Science in Pictures: Ecosystems as Autocatalytic Sets

In a previous post, a small example of an ecosystem consisting of three different species was given, arguing that it was an instance of an autocatalytic set. The claim that ecosystems can be viewed as autocatalytic sets was originally (and more formally) made in a scientific publication a few years ago (a free pre-print is also available). In that publication, a specific example of an ecosystem consisting of five different species was given. Recently, I had the pleasure of actually observing this example ecosystem in action for real, in nature itself.

Briefly, an autocatalytic set is a chemical reaction network in which the molecules mutually catalyze each other’s formation, starting from a basic “food source”. As such, the reaction network as a whole is self-sustaining, as it is able to (re)generate (from a given food source) its own components through chemical reactions that are catalyzed (i.e., “facilitated”) by these very same components. For more details, see the last part of a series of popular science articles I wrote on the origin of life.

The claim that ecosystems can also be viewed as autocatalytic sets rests on generalizing the notions of chemical reactions and catalysis. For example, in an ecosystem some species eat other species. This could be viewed as the equivalent of a chemical reaction: “transforming” one species into another. Furthermore, some species cooperate, without one eating the other. As in the three-species example given in the previous post, flowers produce nectar that is collected by insects, which in turn help the flowers to be pollinated. This can be viewed as the equivalent of catalysis (or “facilitation”).

The five-species example from our scientific publication to illustrate the claim that ecosystems can be viewed as autocatalytic sets is reproduced below. The food source consists of bacteria (f1) and plants (f2). Aphids (p1) eat plant sap (“reaction”, or transformation r1), which lacks certain essential amino acids. However, the aphids have acquired gut bacteria that produce those missing amino acids. Thus, these bacteria act as a catalyst (indicated by a dashed arrow) for the “transformation” of plants into aphids (the bacteria are not consumed by the aphids). Aphids, however, are eaten (r2) by ladybugs (p2). Plants infested with aphids produce a certain chemical that attracts ladybugs, and thus act as a catalyst for the “transformation” of aphids into ladybugs. In response, aphids produce a sweet substance that is harvested by ants (p3), which in return provide protection against ladybugs, which they sometimes attack and eat (r3). Aphids thus act as a catalyst for the “transformation” of ladybugs into ants, closing the catalytic loop. Also, the ants form an additional catalyst for the transformation of plants into aphids.

On a recent hike in the woods I had the pleasure of observing this specific ecosystem (and autocatalytic set) play out in real life. At first, I noticed some plants covered in aphids, and a few ants running around harvesting the sweet substance produced by the aphids.

Everybody seemed to be just going about their usual business.

But then, to my own surprise and delight, a ladybug appeared from behind a leaf on a nearby branch.

I watched the ladybug as it tried to approach the aphids, only to be literally chased away by some ants.

So, the aphids were indeed protected by the ants, and the ladybug had a lucky escape. The only species not visible in these images are, of course, the bacteria in the guts of the aphids. But it sure is a wonderful experience to see some of our own theoretical musings about ecosystems and autocatalytic sets playing out for real, in nature itself. And a great example of how knowing a little more about nature from a scientific point of view can make observing natural phenomena and processes even more enjoyable!

No species exists on its own. We’re all connected…