Despite some general concordant patterns (i.e. the latitudinal richness gradient), species richness and composition of different European beetle taxa varies in different ways according to their dispersal and ecological traits. Here, the patterns of variation in species richness, composition and spatial turnover are analysed in European cryptocephaline and chrysomeline leaf beetles, assessing their environmental and spatial correlates. The underlying rationale to use environmental and spatial variables of diversity patterns is to assess the relative support for niche- and dispersal-driven hypotheses. Our results show that despite a broad congruence in the factors correlated with cryptocephaline and chrysomeline richness, environmental variables (particularly temperature) were more relevant in cryptocephalines, whereas spatial variables were more relevant in chrysomelines (that showed a significant longitudinal gradient besides the latitudinal one), in line with the higher proportion of flightless species within chrysomelines. The variation in species composition was also related to environmental and spatial factors, but this pattern was better predicted by spatial variables in both groups, suggesting that species composition is more linked to dispersal and historical contingencies than species richness, which would be more controlled by environmental limitations. Among historical factors, Pleistocene glaciations appear as the most plausible explanation for the steeper decay in assemblage similarity with spatial distance, both in cryptocephalines and chrysomelines.

Beta diversity, biogeography, Chrysomelidae , Chrysomelinae , Cryptocephalinae , species richness

The assessment of large-scale biogeographic patterns has proven a fruitful research discipline for understanding the ecological, evolutionary and historical processes that have determined the current distribution of biological diversity (Brown and Maurer 1989; Farrell 1998; Gaston and Blackburn 2000). In recent years, several contributions have considerably advanced in our search for a general explanation about the factors driving species distributions and clade diversification, using both empirical (Hawkins et al. 2003; Svenning and Skov 2007a; Wiens 2007) and theoretical approaches (Hubbell 2001; McGill 2010). However, probably because of the complexity of the problem and the difficulty to perform controlled experiments due to the large temporal and spatial scale of the processes potentially involved, a general comprehensive theoretical framework to explain biological diversity and its distribution on Earth is still missing. From the empirical point of view, one important shortfall is the under-representation of invertebrates in biogeographical studies, caused by the difficulty to sample and identify invertebrate species, compared to vertebrates or plants. This has led to a marked scarcity of invertebrate distribution data that prevented biogeographers to assess invertebrate biodiversity patterns. As a result, most biogeographical and macroecological theories have been tested using vertebrate and plant data, a circumstance that does not fit very well with the fact that invertebrates are the largest fraction of local, regional or global biodiversity (Erwin 1982; Gaston 1991; Odegaard 2000; Storck 1997). Therefore, a potential avenue for progressing towards a full understanding of biodiversity patterns would be to gather invertebrate distribution data and assess large-scale biodiversity patterns.

Recent examples of this approach have proven particularly successful. A few invertebrate taxa (e.g., diurnal Lepidoptera) have been particularly well sampled, so diversity patterns are relatively well known (Hawkins 2010; Hawkins and Porter 2003). In other cases, the acquisition of reliable distributional data requires considerable sampling effort (e.g., Baselga et al. 2013; Papadopoulou et al. 2011), but an alternative solution to go around the impediment caused by the lack of accurate distributional data is to use country level inventories. Despite its limitations derived from the coarse spatial resolution and the unequal country area (Keil and Hawkins 2009), this kind of data has proven robust enough to unveil the major correlates of species richness and turnover patterns, as shown for water beetles (Ribera et al. 2003), longhorn beetles (Baselga 2008), ground beetles (Schuldt and Assmann 2009; Schuldt et al. 2009), springtails (Ulrich and Fiera 2009), Odonata and Lepidoptera (Keil and Hawkins 2009), and darkling beetles (Fattorini and Baselga 2012). Regarding beetles, the important contributions of databases as Fauna Europaea (Fauna Europaea version 2.6, available online at http://www.faunaeur.org) and catalogues as the Catalogue of Palaearctic Coleoptera (Löbl and Smetana 2003; 2004; 2006; 2010) have been crucial for making possible the aforementioned analyses, including the comparative assessment of biodiversity patterns across multiple beetle taxa (Baselga et al. 2012b; Gómez-Rodríguez et al. 2015).

The studies cited above have shown that different beetle taxa have different diversity patterns across continental Europe. Regarding species richness, the steepness of the negative latitudinal richness gradient has been shown to be related to the dispersal capacity of taxa (Baselga et al. 2012b), suggesting a causal link between the loss of species richness to the North and the incomplete re-colonization of northern regions after Pleistocene glaciations (Baselga et al. 2012a; Svenning et al. 2008; Svenning and Skov 2007a). Regarding the variation in species composition (i.e. beta diversity), available evidence suggests that both dispersal capacity and niche traits are responsible for differences in beta diversity patterns among beetle taxa (Gómez-Rodríguez et al. 2015). Within this context, the aim of this study is to assess the biodiversity patterns across continental Europe in two major clades of leaf beetles: Cryptocephalinae (excluding clytrines) and Chrysomelinae. To this end, the variation in species richness, species composition and species spatial turnover has been assessed in both leaf beetle groups, and their relationship with two sets of environmental and spatial variables. The rationale behind is to confront the explanatory capacity of current environmental factors that would reflect the effect of niche-based processes (Hawkins et al. 2003) with purely spatial variables that could reflect alternative dispersal-based processes, as historical colonization events or neutral dynamics (Lobo et al. 2001; Svenning and Skov 2007a).

The most prominent pattern in species richness of both cryptocephalines and chrysomelines is the existence of a clear latitudinal gradient. This is an almost universal macroecological pattern (Willig et al. 2003), that in Europe has been thoroughly documented in numerous taxa, especially vertebrates and plants (Araújo et al. 2008; Montoya et al. 2007; Rodríguez et al. 2005; Svenning and Skov 2007b; Whittaker et al. 2007) and some invertebrates such as butterflies (Hawkins and Porter 2003) and springtails (Ulrich and Fiera 2009), for example. Within Coleoptera, the situation is not very different and most taxa display a clear latitudinal gradient (Baselga 2008; Fattorini and Baselga 2012; Hortal et al. 2011; Schuldt and Assmann 2009). However, the steepness of the latitudinal richness gradient is subject of a great variation among different beetle taxa (Baselga et al. 2012b), with some taxa as families Scolytidae and Silphidae presenting weak (almost flat) gradients, while other taxa as genera Trechus (Carabidae) and Otiorhynchus (Curculionidae) having steep latitudinal gradients. This variation is partially explained by the differences in dispersal ability among beetle taxa, suggesting that limited post-glacial re-colonization processes are a major determinant of beetle richness gradients in Europe (Baselga et al. 2012b).

Other significant variables of species richness in both cryptocephalines and chrysomelines were country area, temperature, altitude and the range of precipitations. The major difference between both groups was the significant negative relationship between richness and minimum longitude in chrysomelines but not in cryptocephalines. Therefore, species richness models were qualitatively similar in both groups, although a relevant quantitative difference was also observed. While in cryptocephalines the environmental variables were relatively more important than the spatial ones (unique contributions of 31% vs. 2%, respectively), in chrysomelines the situation was reversed, with a largest contribution of spatial rather than environmental factors (unique contributions of 24% vs. 14%, respectively). These figures may be interpreted as cryptocephaline richness being more determined by environmental conditions (mostly temperature), while chrysomeline richness being more subject of undetermined factors causing a spatial structure independent of climatic factors, likely historical factors linked to diversification and limited dispersal. This interpretation is in accordance with the fact that flightless species are common among chrysomelines but not among cryptocephalines. In a wider context, the differences between species richness models for cryptocephalines and chrysomelines suggest that the preponderance of niche-based processes related to energy and water availability dynamics (Currie et al. 2004; Hawkins et al. 2003), niche conservatism and historical climate change (Hortal et al. 2011) or dispersal-based processes (Baselga et al. 2013; Svenning and Skov 2007a) is probably taxon-dependent.

The patterns of variation in species composition were, in contrast, very similar in both groups: several European southern regions, namely Iberian and Greek peninsulas and southern Russia, presented singular species composition differing from the rest of the continent, which harboured a relatively homogeneous fauna. Therefore, the environmental correlates of variation in species composition were very similar in both groups, including altitude and several temperature-related variables. The only difference was the significant relationship between species composition and precipitation of the driest quarter in cryptocephalines but not in chrysomelines, suggesting that cryptocephaline species are filtered by water availability but chrysomeline species are not. Likewise, the spatial determinants are similar in both groups, including both latitudinal and longitudinal components. The partition of variation in species composition between environmental and spatial variables revealed that a strong spatial structure (independent of environmental factors) exist in both groups, as also found in other beetle taxa (Baselga 2008; Fattorini and Baselga 2012). This suggests that, besides the effect of environmental factors causing species turnover as a result of differential physiological requirements, other spatially-structured processes have deeply influenced the replacement of cryptocephaline and chrysomeline faunas across Europe. Speciation processes and limited dispersal are likely behind such spatial patterns, and the effect of Pleistocene glaciations appears as one of the most plausible explanations. Indeed, while northern European faunas were obliterated during Pleistocene glaciations, southern European regions have acted as faunal refugia during glacial cycles. This has impacted southern European faunas in two ways. In first place, pre-Pleistocene assemblages were not obliterated, providing more time for in-situ diversification (Hewitt 2004), compared to northern regions where beetle assemblages are necessarily recent (i.e. arrived there since last glaciation). Indeed, several studies have reported the pre-Pleistocene origin of Mediterranean endemics (Andújar et al. 2012; Condamine et al. 2013; Hidalgo-Galiana and Ribera 2011; Ribera et al. 2010). In second place, glacial refugia have also acted as speciation centres (Gomez-Zurita et al. 2012; Ribera and Vogler 2004), increasing the singularity of southern faunas. Among the different glacial refugia that could serve as sources of re-colonization of northern regions, the Italian peninsula is the only one that does not present a markedly singular fauna (despite its high species richness), a fact also observed on longhorn and darkling beetles (Baselga 2008; Fattorini and Baselga 2012). This might suggest that the Italian peninsula was one of the major re-colonization sources, making most of the European faunas similar to the Italian one, but less so to the Iberian and Greek assemblages, in which re-colonization events seem to have been much more limited. Finally, the higher relevance of spatial factors for species composition compared to species richness suggests that species composition is more linked to dispersal and historical contingencies than species richness, which would be more controlled by environmental limitations.

The effects of Pleistocene glaciations seem also to be behind the differences in the patterns of distance decay of similarity between northern and southern regions. As it could be predicted from the visual inspection of maps illustrating the distribution of major clusters, the spatial turnover of cryptocephaline and chrysomeline assemblages with spatial distance is much steeper in southern Europe compared to northern Europe. As previously suggested elsewhere (Baselga 2010; Baselga et al. 2012a), this difference could be interpreted as a consequence of Pleistocene glaciations. Southern regions have retained beetle assemblages during larger periods of time, allowing speciation processes to accumulate and thus increasing the differences between assemblages. In contrast, beetle assemblages in northern regions were obliterated during glaciations, and the species currently present there are all generalist ones with high dispersal ability (which have allowed them to colonize the continent from southern refugia). Interestingly, differences between northern and southern slopes of the distance decay pattern are much larger in both leaf beetle taxa than in longhorn beetles. The reason for this larger difference is that, in southern Europe, leaf beetles present a much steeper distance decay of similarity than longhorn beetles. In contrast, the decay of similarity with spatial distance in northern Europe is more similar for the three taxa. In other words, differences in diversity patterns between the two leaf beetle taxa and the longhorn beetles are more marked in southern than in northern Europe, suggesting again that the relative homogeneity of northern faunas is a phenomenon resulting from a common process in all taxa, likely post-glacial re-colonization by only those species with high dispersal ability, in line with previous evidence (Baselga et al. 2012b). In contrast, southern faunas would be composed by older assemblages and differences in diversity patterns of taxa had more time to accumulate, reflecting dispersal, ecological and evolutionary differences.

This work was supported by the Spanish Ministry of Economy and Competitiveness, and the European Regional Development Fund (2007–2013) through grant CGL2013-43350-P.