Temperature-dependent competition hierarchy: a mechanism stabilizing the phenological strategy in the scorpionfly Panorpa communis L.

Previous studies of the phenologies and the different microclimatic patterns of the distribution of the scorpionflies Panorpa communis L. and Panorpa vulgaris Imhoff and Labram 1836 showed different phenological strategies. In P. communis , a species foraging at shadowy and cool places only, a majority of 90% of the individuals are univoltine; however, approximately 10% of the offspring of the first annual generation are bivoltine. This proportion remained unchanged in the Freiburg population over 8 years. Differently, all individuals of P. vulgaris foraging equally frequent at sunny and warm as at shadowy and cool places are bivoltine. The proximate cause of bivoltinism in both species is a heritable variation of different ‘day length thresholds’ triggering diapause‐free development if natural day length exceeds these thresholds. As selection favours maximal temporal exploitation of food availability it remains obscure why in P. communis the number of diapause‐free developing individuals does not increase continuously from year to year although this phenotype reproduces twice a year. Therefore, in the present paper, we focus on the following main questions. Does the competitive inferiority of P. communis in the presence of P. vulgaris at the temperature regime of the late summer function as a mechanism maintaining the majority of individuals of P. communis univoltine, by dramatically reducing the fitness of the bivoltine ones? As a long‐term evolutionary change in the frequency of bivoltine individuals in P. communis solely depends on the lifetime reproductive success of the females, we here consider the influence of interspecific competition and temperature conditions on the reproductive success of the females of P. communis only. Five lines of evidence suggest that the mechanism of maintaining univoltinism in P. communis is primarily because of differences in the ability of each species to exploit dead arthropod resources: (1) these species show complete diet overlap; (2) dead arthropods are limiting resources for both species of scorpionflies as indicated by positive demographic effects with increased food availability; (3) in competition with P. vulgaris at high temperatures, P. communis is competitively inferior in the ability to detect and exploit dead arthropods; (4) this reduced resource acquisition of female P. communis translates into significant reductions in the survivorship, body condition, fecundity and lifetime reproductive success; (5) exploitation competition does account for these negative demographic effects on second‐generation females of P. communis more than interference competition.