Just what is the thermal niche?

The term 'thermal niche' is frequently used in ecology. Using the Web of Science database, I reviewed how the term has been used in current literature (2000-2015). While the use of 'thermal niche' has increased notably over the last 15 years, its meaning differs between studies. Only 10% of studies measured thermal niche directly as the range of body temperatures enabling non-zero population growth. Indirect thermal niche estimates were based on distributional data (47%), thermal reaction norms (36%) and preferred body temperatures (7%). Here, I present a new conceptual unification model for the term in which I propose that the choice of commonly used thermal physiology traits for indirect thermal niche estimates depends on behavioural ability to buffer variation in thermal environments during both activity and inactivity (diel or seasonal). The resulting body temperature distribution determines the relative importance of key thermal physiology traits, thermal tolerance, thermal preferenda and temperature dependence of metabolic rate. This framework provides a guide for using the term 'thermal niche' in ecological literature and identifies key areas for further research.