Warming threatens aquatic–terrestrial linkages: evidence from tropical geothermal streams

Terrestrial and aquatic ecosystems are connected through energy fluxes partially driven by aquatic organisms that migrate to terrestrial environments during adulthood. According to the thermal performance curve (TPC) framework the physiological performance of organisms is limited at low temperatures, which can affect the body mass of freshwater insects, stoichiometric imbalances, abundances, and emergence frequency to riparian systems. As temperatures approach the optimal point on the TPC, energy fluxes to terrestrial ecosystems are expected to accelerate. However, in warmer environments, where temperatures exceed the optimal point, organism performance can be severely compromised, restricting the occurrence of aquatic species and potentially leading to a collapse of energy fluxes between ecosystems. Therefore, we predict that terrestrial predators that rely on aquatic subsidies (e.g. spiders) will benefit by living near water sources with intermediate temperatures, where aquatic insect emergence is expected to be highest. We tested these predictions by using natural experiments in several geographically independent tropical geothermal streams, where we surveyed aquatic and riparian terrestrial arthropods upon plants, along contrasting levels of water temperatures (range 25.9–41.9°C). Structural equation modeling detected a direct, hump‐shaped impact of temperature on the abundance of aquatic insects and an indirect effect on spiders' abundance and richness, which declined in response to the reduction of aquatic insects. This result is substantiated by δ 13 C and δ 15 N isotopic analyses, which detected a higher consumption of aquatic insects in locations with temperatures below 34°C. These results highlight that an increase in temperature can disrupt the trophic dynamics between aquatic and terrestrial ecosystems. Furthermore, our findings reinforce the role of geothermal systems as natural laboratories for studying the effects of warming on trophic interactions, providing novel insights into how temperature influences cross‐ecosystem energy transfer and predator–prey dynamics in interconnected aquatic and terrestrial food webs.