Butterflies in the city: ecological filtering of urban landscape

Abstract

With the expansion of urban areas and the increasing valuation of ecosystem services for the citizenship, promoting biodiversity conservation in cities is now a priority. However, urban environments represent one of the most modified landscapes that species are facing, and this affects their distribution and occupancy, which potentially can lead to biodiversity homogenisation. Using butterflies as study system, we evidenced an ecological filter of species traits by the urban landscape and therefore a biotic homogenisation from a functional point of view. Our first scan evidenced that butterfly species in the city were mostly those highly dispersive, highly reproductive and generalist feeders. This ecological filter was mostly driven by the density of the built matrix, the garden isolation and the abundance of semi-naturalgrassland vegetation. Taking in account the different effect of the urban landscape on the species traits we modelled potential realistic management scenarios for three species models: low dispersive, low reproductive specialist species (Type 1, T1); medium dispersive, medium reproductive generalist species (T2); high dispersive, high reproductive generalist species (T3). Management scenarios were based on increased habitat quality of the gardens and increased connectivity. To do so we used spatially explicit individual-based modelling coupling demographic and mechanistic dispersal models in RangeShifter. Increasing the garden habitat quality increased the occupancy of all three type of species but to a contrasting degree (28, 57 and 90% occupancy probability for T1, T2 and T3 if tripling habitat quality); being T3 the most frequent across the city at all management scenarios (3.2 and two times more than T1 and T3). Reducing the built density and the garden isolation have a stronger positive effect on T1 and T2 with double and 1.2 times increases of their occupancy with reductions of 10% of the built matrix. Our results highlight the primary negative effect of the built matrix on urban biodiversity and the need to reduced the built density and the garden isolation via urban planning to promote biodiversity in the cities.