Patch Size, Pollinator Behavior, and Pollinator Limitation in Catnip

Abstract

We examined the effects of the patch size of catnip, Nepeta cataria, on pollinator visitation rates and pollinator limitation. The most important floral visitors were honey bees (Apis mellifera), solitary bees (Halictidae), and bumble bees (Bombus spp.). Our first goal was to see how spatial variation in patch size affected the rate at which individual flowers received pollinator visits (visitation rate). Visitation rate was higher in larger patches for honey bees and bumble bees, but lower for solitary bees. Patch size explained 74-83% of the variation in visitation rate. Intraspecific isolation also had an effect: isolated patches received relatively few visits. Visitation rate depended both on visitor abundance and on the proportion of flowers entered during one visit. All three visitor types wee more abundant in larger patches, i.e., flowers showed mutual attraction of pollinators. Relative to a visit to a small patch, during a visit to a large patch, honey bees visited more flowers but a lower proportion of flowers; solitary bees visited fewer flowers and thus a lower proportion of flowers; and bumble bees visited not only more flowers but a higher proportion of flowers as well. Thus within patches, flowers competed for visits from honey bees and solitary bees but showed facilitation regarding bumble bee visits. Our second goal was to relate patch size and visitation rates to immediate pollinator limitation. Comparisons of the percent of capsules setting seed (fruit set) of open-pollinated, hand-pollinated, and bagged flowers were used to evaluate pollinator limitation. The ranking of fruit set was: hand-pollinated > open-pollinated > bagged. Patch size did not affect fruit set in hand-pollinated or bagged flowers; however, for open-pollinated flowers, fruit set was lower in smaller patches. patch size explained 63% of the variation in pollinator limitation. The effect of patch size comes through its effects on visitation rates. A multiple regression model using visitation rates of the three types of visitors as independent variables explained 67% of the variation in pollinator limitation.