Changes in natural enemy assemblages across an urban to rural landscape gradient

Background/Question/Methods One of the most significant changes to the landscape in the last 100 years is the increasing dominance of urban areas.  Creating new urban areas converts natural habitat into human-dominated ecosystems, resulting in high levels of habitat removal and loss of biodiversity. Little is known about the effects of urbanization on insect natural enemy populations and whether landscape plantings in urban areas harbor natural enemy populations comparable to those in rural areas.  Insect natural enemies are important in urban landscapes because they may suppress insect pests of ornamental plants. This study looked at the effects of large and small scale land cover change on natural enemy assemblages.  At a large scale 500 meters surrounding study sites, we hypothesized that as urbanization increased natural enemy abundance and diversity would decrease. At the scale of individual parcels, which included residential yards and businesses, we expected ornamental landscaping features such as flower diversity would be positively correlated with natural enemy populations.  To test these hypotheses, we monitored natural enemy abundance and diversity at 50 sites across Dane County, Wisconsin.  Urbanization was measured as the percentage of impervious cover within a 500 meter radius around each study site.  We used GIS and land cover maps of impervious surface from the National Land Cover Database to calculate impervious cover.  We selected 15 sites in the city of Madison that had greater than 60% surrounding impervious cover and 15 sites in rural Dane County that had less than 5% surrounding impervious cover.  The remaining sites had intermediate proportions of impervious cover, 6-59%, surrounding the site. Within each parcel, we measured size and spatial arrangement of ornamental plantings, proportion of perennial vs. annual flowers, and proportion hardscape (buildings and pavement).

Results/Conclusions Our results showed natural enemy diversity was 5% lower and abundance was 12% lower in urban sites compared to sub-urban and rural areas.  However, urban sites with a greater proportion of flowering plants had up to 25% more beneficial insects. These results suggest the large scale impacts of urbanization may be partially mitigated by manipulating the design of landscape plantings.