rid of parasites.

How would such animals affect the ecosystem? Would it upset the balance completely? How would the ecosystem adapt?

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  • $\begingroup$ i immediately am thinking of a panda bear. with beaver like teeth. because i eat liquorice trees i wouldn't bother with giving it thick skin. i'd imagine them more ripping the fibres off rather than straight up biting in. $\endgroup$ Sep 15 '21 at 15:35
  • $\begingroup$ @Postlim Fort Haha I like it! But honestly thick skin is optional, it doesn't even need to be an inch thick. I guess it’s just in case their skin gets scratched a lot like human hands during labour. $\endgroup$ Sep 15 '21 at 15:40
  • $\begingroup$ @PcMan Instead of wording it like that you could have suggested an edit. Fine, I'll fix it. $\endgroup$ Sep 15 '21 at 17:49
  • $\begingroup$ We live in such a world. It wasn't always so... and the world in which nothing digested wood gave us abundant coal deposits. But soon after those were buried, wood digestion evolved, and apparently no one noticed. $\endgroup$
    – John O
    Sep 15 '21 at 18:14
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    $\begingroup$ about the same calories as the same weight of potatoes but ten times longer to digest, even longer if you don't chew it to fine pulp. $\endgroup$
    – John
    Sep 16 '21 at 0:09
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Plausibility

Is there energy in trees? yes. Wood fires are a thing. Is there much completion for trees as food source.Not much. Only some insects.

The most plausible is a ruminant switching to a progressively woodier diet, It would require getting bacteria that can digest wood fiber and lignin, that are able to live in a ruminant digestive system. More challenging is the teeth needed to be able to grind or chip the wood. rodents in the form of beavers have the teeth to chip wood. But they don't have the ruminant digestive system.

Combining the ruminant digestive system and rodent teeth system is the most likely fast path to creating such a creature.

Also they don't need particularly tough skins, They would however need a tough and or resilient upper digestive system. Pointy sticks are known to puncture bodies.

Impact on ecosystem.

Closest animal that we know of are elephants. They have large impact on the ecosystem by knocking over trees. Thus reducing forest cover and increasing grassland. This is seems to have been similar for the mammoth steppe of the Pleistocene, where mammoths kept the trees down, keeping the steep as grasslands, which were more productive then forests.

A tree eating creature would amplify this change. I could easily see the more juvenile creatures eating saplings/smaller trees. while the older larger animals eating larger trees, sometimes pushing over large trees to get at the smaller upper branches.

Some areas this would be beneficial as grasslands in some climates are more productive, other regions it would be very destructive as trees are essential to collect and keep moisture for the trees and other vegetation.

This however would be mitigated by the slow consumption and digestion of the wood. It takes time to chip and or grind the food up. Additionally while large adults with teeth and jaws more then capable of crushing bone would keep predators very cautious. Their young would be vulnerable to predators.

How easy would it be for these creatures to overpopulate and deforest whole continents/forests? Depends on many factors but seems likely on a regional basis only. The slow growth rates and regional variation would ensure over population will not happen suddenly or at all places at the same time. I would suspect it would have affect of decreasing average size and age of most trees, and encourage trees to produce less tasty wood.

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  • $\begingroup$ nice analysis, not bad $\endgroup$
    – MolbOrg
    Sep 16 '21 at 1:34
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On average wood contains around ten times more calories per kilogram than fruits or leaves

Having ten times the potential calories isn't very informative, if the energy required to break down those complex structural carbohydrates is more than ten times the energy required to digest fruits or leaves.

Cellulose is already a problematic material to break down... lignin seems even worse. I'm not sure your plan would work particularly well.

Perhaps more importantly, the nutritional value of wood is quite low... the live bit (the cambium) has all the good stuff in, but the heartwood that makes up much of the mass of the tree is much less tempting. Contrast this with eating leaves or soft stems where a much larger proportion of the food being ingested is made of actual living cells, and therefore contains a wider range of nutrients.

How would such animals affect the ecosystem? Would it upset the balance completely? How would the ecosystem adapt?

If you suddenly introduced animals that could eat mature trees into a new ecosystem, they would indeed be pretty destructive, but all kinds of invasive species are destructive... this isn't a unique property of a tree eater. Vegetation and herbivores evolve alongside each other, and if your tree-eaters did exist then suitable trees must necessarily be able to survive alongside them.

Remember that there are and have been plenty of animals that can fell mature trees (such as beavers) and animals that can eat large quantities smaller branches of trees (such as elephants) or even push quite large trees over. No doubt there were large dinosaurs that had similar diets. Bad weather can also fell trees, of course.

Trees have had plenty of time to evolve appropriate responses to being knocked down or nibbled by megafauna. You can see this in the way trees respond to coppicing or pollarding... humans discovered the regenerative abilities of some kinds of tree, and took advantage of them.

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  • $\begingroup$ I did some research and it seems that wood has 4 Calories per gram like dried mashed potatoes. Eating wood is a niche that not many animals can fill, so there's little competition there as wood requires specific adaptations to eat. $\endgroup$ Sep 15 '21 at 16:09
  • $\begingroup$ @LiveInAmbeR but what does that figure actually mean? what is the energy cost of the enzymes required to break it down, for example? $\endgroup$ Sep 15 '21 at 16:14
  • $\begingroup$ I've searched but haven't found an answer to the energy cost. No one asks the energy cost of digestive enzymes. That's because stomachs don't refill themselves for every meal, they reuse enzymes and mix food. $\endgroup$ Sep 15 '21 at 17:04
  • $\begingroup$ @LiveInAmbeR TANSTAAFL. $\endgroup$ Sep 15 '21 at 17:17
  • $\begingroup$ the enzymes require similar amounts of energy, but they are slower, the cellulose molecule has to be slowly peeled apart which drastically increases the time needed to break it down. the cost is time not energy. $\endgroup$
    – John
    Sep 15 '21 at 20:48
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Evolutionary history would seem to indicate that the trees get bigger and faster growing. The sauropods were the huge tree eating dinosaurs that seem to have driven the evolution of species like the redwood and giant sequoia which simply outgrow the ability of even those multi-tonnes browsers to reach their foliage.

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Growing wood takes also lot of energy. If it would be consumed easily, it probably wouldn't be worth growing, because the tree needs time to harvest the needed energy.

Grass and leaves can be grown quickly, and praire grass for example has evolved to be grazed, but the growth rate of grass is not comparable to the one of wood.

Unless the trees could grow protection like tornes to discourage the eaters.

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