COS 162-5 - Planting complex, native understorey on inner city streets significantly increases bee and butterfly abundance and diversity

Background/Question/Methods

Increasing native understorey in urban landscapes can have positive effects on a range of fauna. However, moving away from ubiquitous trees and mown grass to more complex vegetation is challenging, particularly on streets. To help city managers do this, we developed a streetscape understorey planting guide designed explicitly to support local biodiversity. More than 80 plant species were selected based on horticultural attributes, tolerances and the biodiversity resources they provide native bees (nectar, pollen, nesting locations), butterflies (nectar, larval food plants) and birds (nectar, seed, protected nesting locations). Four streetscapes within the City of Melbourne were then planted with the palette in 2018.

Using a Before-After-Control-Impact (BACI) approach, with four control sites for every impact site, we experimentally tested the ability of streetscape biodiversity plantings to increase the abundance and diversity of bees, butterflies and birds. Sites were surveyed using standard techniques four times in spring and summer in 2017 (‘Before’ impact), 2018-19, 2019-20 and 2021-22. Results were analysed using negative binomial GLMMs with treatment, survey year, and their interaction as fixed effects, and random effects for site (nested within landscape (four landscapes, each being a cluster of four sites including one impact and its associated controls) in R.

Results/Conclusions

Total bee abundance and richness changed across years and the direction of change differed between control and impact sites. The year-by-treatment interaction was highly significant in both cases), with abundance and species richness decreasing moderately over time in control sites, but increasing in impact sites.

Butterfly abundance and species richness was higher at all impact sites but the longer-term trends appear to be influenced by seasonal variation and more detailed statistical analysis is required. Community response may be limited by a much-depleted local species pool available to colonise the plantings and a lack of connectivity caused by small, isolated populations of many larval food plants.

Bird species richness and abundance have been very variable across sites and over time, with no consistent patterns observed. The bird resources at sites, in particular refuge habitat, are still developing as plants grow. Consequently, a response may take a number of years to become apparent. Alternatively, the bird community may be driven by landscape scale factors and the scale of the streetscape interventions may be too small to be impactful. Longer-term monitoring at reduced frequencies is recommended to better understand the impact of the streetscape biodiversity palette on the local bird communities.