Shaken, not shifted: Genotypic variation tunes how interspecific competition shapes niches

Individual variation in resource use as well as in the response to competitors has been recognized as playing an important role is species interactions. Still, we have as yet little information on whether such responses have a genetic basis as well as on how they affect each other. Here, we tested whether 20 genetically inbred lines of the spider mite Tetranychus evansi vary in their response to a gradient of cadmium concentration within plants as well as in their propensity to reshape niches when facing interspecific competition along this gradient. In absence of interspecific competitors, most lines were negatively affected by cadmium, albeit often in a non-linear fashion. Morevoer, half of the lines exhibited changes in the curvature of the relationship between number of females and cadmium concentration when facing competition with the congeneric T. urticae. Inbred lines also showed a shallower decay in offspring number along the cadmium gradient in presence of interspecific competition. Our findings provide evidence for large, partly genetic, variation in resource use and in the response to interspecific competition in heterogeneous environments. Moreover, we show that genotype responses to interspecific competition is contingent upon their response to an environmental gradient. Together, our results thus emphasize the importance of considering intraspecific variation in responses to interspecific competition, providing novel insights to link intra- and interspecific levels of biodiversity.