Variation of pollinator assemblages and pollen limitation in a locally specialized system: the oil-producing Nierembergia linariifolia (Solanaceae)

Abstract

Background and aims: Few studies have examined the dynamics of specialist plant-pollinator interactions at a geographical scale. This knowledge is crucial for a more general evolutionary and ecological understanding of specialized plant-pollinator systems. In the present study, variations in pollinator activity, assemblage composition and pollen limitation were explored in the oil-producing species Nierembergia linariifolia (Solanaceae).

Methods: Pollen limitation in fruit and seed production was analysed by supplementary hand pollination in five wild populations. Pollinator activity and identity were recorded while carrying out supplementary pollination to assess the effect of pollinators on the degree of pollen limitation. In two populations, pollen limitation was discriminated into quantitative and qualitative components by comparing supplementation and hand cross-pollination in fruit set and seed set. The effect of flower number per plant on the number of flowers pollinated per visitor per visit to a plant was examined in one of these populations as a possible cause of low-quality pollination by increasing geitonogamy.

Results and conclusions: Although pollen limitation was evident along time and space, differences in magnitude were detected among populations and years that were greatly explained by pollinator activity, which was significantly different across populations. Floral display size had a significant effect on the visitation rate per flower. Limitation by quality clearly affected one population presumably due to a high proportion of geitonogamous pollen. The great inter-population variation in plant-pollinator interaction (both in pollinator assemblages composition and pollinator activity) and fitness consequences, suggests that this system should be viewed as a mosaic of locally selective processes and locally specialized interactions.

Fig. 1.

Mean number of visits per flower per hour (V_f), pollinator assemblage composition, and relative contribution of the pollinator species to the total number of visits recorded in five populations of N. linariifolia during two years. Populations are ordered by geographic proximity. Key: white for pollen-collecting bees; light grey and hatched grey for small- and medium-sized oil-collecting bees, respectively; dark grey and black for large-sized oil-collecting bees.

Fig. 2.

Oil-collecting bee species visiting the flowers of N. linariifolia: (A) the large-sized Tapinotaspis chalybaea sweeping the entire oil-producing area with the oil-collecting brushes on its long middle legs, with the anthers and stigma in contact with the ventral part of the thorax; (B) the small-sized Chalepogenus parvus collecting oil with its forelegs on a sector of the corolla without pollinating; (C) the medium-sized Ch. nigripes collecting oil with its forelegs, with the anthers and stigma in contact with its forehead; (D) the large-sized Centris tricolor seizing the stamens with its mandibles and sweeping a large area of the corolla with the foreleg oil-collectors; similarly to Ch. nigripes with the anthers and stigma in contact with the forehead.

Fig. 3.

Mean (± s.e) seed set following open-pollination in treated and control plants in N. linariifolia populations studied in 2002 and/or 2003. No significant differences were observed (Tuckey's test: P > 0·05 in all cases). Abbreviations as in Table 1.

Fig. 4.

Pollen limitation in N. linariifolia populations studied in 2002 and/or 2003. (A) Median fruit set, (B) mean (± s.e.) seed set and (C) mean (± s.e.) seed produced per plant. Asterisks indicate a significant increase in fruit, seed set or seed production with supplemented pollination (P < 0·05). Abbreviations as in Table 1.

Fig. 5.

Effect of pollen quantity and quality in female fitness of N. linariifolia. (A) Median fruit set and (B) mean (± s.e.) seed set, following open-pollination, pollen supplementation and cross-pollination in ACH and CU populations. Columns with different letters indicate significant intra-population differences (P < 0·05). Abbreviations as in Table 1.

Fig. 6.

Regression between mean number of visits per flower per hour (V_f) and pollen limitation index (L). L = 1 – (P_o/P_s) where P_o is the seed set of open-pollinated flowers and P_s is the seed set of supplemented flowers; a value of L = 0 indicates no pollen limitation. Regression equation y = 0·52 – 0·1x; R² = 0·67; P < 0·001.