Why is the Shaw's Agave (Agave shawii subsp. Shawii) Not Reproducing? Investigating a Rare Species' Pollination and Germination in a Fragmented Urban Park

Abstract

Shaw's Agave (Agave shawii subsp. shawii) is a rare plant found in only a handful of occurrences within the extreme southwestern corner of the USA in a highly urbanized region. Seedling recruitment of Shaw's Agave in California is visibly low, and viable seed production appears to have been very low in recent years, and has been identified as a primary threat to the persistence of these populations. We performed pollination trials, seed germination studies, and inventoried floral visitors and potential pollinators (including the collection of invertebrates) to determine if pollination and/or germination failure is limiting reproduction. We placed bat detectors and camera traps near blooming agaves, and observed bird and insect visitation to the flowers. Pollination treatments included hand pollination with cross-individual and selfing trials. Our results found a low amount of seed produced per fruit across the pollination trials. Seedling herbivory was also very high (85% and 100%) in the two treatments. The combination of low seed set and high herbivory may help explain the lack of recruitment observed at multiple sites in recent years. No nectar feeding bats utilized the Shaw's agave in our study area. The only significant insect visiting flowers was the honey bee (Apis mellifera), which gathered nectar without touching the anthers. Hummingbird species were also frequent visitors but generally avoided touching the agave anthers. Only the Hooded Oriole (Icterus cucullatus), a migratory bird, appeared to regularly visit flowers for their nectar, while standing on the flowers and brushing against the anthers, potentially providing some pollination but only for plants blooming during the period the oriole is present in the region. Seedling predation was significant, and poses a considerable threat to this population. Further research into seed set and pollination treatments is recommended.