Prediction and test of the effects of interspecific competition

Abstract

Although interspecific competition is of major importance in ecological and evolutionary theory¹, there has been no critical study of whether indirect estimates of niche overlap accurately reflect the effects of competition on fitness as indicated by reproductive output. I report here that competition from jackdaws, Corvus monedula, significantly reduced reproductive output of magpies, Pica pica, due to starvation of nestlings and a high rate of nest predation. The competition coefficient as estimated from these direct effects (α = 0.43) was only slightly smaller than predicted by niche overlap values based on morphological similarity, habitat distribution and feeding technique (α = 0.58), or food composition (α = 0.54).