Global average temperature, frequency of temperature extremes and amplitude of daily fluctuations are all expected to change under prospected scenarios of climate change. The amplitudes of temperature variation and the ability of species to tolerate thermal stress are crucial for species fitness and survival in rapidly changing environments. Small differences in the life histories of species or in their responses to environmental variation can differentiate their niche and support coexistence; hence, even small changes in environmental conditions may not only affect the fitness of individual species, but also the outcome of species interactions. In the present experiments, the free-living nematode species Halomonhystera disjuncta is used as a model organism to investigate physiological and behavioural responses to environmental change in an ecological context of species interactions. We examine population fitness and food-taxis behaviour of H. disjuncta under different temperature regimes in the presence and absence of a competing species. Both stressors – temperature and competition – affected life-history traits and population development of the model species in different ways; the nematode populations exhibited higher fitness under a daily fluctuating temperature regime, but a lower fitness under an increased constant temperature. The presence of a competing species had a negative effect on its population fitness and food-finding behaviour at all temperature regimes, yet the most pronounced effects of competition were observed at the highest temperatures, suggesting that interspecific interactions such as competition may weaken a species’ intrinsic tolerance to environmental change. Thermal stress thus not only affects species fitness but also species interactions, which in turn will likely have repercussions for species coexistence.
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