Pollinator preference for large flowers persists when climate induces the expression of smaller flowers

Abstract

Premise: In arid environments, large flowers can be disadvantageous owing to excess transpiration through petals and other floral organs, often resulting in the expression of smaller flowers. Larger flowers, which serve as visual attractants, can provide greater rewards compared to smaller flowers resulting in a greater number of floral visitors. Environmental conditions can also shift community compositions of pollinators across landscapes resulting in different preferences. Therefore, pollinator preferences and abiotic factors may conflict or act synergistically to shape floral size across elevation. Methods: For four years, we examined how Iris missouriensis (Iridaceae) flower size changes across dynamic abiotic and biotic conditions. We monitored populations along a steep environmental gradient in the Colorado Rocky Mountains to investigate the relationship of flower size, elevation, and climate. For two of the years, we also established arrays of artificial flowers across the same gradient to evaluate the flower size preference of floral visitors, while controlling for floral traits other than size. We recorded visitations to these arrays across two years to capture interannual variation in the preferences of floral visitors. Results: Consistent with expectations based on clinal gradients in precipitation, we found that flower size increased with elevation in most years, resulting in smaller flowers in arid low elevation sites. However, that pattern was lost in 2023 when temperatures were the coolest and early season precipitation was high. In our artificial floral arrays, larger flowers received more visitors relative to smaller flowers even in sites where naturally recruiting plants had much smaller flowers. These results are consistent with climate-induced clinal variation in I. missouriensis floral size where floral visitor preference does not track local flower size. Conclusions: As climate change progresses, conflicting effects from pollinators and abiotic forces could alter the expression and evolution of flower size, potentially disrupting plant reproduction for animal-pollinated species. Consistent pollinator preference for large flowers, even in sites with naturally small flowers, indicates a potential mismatch between biotic and abiotic forces on floral trait expression.