The Species Concept

A mushroom is a living, growing organism. When you find one, you get a snapshot of what it looks like at that moment in its life. If the snapshot in your field guide was taken at the same time as the mushroom in your hand, you can identify it that way, instantly! But that’s not how life usually works. Nor should it be.

When you see a mushroom, it’s better to use what you observe to build up an idea of what changes it goes through in life, why it grows where it does, until you have a fuller concept of how its life goes. Then you’ll be able to recognize it at more stages of life, you’ll understand better what’s going on with it at the time you find it, and you’ll be able to account for how unusual factors in the environment might be affecting it. This sort of full-life narrative is called a species concept.

Take for example the photo above: it shows several individuals of the same species, gathered at the same time, at the same place. Thus these mushrooms constitute a collection, and gathering them is called making a collection. Hopefully, as was done here, you can collect the mushroom in different stages of its development, thus making this a good representative collection. A representative collection helps you understand the mushroom’s life history, helping you to build up a good species concept.

This is why field guides tell you to try and gather a new mushroom in several stages of its life. It’s not in the hope that if you gather enough, there will be one that matches the photo in the book! It’s because species concepts are what you should be extracting from books in the first place; and likewise, you should be trying to discover the species concept for the collections you make in the wild, and matching the concepts rather than the pictures.

Okay, so what can we say about this collection? (Here it is again, so you don’t have to scroll around) In case you’re not sure, size is roughly correlated with age here: the smaller mushrooms on the left are the youngest. Some features of this collection stay the same no matter the age of the mushroom while others change. What changes and what stays the same? Well, the cap surface is white in all of them. The stem is also usually white. There are a few that have some brown on the stem, but that doesn’t change with age, so we’ll conceptualize that as something that varies randomly. What does change (besides overall size) are the shape of the cap and the color of the gills. The cap starts out in a ball, unfurls until it’s slightly rounded (the center mushroom) and eventually flat or even with the edges lifted a little (next to last on the right). The gill color changes from white to brown in a parallel process.

This is because mushrooms are formed in order to grow and spread spores. In the case of most mushrooms, the spores blow away in the wind and hopefully land somewhere compatible, germinate, and eventually start growing another mushroom. Spore color is important in identifying mushrooms, and the spore color of this mushroom seems to be dark brown. That’s why the gills change color: the gills themselves are white, but the spores that develop on them are brown; so as the spores mature, they change the color of the gills from white to brown. Likewise, the cap opens up as the spores mature to give the gills free rein to drop their spores without hitting another part of the mushroom. This is all part of the species concept for this mushroom.

Let’s take a look at a very different kind of mushroom now. This is a kind known as a stinkhorn:

Stinkhorns start out completely enclosed in a fibrous membrane called a universal veil. So as the mushroom first breaks out of the ground, it looks like a white egg (top center). The stinkhorn then breaks through the universal veil, leaving it like a broken eggshell at the bottom of the stalk.

A stinkhorn’s cap is covered with a foul-smelling greenish slime which contains the spores (bottom center). The bad smell attracts flies which eat the slime and end up getting it on their feet and bodies. When they land elsewhere, some of the slime sticks, and if this new place is hospitable for stinkhorns, they germinate and we end up with new stinkhorns.

This head of this particular stinkhorn has a randomish pattern of raised ridges on its cap. As the slime is eaten by the flies, the white ridges start to show through, as you can see in the photo at right. Nees’s illustration also tries to show this development by the difference between the young mushroom in the bottom center and the older one on the left. In the photo, you can see the ridges beginning to emerge at the top of the cap. You can also see the spongy texture of the stalk, which Nees also tries to show. This is all part of the species concept for how the mushroom develops. The stem grows through the expansion of this foam-like structure, so it can do so quite quickly. If you take one of the eggs home and put it on some moist paper towel, the stinkhorn will often grow out of its egg (and stink up your house). If you try this, you may want to put the stinkhorn inside a closed jar – both to contain the smell and help it stay moist.

Nees also shows the rootlike rhizomorphs at the base of the stalk; and the cross-section on the right shows that the stalk has a hollow center that extends all the way through the top of the cap. All in all, the stinkhorn has a very different sort of species concept from gilled mushrooms.