Deconstructing complexity: Towards a theory of whole food webs

Background/Question/Methods

The growing realization of a looming biodiversity crisis has inspired considerable recent progress in the quest to link biodiversity, structure and ecosystem function. Nonetheless, simple low-species food web theory often stands in contrast to complex high-species food web theory on the role diversity and structure play in mediating the stability of nature’s complex networks. Of considerable concern is the recent discovery that species-rich predator-prey networks seem to be stabilized by increasing interaction strength, a result that overturns 30 years of ecological research. The reasons for this disconnect remain elusive.

Results/Conclusions

We construct a simple method that bridges low and high species approaches to food web theory by elucidating the connection between the stability of the basic building blocks of food webs (i.e., the consumer-resource interaction) and the mean stability properties of large random food web networks. We show how simple biological constraints removes the contradictory results and explains when we expect one stability result versus another – independently of the number of species.

Applying this theoretical framework to common food web models reveals two key findings. First, that in almost all cases, large species-rich, random food web models yield a stability tradeoff between weak and strong interactions compatible in every way to simpler consumer resource models. And second, the models that generate the more recent, contradictory, phenomena of total stabilization correspond to biologically implausible configurations of perfect consumer consumption efficiency. Importantly, when time series data from microcosms, aquatic and terrestrial ecosystems are analyzed I found the predicted trade-off between interaction strength and stability, where systems with stronger interactions are more variable. This empirical finding implies, as our theory predicts, increasing interactions strengths will tend destabilize systems no matter the size.