The project aims at understanding the consequences of (a) individual variation for traits associated to competition and to the interaction between mites and plants and (b) evolving with competitors, for trait evolution and the probability of coexistence. In particular, we investigate the consequences of niche construction in spider mites, as one species is able to suppress plant defences, thus making resources available to themselves but also to competitors. So far, we found large variation for the ability of spider mites to suppress plant defences, but less so for their ability to benefit from this suppression. These traits were found not to be correlated, which raises important questions concerning the maintenance of such niche construction.
We also found large variation for intra and interspecific competitive ability, but again no correlation among traits. We are currently investigating whether specific combinations of these traits facilitate coexistence among these competitors, based on theoretical predictions.
Moreover, we found that tomato plants accumulate cadmium and that the response of the two species to this accumulation varies in shape and intensity and is altered by the presence of interspecific competitors. However, the change induced by the presence of the competitor is not in niche location or width, as expected by classical theory, but in the shape of the response.
Furthermore, we performed experimental evolution of spider mites on tomato plants with or without cadmium and with or without competitors, then tested how this evolutionary history affected the probability of coexistence in environments with and/or without cadmium. We found effects of the evolutionary history in some, but not all, cases, and a strong dependence of this effect upon the environment tested. These findings have important consequences for the effect of evolution in species interactions.
Finally, we performed experimental evolution of spider mites on tomato plants in which defences can be induced or suppressed or in plants where defences cannot be altered by spider mites, in presence or absence of competitors. We found that T. evansi, the suppressor, stopped suppressing when (a) evolving on plants where suppression has no effect and (b) evolving on plants with competitors. This suggests that suppression has ecological and physiological costs, a key finding for the understanding of herbivore-plant interactions.
