Alladi Ramakrishnan Hall

Local interactions, spatial patterns and ecosystem stability

Sumithra Sankaran

CES, IISc, Bangalore

Many ecosystems exhibit striking patterns in the spatial distribution of organisms, for example, patterns of clumping and dispersion in semi-arid vegetation, mussel in inter-tidal beds, and even marine sea-grass and macroalgae. Elucidating local-scale processes that generate these macroscopic patterns is of fundamental ecological importance. In addition, these patterns may provide insights and tools to quantify ecosystem stability and forecast future dynamics. We now know that several ecosystems may undergo abrupt and irreversible changes in the density of their dominant communities, potentially resulting in local extinctions. Discerning the vulnerability of ecosystems to such regime shifts has become an important focal area of research in recent times. However, our capacity to predict, as well as the accuracy of our predictions are constrained by a) how we quantitatively characterise these patterns b) our understanding (or lack thereof) of how local interactions between organisms scale up to their spatial distribution and ecosystem stability.



In this talk, I discuss spatial signatures of ecosystem stability, particularly those based on patterns of clustering by organisms. I use a spatially explicit model of regime shifts to investigate how local facilitative interactions in an ecosystem shape its response to stress. My main finding is that cluster size distribution, as a metric of ecosystem stability, can be misleading and is theoretically unfounded. Clustering is driven by local interactions and does not reflect changes in the ecosystem's dynamics. Clustering patterns can thus, instead, be used to infer local facilitative interactions operating on the landscape. I also demonstrate these theoretical results with vegetation data from semi-arid ecosystems.