Birds
Urban sites had a greater abundance and species richness of birds compared to rural sites (Figure 3), which could be attributed to the addition of species accustomed to urban environments, termed “urban exploiters” (Crooks et al., 2004). For example, Rock Doves (Columba livia ), and House Sparrows (Passer domesticus ) are dexterous at exploiting discarded food, utilising human made nesting sites (roofs) and other resources in urban environments, and consequently achieving higher densities in developed areas (Blair et al., 1996). Indeed, sparrows were abundant in urban locations in our study (559 individuals) and were much less common in rural areas (29 individuals), echoing the finding of Belinsky et al, (2019). This dominance of adaptable bird species in urban locations underpinned the disparity in community composition compared to rural habitats (Figure 5b), with urban areas usually supporting fewer species from ecologically sensitive groups, e.g., ground nesters, migratory birds, and dietary specialists (Dale, 2018; Blair et al., 1996). Our findings differ from prior research suggesting that species richness is lower in urban areas due to the prevalence of buildings over vegetation (Kark et al., 2007, Tzortzakaki et al., 2018). Several studies have found that species richness peaks with intermediate levels of urbanisation which resonates with the low-intensity urbanisation found on Lipsi Island (Crooks et al., 2004, Blair et al., 1996). Indeed, eight of our nine urban sites are naturally vegetated and underdeveloped, consistent with the findings from White et al. (2005), showing that underdeveloped areas had a greater abundance and species richness of birds compared to recently developed locations.
Nevertheless, the greater abundance and species richness of birds in urban areas did not seem to elevate the predation pressure on butterflies, which had similar abundance and diversity in urban and rural environments. This could be due to increased dominance of non-predatory functional groups of birds, with Nason et al. (2021) finding a greater abundance of granivorous and omnivorous birds rather than insectivores in urban areas, perhaps due to the abundance of discarded food available there, causing a decline in overall bird attacks on animal prey. Indeed, urban dominance by omnivorous House Sparrows, Hooded Crows, and Yellow-Legged Gulls on Lipsi may have reduced predation pressure on butterflies in urban areas. However, the insectivorous Barn Swallows and Common House Martins were more abundant in urban (93 individuals) compared to rural (32 individuals) sites, suggesting complex effects of urbanisation on food web interactions that would require dietary studies to disentangle.
Changes in predation pressure are further complicated by responses of non-avian predators to urbanisation. For example, robber flies (Asilidae spp), which are important predators of butterflies (Londt, 1999; Lehr et al., 2007), were seen in almost every shrubland site, but rarely in urban areas. Aposematism defence (use of vibrant colours) acts as a warning for predators (Pinheiro and Cintra, 2017), and butterflies with intricate camouflage such as meadow brown and graylings were observed to be more abundant in rural habitats (63 individuals) compared to urban locations (13 individuals). This may point to the greater predation pressure experienced by butterflies in rural areas and could help to explain the surprising similarity in abundance and diversity of butterflies in rural compared to urban environments.