Introduction:
In the debate regarding global biodiversity decline, urban areas have sometimes been suggested to promote diversity (Cardinale et al., 2018). Urbanisation involves changes in the landscape, soil modifications, climates changes, and biodiversity loss, resulting in a distinct ecosystem (Rodrigues et al., 2018). Fragments of semi-natural 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). But they also host ornamental or non-native plants which fail to support native wildlife. Urban habitats place stronger environmental constraints on plant and animal communities than rural ecosystems 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). 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). Moderate levels of urbanisation lead to varying patterns of species diversity across taxonomic groups, particularly when there are sufficient corridors of green space to allow colonisation from the regional species pool (Rega-Brodsky, et al., 2022). 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.
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 and noise pollution (Rodrigues., 2018). 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 ), and cats (Felis silvestris catus ). 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 ), house sparrows (Passer domesticus ), 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. 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). Furthermore, urban vegetation is necessary for mitigating urban heat islands, floods, increasing carbon storage, improving biodiversity, and benefitting human health (Chen et al., 2022). Plant biodiversity is greatly affected in urban areas by the introduction of exotic, non-native species, changes in microhabitat availability, and altered landscape patterns (Peng and Liu, 2007). The introduction of non-native plant species in urban areas degrades habitats and shifts community composition, which can influence ecosystem services and habitat resilience (Dolen et al., 2011). 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 (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). 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 the stony and rocky area (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. As butterfly ecology has been seldom studied within the Aegean, an investigation of the urban and rural habitats adopted by butterflies in island ecosystems is of high ecological importance.
This study aims 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.