FLOWER‐TO‐SEEDLING CONSEQUENCES OF DIFFERENT POLLINATION REGIMES IN AN INSECT‐POLLINATED SHRUB

Insect pollinators of the Mediterranean shrub Lavandula latifolia (Labiatae) differ in components of pollinating effectiveness that may influence seed production and progeny quality. This study tests the prediction that L. latifolia flowers exposed to pollination regimes differing in pollinator composition should differ in their expected contribution to population recruitment in the field, as measured in terms of established seedlings, because of differences in seed production, progeny quality, or both. Using a randomized complete block design, two pollination regimes were induced during a 3‐wk period on each of 15 L. latifolia plants by alternately exposing flowers in the two halves of each shrub to natural pollinators during two nonoverlapping diurnal periods (0930–1630, MD regime; dawn to 0930 and 1630 to dusk, R regime). The two pollination regimes differed in pollinator composition. The importance as flower visitors of butterflies and small bees was greater during the MD regime, which was also characterized by higher flower visitation rate and higher probability of visitation per time unit of individual flowers. MD flowers set proportionally more fruits, and each fruit contained more seeds, than R flowers. Seed production per inflorescence was also greater in MD than in R half‐plants. Seeds from the MD and R regimes did not differ significantly in either mean mass, probability of germination under laboratory conditions, or probability of producing a seedling when planted in a greenhouse. When planted in the field, seeds originating from MD flowers had a greater probability of producing a seedling than those originating from R flowers, the difference being greatest where soil characteristics were most adverse for early seedling growth. In the field, MD and R seedlings did not differ in post‐emergence survival rate and size of eventual survivors. The estimated average probability (±1 se ) of one ovule eventually producing one third‐year, established seedling in the home environment was significantly greater for MD (0.0192 ± 0.0041) than for R (0.0085 ± 0.0020) flowers. This demographic advantage of MD ovules was mainly accounted for by the greater seedling emergence rate of MD seeds, and only secondarily by the greater fruit‐ and seed‐set of MD flowers. Observed patterns may be explained by a combination of increased outcrossing derived from the greater relative importance of small bees and butterflies as pollinators, and increased gametophytic competition resulting from increased overall visitation rates to individual flowers during the MD pollination regime. This study suggests that even relatively minor differences in pollinator composition and abundance may have a measurable demographic impact on plant populations.