Climate variability and aridity modulate the role of leaf shelters for arthropods: A global experiment.

Current climate change is disrupting biotic interactions and eroding biodiversity worldwide. However, species sensitive to aridity, high temperatures, and climate variability might find shelter in microclimatic refuges, such as leaf rolls built by arthropods. To explore how the importance of leaf shelters for terrestrial arthropods changes with latitude, elevation, and climate, we conducted a distributed experiment comparing arthropods in leaf rolls versus control leaves across 52 sites along an 11,790 km latitudinal gradient. We then probed the impact of short- versus long-term climatic impacts on roll use, by comparing the relative impact of conditions during the experiment versus average, baseline conditions at the site. Leaf shelters supported larger organisms and higher arthropod biomass and species diversity than non-rolled control leaves. However, the magnitude of the leaf rolls' effect differed between long- and short-term climate conditions, metrics (species richness, biomass, and body size), and trophic groups (predators vs. herbivores). The effect of leaf rolls on predator richness was influenced only by baseline climate, increasing in magnitude in regions experiencing increased long-term aridity, regardless of latitude, elevation, and weather during the experiment. This suggests that shelter use by predators may be innate, and thus, driven by natural selection. In contrast, the effect of leaf rolls on predator biomass and predator body size decreased with increasing temperature, and increased with increasing precipitation, respectively, during the experiment. The magnitude of shelter usage by herbivores increased with the abundance of predators and decreased with increasing temperature during the experiment. Taken together, these results highlight that leaf roll use may have both proximal and ultimate causes. Projected increases in climate variability and aridity are, therefore, likely to increase the importance of biotic refugia in mitigating the effects of climate change on species persistence.

Allochthonous aquatic subsidies alleviate predation pressure in terrestrial ecosystems.

The input of external energy and matter in recipient ecosystems can act as a bottom-up force that subsidizes consumers, with subsequent cascading effects throughout the food web. Depending on the amount of input, dietary preference, and the strength of trophic links, allochthonous resources generally play a stabilizing role on food webs. In this study, we investigated the stabilizing role of allochthonous aquatic resources on intraguild predation (IGP) and their consequences on shared prey in a terrestrial ecosystem. To this end, we manipulated the input of emergent aquatic insects (the allochthonous resources) from streams to land, and predation pressure by bats and birds (the top predators), in a multitrophic food web using an orthogonal exclusion experiment. Using stable isotope metrics, we found that bats, birds, and spiders (the mesopredators), were highly subsidized by emergent aquatic insects. Moreover, among terrestrial prey, top predators fed more on spiders than insects. As predicted, spiders were strongly affected by the presence of top predators when allochthonous resources were excluded. Consequently, in this scenario terrestrial insects were two times more abundant. Because spiders showed a higher preference for consuming aquatic resources, we suggest that nonconsumptive effects of spiders upon terrestrial insects could be mediating the strong response of those shared prey. We demonstrate that the input of allochthonous aquatic resources can play a fundamental role in stabilizing terrestrial trophic interactions and trophic cascades in riparian zones via decreasing predation pressure.