Small Friends of Fungi

This article first appeared in the splendid Cornell Mushroom Blog. Reading it, we were jolted into realizing that we had a distorted view of the living world, no less. Like most people, we believed that in natural habitats, some animals eat plants, those animals are eaten by predators, and that's about it. We thank Bob Mesibov for setting us straight. We are delighted to share his insights with you by reproducing his article here, with permission from him and from Kathie Hodge, editor and blogificator of the Cornell Mushroom Blog.

by Bob Mesibov

The millipede Narceus americanus photographed by
Kent Loeffler, copyright Cornell University. You can
find this borescopic image among the many in the
book Beneath Notice: Adventures with a Borescope
by Kent Loeffler, with "Fungal Outbursts" by Kathie
Hodges.

If you studied the traditional sort of biology, you're probably carrying around an unfortunate prejudice.

You see terrestrial habitats as a simplified nutrients-and-energy pyramid. At the bottom are green plants, feeding on sunlight, carbon dioxide and soil water and minerals. Next layer up on the pyramid is the herbivore mob: leaf and stem eaters, sapsuckers, root nibblers, seed and fruit gobblers. Above these green feeders are a couple of layers of predators. And that about sums up the world, right?

Wrong. That's only part of the world, and a small, very specialised part of it, too.

To begin with, most animals can't eat green food. Herbivores are dietary specialists among the insects, mollusks, birds and mammals. Your average green leaf or stem doesn't show much herbivore damage, and for good reason. It's mainly water boxed in by cellulose and other structural carbohydrates, which are impossible or extremely hard for animals to digest. Other nutrients are present, but at low concentrations. You need to eat a great mass of indigestible green stuff to get a decent return of elements like nitrogen, phosphorus, potassium and calcium. As for animals eating wood, which makes up most of the biomass in a forest – well, there are termites, and…um…termites…

The truth is that in the real world outside the biology classroom, only a tiny proportion of terrestrial primary production goes through the stomachs of the few evolutionary lineages brave enough to tackle what green plants produce. In any terrestrial habitat, the great bulk of primary production just does not get eaten. It sits, instead, at the bottom of a very different food pyramid. I call it the Dead Plants Society (DPS), as opposed to the Green Feeders Guild (GFG).

In the absence of fire, all that uneaten primary production is first attacked by fungi and bacteria. By 'attacked' I mean 'converted from low-nutrient indigestibles to concentrated yummies', i.e. fungal and bacterial bodies. Stacked on top of this microbial layer in the pyramid are microbivorous layers of nematodes, mites, springtails, earthworms, millipedes and other soil animals. On top of those are predators – but picture 'centipede', not 'eagle'.

The GFG and DPS animal communities differ in many ways. To begin with, in any given habitat the GFG has very high species diversity (think of plant-eating insects) but low higher-taxon diversity, while the DPS has great higher-taxon diversity (lots of strange sorts of animals), but low species diversity. Next, GFG herbivores tend to specialise on particular plants, while DPS microbivores will eat anything that's rotting nicely. There are also a lot of winged GFG members ('gotta find that particular plant I like…'), whereas almost no DPS members have wings, at least in their younger, feeding stages. There's an architectural difference, too. The GFG extends well up in the air, to ca. 100 m in some tall forests, while the DPS is largely confined to the ground.

Then there's the matter of heritage. The earliest DPS fossils are of mites, springtails and millipedes, and they're more than 400 million years old, from a time when terrestrial vegetation was mainly mossy and ground-hugging. The first solid evidence for green feeding (early insects with spores in their guts) appears much later in the fossil record, from coal swamp times. The DPS is vastly older than the GFG, and when you handle richly organic soil you're holding animal communities which are spectacularly ancient and robust. You can almost imagine a springtail thinking: 'Seen the dinosaurs come and go, mammals are nearly done. Wonder what great lumbering dopes we'll see in the next 100 million years? Yum, love these hyphae with yeast sprinkles!'

Dr. Robert Mesibov is a taxonomist from down under who studies seriously fungophilic animals: millipedes. Check out his other many-legged doings via his website. A previous article on the Dead Plants Society is available here.

Comments

[Karen Vaughan]

Fascinating reworking of the common paradigm, but there is also the consumption of carnivores and other animals by mites, fungii, and other microbiota. They are consuming me as I write, which makes most of the vegetarian arguments, based on that inaccurate pyramid irrelevant.

[Eric Johnson]

Interesting stuff. Is this as true in grasslands as it is in the forest? It seems like tons of ruminants manage to make the economics of grass cellulose work for them. There are also folivores in the proper sense, which eat trees, but they seem to be fewer in biomass and diversity if my nonscientific impressions are right. Are they more predation-limited for some reason than grazers? Do grasses bleed more energy to the Green Feeders than trees do? Are any herbivores limited malthusianly (by parasites and intraspecific competition & violence)? Or is there always an apex predator that can trim them down? (Lions of course used to exist in the New World, until quite recently.)

One theory, which I just made up after reading wikipedia[folivores], is based on the observation that flying animals are rarely folivores because the amount of cellulose digesta they'd need to hold is so heavy. It says that folivorous bats chew leaves and soon eject the recalcitrant material such as cellulose, which tallies with this. My suggestion is that the same thing may apply more weakly to climbers, so there are less folivores than grazers. Cows are much freer than monkeys to carry around many kg of digesta, as well as the more massive digestive tract needed to deal with that stuff.

This post of John Dennehy's might be relevant:
http://evilutionarybiologist.blogspot.com/2008/06/this-weeks-citation-classic-world-is.html