The climate variability hypothesis
According to the climate variability hypothesis, the species range size expands as the stability of climatic conditions decreases (Stevens, 1989, 1992). Many studies have found evidence to support the climate variability hypothesis (latitude: Letcher & Harvey, 1994; Köckemann, Buschmann, & Leuschner, 2009; Morin & Lechowicz, 2013; Pintor, Lin, & Krockenberger, 2015; elevation: Pintor, Lin, & Krockenberger, 2015; Chan et al., 2016; Liang et al., 2020). Studies argued that this positive relationship was probably caused by the physiological tolerance of species (Kevin J. Gaston & Spicer, 2001). Species with wider physiological tolerances are generally considered to have a wider niche breadth, so the species can adapt to more habitats and thus have a wide range of distribution (Brown, 1984).
Our result was inconsistent with the prediction of the climate variability hypothesis (the annual temperature range was negatively correlated with species mean elevational range size along the elevational gradient in Lebu Vally). There are many indicators to measure climate variability and using different indicators to access the climate variability hypothesis may lead to changes in the results. For example, seasonal temperature range was also considered as a good indicator of climate variability, and studies have shown that species range size could be positive correlated with seasonal temperature range (e.g., Köckemann, Buschmann, & Leuschner, 2009; Whitton, Purvis, Orme, & Olallatarraga, 2012; Chan et al., 2016; Liang et al., 2020). Hence, we correlated seasonal temperature range (additional to annual temperature range) with the species range size, and we also found that seasonal temperature range was negative correlated with species range size (standardized beta coefficient=-0.661, R ²=0.436, P <0.01), which was also inconsistent with the prediction of climate variability hypothesis.
Climate variability is closely related to many other environmental factors (e.g., productivity, habitat). The influence of climate variability on species range size may have different manifestations under different situations (Letcher & Harvey, 1994). For example, the impact of unfavorable climate could be mediated by high productivity or resources. Moreover, climate variability also influences species range size in an evolutionary time (Kevin J Gaston, 1996). However, studies have struggled to take all these factors into account. Therefore, whether the climate variability hypothesis is applicable to explain the mean range size of species pattern (particularly regarding its generality when considering elevation) remains further tested in different regions and taxa.