This paper in Urban Forestry and Urban Greening suggests three ways to increase biodiversity in cities.

Consideration of the three key factors influencing biodiversity identified here (grassland extent, land-use in the surroundings, and management intensity) would provide the optimal options for maintaining city biodiversity. Protecting current urban grasslands from development and restricting construction in their surroundings, restoring city wilderness areas using urban spatial planning, and setting up butterfly-friendly management regimes (e.g., mowing in mosaic) could all be future options to help enhance biodiversity in cities.

Urban Forestry and Urban Greening

These sound like measures the average U.S. homeowner’s association will gleefully embrace (#irony).