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.