Astragalus ampullarioides (Shivwits milkvetch) is a federally endangered long-lived herbaceous perennial restricted to soil of the Chinle formation. Adults can live up to 15 years and persist in the ground with no above-ground shoot growth in drought years. Populations occur at the edge of the Mojave Desert in the Southwest corner of Utah, USA. Previous work revealed population decline and a constant risk of mortality through time. Unlike many Astragalus, this species is palatable and susceptible to herbivory of the flowering stalks. Reproductive success is a concern in terms of the loss of flowers, seed parasitism, and possible inbreeding depression. In the spring of 2019, three populations on federal land were visited; Pahcoon Wash (PW), Harrisburg (HB), and Silver Reef (SR). For each population at least 57% of the adult plants were visited to asses herbivory damage. Reproductive output data consisted of sampling one fruiting stalk from individual plants. In a conservative approach only 15% of flowering adults were sampled at two populations, whereas at the smallest of the three populations only 12% were sampled. Individual stalks were evaluated for fruit set and viable seeds, unfilled ovules and insect damage were evaluated for each fruit.
Results/Conclusions
Herbivory effect was similar for all populations with only 10% on average of flowering stalks damaged. Fruits per stalk averaged at 17 between HB and SR, whereas PW averaged higher at 26. Fruit set was relatively high (61%) across the populations; HB having the highest (67%). Viable seed number varied across populations, HB having the highest percent of fruits containing good seed (PW 38%, HB 44%, and SR 16%). A small fraction of pods contained potentially viable seeds (funiculi count) and small nonviable seeds at an average of 8% and 6% respectively. Unfilled ovules made up a significant percent within the fruits (PW 42%, HB 45%, and SR 71%). Impact of insect parasitism was relatively low for two populations (PW and HB), whereas for the largest population (SR) 38% of pods were affected by insect damage. These results indicate even with high fruit set this species is experiencing a significant problem with unfilled ovules across the three populations, particularly at Silver Reef. This may indicate a pollinator problem or inbreeding depression. Because the populations are small and isolated this is an additional threat its persistence.