Standardised measures of TFLs may help to predict species at risk under climate change.

To understand these risks, we need a cross-species measure of thermal fertility loss.

Rising global temperatures are threatening biodiversity. Studies on the impact of temperature on natural populations usually use lethal or viability thresholds, termed the ‘critical thermal limit’ (CTL). However, this overlooks important sublethal impacts of temperature that could affect species’ persistence. Here we discuss a critical but overlooked trait: fertility, which can deteriorate at temperatures less severe than an organism’s lethal limit. We argue that studies examining the ecological and evolutionary impacts of climate change should consider the ‘thermal fertility limit’ (TFL) of species; we propose that a framework for the design of TFL studies across taxa be developed. Given the importance of fertility for population persistence, understanding how climate change affects TFLs is vital for the assessment of future biodiversity impacts.

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Glossary

1 Overgaard J.

et al. Validity of thermal ramping assays used to assess thermal tolerance in arthropods. a suite of commonly used measures of the maximum and minimum temperatures at which organisms can viably function. Individuals are exposed to either static stressful temperatures or gradually ramping temperatures and observed for physiological failure (e.g., uncoordinated movement, heat coma, death []). Typically, either the duration of exposure or the temperature at which loss of viability is observed is recorded as the thermal limit.

the total number of offspring an individual can produce across a set interval or lifetime.

the ability of an organism to produce viable offspring. Fertility can be measured in a number of ways but always reaches its lower limit when conditions prevent an individual from producing any offspring (i.e., sterility).

2 Loeschcke V.

Hoffmann A.A. Consequences of heat hardening on a field fitness component in Drosophila depend on environmental temperature. increased thermal tolerance shown by organisms after a short period of exposure to a stressful but nonlethal temperature within the same life stage. Hardening tests are one component of a species’ plastic response when exposed to stressful temperatures [].

describes an individual that cannot produce any offspring over a defined period and thus is synonymous with complete infertility.