Short-term heat pulses as demographic bottlenecks for Amynthas earthworms: temperature–time mortality across life stages

Increasing summer temperatures and episodic heat waves may strongly influence the survival, activity, and seasonal dynamics of near-surface soil fauna, yet thermal mortality thresholds across life stages remain poorly resolved for many earthworms, including Amynthas species introduced to North America. We quantified mortality responses of adults and embryos of Amynthas agrestis and A. tokioensis across factorial combinations of temperature (35–43°C) and exposure duration (20–300 min) to estimate the exposure times required to reach ≥99% mortality (LT₉₉). Cocoons (n = 2,040) and adults (n = 186) were exposed to short-term heat pulses in soil and submersion conditions. Mortality remained low at 35°C but increased steeply at ≥38°C, with ≥99% mortality occurring within 40–80 min at 40–43°C. Adults were consistently less heat tolerant than embryos within cocoons: at 35°C about half of adults died within ∼160 min while embryos persisted beyond 200 min, and at 40°C adults reached complete mortality within ∼80 min whereas embryos persisted for ∼90 min. Substrate effects were life‑stage specific, as submersion significantly increased adult mortality. These findings suggest that episodic summer heat pulses may periodically exceed thermal safety margins in epi-endogeic Amynthas populations, particularly for cocoons, and could contribute to localized mortality or demographic bottlenecks during extreme warming events. By resolving how mortality scales jointly with temperature and exposure time across life stages and substrates, this study highlights how short-term thermal extremes can shape survival, phenology, and local persistence of soil-dwelling organisms under increasingly variable climates.