Butterfly populations are declining globally, and habitat degradation due to increasing urbanization is a major contributing factor. Butterfly gardens may be important for minimizing the loss of butterfly diversity as urbanization increases, especially considering that in many cities the amount of privately-owned green space far exceeds that of public parks. The objective of this study was to determine which characteristics of butterfly gardens have the greatest influence on adult butterfly diversity and abundance. Butterfly surveys were conducted at 26 garden sites in and near Louisville, KY from May–October 2018. Sites included school gardens, private gardens, and gardens in public parks. The following variables were measured at each site: Percent impervious surface within 0.5km (3–87%), garden area (18–2,165m2), plant species richness (8–65 species) and plant biovolume density (0.02–0.65m3/m2). Surveys were conducted by walking slowly throughout the site and recording each butterfly seen, pausing the timer while identifying individuals to species. Surveys used a rule-based stopping method with a base length of 20 minutes. New species appearing in the final five minutes extended the survey 10 minutes. Surveys ended when no new species appeared in the final five minutes. Regression analyses were conducted using glm in R.
Results/Conclusions
Analysis of butterfly species richness showed a significant interaction between impervious surface percentage and plant species richness (p=0.035). Butterfly species richness declined with increasing impervious surface percentage when garden plant species richness was low, and increased with increasing plant species richness when impervious surface percentage was high. There was also a significant main effect of garden area (p=0.035). As garden area increased so did butterfly species richness. Analysis of butterfly abundance showed three significant interactions: impervious surface percentage with garden area (p=0.01), impervious surface percentage with plant biovolume density (p=0.001), and plant biovolume density with plant species richness (p=0.03). Butterfly abundance decreased with increasing impervious surface percentage when garden area and plant biovolume density were low, and increased with increasing garden area and plant biovolume density when impervious surface percentage was high. Butterfly abundance increased with increasing plant biovolume density or plant species richness when the other variable was low. These results indicate that large, plant-diverse gardens are able to somewhat mitigate the negative effects of high impervious surface percentage on butterfly diversity and abundance. Recommending to landowners that they increase their garden plant diversity or expand their gardens by replacing some lawn area could aid urban butterfly conservation considerably.