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

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

Natural floral herbivory

Flower petals were naturally damaged both at the bud stage (whose impact was studied here; assessed after corolla opening) and following corolla opening (not studied) by scarab beetles and other insect herbivores (unidentified). To determine the effect of bud stage floral herbivory on subsequent fruit set, flowers were categorized as either damaged or not damaged and fruit was categorized as either matured or uninitiated for analysis. To determine if floral herbivory was randomly distributed among the treatments, data were analyzed by contingency table (2 X 4). Two-sample t tests were performed individually on the floral traits measured to compare flowers with and without natural floral herbivory. The probability of fruit maturation was compared between damaged and undamaged flowers in a 2 X 2 contingency table to determine if fruit development was independent of floral damage. Two-sample t tests were performed to compare total ovule number and seed set of damaged and undamaged flowers.

Pollinator observations

Hawk moth visitation was quantified on three separate nights, totaling 16 person hours of observation. Patches of at least 10 flowers, from naturally occurring plants on the glade (located outside of experimental transects), were observed by one person. Each flower was marked with a numbered flag prior to opening so that pollinators were presented with a range of flower sizes to visit, all from unmanipulated plants. Observations began [tilde]1 h after twilight and lasted for 1.5-2 h, during peak hawk moth activity. All flowers were measured at the end of the observation period for corolla diameter and floral tube length. Logistic regression was used to determine if floral traits explained whether a flower received a visit (response variable) from a hawk moth.

Path analysis

We used path analysis to determine whether the observed impact of experimental leaf damage on seed production was due to a direct effect on plant resources, or instead an indirect effect on floral traits and subsequent plant pollinator interactions. Path analysis was performed using PROC REG (SAS Institute 1991; see Schemske and Horvitz [1988] and Mitchell [1993]). Individual path analyses were performed for plants in the control (natural pollination natural leaf damage) and leaf damage (natural pollination 25% leaf damage) treatments. Corolla diameter was used in path analysis because this trait is potentially an important visual cue for pollinator attraction (Herrera 1993b, Johnson et al., 1995), whereas O. macrocarpa floral tubes are not visible to pollinators. Initial leaf area was the independent variable in the diagram. For each dependent plant variable (mean percentage damage at end of flowering [June census], corolla diameter, and seed number), standardized partial regression coefficients (=path coefficients) were calculated using the STB option in PROC REG (SAS Institute 1991). In path diagrams, one-headed arrows represent a causal effect of one variable on another and U represents unexplained causes. The variable U was calculated as the square root of (1 - [R.sup.2]).