Environmental correlates of food chain length

Ecosystem flow networks: loaded dice?

An information-theoretic comparison of the topologies of observed ecosystem transfers and randomly constructed networks reveals that it is not easy to separate the members of the two sets. The distribution of ecosystem flow magnitudes, however, is seen to differ markedly from ordinary probability functions and to resemble the Cauchy or Pareto distributions. The agencies that impart such structure to ecological flow networks are not obvious, but one strong possibility is that autocatalysis, or indirect mutualism, promotes certain pathways at the expense of others, thereby enlarging the tail of the distribution of flow magnitudes.

Scale invariance in food web properties

The robustness of five common food web properties is examined by varying the resolution of the data through aggregation of trophic groupings. A surprising constancy in each of these properties is revealed as webs are collapsed down to approximately half their original size. This analysis of 60 invertebrate-dominated community food webs confirms the existence of all but one of these properties in such webs and addresses a common concern held by critics of food web theory that observed food web properties may be sensitive to trophic aggregation. The food web statistics (chain length; predator/prey ratio; fraction of top, intermediate, and bottom species; and rigid circuits) are scaling in the sense that they remain roughly invariant over a wide range of data resolution. As such, within present standards of reporting food web data, these statistics may be used to compare systems whose trophic data are resolved differently within a factor of 2.

The stability of ecosystems

Stability criteria and phase boundaries for complex ecosystems are obtained and contrasted with previously studied scenarios. The stability of such systems is determined by the behaviour of the largest eigenvalue of matrices governing the response of the system to small perturbations. As a result we show that ecosystems with unstructured cooperative interactions between arbitary species can be less stable than had been previously determined. We also examine hierarchical ecologies, and demonstrate their increased stability under certain conditions.