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 to 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 δ13C and δ15N 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.