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.