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 4C 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.