Entomogamy in wildflowers: What level of pollinator biodiversity is required?

Wildflowers are gaining increasing importance due to their beneficial impact on biodiversity and thus the health of ecosystems. Unfortunately, their mutualism with pollinator vectors that transport pollen to stigmas makes their survival vulnerable. The aim of this experiment was to verify the inbreeding depression in terms of seed-set of sixteen rare wildflower species, in the complete absence of pollinators. Despite being strongly species-dependent, this inbreeding depression was found to be consistent in perennial species. Conversely, the effects of self-pollination on other biological parameters such as 1,000 seed weight, germinability and viability were decidedly weaker and non-converging in the various wildflowers species. Each had differing flowering calendars for attracting a species-dependent intensity and biodiversity of visits by pollinators grouped into bees, bumblebees, solitary bees, hoverflies, bee flies, and butterflies. By correlating pollinator biodiversity during open pollination (Shannon index, H’) with the inbreeding depression (δ) caused by self-pollination (bagged flowers), a statistically significant inverse linear relationship (p < 0.05) between these two parameters was found. This linear regression highlights the lower need for biodiversity by the “generalist” wildflower species and a greater need in the “specialized” species characterized by a closer evolutionary mutualism with more restricted pollinator taxa. These data are discussed in terms of the risk of feed-back rarefaction of wildflowers and pollinators in both natural and agricultural ecosystems. Finally, strategies are suggested for promoting the ecological sustainability of wildflower biodiversity.