Projected climate change threatens significant range contraction of Cochemiea halei (Cactaceae), an island endemic, serpentine adapted plant species at risk of extinction

ABSTRACT
Aim: Threats faced by narrowly distributed endemic plant species in the face of the Earth’s sixth mass extinction and climate change exposure are especially severe for taxa on islands. We investigated the current and projected distribution and range changes ofCochemiea halei , an island endemic cactus. This taxon is of conservation concern, currently listed as vulnerable on the International Union for the Conservation of Nature Red List and as a species of special concern under Mexican federal law. The goals of this study are to 1). identify the correlations between climate variables and current suitable habitat for C. halei ; 2). determine if the species is a serpentine endemic or has a facultative relationship with ultramafic soils; 3). predict range changes of the species based on climate change scenarios.
Location: The island archipelago in Bahía Magdalena on the Pacific coast, Baja California Sur, Mexico.
Methods: We used temperature and precipitation variables at 30 arcsecond resolution and soil type, employing multiple species distribution modeling methods, to identify important climate and soil conditions driving current habitat suitability. The best model of current suitability is used to predict possible effects of four climate change scenarios based on best case to worst case representative concentration pathways, with projected climate data from two general circulation models, over two time periods.
Main conclusions: The occurrence of the species is found to be strongly correlated with ultramafic soils. The most important climate predictor for habitat suitability is annual temperature range. The species is predicted to undergo range contractions from 21% to 53%, depending on the severity and duration of exposure to climate change. The broader implications for a wide range of narrowly adapted, threatened and endemic plant species indicate an urgent need for threat assessment based on habitat suitability and climate change modeling.
Keywords : climate change, species distribution modeling, biodiversity, biogeography, range shifts, serpentine adaptation, island endemism, Cactaceae, endangered species

INTRODUCTION

Cactaceae are the 5th most endangered plant or animal family to be globally assessed to date by the International Union for the Conservation of Nature (Goettsch et al., 2015). The primary known threats to populations of cactus species are poaching of wild populations for the horticultural trade, small scale farming and ranching, mining operations, and the effects of climate change (Anderson, Taylor, Montes, & Cattabriga,1994; Hernández & Godínez-Álvarez 1994; Oldfield, 1997; Bárcenas-Luna, 2003; Godínez-Álvarez, Valverde & Ortega Baes, 2003; Téllez-Valdés & Dávila-Aranda, 2003; Martorell & Peters 2005; Ureta & Marti 2012; Goettsch et al.). Aside from these threats, increased risk of extinction in Cactaceae due to climate change exposure is not well understood (Goettsch et al.). This study aims to help fill this gap, using predictive modeling to anticipate the extinction risk due to current conditions and climate change impacts, faced by an island endemic, threatened cactus.
Temperature and precipitation have been shown to be strong correlates for the distribution of plant species (e.g., Guisan & Zimmerman, 2000; Guisan & Thuiller, 2005; Hawkins et al., 2003; Elith & Franklin, 2013). Cacti are often narrowly adapted to specific thermal niches, as well as highly sensitive to seasonal precipitation patterns; for example, the bi-seasonal winter/summer precipitation cycles of the Sonoran Desert region, as well as longer precipitation cycles caused by shifts in the California current and other factors (Gibson & Nobel, 1986; Anderson, 2001). Islands often have both thermal and precipitation differences from their nearest peninsular or continental land masses, and these factors have been shown to contribute to island endemism and increased risk to island biodiversity (Kreft, Jetz, Mutke, Kier, & Barthlott, 2008; Humphreys, Govaerts, Ficinski, Lughadha, & Vorontsova, 2019). Island climates are often significantly distinct from those of adjacent land masses, and, as a result, islands harbor a significant amount of plant biodiversity (e.g., Kreft et al., Kier et al., 2009), yet island ecosystems also host endemic plant species subject to a rate of extinction 500 to 1000 times higher than the background rate (Humphreys et al.).
Ultramafic soils, predominant in the distribution of C. halei , such as ophiolite, amphibolites, serpentine, and gabbros, have been shown to drive plant endemism (Kruckberg 1951; Kazakou, Dimitrakopoulos, Baker, Reeves, & Troumbis, 2008; Botha & Slomka 2017). These soils contain high proportions of heavy metals and low quantities of plant nutrients, and are toxic to most plant species. Consequently, species adapted to these soils have a competitive edge and are able to colonize areas that other plants cannot (Brady, Kruckberg, & Bradshaw, 2005; Harrison, Safford, Grace, Viers, & Davies, 2006; Anacker, Whittall, Goldberg, & Harrison, 2011). To date, no studies of habitat suitability of cacti associated with ultramafic soils have been done, and the importance of this substrate to the distribution of C. halei is unknown.
Climate change is likely to affect the future distribution of many plant species due to shifts in temperature and precipitation (Bakkenes, Alkemade, Ihle, Leemans, & Latour, 2002; Walther et al., 2002; Kelly & Goulden, 2008; Urban, 2015; Warren, Price, Vanderwal, Cornelius, & Sohl, 2018). Prior to 2019, there were very few studies of habitat suitability and the potential effects of climate change in Cactaceae (Téllez-Valdés & Dávila-Aranda, 2003; Martorell & Peters, 2005; Butler, Wheeler, & Stabler, 2012; Albuquerque, Benito, Rodriguez, & Gray, 2018). In particular, climate projections under all representative concentration pathways (RCPs) of atmospheric “greenhouse gasses” and particulates, show increased mean temperatures ranging from 1.5 C to 4.5 C globally, and increased aridification of existing deserts due to larger areas subject to lower amounts of annual rainfall (Collins et al., 2013). Although cacti are adapted to arid conditions, prior studies have shown that they are vulnerable to projected changes in both temperature and precipitation under climate change scenarios (Téllez-Valdés & Dávila-Aranda, Martorell and Peters, Butler et al., Albuquerque et al.). The effect of climate change on the future suitable habitat of C. halei is unknown.
Our investigations include identifying the environmental variables that determine the habitat suitability of Cochemiea halei . Abiotic correlates for the distribution of rare, narrowly restricted endemic species can provide important insight into suitable habitat, possible threats to the persistence of populations, and the potential effects of future climate change (Hawkins et al., 2003; Hijmans & Graham, 2006; Benito, Martínez-Ortega, Muñoz, Lorite, & Peña, 2009; Franklin 2010; Albuquerque, Castro-Díez, Rodríguez, & Cayuela, 2011; Albuquerque, Astudillo-Scalia, Loyola, & Beier, 2019; Albuquerque, Rodríguez, Búrquez, & Astudillo-Scalia, 2019).
Our specific goals are to investigate: 1). the environmental correlates to the distribution of C. halei ; 2). whether populations ofC. halei are more likely to occur on ultramafic soil; 3). whether the species is likely to colonize the peninsula, or if it is more likely to remain isolated on the islands; 4). the effect of varying levels of climate change on the future range and as a contributor to the risk of local and global extinction of C. halei over the next 30 to 50 years. This study will help provide background for urgently needed future analyses of the specific risks faced by narrowly distributed, endemic and endangered cacti and other island endemic plant species.
METHODS