Modelling stable multispecies assemblages

The continuous functioning of ecosystems despite fluctuating environmental conditions is mainly based on redundancies: the same ecosystem function is carried out by a variety of different species. Thus, even if a large number of species go extinct, it is highly probable that the ecosystem is still fully functional. This raises the question how a large number of very similar species can coexist without one superior species driving the others extinct. This aspect is particularly puzzling in homogeneous environments: For example in the upper ocean, photosynthesis is carried out by phytoplankton, of which all species require light and the same inorganic nutrients for growth. Recent modelling approaches have shown the difficulty of modelling sustained phytoplankton diversity. One important aspect in modelling multispecies assemblages is the concept of the one limiting factor: Assuming sufficient light, a species' growth rate is determined only by its most limiting nutrient. For coexistence to be possible, each species must be limited by a different nutrient, otherwise the best competitor will drive the others to extinction. This condition for the existence of a multispecies equilibrium is albeit not sufficient to allow for sustained diversity: For the given equilibrium to be stable, the amount of nutrients consumed is crucial: By consuming resources, any species removes them from the common pool and thus limits every species' growth rate. It is shown that if all species consume all nutrients in the same ratio, as is commonly assumed in plankton models, stable coexistence is not possible. If, however, each species consumes most of the resource by which it is limited, the multispecies equilibrium is stable even when subject to fluctuating nutrient supply; i.e. for stable coexistence to be possible, each species' consumption pattern needs to limit itself more than it limits others.

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