Life begets life: the diversity of species on Earth is generating itself
A new research hypothesis suggests that biodiversity is autocatalytic
If competition is the main evolutionary driver, why can so many species coexist within the same ecosystem instead of having a few that dominate? This a longtime and central question in ecology. Many ideas have been suggested in an attempt to explain this evolutionary paradox. Most of them are based on the importance of ecological niches for the maintenance of differentiated ecosystems against dominated ones. In 2011, Dr. Roberto Cazzolla Gatti, an associate professor of ecology and biodiversity at Tomsk State University (Russia), proposed the “Biodiversity-related Niches Differentiation Theory” (BNDT), arguing that species themselves are the architects of biodiversity, by proportionally increasing the number of potentially available niches in a given ecosystem. Along similar lines, but independently, the idea of viewing economics, biology, and ecology as emergent autocatalytic sets (a self-sustaining network of mutually catalytic entities) was suggested by Dr. Wim Hordijk, a researcher at the Konrad Lorenz Institute for Evolution and Cognition Research, Klosterneuburg (Austria), and the well-known McArthur Fellowship winner, Prof. Stuart Kauffman, of the Institute for Systems Biology, Seattle (USA).
Now, in a paper published this week in Ecological Modelling (Volume 346, 24 February 2017, pp. 70–76) with the title “Biodiversity is autocatalytic”, the three scientists merged their ideas in a new hypothesis to explain why and how such a great number of species can live together in the same environment. They suggest that one group of species enables the existence of (i.e., creates niches for) other species. This means, the authors say, “that biodiversity can indeed be considered a system of autocatalytic sets, and that this view offers a possible answer to the fundamental question of why so many species can coexist in the same ecosystem”.
The variability among living organisms in terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are a part have been described by the term biodiversity. Apart from the formal definition and the different ways to measure it, the central question about biological diversity on Earth is how so many species can coexist within the same ecosystem.
However, the idea that interactions between species are important catalysts of the evolutionary processes that generate the remarkable diversity of life is gaining interest among ecologists. For instance, it has been shown that symbiosis between gall-inducing insects and fungi catalyzed both expansion in resource use (niche expansion) and diversification. Indeed, facilitation (a process that allows the colonization and presence of new species taking advantage of the presence of other species by expanding the ecosystem hypervolume), plays a major role in species coexistence, greatly increasing the biodiversity of an area. A species emerges from this environment and is an expression, in fact a historically contingent expression, of those interactions. In other words, species are expressed and maintained by a complex interacting ecological network.
An autocatalytic set, a chemical process associated by Cazzolla Gatti and his co-authors with biological systems, is a group of entities (e.g., molecules and the chemical reactions between them) of which each can be produced catalytically, that is, triggered by other entities within the set, such that the entire set is able to sustain and reproduce itself from a basic food source. In other words, the set as a whole is self-sustaining and collectively autocatalytic.
Autocatalytic sets were originally defined in the context of chemistry (in particular polymer systems), but Cazzolla Gatti, Hordijk, and Kauffman show that biodiversity can also be considered a system of autocatalytic sets. Dr. Cazzolla Gatti explained, “Species themselves, creating favorable conditions for the colonization of other species, allow their concurrent presence, and the fundamental mechanism that supports the coexistence of species is the creation of diversity-related niches.”
A species must not only preserve itself but also engage in autocatalytic feedback cycles (e.g., reward loops, function circles, and autopoiesis) that contribute to the overall functioning of the local environment. The phenomenon of autocatalysis can therefore be considered a process of internal and external selection.
These ideas open up many new and interesting research questions. For example, we can ask whether there is a limit to the growth of biodiversity, and if so, whether this limit coincides with the ecosystem carrying capacity. Another open question that derives from this theory is: If biodiversity is indeed autocatalytic, does it follow a sigmoidal growth over time (as does every autocatalytic reaction)? Considering, for example, the trends of number of genera during the Phanerozoic Eon, which follow an exponential growth curve, the authors argue that, if the answer to the above question is positive, global biodiversity should – in absence of catastrophic events – eventually reach a plateau and show a sigmoidal trend (as predicted by the differential equation of the BNDT).
Finally, a more practical and empirically interesting question, which could be answered by the BNDT is whether we can estimate the (possible or existing) number of species of a particular group from ecological variables (mainly influencing the autocatalysis of that group), such as biomass. For example, in 1993 Prof. Kauffman calculated the number of cell types as the square root of the number of genes of an organism. Similarly, Cazzolla Gatti and his co-authors attempted to calculate the number of vascular plants by taking the square root of the estimated total live plant biomass of 550 BTC (billion tons of carbon). Their captivating result estimated a maximum of 741,620 vascular plant species. As of 2013, there are approximately 350,000 accepted plant species names and over 240,000 names that remain to be resolved as accepted names or as synonyms . Considering that an unknown number of plant species have yet to be discovered, and summing the number of accepted species to a mean number of unresolved names, the final sum is surprisingly close to authors’ estimate. Is this result just a coincidence or is it truly a consequence of the theory? Dr. Cazzolla Gatti and colleagues hope that in the near future we will be able to perform a similar calculation for animal groups and resolve this question.
With this hypothesis on the autocatalysis of biodiversity (i.e., biological diversity itself begets diversity), the authors provide a response to Darwin's pondering about the view of life "that with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning, endless forms most beautiful and most wonderful have been, and are being, evolved".
However, Dr. Cazzolla Gatti and colleagues showed that, in his wonderment, Darwin was wrong about the causes of this great biological diversity. Biodiversity, declared Dr. Cazzolla Gatti, does not originate "from the war of nature, from famine and death" as Darwin thought, but from the power of life to enable other life; not from war, but from searching for peace; not from competition but from the avoidance of it, that is, from cooperation and facilitation (i.e., by autocatalysis).