Fitness Impacts Of Herbivory Through Indirect Effects On Plant-Pollinator Interactions In Oenothera Macrocarpa

Ecology, Jan, 2000 by Kristine Mothershead, Robert J. Marquis

RESULTS

Pollinator observations

Flowers with larger corolla diameters were more likely to receive a visit than those with smaller corolla diameter (logistic regression, P [less than] 0.05, Table 1). Floral tube length, however, did not significantly explain variation in flower visitation (logistic regression, P = 0.81, Table 1).

Plant leaf area and leaf damage

Mean initial leaf area ([plus or minus] 1 SE) across all plants was 131.8 [plus or minus] 7.3 [cm.sup.2] (with a range of 29.9-720 [cm.sup.2]). For natural leaf damage plants there was no relationship between initial leaf area and percent damage at the 14-19 June census (df = 72, F = 2.3, P = 0.14) or the 16-19 July census (df = 72, F = 0.04, P = 0.84). The mean [plus or minus] 1 SE percentage leaf damage (June) was 6.5% [plus or minus] 0.01 for natural leaf damage plants and 33.4% [plus or minus] 0.01 for increased leaf damage plants (ANOVA, [F.sub.1,143] = 1149, P [less than] 0.0001). The seasonal range of leaf damage to plants was 0-59% for natural leaf damage plants and 21-44% for increased leaf damage plants. Thus, experimental levels of leaf damage fell well within measured natural levels of damage.

Female plant fitness

Hand pollination significantly increased fruit set per plant (ANCOVA, P [less than] 0.0001, Table 2), while damage significantly decreased fruit set (ANCOVA, P [less than] 0.05, Table 2). Comparisons of least-square means (Fig. la) revealed the difference in fruit set between control and damage treatment was marginally significant (P = 0.063); hand pollination and hand damage fruit set did not differ significantly (P = 0.142). Number of seeds per plant was significantly affected by pollination, damage, and initial leaf area, with hand pollination plants producing more seeds than natural pollination plants, and natural leaf damage plants producing more seeds than increased leaf damage plants (Table 2, Fig. 1b). Hand pollination did not affect the number of flowers per plant (ANOVA, [F.sub.1,144] = 1.04, P = 0.30). There were no significant treatment effects on total ovule number, aborted seeds, or seed mass per plant (ANCOVA, [F.sub.3,130] [less than] 2.9, P [greater than] 0.15).

Data from naturally pollinated study plants provide further information about plant-pollinator interactions in the system. For these plants, pollinators apparently visited with greater frequency flowers with larger corolla diameters, as corolla diameter was a significant positive predictor of fruit maturation in these plants (logistic regression, P [less than] 0.05, Table 1). In contrast, floral tube length did not significantly predict flowers that produced a mature fruit (logistic regression, P = 0.25, Table 1). However, there was a significant positive relationship between floral tube length and the mean number of seeds per fruit (df = 63, [R.sup.2] = 0.24, F 19.35, P [less than] 0.0001) in naturally pollinated flowers. This latter result suggests that once a flower is visited, pollinator efficiency increased with longer floral tube lengths.