*4.1. Climatic Gradients*

Climate overrides the effects of land use and human population density on shaping the functional diversity and species richness patterns of amphibians, reptiles, and mammals across Europe. The land–human model exhibited low explanatory power, but the land use variables and human population density slightly reinforced the explanatory power of the overall model in almost all cases (see the reptile Rao's quadratic entropy). Therefore, climate is the prevalent driver of functional diversity and species richness patterns [15,16], and although land use and human population density act at local scales [23,26], their imprint can be detected at broader scales [22].

Amphibian and reptile functional and taxonomic diversity varied along the climatic gradients, with different aspects of diversity within each taxon responding similarly to factors. The amphibian and reptile communities were species-poorer and functionally less diverse in Northern Europe, while reptile communities were species-richer and more diverse than amphibians in Southeastern Europe. The critical thermal minima of amphibians are lower than those of other ectothermic vertebrates, such as reptiles [44–46]. Therefore, the cooler temperatures of Northern Europe seem to exceed these thermal ranges and only a few species can survive there, which is reflected in their distribution [47] and functional diversity patterns [29]. In Southeastern Europe (e.g., along the coastline of the Mediterranean region), areas are characterized by high temperatures and moderate levels of precipitation, but higher seasonality. Amphibians and reptiles, as ectotherms, depend on the ambient temperature to thermoregulate, and other aspects of their physiology and behavior (e.g., reproduction) depend on the temperature and precipitation [48]. Amphibian distribution is associated more strongly with precipitation at the fine spatial scale [49]. Their body structure (i.e., the water permeability of their skin) is linked to thermoregulation, and the precipitation-dependent aspects of their ecology (e.g., most amphibians reproduce in the water) renders precipitation a crucial factor for their distribution [49,50]. Therefore, lower precipitation combined with higher temperatures (i.e., higher aridity) seems to restrict the number and range of the amphibian traits, resulting in higher trait similarity, and thus lower functional diversity [29]. In contrast, reptiles depend more strongly on temperature [49] and seem to be equipped with specific traits to cope well with high aridity [33].

Mammalian diversity patterns seem to tell a slightly different story. Their species richness varied along the climatic gradient, but mammalian functional diversity showed little variation and higher values compared to amphibians and reptiles across all of Europe. Some functionally homogeneous communities were detected scattered in Southern Europe but also in Northern Europe (particularly Norwegian coastal areas) and Ireland. The climatic and overall models performed less well for this taxon, implying that mammalian diversity is more weakly associated with climate compared to ectothermic taxa [49]. Furthermore, mammalian species richness is driven by environmental factors related to water–energy dynamics, for example, actual evapotranspiration and primary productivity in Europe [51] and globally [52], which were not included in the present study. On the other hand, functional diversity depends mostly on evolutionary time [52], and this perhaps is reflected in the lower predictive performance of our models. In temperate areas, the available time for niches to evolve, along with competition, energy availability, and adaptations to the environment, have resulted in functional divergence [19].

Seasonality was significantly related to the species richness and functional diversity of amphibians, reptiles, and mammals of Europe, confirming previous broad-scaled research on other taxa [14,16,19,29,53]. The species richness of ectothermic taxa had a unimodal relationship with temperature seasonality, while both species richness and functional diversity exhibited an approximately inverse unimodal relationship with precipitation seasonality. Amphibians are favored by low levels of precipitation seasonality that are observed in Central Europe, but also by high levels of precipitation seasonality that are observed in Southern Europe; however, in the latter region, temperature was a strong constraining

factor. Ochoa-Ochoa, Mejía-Domínguez, Velasco, Marske, and Rahbek [53] explored the amphibian functional diversity in America and reported a positive association between amphibian communities with precipitation and low precipitation seasonality, while temperature was not a significant driver. It is possible that precipitation seasonality acts together with temperature, resulting in functionally even amphibian communities in areas with low precipitation seasonality in Europe [29] and low to moderate temperatures. Low to moderate levels of temperature seasonality favored reptile species richness and functional diversity, while although the effect of precipitation seasonality was significant, reptile functional diversity varied little with it. Considering the positive strong effect of temperature on reptile communities and that temperature seasonality is higher at higher latitudes [54], climatic requirements were mirrored in the higher functional diversity in Southern Europe. At higher northern latitudes, reptile assemblages are more functionally constrained by seasonal changes, and the species pool consists of more similar traits [29]. Mammal species and functional richness increased with temperature seasonality and tended to decrease with precipitation seasonality, that is, Central Europe offers more suitable climatic conditions. Interestingly, the role of seasonality in mammals suggests either that species with a variety of trait values could co-occur regionally at higher latitudes [19] or that there is a potential discrimination between coastal and mountainous areas in Southern Europe associated with high temperature seasonality (dry/warm vs. cold/wet).
