The music of the networks

The lines that separate human cultures exist only in our minds. Science, art, and philosophy can contribute to one another with inspiration and insight. This is how our recent paper was born.
The music of the networks

A harpsichord concert has the power of transporting you to a dream dimension.

During a conference dinner in Brazil in 2014, right before playing, the harpsichordist lectured us on “the music of the spheres." This ancient Greek theory proposes a relationship between planet orbits and musical harmony. Art and philosophy, in harmony, made my mind wander. It was like a dream of spheres rotating on different colorful axes. A vision of species entangled in a web of life woven by ecological interactions.

Figure 1: the music of the spheres, Renaissance engraving. Source: Wikimedia Commons.

Recent developments in network science allowed my colleagues and I to translate this “music of the networks” into our paper on multilayered interactions in Oikos in 2016. Mammals and plants were bound together in a dual relationship of seed dispersal and destruction. Mutualism and antagonism. Positive and negative.

Figure 2: the music of the networks: seed dispersal (blue) and destruction (red) by small mammals (orange) feeding on plants (green) in an urban park in Brazil. Graph re-drawn using the same data as in Genrich et al. (2017, Oikos), available on the Interaction Web Database.

Despite its beauty, this duality proved to be an illusion. A seed predator can turn into a seed disperser or the other way around. Rodents are not always the bad guys, and marsupials, the good guys. It shows that labels attributed to species mean little compared to the wealth of information gained by focusing on the interactions. 
Looking at interaction networks from this new perspective, later in 2016, during a conference lunch in South Africa, we took the first step that led us to our paper published in Nature Ecology and Evolution. We faced constraints related to the food items we could eat at lunch, such as the meat vs. pasta dilemma. Many bats also face constraints associated with feeding on fruits or nectar. It is another kind of duality at a much larger spatial scale: the entire Neotropics.

Figure 3: the music of vast networks: bats (orange) feeding on the fruits (blue), flowers (yellow), or both (purple) resources from plants (green) in the Neotropics. Graph re-drawn using the same data as in Mello et al. (2019, NatEcoEvo), available in the paper’s supplement.

What we found was a vast network whose parts do not resemble the whole. In other words, a compound structure assembled by different processes, from evolutionary constraints to functional morphology. In this compound structure, what happens in one layer echoes in the other. If a bat gets really good at living off fruits, it probably cannot consume nectar as efficiently. It depends mostly on the type of skull it has. The skull determines which bats form the core or the periphery of the network, and which bats become bridges between layers. 

Figure 4: the hierarchical assembly of a continent-wide, multilayer network of bats and plants.

Bridges should also be built between topics and disciplines. For instance, the duality between frugivory and nectarivory is another illusion. Frugivores and nectarivores, as well as seed dispersers and destroyers, are possibilities in a continuum of interaction scenarios. They belong to the same symphony. 

The music of the networks is helping us achieve theoretical harmony. The borders between disciplines seem to be as illusionary as the borders between interaction types. So, the dream of building integrative theories may be more feasible than we usually imagine.

Acknowledgments: Regina Alonso kindly made a critical reading of an earlier version of this post.

(Poster image by Marco Tschapka)

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