Introduction:
In the debate regarding global biodiversity decline, urban areas can
both be a filter and enhancer of biodiversity, with different factors
and context determining the outcomes (Cardinale et al., 2018; Uchida et
al., 2021; Lepczyk et al., 2023). Fragments of artificial spaces such as
parks, gardens, and other green areas may provide a diverse plant
composition and fulfil ecosystem functions needed to maintain urban
wildlife (Townsend, 2008; Swan et al., 2021), but they also host
ornamental or non-native plants which may fail to support native
wildlife (but see Harrison and Winfree 2015; Padovani et al., 2020).
Urban habitats place stronger environmental constraints on plant and
animal communities than rural habitats (e.g. air pollution, noise,
artificial light; Isaksson, 2015) and may disrupt ecological
interactions between plants and pollinators via habitat fragmentation
(Hennig and Ghazoul, 2011). Impacts on species richness in urban areas
are dependent on the specific taxonomic group, the spatial scale of
analysis, and the intensity of urbanisation (McKinney, 2008; Fournier et
al., 2020; Theodorou et al., 2020). A greater species richness in urban
areas may be due to the increased number of both native and non-native
species, due to the larger species pools that urban areas maintain
(Dolan et al., 2011), particularly when there are sufficient corridors
of green space to allow colonisation from the regional species pool
(Rega-Brodsky, et al., 2022). This could also be underpinned by the
mosaic of habitat patches in urban ecosystems, and the associated
heterogeneity of plant communities that could support biodiversity at
higher trophic levels (Swan et al., 2021). Effects of urbanisation on
top-down control (e.g., altering predation by birds) or bottom-up
control (e.g., altering vegetation structure) could also lead to
indirect effects on abundance, species diversity, or community
composition throughout the food web (Theodorou, 2022).
Urban areas can be characterised as a spatial assemblage of people whose
lives are structured around non-agricultural activities, with rural
areas defined as any place that is not classified as urban (Weeks,
2010). Urban areas can also be classified as land that is built over,
while rural areas consist of land that is not built over and with a much
smaller population size (Bibby and Shepherd, 2004). Rapid urban
development and expansion in recent years have altered many wildlife
assemblages, especially invertebrates (Van Swaay and Warren., 1999).
Perhaps the most well studied group is butterflies, as they are popular,
easy to identify, and have been used as model insects for many years
(Warren et al., 2021). But butterflies are also in decline due to severe
habitat loss and climate change (Zografou et al., 2009). More generally,
butterflies are important indicators of ecosystem health due to their
susceptibility and sensitivity to changes in the environment (Ghazanfar
et al., 2016). Butterflies have a high reproductive rate and occupy low
trophic levels; thus, they respond quickly to environmental stressors
and could be utilised as a proxy for general reductions in wildlife
(Ghazanfar et al., 2016). Here, we focus on butterflies as indicator
taxa, whilst considering the impact of urbanisation on their predators
and resources for a multitrophic perspective.
Urbanisation has been shown to degrade bird communities through species
decline and functional homogenisation (Tzortzakaki et al., 2018). The
main factors affecting bird species assemblages are green space
availability, noise pollution, interspecific competition, and habitat
heterogeneity (Rodrigues., 2018; Martin et al., 2018; Chiron et al.,
2024). Collisions with buildings in urban areas also heavily affects
bird populations, including species of conservation concern (Hager et
al., 2017). Vincze et al. (2017) found that in urbanised areas there was
an increase in predation of bird nests by urban exploiters such as crows
(Corvus spp.), magpies (Pica pica L., 1758), and cats
(Felis silvestris catus L., 1758). However, it is also suggested
that prey populations of birds thrive in urban areas as these habitats
are low in abundance of larger predators (Vincze et al., 2017). Cities
and towns have variability in terms of the activity or usage of areas,
thus bird species distribution in urban areas is related to the degree
of urbanisation and habitat features such as tree and shrub cover and
the density of buildings (Rodrigues et al., 2018). Moreover, human
landscape characteristics favour species that can exploit novel
resources and adapt to new habitats, such as hooded crows (Corvus
cornix L., 1758), house sparrows (Passer domesticus L., 1758),
and pigeons (Columbidae spp.) (Kark et al., 2007).
The high abundance of adaptive birds in urban environments could thus
have negative impacts on invertebrates, specifically butterfly
populations compared to rural habitats. For example, birds often achieve
higher population densities in urban environments due to the lack of
natural predators and abundance of food, which could lead to greater
top-down control on butterflies (Shochat et al., 2010). However,
butterflies have developed various defensive traits against birds, such
as chemical cues and aposematic or cryptic colouration, i.e., bright
colours in conspicuous patterns on the wings (Paladini et al., 2018).
Additionally, many butterflies have adopted fast, unpredictable flight
and weak, fragile wings that allow escape by tearing when pecked by
birds (Pinheiro and Cintra, 2017). Brighter colouration signals are
commonly associated with potent defence and greater reproductive
success, as predators are naturally deterred, within-species rivals are
more cautious, and potential mates are more interested (Yeager and
Barnett, 2021). Due to the high frequency of beak marks on the wings of
butterflies, birds are likely their most significant predator (Pinheiro
and Cintra, 2017). Nonetheless, small mammals, toads, and lizards also
feed on adult butterflies, and there may be significant predation by a
variety of invertebrates (Londt, 1999).
Changes in the patterns of vegetation composition and structure in urban
areas, can lead to a reduction of bird species richness and selection
for omnivores, carnivores, and species which nest in cavities (de Toledo
et al., 2012). But native vegetation diversity within green spaces can
strengthen the abundance and richness of specialist and insectivorous
bird species (Silva et al., 2021). Plant biodiversity often increases in
urban areas through the introduction of exotic (non-native) species
(Peng and Liu, 2007), but this is strongly dependent on the influence of
human preferences and management activities (Goodness, 2018; Avolio et
al., 2021). The introduction of non-native plant species in urban areas
degrades habitats and shifts community composition, however, often with
huge turnover of species across urban habitats, which can influence
ecosystem services and habitat resilience (Dolen et al., 2011; Swan et
al., 2021). Urbanisation also alters the timing of important reoccurring
plant phenology events, such as flowering and leaf-out, leading to
cascading consequences on the species within a community and disturbing
important interactions such as pollination and herbivory (Dale and
Frank, 2018; Li et al., 2019). The gross primary productivity of
vegetation also decreases with increasing levels of urbanisation from
loss of green land and changing macro-environment (Chen et al., 2022).
While habitat enhancements of exotic species may increase ecosystem
resilience and integrity, restoration of native communities in urban
areas may increase connectivity to surrounding rural landscapes and
support native ecosystems (de Carvalho et al., 2022).
There is a mutual and historical co-evolution in operation between
plants and invertebrates (Ghazanfar et al., 2016). Co-evolutionary
traits include adaptive radiation of plants that evolved to have
chemical protection from herbivores, followed by adaptive radiation in
herbivores who developed characteristics to counter this defence (Feeny,
1975). For example, the butterfly proboscis attachment has adapted to
reach the nectar at the base of long-tubed flowers (Ghazanfar et al.,
2016). Alternatively, some skippers (Hesperiidae) are only capable of
utilising shallow blossoms, such as flowers in the myrtle family
(Myrtaceae) (Ghazanfar et al., 2016). Increasing urbanisation results in
fewer plant species visited, indicating lower resource use or
availability for pollinators in urban environments (Ellis et al., 2023).
Smaller plant patches found in urban environments tend to receive fewer
pollinator visits and suffer pollen limitation (Barker, 2018). This
reduces genetic exchange and flowering plant diversity, and
consequently, supports fewer pollinator species. Yet, low building
density and the presence of green space within urban areas, may drive
pollinator movement and thus gene flow between patches (Hennig and
Ghazoul, 2011).
Whilst anthropogenic disturbances are fostering negative impacts on
butterfly species, human practices have created agricultural and
woodland management systems such as hay meadows and coppicing that
assist the growth of butterfly populations (Dover and Settele, 2009).
The Mediterranean is one of the world’s 25 biodiversity hotspots, mainly
due to the abundance of endemic species within this area (Lopez-Villalta
et al., 2010). The Aegean Sea is located within the Mediterranean where
butterfly species vary between the islands. In this area, Haahtela et
al. (2019) recorded the highest levels of diversity on Samos Island (64
species) and Lesbos Island (63 species) (Haahtela et al., 2019). The
evolution, extinction, and species migration of animals and plant
species over archipelago islands are reflected in the pattern of species
diversity (Dennis et al., 2000). Therefore, a distinct and endemic
species assemblage of butterflies may be present across the Aegean
islands. This highlights the importance of green space within
Mediterranean urban areas and a demand to assess the butterfly species
within this environment. The study of butterflies within the Aegean
region is severely lacking and mainly focuses on biogeographical studies
(e.g., Dennis et al., 2000; Hammoud et al., 2021; Hausdorf and Hennig,
2005), thus, the specific habitat types that butterflies utilise is not
known. When studying Tuz Lake in Turkey, Seven (2017) compared habitat
preferences of butterflies and observed the highest species diversity
within the steppe habitat (defined as semi-arid grassland) and the
lowest diversity in poorly vegetated areas dominated by rocks,
indicating that species may prefer vegetated and shaded areas. Due to
the global decline of butterflies, the exploration of urban green space
as a possible diversity hotspot is crucial and contributes to current
research.
Increasing urbanisation due to ongoing development of islands in the
Aegean region makes it essential to study the impacts of even
low-intensity urbanisation on butterfly communities and their predators
and resources. This is particularly relevant given the paucity of
research on butterfly ecology within the Aegean. Thus, a key novel
contribution of this study is to compare the ecological communities
found in rural areas and urban green spaces on Lipsi Island, Greece. It
is hypothesised that total abundance, species richness, and Shannon
diversity of (1) butterflies, (2) birds, and (3) vegetation will be
higher in rural compared to urban sites and that (4) urbanisation will
have an impact on community composition of each trophic group.