Climate change is altering the latitudinal and elevational range limits of species, often driving poleward and upward range shifts, causing species that normally do not interact to co-occur. Concurrent with climate change, additional stressors such as the addition of non-native predators are simultaneously structuring communities. However, to date tests of novel community interactions have not been designed to test how trophic interactions modify the assembly of novel communities. Here, we used an aquatic mesocosm experiment in Yosemite National Park with zooplankton (copepods, cladocerans) collected from small shallow lakes across an elevational gradient. We simulated climate change migration scenarios by moving communities higher or lower in elevation and invading recipient communities with novel lower elevation communities; all treatments were then crossed with the presence or absence of juvenile trout. We used growth rates, biomass, turnover and productivity to determine the direct effect of temperature and predation on the persistence of unique focal species that encompass taxonomic and functional differences, and how shifts in the composition of zooplankton communities alter ecosystem functioning.
Results/Conclusions
The effect of novel competitors on the initial growth rates of zooplankton depended on whether the species is tracking climate change and whether fish are present or not (significant third order interaction; P < 0.05). For example, Bosmina longirostris populations only persisted when they invaded a novel community and co-occurred with fish, because fish preferentially predated on the larger high elevation zooplankton (e.g., Daphnia melanica), which allowed the smaller competitively inferior Bosmina species to establish. Other species such as Eucyclops agilis could establish in novel competitive environments when fish were absent, but the presence of fish doubled growth rates. In contrast, Ceriodaphnia laticaudata could only establish in novel high elevation sites when fish were absent. The shift to smaller bodied species in the presence of fish not only facilitated the invasion of novel species, fish predation also decreased herbivore biomass leading to a trophic cascade that increased primary productivity. In sum, our preliminary results suggest that, at least during the establishment phase of a novel species introduction, non-native predators can alter the success of range shifting species and that this change in the identity of invaders has consequences for diversity and ecosystem functioning.