Species abundance and asymmetric interaction strength in ecological networks

Abstract

The strength of interactions among species in a network tends to be highly asymmetric. We evaluate thehypothesis that this asymmetry results from the distribution of abundance among species, so that speciesinteractions occur randomly among individuals. We used a database on mutualistic and antagonistic bipartitequantitative interaction networks. We show that across all types of networks asymmetry was correlated withabundance, so that rare species were asymmetrically affected by their abundant partners, while pairs ofinteracting abundant species tended to exhibit more symmetric, reciprocally strong effects. A null model showsthat abundance provides a sufficient explanation of the asymmetry structure in some networks, but suggests therole of additional factors in others. Although not universal, our hypothesis holds for a substantial fraction ofnetworks analyzed here, and should be considered as a null model in all studies aimed at evaluating theecological and evolutionary consequences of species interactions.