The effects of climate change on the range of Cochemiea
halei
For all projections, loss of between 21%–53% of suitable habitat is
predicted for the species. In the case of the lower representative
concentration pathways (RCPs), range contraction is partially offset by
expansion into previously unsuitable habitat. As the climate change
scenarios increase in RCP, especially over the longer time period to
2070, expansion is reduced significantly. (Table 1) The range maps
showing projected future areas of contraction and expansion for C.
halei indicate the greatest potential loss of habitat is on Isla
Margarita, with regions on that island accounting for 40% to 65% of
the total contraction. Only two scenarios predict expansion to the
peninsula under the effects of climate change, and in both cases, the
predicted new suitable habitat is an isolated patch. (Figs. 7 and 8).
Boxplots of the effect of the two most important predictors, annual
temperature range and mean temperature of the warmest quarter, on future
habitat suitability indicate significant changes for all climate change
projections to 2070. The greater variability of annual temperature range
for range contraction areas is consistent with the species having more
suitable habitat within a narrower temperature range. The significantly
higher mean temperatures of the warmest quarter for all projected future
areas also contribute to habitat loss. Predicted areas of expansion also
feature higher temperatures, which is a result of temperatures across
the study site increasing due to climate change. (Fig. 9).
DISCUSSION
We investigated the effects of environmental and soil variables on the
distribution of Cochemiea halei , as well as the possible impacts
of climate change, using species distribution models. Our results show
significant factors affecting the current distribution of the species
and potential threats to the persistence of the species under climate
change as a result of significant range contraction.
The island endemism of Cochemiea halei is strongly correlated
with both soil and climate effects. The island archipelago in Bahía
Magdalena, the primary suitable habitat for C. halei , has
significantly different soil and climate from the nearby peninsula.
These contrasting conditions are similar in other island habitats near
coastal areas along the Pacific Ocean, where conditions are
significantly different even a short distance inland (Reimann &
Ezcurra, 2005; Bizzarro, 2008; Ratay, Vanderplank, & Wilder, 2014).
Cochemiea halei occupies a narrow range of temperature and
precipitation correlates. The moderating temperature effects of the
California current system (Hickey, 1979; Huyer, 1983; Bakun, 1990;
Robinson et al. 2007) are important to habitat suitability for the
species. The two most influential climate variables in the best model
were annual temperature range and average temperature of the warmest
quarter, accounting for approximately 73% of the model’s predictive
power. Both of these variables show significantly lower values on the
islands than on the peninsula, patterns typical of coastal areas
moderated by the upwelling of the California current system, especially
in summer (Bakun, 1990). The temperature range on the islands is
approximately 10○ C narrower than on the peninsula,
and the mean temperature of the warmest quarter is 4○C cooler. The bi-seasonal precipitation patterns of the Sonoran Desert
region (Shreve & Wiggins, 1964) are represented by the influence on
model performance of the precipitation of both the warmest and coldest
quarters. However, precipitation is lower on the islands than inland,
during both seasons, with the most significant differences occurring
during the warmest quarter. These localized effects have been shown to
drive endemism (Hijmans & Graham 2006; Snyder, Sloan, Diffenbaugh, &
Bell, 2003; Gogol-Prokurat, 2011; Humphreys et al., 2019). C.
halei is an example of a species that has a localized, well-defined
climate response, with the highest probability of suitable habitat
predicted to be within a relatively narrow band of thermal and
precipitation parameters.
In addition to strong climate influences on the current distribution ofCochemiea halei , soil type plays an important role. Narrowly
restricted endemic plant species, including cacti, have been shown to be
strongly dependent on soil types for habitat suitability (Kruckberg,
1951; Kruckberg & Rabinowitz, 1985; Harrison et al., 2006). Several
studies of plant distributions have determined the importance of
ultramafic soils in particular as a driver of plant endemism (e.g.,
Kruckberg, Kazakou et al., 2008; Botha & Slomka, 2017). While C.
halei does not appear to be an obligate endemic to ultramafic soils,
the species is more likely to occur on those soil types, with 60% of
occurrences on ultramafic soil. This is similar to other species in the
Cactaceae that occur on ultramafic soils, in particular on the islands
of Cuba and Puerto Rico (Reyes-Fornet, Fornet-Hernandez, & Martinez
Ondaro, 2019). Obligate and facultative adaptations to ultramafic soils
have been shown to provide a competitive advantage (Brady et al., 2005;
Anacker et al., 2011; Harrison et al., 2006; Pollard, Reeves, & Baker,
2014). The model with soil type had stronger predictive performance, and
indicated a more fragmented, lower habitat suitability for areas in our
surveys where population density was low, suggesting that ultramafic
soils are an important constraint on the distribution of the species.
Cochemiea halei ’s observed establishment on virtually unweathered
ultramafic rock and exposed gravel, in addition to lower precipitation
on the islands than on the peninsula, suggest that the species is
adapted to evaporation of soil moisture and drier conditions, a common
characteristic of cacti distributed in rocky environments (Gibson &
Nobel 1986). In summary, C. halei favors cooler, drier habitat,
on ultramafic rock and soil, with a moderated annual temperature range,
a suite of abiotic predictors that characterize the island habitat in
contrast to the nearby peninsula.
Consequently, Cochemiea halei is not likely to migrate to the
peninsula except for small foothold regions along the peninsular shore.
This characterizes C. halei as a “stranded” endemic, making its
persistence more vulnerable to changes in climate (Crawford & Stuessy,
1997; Cowie & Holland, 2006; Stuessy, Takayama, López-Sepúlveda, &
Crawford, 2014). The only known peninsular population of C. haleiconsists of approximately six individuals limited to a patch of sand
measuring 150 m2. At that site, there is no sign of
dispersal in the surrounding area, in spite of the plants being large,
seed bearing and apparently well-established (Gorelick, 2007).
The suitable habitat for Cochemiea halei is a patchwork of sites
even within its narrow range on the islands. Major geographical
distinctions within the islands that are illustrated on the prediction
map from the model with the best predictive ability (Fig. 4) include two
distinct regions on Isla Margarita, zones of less suitable habitat on
Isla Magdalena, and a narrow zone of suitability at Cabo San Lazaro,
with few areas of on-the-ground connectivity between suitable habitats.
Endemic plant species often occur in fragmented habitat with
geographical barriers and low connectivity between sites (Rabinowitz,
1981; Kotliar & Wiens, 1990). As a narrowly restricted endemic,
essentially stranded on the islands, the species is at increased risk
for stochastic environmental, demographic and genetic setbacks
(Ellstrand & Ellam, 1993; Lande, 1993; Menges, 1992; Matthies, Bräuer,
Maibom, & Tscharntke, 2004; Melbourne & Hastings, 2008; Mubayi, Kribs,
Arunachalam, & Castillo-Chavez, 2019). Even without the impacts of
climate change exposure, the species appears to be at elevated risk for
local extinction events, population bottlenecks and increased
fragmentation.