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

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

To examine whether increased leaf damage and/or flower order (successive seasonal flowering within a plant) affected floral traits (corolla diameter, floral tube length), flowers from natural and increased leaf damage plants were compared using analysis of covariance (ANCOVA). The response variable was the individual floral trait and flower order was the covariate. Flower order was used as a covariate to separate potential effects of a plant's seasonal change in flower size from leaf damage effects on flower size. A significant interaction term (leaf damage X flower order) indicates a difference in the slope of the relationship between floral traits and order for the two levels of leaf damage. The median test (Sokal and Rohlf 1995) was used to determine whether median flowering phenology differed for plants in natural and increased leaf damage treatments, by comparing treatment proportions to each side of the overall median flowering date (28 May for 1996). The effect of increased leaf damage and hand pollin ation on mean number of flowers per plant was analyzed by two-way ANOVA. The relationship between initial leaf area and corolla diameter was analyzed by linear regression.

Measurement and analysis of female plant fitness

Flowers in which no ovules developed were classified as uninitiated fruit. Flowers that became fully formed fruits and had at least one seed developed inside were classified as mature fruits. Mature fruits were collected from plants prior to seed dispersal (29 June-18 July) and number of seeds, undeveloped ovules, and aborted seeds were counted (sum of these variables = total ovule number per fruit). Seed mass was calculated as total seed mass (mg) per fruit per number of seeds per fruit. Plant means were then calculated for analysis for total ovule number, aborted seeds, and seed mass. Fruit set per plant was calculated as the ratio of mature fruit/total number of flowers. Total seed number per plant equals the sum of seeds over all mature fruits. Fifteen plants did not flower during the experiment and therefore were not included in any analyses. The distribution of these nonflowering plants was independent of treatment (df = 3, G = 2.0, P [greater than] 0.5).

Fruit set was transformed [arcsine(square root[fruit set 1])] to meet assumptions of normality. To determine if mature and uninitiated fruits differed in floral characters, logistic regression was performed with corolla diameter and floral tube length as explanatory variables and fruit as the response variable. The relationship between floral tube length and seed number per fruit was analyzed by linear-regression using one randomly chosen floral tube per naturally pollinated plant.

The effects of hand pollination and increased leaf damage (June census) on mean total ovule number, aborted seeds, seed mass, fruit set, and seed number were analyzed using two-way ANCOVA, with initial leaf area as a covariate and leaf damage and pollination as the main effects. The relationships between initial leaf area and number of flowers, initial leaf area and seeds per plant, and number of flowers and seeds per plant were each analyzed by linear regression.