Intensification of agriculture is one of the main drivers of biodiversity declines worldwide (Tilman et al. 2002; Benton, Vickery & Wilson 2003; Tscharntke et al. 2007). An important way in which biodiversity is lost is through biotic homogenization, which refers to the dominance of generalist species resulting from local extinctions of species susceptible to external perturbations (McKinney & Lockwood 1999; Olden et al. 2004; Dormann et al. 2007). Generalists are able to utilize a wide spectrum of vegetation types whereas sensitive species tend to be habitat specialists, with strict demands on habitat quality and landscape properties (Hanski 1999; McKinney & Lockwood 1999; Olden et al. 2004). Simplified landscapes created by intensive land-use can be particularly detrimental to habitat specialists and species with poor dispersal abilities (Hanski 1999; Steffan-Dewenter & Tscharntke 2002). Intensified agriculture can therefore lead to biotic homogenization through loss of habitat specialists and poor dispersers, either as a response to increased use of fertilizers and pesticides, or by increased cover of arable land (Dormann et al. 2007; Kleijn et al. 2008). Thus, when increasing land-use intensity results in homogeneous landscapes, it can be assumed to select for high species mobility and low habitat specificity with a subsequent decrease in beta diversity (Dormann et al. 2007). Therefore, one would expect to find more species with poor dispersal abilities and a lower fraction of habitat generalists in communities characterized by high beta diversity.
Recent studies on farmland biodiversity have addressed the effects of habitat connectivity, landscape heterogeneity and farming systems on species richness, functional diversity and community similarity (Schweiger et al. 2005; Clough et al. 2007; Diekötter, Billeter & Crist 2008; Flynn et al. 2009). The occurrence and spatial arrangement of semi-natural habitats have been shown to play a key role in determining patterns of diversity in agricultural landscapes (Hendrickx et al. 2007; Öckinger & Smith 2007; Diekötter et al. 2008; Holzschuh, Steffan-Dewenter & Tscharntke 2009). However, in contrast to the predictions outlined above, an association between increasing landscape homogeneity and decreased numbers of habitat specialists and poor dispersers in natural communities has hitherto not been demonstrated.
We present empirical evidence on biotic homogenization of butterfly communities through the loss of habitat specialists (i.e. species preferring a given vegetation type) and poor dispersers associated with simplified landscape structure in intensively cultivated agricultural landscapes. We first compare the responses of butterfly and day-active geometrid moth alpha and beta diversity to increased agricultural intensity, measured as the percentage cover of arable fields. We expect both alpha and beta diversity to decrease with increasing cover of arable fields. We then relate mean species’ mobility and percentage of generalists in butterfly communities with arable field cover percentage, expecting an increasing proportion of habitat generalists and higher average mobility with increasing agricultural intensity. Finally, we compare geographical regions with contrasting average agricultural intensity and expect to find the strongest relationship between butterfly diversity partitions and the proportion of habitat generalists and average mobility in the regions with high agricultural intensity.