Predator mass mortality events restructure food webs through trophic decoupling.

Predators have a key role in structuring ecosystems 1-4 . However, predator loss is accelerating globally 4-6 , and predator mass-mortality events 7 (MMEs)-rapid large-scale die-offs-are now emblematic of the Anthropocene epoch 6 . Owing to their rare and unpredictable nature 7 , we lack an understanding of how MMEs immediately impact ecosystems. Past predator-removal studies 2,3 may be insufficient to understand the ecological consequences of MMEs because, in nature, dead predators decompose in situ and generate a resource pulse 8 , which could alter ensuing ecosystem dynamics by temporarily enhancing productivity. Here we experimentally induce MMEs in tritrophic, freshwater lake food webs and report ecological dynamics that are distinct from predator losses 2,3 or resource pulses 9 alone, but that can be predicted from theory 8 . MMEs led to the proliferation of diverse consumer and producer communities resulting from weakened top-down predator control 1-3 and stronger bottom-up effects through predator decomposition 8 . In contrast to predator removals alone, enhanced primary production after MMEs dampened the consumer community response. As a consequence, MMEs generated biomass dynamics that were most similar to those of undisturbed systems, indicating that they may be cryptic disturbances in nature. These biomass dynamics led to trophic decoupling, whereby the indirect beneficial effects of predators on primary producers are lost and later materialize as direct bottom-up effects that stimulate primary production amid intensified herbivory. These results reveal ecological signatures of MMEs and demonstrate the feasibility of forecasting novel ecological dynamics arising with intensifying global change.