High-diversity ecology

Resource competition at high diversity

Organisms modify their environment, which in turn affects their survival: ecological and evolutionary processes are deeply linked. What is more, the real world is very "high-dimensional", with hundreds of metabolites at play. I'm interested in the non-intuitive effects that arise because of this high dimensionality. I use the classic model of resource competition as a simplest example of an ecological feedback: by consuming a resource, an individual changes its availability for others. Classic work of Tilman investigated cases of N = 1 and N = 2 resources, but what happens when the number of resources becomes large?

It turns out that high dimensionality does indeed lead to surprising behavior. Even a purely competitive community can exhibit an effective "cohesion" when interacting with other communities and its environment, in a certan strict mathematical sense. Encouraged by this observation, I have partnered with Remi Monasson to show that the large-N limit of resource competition of MacArthur's classical model can in fact be characterized analytically. We found that high diversity triggers a phase transition into a curious collective regime.

The non-intuitive geometry of high-dimensional spaces

A phase transition at high diversity

Physicists like phase transitions; however, the most exciting aspect is not the transition itself (its relevance beyond a particular model remains to be investigated). What matters is that we now have an analytically tractable model where the ecological consequences of large dimensionality can be studied in a new way. Importantly, resource consumption is a functional aspect. This is in contrast to the traditional "large-N ecology" where, following the classic work of Robert May, the questions are usually compositional: e.g., how many species can coexist?

Going forward, I am using this analytical framework to pursue two directions. First, I intend to go beyond the equilibrium picture, characterizing the response of community to environmental perturbations. I would like to develop an information-theoretic perspective on the classical tradeoff between degree of adaptation (to a given environment) and phenotypic flexibility (in a changing world)

Second, I am investigating the evolutionary aspects, exploring a formal analogy whereby the ecological dynamics of a community can be seen as adaptive evolution of a single ("community-level") individual.

 

MT (2016). Community-level cohesion without cooperation. eLife 5:e15747.
MT and R Monasson (2017). A collective phase in resource competition in a highly diverse ecosystem. PRL 118, 048103.

Ecology without species?

Inspired by recent experimental work on reproducible features of ecological dynamics (“ecomodes”), I have proposed that an eigenmode-based dynamical description of microbial communities could serve as a naturally hierarchical alternative to the ill-defined “species”-based picture. Instead of clustering individuals into species, one can think of this as a more general operation: a change of basis.

Can this perspective be used to ask different questions, or analyze data differently? I am not yet sure. However, the language I propose can, for example, smoothly interpolate between a scenario with "two species" and a scenario with "three species", and so is capable of describing regimes where the number of species is ill-defined. I find this promising. This work is in review; you can find a preprint here.

Eigenmodes as a naturally hierarchical description

The regime 'without species'

 

MT (2016) Theoretical ecology without species. arXiv:1504.02550

Community-level competition experiments

Whole-community encounters are a frequent occurrence in nature ("coalescence events"). They are important medically (cf. the remarkable clinical success of whole-community fecal transplants in treating C. difficile infections) and conceptually (washing out the boundary between two fundamental concepts: competition and genetic recombination). Yet the classical program of investigating community resilience is once again focused on individual “invasive” species; community-level competition remains unstudied.

I am fortunate to have excellent collaborators with whom to embark on this enterprise. Check out Alvaro Sanchez's group at Yale, and Benjamin Wolfe's work on kombucha at Tufts. Experiments are up and running; stay tuned for updates!

Community-level competition experiments

Spatial structure and metabolism

Working with Michael Brenner and Yipei Guo (Harvard), I am studying the interplay between spatial structure of a bacterial colony and the efficiency of its metabolism. The relation between the two has been investigated extensively, for example in the context of intracellular enzyme packing. The novelty of our approach is to focus on the developmental aspect: a multi-species bacterial colony assembling in the absence of an organizing center. Updates coming soon!
Modeling a cross-feeding consortium in 3D