Our study indicated that the species presence and richness of amphibians and reptiles are influenced by crop extent and crop category in agricultural landscapes across Europe. Moreover, we found that the observed spatial patterns of species richness and of crop extent were scale dependent. More specifically, although the number of species occurring in crops did not differed between scales, the geographic location of peaks and troughs of species richness and of crop extent at coarse scale (grain size 50km2) did not coincide with those at finer scale (10km2). At coarse spatial resolution, amphibian species richness in crops was higher in Western-Central Europe, while for reptiles it was higher in the southern peninsulas. This was in general agreement with the distribution and geographic patterns of amphibians and reptiles of Europe (Sillero et al. 2014). The analysis at a finer scale led to less obvious hotspots and cold spots of species richness, due to the lack of homogeneous distribution data at that spatial resolution. In the case of crop extent, the smaller the areas, the lesser the crop categories, thus increasing the dominance of one crop category, especially on islands.
Our results showed that species presence may be affected by crop extent and crop category. Although all crop categories contributed significantly to explain species presence, our results revealed some scale-dependent contrasts in crop influence on species presence. For example, while agroforestry crop extent had a significant positive effect on most species at UTM 50km2, at UTM 10km2 woody crop category positively affected the highest number of species. Agroforestry crop in our study includes land principally occupied by agriculture with significant areas of natural vegetation and agroforestry areas (like the typical montados and dehesas in the Iberian Peninsula; CORINE Land Cover 2018, version v.2020_20u1). Previous studies have shown that the combination of agriculture and forestry enhances the persistence of amphibian and reptile species (Brüning et al. 2018; Warren-Thomas et al. 2020; Fulgence et al. 2021), being in general beneficial for biodiversity (Hartley 2002; Harvey et al. 2007; Torralba et al. 2016). Agroforests are important for biodiversity as they provide a more diverse habitat than a conventional agricultural system and can serve as corridors between habitats (Harvey et al. 2007). Woody crops in our study included vineyards, fruit trees and berry plantations, olive and chestnut groves and walnut groves shrub orchards. Reptile species, such as lizards, can be widespread in vineyards (Biaggini and Corti 2021). Traditionally cultivated olive plantations host an exceptionally high proportion of specialist reptiles (Kazes et al. 2020, but see Carpio et al. 2016). Furthermore, when herbaceous cover exist woody crops may harbour a diverse community of reptiles (Carpio et al. 2017). All above explanations assume that landscape has a hierarchical structure, i.e. regions consist of a mosaic of smaller habitat patches which occur within larger habitat patches (Kotliar and Wiens 1990). Habitat patches vary depending on an organism’s perception (Wiens and Milne 1989) which involves the spatial resolution and the concept of contrast (i.e. “the magnitude of differences in measures across a given boundary between adjacent patch types”, Wiens and Milne 1989). The response of species to a mosaic of habitat patches depends on both spatial scale and contrast levels (Chust et al. 2003). Thus, a multi-contrast levels and scales approach would help to evaluate properly effect of different crop categories on species occurrence and biodiversity and explain the different patterns we found at different resolutions.
Our results provided evidence that the species richness of amphibians and reptiles is in general negatively correlated with crop extent. However, the results also highlighted scale- and group-dependent richness patterns in response to crop extent and crop categories. This emphasises the importance of considering the context specificity of scale- and taxa-dependent responses to crop types when investigating these patterns. Previous studies have indicated that agricultural lands are often not optimal habitats for amphibians and reptiles (e.g. Loman and Lardner 2006, 2009; Ribeiro et al. 2009), both groups being particular sensitive to agricultural activities (Dürr et al. 1999), such as the use of pesticides (Brühl et al. 2013). Further, agricultural lands may even serve as ecological traps (Rotem et al. 2013).
The extent of dry crops may be the main threat to amphibian species diversity in agricultural landscape. Both at coarse and finer spatial resolutions, amphibian species richness was negatively affected by the extent of dry crops. Pond-breeding amphibians are dependent on availability of aquatic habitats, being strongly related to aquatic habitats characteristics (e.g. surface, hydroperiod, water chemistry, predators and cover vegetation) (Skelly et al. 1999; Skelly 2001) and vegetation around them for migratory events and other terrestrial activities (Mazerolle and Desrochers 2005; Prevedello and Vieira 2010). The low spatial and temporal availability of aquatic habitats in drylands, subject to human activities altering their characteristics and the vegetation around them, most likely affect negatively the amphibian species diversity (Gardner et al. 2007).
Species richness of amphibians was also negatively affected both at coarse and finer spatial scale by irrigated crops, possibly due to higher use of pesticides or alteration of aquatic habitats in this crop category. Similarly, at the coarse spatial scale, agroforestry crops had also a positive effect on amphibians richness, probably due to the higher landscape heterogeneity in those crop categories than in monoculture lands (Brüning et al. 2018; Fulgence et al. 2021). Agroforestry crops have less intensive agricultural modes and contain patches of natural vegetation and hedges, providing shelter and food resources. Indeed, ponds are very abundant in the Spanish dehesas and Portuguese montados agroforests, for providing water to cattle. Reptile species richness responded significantly to all five crop categories, decreasing with the extent of all crop categories except irrigated and woody crops at coarse spatial resolution. Previous studies have shown that non-irrigated crops adversely affect reptiles (Atauri and de Lucio 2001). Grazing is an important anthropogenic disturbance on pastures and can have negative and positive effects on biodiversity (Schieltz and Rubenstein 2016). The response of reptile species to grazing has been found to be influenced by arboreality (Neilly et al. 2018). Terrestrial reptiles were generally negatively affected by increasing grazing pressure, unlike arboreal reptiles (Howland et al. 2014). Research work on agroforests indicated that, although agroforests and monocultures contain different reptile assemblages, the species richness is similar (Wanger et al. 2010; Warren-Thomas et al. 2020).
Crop heterogeneity enhances species richness of both amphibians and reptiles. Crop heterogeneity affects positively a wide range of taxa, including amphibians (Collins and Fahrig 2017). Yet, evidence of biodiversity benefits from crop diversity was claimed to strongly depend on spatial resolution. Our results provided support for an increase in the species richness of amphibians and reptiles with crop heterogeneity at both coarse and finer spatial resolutions. However, we measured the compositional heterogeneity (Fahrig et al. 2011), that describes the diversity of crops grown in a landscape, as the Shannon diversity index of crop categories. Configurational heterogeneity (Fahrig et al. 2011) describing the spatial arrangement of fields and measured as mean field size or density of field borders, might play an important role on amphibian and reptile species richness. Therefore, further studies are needed to understand whether crop heterogeneity could be an effective way to increase the diversity of amphibians and reptiles in agricultural landscape.