To what degree do urban gardens, restored to near native conditions, contribute towards biodiversity conservation? In Portland, Oregon, several thousand yards have been deliberately planted with native trees, shrubs and herbaceous plants, with variation in the degree of native plantings, the sizes and groupings of the yards involved. We investigated some of the ecological benefits from these widespread but small-scale urban gardens that have been restored. Portland provides an excellent test of several principles of conservation and restoration as well as a repeatable study in other areas having widespread yard habitats.
We measured the relative success of yard habitats in providing food web maintenance by measuring plant percent cover, arthropod abundance, and bird species richness. Bird species richness was measured 2 times per week over a 4-week period in the spring. We compared two neighborhoods, one having a high tree cover index, and a second with a lower tree cover index. We selected 8 replicate yards, each with a minimum number of native shrubs, in each neighborhood. We studied yards having comparable species of shrubs. Both neighborhoods have comparable property values and both have nearby forested greenspaces.
In a separate study of 147 yards planted with native shrubs, bird abundance, species richness, and species diversity as well as the encounter rates for individual bird species were measured over two summers. A minimum of two surveys per yard were implemented each year.
We described variation in arthropods and shrub percent cover at the spatial scale of urban yards. We found the amount of tree cover predicted the bird species richness; the abundance of arthropods predicted bird species richness. In general, the higher the tree cover index, the higher the arthropod abundance and bird species richness. Neighborhood tree cover was a stronger predictor of provision of food web maintenance than the occurrence of native shrub species in yards.
Analysis of data from the second study also showed thart local yard-scale variables, including both native plantings and vegetation structure, had no effects on abundance, diversity, or richness of birds. This analysis also showed that landscape variables, especially tree canopy cover, as well as human population density, impervious surface had an impact on bird diversity and abundance