Allochthonous chemical cues drive predation by a top carnivore

Abstract Identifying the mechanisms by which mobile predators detect and select prey remains a central challenge in sensory biology and functional ecology. This study provides the first direct evidence that chemical cues associated with allochthonous organic matter (e.g. leaf litter) can restructure chemosensory environments and fundamentally alter consumer‐resource dynamics. Using a combination of remote behavioural observations, field experiments with free‐ranging animals and predator inclusion/exclusion cage treatments, we examined how external organic inputs indirectly modify trophic interactions within a residential stream community. As top predators, newts ( Taricha torosa ) exhibited a strong chemical attraction to patches containing terrestrially derived plant material, whereas natural substrates composed of rock cobble were used at disproportionately lower rates than predicted by availability. In fact, upon detecting waterborne cues from leaf litter, individuals abandoned cobble and moved upstream to colonize chemically enriched areas. Predator attraction was driven by compounds leaching from allochthonous plant matter, not by cues from native insect larval prey. Even when prey densities were standardized experimentally across habitat types, predation rates by newts were significantly higher in leaf litter than in other substrates. Such chemically mediated shifts in behaviour suggest a general mechanism by which spatial subsidies of organic matter can restructure predator–prey dynamics. By redirecting top carnivore attention and effort, infochemicals derived from non‐native sources may play a fundamental role in shaping community organization and regulating energy flow across ecosystem boundaries. Read the free Plain Language Summary for this article on the Journal blog.