1997. Brody, AK, RJ Mitchell. Effects of experimental manipulation of inflorescence size in the hummingbird pollinated plant Ipomopsis aggregata. Oecologia 110:86-93
Abstract
The role of flowers in excess of those that set fruit remains a question of considerable interest and debate. Pollinators often respond positively to flower number, but studies that demonstrate such a response in the absence of other traits that may be correlated with flower number are rare. If pollinators forage optimally, theory predicts they should prefer plants with large floral displays. By manipulating flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators. In addition, we quantified the ultimate effects of flower number on female reproductive effort. Plants with larger floral displays were most likely to be visited first in any given forgaging bout (P<0.01). Flower number also had positive effects on the number of flowers probed/hour (P<0.01) and the number of flowers probed/approach/hour (P<0.05). However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. Plants did not compensate for producing few flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or increasing seed weight. Pre-dispersal seed predation rates were greater for many- than for few-flowered plants (P<0.001), but this did not offset the potential fitness gains of producing large displays. Our data supports the hypothesis that large floral displays function in enhancing maternal success. Hummingbirds did not behave as the rules of optimal foraging would predict.
Key words: floral display size, optimal foraging, female reproductive success, fruit set, seed set, compensation.