The study site
Bahía Magdalena is an ecologically significant embayment along the Pacific coast of the southern Baja California peninsula (Bizzarro, 2008). In contrast to the adjacent coastal plains, the island archipelago in Bahía Magdalena is part of the North American cordillera, and has mountainous, rocky terrain as a result (Rangin, 1978; Blake, Jayco, & Moore, 1984; Sedlock, 1993). (Fig. 1).
The islands range in elevation from sea level along the bay coastline to nearly 1000 m. (Blake et al., 1984). Several topographical variations on the islands create heterogeneous terrain, including flats, sand dunes, gravel coastlines and highly eroded arroyos. Westerly cliffs drop to the Pacific, at angles as steep as 90. These cliffs are predominantly exposed ultramafic rock and gravel (Rangin, 1978; Blake et al., 1984).
The main soil mineralogical composition is serpentine rock and its eroded derivatives or non-ultramafic basalt and sand (Rangin, 1978; Blake et al., 1984; Sedlock, 1993). The mountainous ridges consist of ultramafic, oceanic crustal rock formed through tectonic plate collisions estimated to have occurred from the Late Jurassic to the Late Cretaceous periods (Sedlock, 1993, Zaitsev, Sánchez-Montante, & Robinson 2007).
Climate data from WorldClim v. 2.0 (Fick & Hijmans, 2017) shows biseasonal summer and winter precipitation, with autumn and spring being the dry months of the year. The moderating effects of the California current system create narrower diurnal and annual temperature ranges, increased precipitation and cooler seasonal averages for the islands than for the adjacent peninsula (Hickey, 1979; Bakun, 1990; Robinson, Gómez-Gutiérrez, & Gómez-Aguirre, 2007; Bizzarro, 2008). From 2010 to 2018, 13 tropical storms or hurricanes occurred in the study region, with the majority occurring in the hottest month, September (National Hurricane Center; Hurricane Research Division; Central Pacific Hurricane Center).
The vegetation of the Bahía Magdalena region features 18 endemic angiosperm taxa; the endemic cacti represent 33% of the plant endemism in the area (León de la Luz, Medel-Narvez, & Dominguez-Cadena, 2015). The area is recognized as one of nine regions of high plant endemism in Baja California (Reimann & Ezcurra 2007). The primary vegetative regime is fog crassicaulescent and sarcocaulescent scrub, that is, a combination of leaf and stem succulents, such as the endemic Agave margaritae Brandegee and scrub vegetation, generally less than 8 m high (Rebman & Roberts, 2012, León de la Luz et al.).
Cochemiea halei Walton, the study species, is a mat forming stem succulent with straight spines and presumably hummingbird pollinated flowers (Craig, 1945; Pilbeam, 1999). C. halei is of conservation concern, assessed as vulnerable by the IUCN, and protected by Mexican law. The factors leading to its classification as vulnerable are the narrow geographic range, the low overall population size, and evidence of declining populations (IUCN). No formal studies of its population viability or quantified risk of extinction have been conducted previously, however.