Do we need individual organisms in ecosystem models?

Background/Question/Methods

The ecosystem perspective is central to our mission of understanding and modeling how humans are impacting the earth system.  The perspective has a long history and has developed through efforts to simplify complex, real-world ecosystems into abstractions that are conceptually and analytically tractable.  Such abstraction relies upon establishing “functional groups” or “functional units” based on trophic divisions (producers and consumers within food webs), biogeochemical divisions (organisms that catalyze a particular nutrient transformation), or chemical-thermodynamic divisions (organisms that can utilize specific electron donors and acceptors).  More recent efforts have sought to resolve the specific influence of species by representing “diversity” as an explicit metric - either by applying a statistical-mechanics approach of analyzing large numbers of species in communities, or by combining the ideas of diversity and functional groups.

I will here ask a question that naturally bridges ecosystem biology, population ecology, and evolution: “Do individual organisms matter in ecosystems, and, if so, how should we represent them?”  I will take a different tact than traditional approaches by anchoring my analysis in experimental observations of organismal strategies and organism-ecosystem feedbacks.  My analysis relies on findings from ecosystems and environments worldwide (many collected by members of my laboratory group), combined with  analytical models. 

Results/Conclusions

My analysis considers organismal traits and strategies that are expressed at the level of individuals, that interact with local environments in direct competition with other species, that evolve through Darwinian selection, and that ultimately influence emergent dynamics at the ecosystem scale.  I show that two classes of strategies – accretive and dilutive – are particluarly strong candidates for determining emergent properties at the ecosystem level, including net carbon exchange, nutrient limitation, and organismal stoichiometry.  I will summarize our present knowledge about nitrogen fixers and establish conditions under which an accretive strategy can emerge at the ecosystem scale.  The results imply that spatial and temporal dynamics at the ecosystem level are causally determined by the strategies and traits deployed by individual organisms in local environments.  A very different result emerged when I considered ecosystems influenced by dilutive strategies.  While such strategies can profoundly affect emergent patterns and dynamics, my analysis raised fundamental questions about how these strategies may evolve and persist at the ecosystem scale.  I will conclude by briefly considering the urgent need to bring evolution into this analysis at two scales: i) selection on traits/strategies in individuals, and ii) selection of emergent feedbacks at the ecosystem scale.