The Consequences of Pollination Competition on the Reproduction of Astragalus bibullatus

Abstract

Morphological and phenological similarities of flowers may result in species competing for pollinators, and this competition may be particularly intense for rare species. Astragalus bibullatus is a federally endangered legume endemic to the cedar glades of middle Tennessee. It co-occurs with a nearby flower homolog, Pediomelum subacaule, which is abundant and significantly overlaps in flowering time. In this study, the competitive effects of P. subacaule on the reproduction of A. bibullatus were assessed by comparing pollinator visitation rates, quantifying the constancy of pollinator taxa, estimating the frequency of interspecific pollination via field stigma collection, and determining the effect of foreign pollen on seed set. The most common pollinator in 2022 for both plants was the eastern bumble bee, Bombus impatiens, which visited P. subacaule significantly more frequently, indicating that P. subacaule may be favored by Bombus impatiens and thus reducing pollination of A. bibullatus. Despite its low abundance in 2023, blue solitary bees, Megachilidae, had a significantly higher visitation rate to A. bibullatus, indicating that blue solitary bees may be essential to A. bibullatus pollination. I used a Constancy Index with a scale of 0 (completely inconstant) to 1 (completely constant) and determined the average constancy of all pollinators was 0.899 in 2022 and 0.982 in 2023. To determine the frequency of foreign pollen deposition, 100 stigmas of A. bibullatus and P. subacaule were collected, and the number and species of pollen grains were recorded. The frequency of foreign pollen grains present in samples was significantly higher in A. bibullatus (22% of stigmas) than in P. subacaule (4%), which is likely affecting the reproduction of A. bibullatus. Racemes of A. bibullatus were subjected to treatments of conspecific pollen, mixed pollen, and foreign pollen. Although the sample size was drastically reduced in 2022, the experiment was repeated in 2023 and racemes treated with conspecific pollen had a significantly higher frequency of fruit production (60.71%) than mixed (28.13%) or foreign pollen treatments (3.5%). The results of my study indicate these two plant species are competing for pollination services, consequently reducing pollinator visits to Astragalus bibullatus and increasing the frequency of foreign pollen deposition. Future management plans should consider the possible negative effect of P. subacaule on pollination and aim to monitor the essential pollinators of A. bibullatus, such as eastern bumble bees and blue solitary bees.