There is ongoing debate about the vulnerability of arthropods to climate change1,2. Long-term impacts of climate change on arthropod communities could manifest through short-term weather patterns3. Arthropods in the tropics are hyper-diverse4,5 and contribute many crucial ecosystem functions6,7, but are comparatively less studied than in temperate regions1,8,9. Tropical forest arthropods and the functions that they provide may be vulnerable to intensified El Niño events under climate change10–12. Here we perform time-series analysis of data from primary tropical forests, which reveal long-term declines in arthropod diversity and function that were linked to El Niño occurrence. In the Americas, species losses correlated with El Niño sensitivity, and abundant species fluctuated according to feeding traits and level of ecological specialization. Parallel declines in butterflies in Southeast Asia suggested that impacts spanned continents. Predicted arthropod diversity changes correlated with observed rates of invertebrate-mediated decomposition and leaf herbivory, which were oscillating and crashing, respectively, across the tropics. Our analyses suggest that an intensified El Niño immediately threatens tropical forest arthropods and the ecosystem functions that they provide. The broader consequences remain unknown, but such widespread changes could fundamentally alter tropical forest ecosystems13. Long-term monitoring of arthropod diversity and forest functioning across the tropics is paramount, as is researching the potential mechanisms that underly this novel threat.
