What drives global patterns in the strength of species interactions? A distributed experiment from the Arctic to Patagonia

Background/Question/Methods

A long-standing hypothesis predicts that species interactions become more intense toward low latitudes and elevations. Large-scale experiments have recently supported this prediction for predation, a universal interaction that greatly affects community structure and evolution. However, it remains unclear a) which features of the environment cause geographic patterns in predation strength and b) at what scale patterns are consistent across trophic levels, limiting our ability to explain large-scale patterns and predict interaction strength at finer scales. Stronger predation could be caused by a direct metabolic effect of warm temperatures on predators, notably invertebrates, or by a greater density of predators via a greater number of habitat niches and/or predator diversity. Here we quantify the intensity of predation on seeds (14,000 depots over 6 years along 31 mountains) and clay caterpillars ( >1700 caterpillars over 4 years along 18 mountains) from Alaska to Argentina. First, we assess whether seed predation rates are best explained by daily or monthly climate (which could influence feeding rates per predator), or by annual climate, productivity (NPP), or animal biodiversity (which could influence predator density over evolutionary timescales). Second, we test whether predation rates on seeds and caterpillars are positively correlated across sites and dates.

Results/Conclusions

Results/Conclusion: Predation on seeds was significantly stronger towards low latitudes and elevations, as predicted and as found in an earlier study, but predation on clay caterpillars did not vary geographically, in contrast to previous results. The intensity of seed predation was best predicted by mean annual temperature, with predation increasing by 25% for every 10°C increase. The intensity of caterpillar predation did not covary with the intensity of seed predation across sites and dates. These findings suggest that temperature is driving large-scale patterns in species interactions through its effect on predators, especially invertebrates. Our results improve our understanding of patterns in the ecological and evolutionary importance of species interactions.