Population size, sex-ratio and sexual dimorphism in Fuchsia parviflora (Onagraceae) an endemic dioecious shrub

Background: In small dioecious populations stochastic events may increase biased sex ratios decreasing the viability of populations. Moreover, pollinators may promote pollen limitation if sexual dimorphism related to pollinator attraction, such as flower size and number of flowers are present. Questions: 1) In order to estimate the viability of natural populations of Fuchsia parviflora, population size, sex ratio and fruit production were estimated in natural populations. In addition, number of flowers and floral traits were compared between genders as a way to predict pollinator preferences to particular gender. Studied species: Fuchsia parviflora is an endemic Mexican dioecious, animal-pollinated shrub. Methods: In 2012 and 2013 population sizes and sex ratios were estimated in 12 populations in the states of Michoacan and Jalisco. In addition, the number of flowers and floral traits were estimated between genders. Results: Population sizes varied widely from 14 to 550 individuals and an overall significant male-bias was found. Male frequency varied from 50 to 78 % among populations and some male-biased populations were also the small populations. A significant negative correlation between population size and male frequency was found, but no relationship was found between population size and number of flowers or fruit production. Reproductive plant height varied among populations but not between genders within populations. Male plants had larger number of flowers but floral longevity did not differ between genders. Consistently in all evaluated populations, staminate flowers had longer floral tubes and wider corolla diameter than pistillate flowers. Conclusions: In accordance to stochastic sex ratio fluctuations by small population sizes, it was expected to find populations with male and female-biased sex ratios. However, the absence of female-biased sex ratio suggests that male plants may have a higher survival chance but this hypothesis has to be tested. Although populations seem not to be seed-limited, some populations were cut during this study to plant avocado trees, suggesting the need to implement conservation efforts to preserve populations of F. parviflora.