Pollination ecology of Digitalis purpurea : Patterns and processes

Abstract

In this thesis I report on several aspects of the reproductive ecology of the short-lived, monocarpic plant Digitalis purpurea L. (Plantaginaceae). Observational field studies are combined with controlled crosses and greenhouse experiments to address questions on pollinator attraction, pollinator behaviour, and possible consequences thereof on mating patterns. Fitness consequences of different matings are investigated through the entire lifetime of progeny. In addition, I focus on the effects of display size and local plant density on pollination and florivory.<br> Bumblebee visitation rates at the plant level increased with display size and plant density, and although bout lengths also increased, the increase was too slow to balance the increased display size. The result was a decrease in per flower visitation rate with display size, indicating that pollinators do not mediate selection for larger display sizes in this population. A significant interaction between density and display size for the proportion of flowers visited per display suggests that density may modify the selective impact of pollinators. The floral herbivore Eupithecia pulchellata Stephens (Lepidoptera: Geometridae) also showed an affinity towards large inflorescences and dense patches, but overall attack rate was low. However, the proportion of flowers attacked increased with inflorescence size, resulting in size-dependent floral damage that could select for smaller display. However, since overall attack rates were low, it is possible that the plants are capable of compensating floral loss at the observed attack rates.<br> The pollination system of D. purpurea (acropetal flower maturation of protandrous flowers in vertical racemes combined with pollinators that start foraging bouts at the base of the inflorescences), was shown to be effective in preventing geitonogamy, only 2.1% of plant visits of bout length two or more involved visitation to a male flower before a female flower. This system was also efficient in terms of both pollen import and export, independent of display size. In view of the short bout lengths (ca 2 flowers) it was surprising that pollen export did not decline with display size. This was due to increased skipping of flowers by pollinators on larger displays, resulting in high visitation rate to male flowers.<br> Inbreeding depression was significant for several life history traits. The 21% reduction in cumulative fitness was relatively small in view of the high outcrossing rate (estimated as 0.96) and may indicate that this population has experienced more selfing during periods of lower pollinator activity. Increased masking of deleterious alleles by the tetraploidy of D. purpurea may also contribute. However, when progeny with different inbreeding levels were grown in the presence of competitors, inbreeding depression was increased, indicating that inbred progeny will suffer larger fitness loss in natural environments. In this population, an optimal outcrossing distance was detected as a consequence of the outbreeding depression found at several life history traits in progeny from 30 m-crossings. Reports on within-population outbreeding depression are rare and my study suggests that outbreeding depression may be more common in autopolyploids than in diploids. The fact that the majority of plant species are believed to be polyploid, merits the inclusion of polyploids in future studies of mating system evolution.