**1. Introduction**

The multiple processes and factors acting simultaneously and shaping biodiversity patterns at different scales constitute a long-standing puzzle for ecologists and biogeographers [1,2]. Taxonomic diversity has been extensively used to unravel underlying mechanisms that structure communities and drive diversity patterns across scales [3]. However, other aspects of biodiversity such as functional diversity reflecting species' functional traits within communities and ecosystems [4] might provide a more detailed and integrated interpretation of diversity patterns and species composition [5,6]. Functional traits can be defined as the main dimensions of the real ecological niche [7], mediating species responses to environmental conditions and reflecting the way organisms respond to environmental variation (e.g., habitat or climate preference) [8]. Functional diversity is quantified by arranging species in a functional trait space according to their functional trait values [9]. This representation allows us to measure various aspects of functional diversity such as

**Citation:** Tsianou, M.A.; Lazarina, M.; Michailidou, D.-E.; Andrikou-Charitidou, A.; Sgardelis, S.P.; Kallimanis, A.S. The Effect of Climate and Human Pressures on Functional Diversity and Species Richness Patterns of Amphibians, Reptiles and Mammals in Europe. *Diversity* **2021**, *13*, 275. https:// doi.org/10.3390/d13060275

Academic Editor: Michael Wink

Received: 27 May 2021 Accepted: 16 June 2021 Published: 18 June 2021

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functional richness (i.e., the overall volume of trait space (niche) occupied by species in a community) [10] or Rao's quadratic entropy (i.e., the functional distance between all pairs of species within a community) [9]. Therefore, the analysis of functional diversity patterns along environmental gradients, including land-use changes and human pressures, is a valuable tool to unlock the role of different factors shaping biodiversity patterns and to predict possible shifts in ecosystem functioning under the prism of global change [11].

Climate is a strong driver of species distributions and diversity patterns [10,12–15]. Research has shown that climatic stress gradients can limit functional diversity; for example, only species with certain adaptations can exist in harsh environmental conditions, and as a result, more functionally similar species coexist there [13,14,16,17]. On the other hand, climate seasonality can facilitate species coexistence despite their different ecological niches [18,19] or limit functional diversity when species with narrower niches coexist in areas of higher energy availability (and/or of lower environmental seasonality) [20,21]. Beyond climatic variables, landscape and contemporary human imprint play important roles in shaping species distributions [22] and functional diversity patterns [11,23]. A negative association between human imprint and functional diversity has been reported [24,25]; for example, highly urbanized communities show significantly decreased functional diversity in contrast to natural environments [26,27], while higher vulnerability characterizes functionally dissimilar species of intensified agricultural areas [28]. However, there are counterexamples which show that urban and agricultural land changes influence community structuring and thus functional diversity by favoring specific adaptations of species to cope with the new environments [11]. Yet, our understanding of the contribution of human pressures to large-scale patterns of functional diversity and their relative importance compared to other mechanisms such as climate remains fragmentary [11,23,29]. Furthermore, functional diversity patterns have generally been investigated in single taxonomic groups, despite the fact that different taxa may play similar and/or complementary ecological and functional roles [11,30]. Comparative analyses of the functional roles and functional diversity patterns of different taxonomic groups are scarce and focused primarily on local scales [11,26], while the same question for broad scales is still in its infancy (see [30]).

Functional traits and functional diversity infer a linkage between biodiversity and ecosystem functioning [31,32]. In this context, examining how functional diversity changes with environmental conditions can shed light on the impacts of climate, landscape, and human imprint on ecosystem processes. Here, we examine the taxonomic and functional diversity patterns of three taxonomic groups (amphibians, reptiles, and mammals) across Europe and explore the effect of climate (temperature, precipitation, temperature seasonality, and precipitation seasonality) and human pressures (agricultural and urban land area, land-cover diversity, and human population density) on their diversity patterns. Amphibians, reptiles, and mammals might either have similarities in key functional roles (e.g., amphibians and reptiles) or differ in ecological roles (mammals) while also having a direct link (e.g., one taxonomic group as a feeding resource for another). Given the scale and extent of our study, we expect that climate will have a stronger influence on these patterns [15,16] in contrast to the human pressures which mainly act at local scales ([11,23]; see [33]).
