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.