The connectance of real ecosystems

Abstract

In the work of Gardner and Ash by¹, and May² it has been shown that ecosystem stability (in the sense of Liapunov stability of equilibria³) imposes nontrivial constraints (summarised below) on ecosystem structure, but little has been done to relate this work to observations. One such study was undertaken by Rejm´nek and Starý⁴, who found in a collection of plant–aphid–parasitoid food webs that connectance C (roughly speaking, the fraction of pairs of species that directly interact) decreases like S⁻¹ as species richness S (the total number of species) increases. Here I analyse a sample of community food webs (the webs of ref. 4 are sink webs⁵), and find a somewhat slower decrease of connectance with increasing species richness. This behaviour suggests, within the context of May's theory², that complex ecosystems will tend either to be more fragile or to have weaker interspecific interactions than simpler ones.