Calescent Drought, Fortuitous Climates and Decline of a Sonoran Desert Cucurbitaceous Vine

Abstract: Tumamoca macdougalii, the Tumamoc globeberry, is a delicate vine of Arizona, Sonora, and northern Sinaloa. The property of the Desert Laboratory on Tumamoc Hill, Pima County, Arizona, is the type locality of both the genus and T. macdougalii. T. macdougalii was listed as an endangered species in 1986 and then delisted in 1993. Three T. macdougalii populations have been monitored during the summer growing season from 2015 to the present. Populations at two of the three sites had been documented at least once in the 1980s–1990s, and at one site individual plants from 1984–1995 and 2007–2014, and then every 2–4 weeks from 2015–2022. The third site was discovered in 2007 and then monitored only from 2015–the present. Specific field procedures evolved over the years, but since 2015 plant size, stage class, flower bud, flower, and fruit counts, and plant condition were recorded. A total of 205 visits among the three sites were made from 1984–2022. Populations declined dramatically between 1996–2006, remained remarkably resilient at very low numbers from 2007–2014, and then experienced a partial rebound in 2015-2018. Survival analysis and population viability analysis indicate very slow recovery. Analyses of climate indices in relation to T. macdougalii population declines show that drought and warming temperatures began immediately following the completion of required post-delisting monitoring and are likely responsible for dramatic population declines from 1996–2006. A brief upswing in favorable climate parameters from 2015–2019 resulted in partial recovery, but since then numbers have declined again, though not as steeply. A recent effort to evaluate status of sites known to support T. macdougalii in Sonora in the late 1980s indicates significant decline in populations paralleling declines in the Tucson area. If T. macdougalii populations are declining in all or a major part of the range, there may be reason to consider putting it back on the list of endangered and threatened species.