The degree of synchrony in reproductive activity has long interested plant ecologists and the effects of fragmentation on synchrony are not generally known. Fragmentation could result in patches that are either more or less synchronous than an unfragmented population. Thus fragmentation, via its effect on synchrony, could affect interactions with pollinators, plant reproduction, gene flow, and selection on flowering time. Echinacea angustifolia, a long-lived species native to the tallgrass prairie, exists primarily in heavily fragmented habitat that contributes to substantial mate limitation in combination with temporal asynchrony. However, high variation in the number of flowering individuals each year makes comparisons of flowering synchrony difficult. We used coefficients of variation to quantify the variability in the number of flowering individuals among years in clusters of E. angustifolia between 1999 and 2015. We defined clusters using a weighted average of the maximum foraging range (MFR) of the eight most common genera of E. angustifolia pollinators. MFR was estimated using a model that accounts for approximately 78 percent of the variation in MFR based on the intertegular span of the pollinator. We used the weighted average of MFR, 240m, to distinguish clusters. Using this threshold, we divided the 5km x 5km study site in west central Minnesota into 20 clusters of E. angustifolia individuals.
Results/Conclusions
Defined clusters exhibited high variability in mean number of flowering individuals per year over the 16-year observation period; 1.6 in the smallest cluster and 278 in the largest cluster. Coefficients of variation also ranged widely (0.40 to 0.96). Clusters differed from each other in total reproductive synchrony over the course of the study. The coefficient of variation for all clusters was larger than the coefficient of variation for the clusters in aggregate (0.38). Additionally, the coefficient of variation for the annual number of flowering plants between 1999 and 2015 per cluster declined with the mean number of flowering plants per cluster. This relationship suggests that the year an individual reproduces influences its mating opportunities to a greater extent when that individual is in a small cluster compared to a large cluster.