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