Colourful wings in a grey landscape: Butterfly species richness and abundance along an urban imperviousness gradient
Species respond to increasing urbanization differently. For some taxa, such as frogs, richness or abundance often decreases with increasing urbanisation due habitat loss, while others, such as birds, often reach a maxima at intermediate urbanization levels due to greater resource heterogeneity. Urban-rural gradients using a single land use are most often used to test species response to increasing urbanization. Rarely is the urban matrix itself considered as viable habitat and, subsequently, is comparatively understudied. Furthermore there are relatively few studies on urban invertebrate assemblages, especially in southern hemisphere cities. This study aimed to understand the impact of increasing impervious surface cover on butterfly species richness and abundance within the urban matrix Melbourne, Australia.
Using GIS, a 500 x 500 m grid was overlaid onto the study area and percent cover of impervious surfaces was calculated. Each cell was then categorized into five levels of imperviousness (0-20%, 20-40%, 40-60%, 60-80%, and 80-100%), where one hundred sampling cells, 20 from each category, were randomly selected for inclusion in the study. Butterfly species richness and abundance was recorded along a 1 km transect in each cell using a Pollard walk. All cells were sampled twice between mid-November 2014 and mid-March 2015.
Eleven species were identified from the 3052 individuals counted. One species, Zizina otis labradus (Grass Blue), comprised the majority of sightings (75%), followed by the non-native Pieris rapae (Cabbage White) comprising 16%, and Heteronympha merope merope (Common Brown) comprising 6%. Preliminary results suggest total abundance of butterflies was negatively correlated with increasing imperviousness (p<0.01), where 8.9% of the variability in total abundance was explained. Butterfly species richness was also negatively correlated with increasing imperviousness within the study area (p<0.001), where one third, 31.5%, of the variability in species richness was explained.
Our study suggests that, overall, butterflies decline in response to increasing urbanization but individual species’ responses need closer evaluation. While the urban matrix may not be the most hospitable habitat for most butterfly species, it does appear to contain critical resources for species with greater plasticity in adult and larval food requirements. Factors relating to food or sheltering resources such as amount of native vegetation and floral abundances also warrant further investigation.