Experimental evidence for niche segregation in a sister species pair of non-biting midges

The principle of limiting similarity states that closely related species need to partition resources of the habitat in order to coexist in the same general area. We tested this hypothesis experimentally with a sister species pair of non-biting midges (Chironomus riparius and C. piger) by assessing their relative larval fitness under several concentrations of nitrite and temperature regimes, as suggested by the observed habitat segregation in a previous field study. Both chironomid species often occur in eutrophic habitats like agricultural areas or industrial point source effluents. Based on field observations, we hypothesised C. piger to tolerate higher nitrite concentrations, higher temperatures and larger temperature ranges than C. riparius. As predicted, C. piger coped better with higher nitrite concentrations. Against the expectations, C. riparius had a tendentially higher fitness at both higher constant temperatures and larger daily temperature ranges. However, the interaction of both stressors favoured C. piger in warm high-nitrite habitats thus concurring to the field observations. The complex interaction of candidate environmental factors with antagonistic effects found here emphasises thus the necessity to experimentally assess field observations of niche segregation.