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

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

The number of flowers per plant was positively related to initial leaf area (df = 144, [R.sup.2] = 0.45, F = 72.8, P [less than] 0.001). However, this relationship between plant size and flower number did not translate into an effect on plant fitness: neither naturally pollinated plants with greater initial leaf area (df = 64, F = 0.96, P = 0.33), nor greater flower number (df = 64, F = 0.36, P = 0.55) produced more seeds per plant. Additionally, there was no relationship between plant size (initial leaf area) and corolla diameter (df = 143, F = 0.15, P =0.70).

Effects of increased leaf damage on floral traits

Median flowering phenology per plant did not differ significantly between natural leaf damage and increased leaf damage treatments (median test, df 1, [X.sup.2] = 0.90, P = 0.34), or between natural pollination and hand pollination treatments (median test, df = 1, [X.sup.2] = 1.29, P = 0.26). However, mean number of flowers produced per plant was significantly lower in increased leaf damage plants compared with the natural leaf damage plants (ANOVA, [F.sub.1,144] = 12.69, P [less than]] 0.001). Mean [plus or minus] 1 SE number of flowers was 3.0 [plus or minus] 0.1 and 4.3 [plus or minus] 0.2 for increased leaf damage and natural leaf damage plants, respectively. Both leaf damage and flower order (successive flowering within a plant) affected floral traits throughout the flowering season (Table 3). Corolla diameter significantly decreased (ANCOVA, P [less than] 0.05) and floral tube significantly increased (ANCOVA, P [less than] 0.0001) as subsequent flower order increased (Table 3). Increased leaf damage significantly decreased mean corolla diameter and floral tube length compared with natural leaf damage plants (Table 3, Fig. 2). In summary, increased lea f damage reduced the total number of flowers produced, and decreased corolla diameter and floral tube length. However, increased leaf damage did not change flowering phenology.

Path analysis

Our hypothesized path diagram (Fig. 3A) shows the relationship between initial plant leaf area, percentage leaf damage at end of flowering time (June census), corolla diameter, and seed number. We included a direct effect of leaf damage on seed number (female fitness) and an indirect effect of damage on seed number through the path from corolla diameter to seed number. We also hypothesized that leaf area might have a potential direct effect on total plant seed number (i.e., larger plants would have more resources for seed production) and an indirect effect through the paths percentage leaf damage and corolla diameter (i.e., larger plants might attract more herbivores and/or produce larger flowers).

Results of path analysis of control plants (N = 34) are shown in Fig. 3B (only path coefficients and variables P [less than] 0.05 are shown). Percentage leaf damage was negatively correlated with corolla diameter (P = 0.01). However, the paths from leaf area to percentage leaf damage (P = 0.88) and seed number (P = 0.26), percentage leaf damage to seed number (P = 0.75), and corolla diameter to seed number (P = 0.34) were not significant.