Dynamics of a complex community with and without dynamic population stage structure. (Caption below)*

A new study by ecologist and SFI External Professor André de Roos** shows that differences between juveniles and adults of the same species are crucial for the stability of complex ecological communities. The research, published in Proceedings of the National Academy of Sciences, represents a major advance in ecological modeling at a time when biodiversity is declining and species around the world are rapidly going extinct.

Up to now, ecological models have focused exclusively on the interactions between species, ignoring the variations within them. The dragonflies, frogs, trout, and phytoplankton in a freshwater pond, for example, would be represented as nodes in a network, connected by edges that represent how each species feeds on the others. 

Using computer simulations, de Roos was able to model both the number of total species in a community and key variations within the same species, accounting for differences between juveniles and adults. He says these differences arise not only because adults reproduce, while juveniles grow and mature, but also because juveniles, being of smaller body size, are generally more limited by food availability than adults and run a much greater risk of being captured by predators. These differences lead to variations over time in the ratio of juveniles to adults, which tip the entire community from instability to stability.

“On the basis of the network of species interactions alone the simulated communities are predicted to be wildly unstable,” he writes, “but these destabilizing forces are swamped and fully attenuated by the stabilizing effects of the dynamics of juvenile and adult densities.”

The study advances a long-running debate in ecology over the relationship between species diversity and ecosystem stability. (A debate which, de Roos notes, was fueled by a 1972 paper by the late SFI Science Board member Robert May).  

“Ecologists have long intuited that diversity can stabilize ecosystems, and even protect them against collapse,” de Roos says. “Theoretically, however, we struggled with a good explanation for that idea. What we’re now discovering through computation and quantitative analysis is that our existing theories fall short, because diversity is more than just a number of species connected by a network of interactions. The interplay between different types of complexity is what determines the function of the system.”

Read the paper, “Dynamic population stage structure due to juvenile–adult asymmetry stabilizes complex ecological communities,” in PNAS (May 25, 2021)

*Figure 1 – Dynamics of a complex community with and without dynamic population stage structure. The top panel at the right illustrates for the community shown at the left that simulations, which only track the total abundance of all species and assume a constant ratio between juvenile and adult density, quickly give rise to wild fluctuations and extinction of all but a handful of species, even though the community starts off in an equilibrium state. In contrast, the bottom panel at the right illustrates that simulations with dynamic changes in the ratio between juvenile and adult density of all species predict this same community to be resilient against perturbations as it quickly returns to its equilibrium state even after a sudden decrease in the densities of all species in the community by 50%. (André de Roos, PNAS, 2021)

** De Roos is Professor of Theoretical Ecology at the University of Amsterdam’s Institute for Biodiversity and Ecosystem Dynamics; the King Carl XVI Gustaf Professor in Environmental Science for 2021-22 at Umeå University; and External Professor at the Santa Fe Institute. He performed much of the analysis for this paper during his three-month visit to SFI in 2019.