A new scientific study shows that differences between life stages within populations —a dimension often overlooked in theoretical ecology— promote ecosystem stability and help explain why some highly complex systems remain surprisingly stable. The research, carried out by a team from the University of Oxford, IFISC (CSIC-UIB) and CEAB-CSIC, presents a new mathematical model that incorporates age structure and reveals that not all interactions between life stages have the same effect: some asymmetric relationships —such as adults preying on juveniles of other species— tend to stabilise communities, whereas others —such as direct competition between juveniles and adults for the same resources— can destabilise them.
The study, published in the scientific journal Ecology Letters, offers a fresh perspective on the long-standing debate between complexity and stability. Understanding why diverse ecosystems —with many species and interactions— can maintain a degree of stability is one of ecology’s major challenges. Since the classic works of the 1970s, ecological theory has predicted that the more complex an ecosystem is, the more unstable it should be. Yet natural systems do not always follow this pattern: forests, coral reefs and other communities with hundreds of interacting species often remain functional and “stable” for decades.
The authors of the study suggest that part of the answer may lie within populations themselves: not only in the structure of the interaction network between species —already known to increase resilience— but also in each species’ internal structure. Not all individuals are the same, and this heterogeneity —in age, size or developmental stage— may be advantageous for maintaining collective stability. Differences in how individuals use resources, avoid predators or compete with other species generate a kind of “internal buffer” that most ecological models have so far overlooked.
“We know that the structure of ecological networks is key to understanding why some ecosystems are surprisingly stable, but the role of different life stages has historically been ignored in most ecological models. Our study shows that this level of organisation is essential to understanding how biodiversity is maintained,” explains Rob Salguero-Gómez, Professor at Ecology at the University of Oxford and author of the study.
A new mathematical model to incorporate population structure
To assess the effect of population structure on ecosystem stability, the team developed a general mathematical model, the Structured Community Matrix, which expands the classical framework used to analyse the stability of ecological communities.
This new theoretical approach makes it possible to consider not only interactions between species but also the internal relationships within each population, that is, differences between life stages such as juveniles and adults.
The model is flexible enough to incorporate any number of species and life stages, and can be applied to food webs as well as plant or animal communities.
According to the authors, this tool opens the door to redefining ecological resilience and persistence:
“Populations are not uniform blocks. When we incorporate structure within each species, we see that the system has more compensatory mechanisms and can respond better to disturbances. This variable has not been usually considered by established approaches in the discipline, but now, with this research, we propose a model that does include it,” says Àlex Giménez-Romero, CEAB-CSIC researcher and first author of the study.
The researchers applied the model to 33 real food webs from ecosystems around the world, from lakes to marine systems. In all cases, simulations show that introducing differentiation between life stages tends to stabilise systems that, according to traditional models, would be expected to collapse. This strengthens the idea that interaction mechanisms between different life stages —such as adults preying on juveniles of other species— may help maintain biodiversity in the long term.
This research repositions population structure as a central element in understanding ecosystem dynamics, previously regarded as a secondary detail.
According to Meritxell Genovart, researcher at CEAB-CSIC and co-author of the study, “The new model highlights that assessing ecosystem stability and resilience requires taking into account population structure and that interspecific interactions may vary across the life stages of the organisms”.
The authors note that the next step is to incorporate other sources of heterogeneity into the model, such as density dependence and spatial distribution of resources. This new approach could improve ecological predictions, such as those relating to the impacts of global change.
