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Biological Journal of the Linnean Society, 2014, 111, 450–461. With 4 figures
Late Pleistocene to Holocene distributional stasis in
scorpions along the Baja California peninsula
MATTHEW R. GRAHAM1,2*, ROBERT W. BRYSON,
JR3
and BRETT R. RIDDLE1
1
School of Life Sciences, University of Nevada Las Vegas, Las Vegas, NV 89154, USA
Department of Biology, Eastern Connecticut State University, Willimantic, CT 06226, USA
3
Department of Biology and Burke Museum of Natural History and Culture, University of
Washington, Seattle, WA 98195, USA
2
Received 5 July 2013; revised 5 October 2013; accepted for publication 5 October 2013
The biota of the Baja California peninsula (BCP) assembled in response to a complex history of Neogene tectonics
and Quaternary climates. We constructed species distribution models (SDMs) for 13 scorpion species from the BCP
to compare current suitable habitat with that at the latest glacial maximum about 21 000 years ago. Using these
SDMs, we modelled climatic suitability in relation to latitude along the BCP. Our SDMs suggested that most BCP
scorpion distributions have remained remarkably conserved across the latest glacial to interglacial climatic
transformation. Three areas of climatic suitability coincide remarkably well with genetic discontinuities in other
co-distributed taxa along the BCP, indicating that long-term persistence of zones of abrupt climatic transition offer
a viable alternative, or synergistic enhancement, to hypotheses of trans-peninsular seaways as drivers of
peninsular divergences. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014,
111, 450–461.
ADDITIONAL KEYWORDS: ecological niche model – Mexico – Quaternary – Scorpiones – species distribution model.
INTRODUCTION
Quaternary climate change has profoundly influenced
the associations of species, communities, ecosystems
and biotas. Cooler climates during the last glacial
maximum (LGM, c. 21 000 years ago) followed by the
period of warming (albeit not a simple linear warming
trend) from the LGM until the present represents an
episode of climatic transition that can be deciphered
for understanding how species and biotas might
respond to current and future climatic warming.
Molecular genetics (e.g. Klicka & Zink, 1997; Avise,
2000) and palaeontology (e.g. Betancourt, Van
Devender & Martin, 1990; Macfadden, 2006) have
provided keen insights into the evolutionary
responses of species to climate change during this
period of time. Species distribution modelling (SDM),
also referred to as ecological niche modelling (see
*Corresponding author. E-mail: [email protected]
450
Saupe et al., 2012), is emerging as an additional tool
for assessing potential species responses to changing
climate. By using climate data to derive SDMs, and
then projecting these models onto climatic simulations of the LGM (‘hindcasting’), investigators seek
to reconstruct the impact of Quaternary climate
change on the distributional responses of species. One
outcome might be the emergence of common response
patterns within and among co-distributed species,
providing a tool for predicting the future assembly
and disassembly of regional biotas.
The Baja California peninsula (BCP) of Mexico is
the second longest terrestrial peninsula in the world,
and possesses a diverse array of desert and tropical
dry forest environments. Originally, the BCP (Fig. 1)
was thought to have a dispersal-dominated biotic
history (Savage, 1960), with desert species leaving
and re-entering the peninsula in concert with glacial–
interglacial climatic cycles. Later advances in our
understanding of plate tectonics, however, facilitated
complex vicariant hypotheses that better explained
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
DISTRIBUTIONAL STASIS IN BAJA CALIFORNIA SCORPIONS
451
Figure 1. Map of the Baja California peninsula with major geographical regions delineated and hypothesized transpeninsular breaks shown: A, mid-peninsular break; B, Loreto break; C, La Paz break.
the distribution of biodiversity along the BCP (e.g.
Hess, 1962; Murphy, 1983; Grismer, 1994). Starting
with seafloor spreading, rifting and volcanic origination of islands that coalesced into the modern peninsula during the mid-Miocene (e.g. Lonsdale, 1989;
Spencer & Normark, 1989; Stock & Hodges, 1989),
geological activity is now widely held to have heavily
influenced the diversification, distribution and composition of the BCP biota (Carreño & Helenes,
2002).
Recent studies have revealed a number of spatially
congruent genealogical breaks across the BCP (Upton
& Murphy, 1997; Riddle et al., 2000; Murphy &
Aguirre-Léon, 2002; Crews & Hedin, 2006; Lindell,
Méndez-de la Cruz & Murphy, 2008; Bryson et al.,
2012). The spatial congruence of molecular breaks
across these disparate taxonomic groups prompted
the hypothesis that vicariant events driven by the
formation of trans-peninsular seaways could have
isolated populations that subsequently reunited at
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
452
M. R. GRAHAM ET AL.
contact zones following a period of evolutionary divergence (Lindell, Méndez-de la Cruz & Murphy, 2005,
2008). Others have suggested that genetic discontinuities can be explained by abrupt habitat transitions
created by latitudinal gradients in climatic patterns,
as an alternative to trans-peninsular seaway
vicariance (Grismer, 2002; further discussed by
Riddle & Hafner, 2006). A third possibility is that
abrupt habitat transitions operated synergistically
with trans-peninsular seaways to reinforce long-term
isolation of populations (Riddle & Hafner, 2006).
To examine the potential impact that historical
climate change has had on biodiversity along the
BCP, we modelled the current and past distributions
of 13 scorpion species representing four families
(Caraboctonidae, Chactidae, Scorpionidae, Vaejovidae) distributed along the peninsula. Scorpions are
particularly diverse and well documented on the BCP
and occupy an array of habitat types (Williams, 1980;
Due & Polis, 1986). Quaternary climate fluctuations,
such as the period between now and the LGM, have
driven profound distributional shifts in many desert
species (e.g. Jezkova et al., 2009; Oláh-Hemmings
et al., 2010; Wilson & Pitts, 2012; Graham et al.,
2013a, 2013b). Based on these studies, we predicted
similar dynamic shifts in the reconstructed distributions of scorpions along the BCP as desert habitats
contracted and expanded. The absence of large shifts
in suitable habitat between the LGM and the present
day would support an alternative hypothesis of stability in climatically mediated distributions.
MATERIAL AND METHODS
SCORPION OCCURRENCE DATA
Occurrence records for 13 species of scorpions from
the BCP (Table 1) were obtained from museum
records (California Academy of Sciences and San
Diego Natural History Museum), private collections
and from the literature (Stahnke, 1968, 1969;
Williams, 1970a, 1970b, 1970c, 1971, 1974, 1980;
Williams & Lee, 1975; Sissom, 1991). Although over
60 species of scorpion are found on the BCP
(Williams, 1980), these 13 species were selected
because of their relatively wide distributions across
Table 1. Families, distributions, sample sizes and measures of niche shifting for 13 scorpion species from the Baja
California peninsula
Species
Family
Distribution
Total no.
of samples
Test
samples
Centroid
distance (km)
Change in
area (km2)
Anuroctonus pococki
Soleglad & Fet, 2004
Hoffmannius puritanus
(Gertsch, 1958)
Kochius hirsuticauda
(Banks, 1910)
Hadrurus concolorous
Stahnke, 1969
Hadrurus pinteri
Stahnke, 1969
Kochius bruneus
(Williams, 1970)
Bioculus comondae
Stahnke, 1968
Hoffmannius diazi
(Williams, 1970)
Hoffmannius vittatus
(Williams, 1970)
Nullibrotheas allenii
(Wood, 1863)
Bioculus caboensis
(Stahnke, 1968)
Hadrurus hirsutus
(Wood, 1863)
Kochius punctipalpi
(Wood, 1863)
Chactidae
Northern
138
34
41.3
Vaejovidae
Northern
38
9
188.9
–62 421.44
Vaejovidae
Northern
51
12
199.0
27 423.26
Caraboctonidae
Central
60
15
120.6
Caraboctonidae
Central
21
5
5.0
–41 827.46
Vaejovidae
Central
30
7
260.6
85 479.16
Scorpionidae
Southern
19
4
33.5
–2672.04
Vaejovidae
Southern
29
7
64.6
12.92
Vaejovidae
Southern
36
9
44.2
–31 697.43
Chactidae
Southern
47
11
101.6
6425.01
Scorpionidae
Cape
18
4
16.4
–33 073.46
Caraboctonidae
Cape
27
6
21.71
–15 927.77
Vaejovidae
Cape
30
7
19.1
–25 568.22
–104 606.4
–12 821.9
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
DISTRIBUTIONAL STASIS IN BAJA CALIFORNIA SCORPIONS
one or more of the northern, central, southern and
Cape regions of the BCP. Although common along the
BCP, we did not include Centruroides exilicauda in
this study because genetic data suggest that it might
represent a complex of cryptic species (Gantenbein,
Fet & Barker, 2001). We used Google Earth (http://
earth.google.com) to estimate latitude and longitude
(Appendix S1) for specimens lacking GPS coordinates
using standard georeferencing techniques (Chapman
& Wieczorek, 2006). We used only records with
maximum error distances under 5 km to match the
resolution of the climatic layers. We adhere to the
taxonomic nomenclature proposed by Soleglad & Fet
(2008), but refer readers to Williams (1980) for alternative classifications that are still commonly used.
SPECIES
DISTRIBUTION MODELLING
We used the program Maxent v.3.1.0 (Phillips, Dudik
& Schapire, 2004) to develop SDMs for the 13 scorpion species. The Maxent algorithm has been shown
to perform well in a comprehensive assessment of
modelling approaches (Elith et al., 2006), and it uses
presence-only species occurrence records and environmental data in the form of GIS layers to approximate
environmental requirements for a species by finding
the distribution that maximized entropy, subject to
constraints imposed by the occurrence records. In
other words, the values of the environmental (in this
case climatic) variables act as constraints on the
unknown distribution, forcing the mean and variance
of the output SDMs to approximate that of the occurrence data values.
Current (0 kya) and LGM SDMs were developed for
each of the 13 scorpion species using the logistic
regression output of Maxent. For 0 kya climatic conditions, models were reconstructed using 19 global
bioclimatic layers (temperature and precipitation
information) downloaded from the WorldClim database (Hijmans et al., 2005) at a resolution of 2.5 min.
Current models were projected onto two general circulation models simulated for the LGM at approximately 21 kya, the Community Climate System
Model (CCSM; Otto-Bliesner et al., 2006) and the
Model for Interdisciplinary Research on Climate
(MIROC v.3.2; Hasumi & Emori, 2004). Since there
can be slight variation in SDMs depending on which
occurrence record is used to seed each run, we created
six current models and projected them on three
CCSM and three MIROC models for each species.
SDMs were then concatenated and averaged in
ArcMap v.9.2 (ESRI, Inc.) to form consensus models
for each species under 0 kya and LGM climates, thus
capturing some of the intrinsic variation between
model runs.
We used random seeding and default settings (regularization = 1; convergence threshold = 0.00001, itera-
453
tions = 500) in Maxent, with the exception of an
additional 0 kya run for each species used for model
evaluation where the random test percentage was set
to 25%. To visually depict models, we used the
average lowest training presence (LTP) threshold
(average among models) for each species (with 100%
of the data used for training) so that nearly all areas
predicted by SDMs would have logistic probability
values greater than that of the occurrence record with
the smallest value. For better visualization of our
models, we averaged the LTP and the highest presence value for each species and imposed this value as
an additional threshold. This latter threshold was
used only to better visually represent SDMs and was
not used in area or centroid calculations (see below).
Models were validated using the Receiver Operating
Characteristic for its area under the curve (AUC)
value.
Our methodology outlined above and SDMs in
general are built on a number of assumptions discussed elsewhere (e.g. Nogués-Bravo, 2009) but that
warrant mentioning again. When projecting presentday models to past climatic landscapes such as the
LGM, a depiction of suitable areas at the LGM is
obtained for a species that matches habitat preferences in the present-day (Peterson & Nyári, 2008).
Accordingly, this method assumes niche stability
within the species being analysed (Nogués-Bravo,
2009). The absence of fossil data for scorpions across
the BCP prevents us from directly testing this
assumption of niche stability. Whereas many studies
have found that species niches have been stable since
the LGM (e.g. Martínez-Meyer, Peterson & Hargrove,
2004; Martínez-Meyer & Peterson, 2006), a growing
number of recent studies have found contrasting evidence that suggests niche stability may vary (Elith,
Kearney & Phillips, 2010), even over relatively short
time scales (Ahmadzadeh et al., 2013). Given the
evolutionary antiquity of scorpions (derived from
amphibious ancestors over 400 Mya) and long history
of several species in south-western North America
(Bryson et al., 2013; Bryson, Savary & Prendini,
2013), we make the critical assumption that ecological
niches of the 13 species in the BCP have been conserved over the past 21 kya, but acknowledge that our
results and discussion are built up from the foundation of this untested and potentially erroneous
assumption.
AREA AND
CENTROID CALCULATIONS
To qualitatively compare SDMs between species, the
total size (km2) as well as the latitude and longitude
of centroid position (the estimated centre of each of
SDM) were calculated for each scorpion SDM in
ArcMap. Area was calculated by converting Maxent
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
454
M. R. GRAHAM ET AL.
output ascii files to rasters consisting of integers and
then transforming the rasters to polygons using the
LTP. All grid cells with values greater than the LTP
were included in the polygons, for which their individual areas were calculated and summed for each
species. Centroids were calculated by converting each
grid cell above the LTP threshold to a point (using the
raster to point function in ArcToolbox) and averaging
the latitude and longitude of each point for each
species. Distances between centroids were calculated
using the Haversine Formula, which incorporates curvature of the earth by giving great-circle distances
between two points on a sphere. Small to no shifts in
suitable habitat from the LGM to the present among
the scorpions suggest stability in the distributions of
those species. All spatial analyses were conducted
using the Albers Equal Area Conic map projection.
RESULTS
MODEL
PERFORMANCE
We compiled 736 total occurrence records (Table 1).
Current SDMs projected to CCSM and MIROC
climate layers yielded different reconstructions for
the LGM. MIROC layers produced SDMs that
depicted more dramatic geographical range shifts
compared with CCSM layers, but often to the degree
that there was no or little suitable habitat anywhere
across the BCP during the LGM. This has been
observed in other studies (e.g. Habel et al., 2010;
Oláh-Hemmings et al., 2010; McGuire & Davis, 2013)
and may be related to sensitivity of the models
to differences in the modelled climate of the LGM
based on sea ice levels (MIROC simulates warmer
and wetter conditions compared with CCSM; OttoBliesner et al., 2006). We therefore chose to use only
the CCSM SDMs and excluded MIROC models from
subsequent analyses (however, MIROC models are
provided in Appendix S2). Current SDMs had high
predictive success rates and performed significantly
better than random in Receiver Operating Characteristic analyses, with AUC scores for testing data
greater than 0.95 (Table 1).
GENERAL
DISTRIBUTIONAL PATTERNS
ALONG THE
BCP
Current SDMs for the 13 species on the BCP fell
primarily into four regional categories: (1) northern
peninsular (extending into coastal California and the
Mojave Desert), (2) mid-peninsular, (3) southern peninsular and (4) the Cape Region (summarized in
Fig. 2; shown fully in Appendices S2–S5). Based on
the models, the northern peninsular group consisted
of three species with relatively large ranges that
extended from the mid-peninsula well into portions
Figure 2. Species distribution models based on current
and last glacial maximum (LGM) conditions for representative scorpion species distributed throughout the
northern (Hoffmannius puritanus), middle (Hadrurus
concolorous), southern (Hoffmannius vittatus) and Cape
(Bioculus caboensis) regions of the Baja California peninsula. The CCSM (our preferred model) and MIROC LGM
models are shown for comparison. Shading represents
probability of occurrence, with darkest shading being most
suitable and lightest shading representing unsuitable
habitat. Black dots indicate occurrence records used in the
modelling and white stars indicate model area centroids.
Species distribution models constructed for the remaining
nine scorpion species from the Baja California peninsula
are presented in Appendices S3–S6.
▶
of central California and southern Nevada. This
group displayed the least congruence among suitable
regions; high climatic suitability in the north
decreased southward at approximately 30.5°N, 25°N
and 26°N in Anuroctonus pococki, Hoffmannius
puritanus and Kochius hirsuticauda, respectively
(Appendix S7). At the mid-peninsula, climatic suitability within all three species quickly peaked at
about 24°N, but then began to drop off at different
locations between 29°N and 30°N (Appendix S8). The
SDMs for the third group, comprising four species
with relatively small ranges in the southern peninsula between Bahia Concepción and Cabo San
Lucas, depicted a nearly linear relationship between
climatic suitability and latitude, with high climatic
suitability found within each species at the southern
tip of the peninsula that then steadily declined to
the north until it reached a sharp decrease at 26°N
for each species (Appendix S9). Models suggested
that the four species in the Cape Region group had
the smallest ranges (all < 23 000 km2) and that each
showed a nearly linear decline in climatic suitability
starting at the southern tip of the Cape to between
24°N and 25°N (Appendix S10). Comparing trends
among all SDMs revealed three areas along the BCP
where climatic suitability tended to quickly shift
with latitude: approximately at 24°N, 26°N and
30°N.
The predicted habitat for all but one of the 13
species was remarkably stable across the BCP since
the LGM. The general trend in the distributions of
these species was high climatic suitability along
a north–south axis of the peninsula flanked by
transitional areas of less than 1° latitude where
suitability quickly decreased from high to low (summarized in Fig. 3; shown fully in Appendices S6–S9).
The predicted habitat of Kochius bruneus was the
only deviation of this pattern. For this species,
habitat at the LGM exhibited a change in centroid
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
DISTRIBUTIONAL STASIS IN BAJA CALIFORNIA SCORPIONS
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
455
456
M. R. GRAHAM ET AL.
Figure 3. Correlation between latitude (x axis) and habitat suitability (y axis) for representative scorpion species
distributed throughout the northern (Hoffmannius puritanus), middle (Hadrurus concolorous), southern (Hoffmannius
vittatus) and Cape (Bioculus caboensis) regions of the Baja California peninsula. Grey diamonds represent individual grid
cells from Maxent species distribution models. The black line is a moving average trendline based on the average of every
200 grid cells. Correlation charts for the remaining nine scorpion species from the Baja California peninsula are presented
in Appendices S7–S10.
position of 260.6 km and a change in area of
85 479.2 km2 (Appendix S4). The change in centroid
position of the other BCP species (Fig. 4) ranged
from 5.04 to 260.6 km (mean = 85.89 km), and
change in area ranged from −104 606.4 to 85 479.16
km2 (mean = −16 251.98 km2).
DISCUSSION
PLEISTOCENE
DISTRIBUTIONAL STASIS
Our results suggest that the suitable habitat for scorpion species in the BCP has remained remarkably
stable since the Late Pleistocene. This finding is in
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
DISTRIBUTIONAL STASIS IN BAJA CALIFORNIA SCORPIONS
Figure 4. Box and whisker plots depicting the change in
centroid position (shaded boxes, km) and change in area
(white boxes, km2) between current (0K) and last glacial
maximum (LGM) species distribution models for 13
scorpion species distributed along the Baja California
peninsula.
stark contrast to dramatic distributional shifts since
the Late Pleistocene inferred for arid-adapted taxa
elsewhere in western North America, including
heteromyid rodents (Jezkova et al., 2009), rattlesnakes and gnatcatchers (Waltari et al., 2007),
phrynosomatid lizards (T. Jezkova, unpubl. data) and
invertebrates (Wilson & Pitts, 2012) including scorpions (Graham et al., 2013a, 2013b). This discrepancy
has several potential explanations. First, the proximity of the BCP to ocean currents may have influenced
this pattern of distributional stasis in scorpions in the
BCP. Oceans are well known to buffer coastal climates
(Rahmstorf, 2002), effectively acting as large sources
and sinks for latent heat. Stewart & Lister (2001)
hypothesize that proximity to the relatively temperate conditions of oceans could have allowed for persistence of coastal coniferous forest refugia in
northern Europe during the Late Pleistocene. In a
similar fashion, the BCP, flanked by the Pacific Ocean
to the west and the Sea of Cortés to the east, could
also be predisposed to such a coastal effect. The SDMs
of Anuroctonus pococki, a widely distributed species
across the northern BCP and into coastal California,
suggest that this species’ suitable habitat remained
remarkably stable from the LGM onward (Appendix
S2), lending additional support to the ocean-buffering
hypothesis. Distributions of mid-latitude species (c.
40–20°N), if flanked by considerable bodies of water,
might therefore be somewhat resistant to the often
severe range shifts and contractions associated with
glacial cycles.
457
The discrepancy between relatively stable distributions of scorpions in the BCP compared with aridadapted taxa in western North America might also be
explained by the more southern latitude of the BCP.
The impacts of Pleistocene climatic fluctuations on
species distributions are generally thought to be
stronger as the distance from the equator increases
(Hewitt, 2004). SDMs for species in the south-western
USA and mainland Mexico at approximately the same
latitudes as the BCP often fail to predict dramatic
distributional shifts since the Late Pleistocene (e.g.
Peterson, Martínez-Meyer & González-Salazar, 2004;
McGuire et al., 2007; Oláh-Hemmings et al., 2010), or
at least do not show the significant loss of habitat in
the north and massive shifts of suitable habitat to the
south as observed in arid-adapted taxa at more northern latitudes (e.g. T. Jezkova, unpubl. data; Waltari
et al., 2007; Jezkova et al., 2009; Wilson & Pitts, 2012;
Graham et al., 2013a). These former studies provide
support for a latitudinal distance-from-the-equator
explanation for distributional stasis in scorpions
in the BCP as an alternative to an ocean-buffering
hypothesis. In fact, the combination of both factors
may explain our observed pattern of stable habitat for
scorpion species in the BCP since the Late Pleistocene.
However, the ocean-buffering and distance-fromequator hypotheses are both in conflict with fossil data
from early Holocene packrat middens, which suggest
that vegetation assembles on the BCP have in fact not
been resilient to range shifts (Rhode, 2002).
SCORPION SDMs
AND BIOGEOGRAPHICAL
HISTORY IN THE
BCP
Emerging genetic information from species along the
BCP indicate that the biogeographical history of the
region is complex (Riddle & Hafner, 2006). Numerous
trans-peninsular genetic discontinuities have been
explained by phenomena such as ephemeral transpeninsular seaways (Minch & Leslie, 1991; Upton &
Murphy, 1997; Riddle et al., 2000; Riddle, Hafner &
Alexander, 2000a, b; Murphy & Aguirre-Léon, 2002;
Mulcahy & Macey, 2009), abrupt ecological and climatic changes (Grismer, 2002), and unusual dispersal
events (Wood, Fisher & Reeder, 2008). Our scorpion
SDMs add another piece to the puzzling biotic history
of the BCP.
Perhaps the most interesting of the genetic divergences across the BCP, especially in light of our
scorpion SDMs, are three trans-peninsular breaks
outlined and described by Lindell et al. (2005): the
mid-peninsular break, the Loreto break and the
Isthmus of La Paz break (Fig. 1). Divergences in these
areas have been substantially debated, with the midpeninsular break being the most provocative, and
differing degrees of genetic divergence in these areas
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461
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M. R. GRAHAM ET AL.
have complicated molecular dating estimates (Leaché,
Crews & Hickerson, 2007). Most authors, however,
posit multiple or reoccurring seaways to explain the
variety of genetic divergences in these regions and the
corresponding array of estimated divergence dates
(Lindell et al., 2005; Crews & Hedin, 2006; Leaché
et al., 2007; Hafner & Riddle, 2011). Although speculated early in discussions of biotic diversification in
the region (Grismer, 2002), relatively little consideration has been given to the hypothesis that vicariance
of terrestrial lineages could be due to climatic constraints, either alone or synergistically with seaways.
Twelve of our 13 scorpion SDMs show sharp transitions in habitat suitability almost precisely across
the three trans-peninsular breaks, providing some of
the first evidence that long-term climatic stability
may have produced patterns of divergence between
species. On the Isthmus of La Paz, for example,
scorpion SDMs reveal two potential sister species
relationships, perhaps associated with the Isthmus
of La Paz break: Kochius punctipalpi and Hadrurus
hirsutus in the Cape Region (Appendix S5), and
Kochius bruneus and Hadrurus concolorous (Appendix S3) found north of the Cape. The SDMs of each
of these species display a sharp decrease in climatic
suitability in the area of the La Paz break. Similar
trends are apparent in SDMs of other scorpion
species in the vicinity of the mid-peninsular and
Loreto breaks (Appendices S6–S8). If patterns of climatic and habitat stability across this admittedly
relatively short time slice from the LGM to present
were expanded to a considerably deeper timeframe,
then we find it conceivable that genetic discontinuities could have arisen in association with localized
adaptation to differential climate regimes. Such a
pattern, however, would be contradictory to the conclusions of Lindell, Ngo & Murphy (2006), who
stated that climate-induced habitat fragmentation
could not adequately explain mid-peninsular genetic
breaks.
Although our SDMs suggest that vicariance across
the BCP could be due to climatic constraints, it seems
difficult to reconcile accumulation of congruent
genetic breaks across multiple taxa (including marine
fishes, Riginos, 2005) with a variety of ecological
traits and presumably niche requirements without
still invoking additional concerted events such as the
previously mentioned trans-peninsular seaways.
Nonetheless, hypotheses like that of the Vizcaíno
Seaway, for which little geological evidence exists
(Lindell et al., 2006; but see Helenes & Carreño, 1999;
Carreño & Helenes, 2002; and Ledesma-Vázquez,
2002), might be due for reappraisal, and the possibility that climate was largely responsible for the
complex phylogeographical patterns along the BCP
should be given more consideration.
ACKNOWLEDGEMENTS
We thank S. C. Williams, V. F. Lee and C. E. Griswold
for granting loans from the California Academy of
Sciences. We are grateful to J. R. Jaeger for providing
assistance with collecting and transporting specimens; T. Jezkova for support with distribution modelling; L. Hancock for georeferencing assistance; M. A.
Wall for kindly allowing access to the scorpions at the
San Diego Natural History Museum; M. E. Soleglad
for scorpion occurrence records; and E. E. Saupe, M.
E. Eckstut and T. Jezkova for helpful reviews of
earlier versions of the manuscript. We thank three
anonymous reviewers for their important comments.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the online version of this article at the publisher’s web-site:
Appendix S1. Coordinates (in decimal degrees) used to generate species distribution models.
Appendix S2. Maxent models created using data from the Model for Interdisciplinary Research on Climate
(MIROC).
Appendix S3. Species distribution models created with Maxent for three scorpion species distributed in
southern California, USA, and northern Baja California, Mexico: (A, B) Anuroctonus pococki, (C, D)
Hoffmannius puritanus and (E, F) Kochius hirsuticauda. Maps A, C and E are present predictions, with black
dots indicating occurrences used in the modeling; maps B, D and F are Maxent predictions at the LGM. White
stars indicate model area centroids and shading represents probability of occurrence, with darkest shading
being most suitable and lightest shading representing unsuitable habitat (see Methods for modelling details).
Appendix S4. Species distribution models created with Maxent for three scorpion species distributed along the
central portion of the BCP, Mexico: (A, B) Hadrurus concolorous, (C, D) Hadrurus pinteri, (E, F) Kochius
bruneus. Maps A, C and E are present predictions, with black dots indicating occurrences used in the modelling;
maps B, D and F are Maxent predictions at the LGM. White stars indicate model area centroids and shading
represents probability of occurrence, with darkest shading being most suitable and lightest shading representing unsuitable habitat (see Methods for modelling details).
Appendix S5. Species distribution models created with Maxent for scorpion species distributed along the
southern portion of Baja California, Mexico: (A, B) Bioculus comondae, (C, D) Hoffmannius diazi, (E, F)
Hoffmannius vittatus, (G, H) Nullibrotheas allenii. Maps A, C, E and G are present predictions, with black dots
indicating occurrences used in the modeling; maps B, D, F and H are Maxent predictions at the LGM. White
stars indicate model area centroids and shading represents probability of occurrence, with darkest shading
being most suitable and lightest shading representing unsuitable habitat (see Methods for modelling details).
Appendix S6. Species distribution models created with Maxent for three scorpion species endemic to the Baja
California Cape Region, Mexico: (A, B) Bioculus caboensis, (C, D) Hadrurus hirsutus, (E, F) Kochius punctipalpi.
Maps A, C and E are present predictions, with black dots indicating occurrences used in the modelling; maps
B, D and F are Maxent predictions at the LGM. White stars indicate model area centroids and shading
represents probability of occurrence, with darkest shading being most suitable and lightest shading representing unsuitable habitat (see Methods for modelling details).
Appendix S7. Correlation between latitude (x axis) and habitat suitability (y axis) for three scorpion species
occurring in northern Baja California. Grey diamonds represent individual grid cells from Maxent species
distribution models, and the black line is a moving average trendline based on the average of every 200 grid
cells.
Appendix S8. Correlation between latitude (x axis) and habitat suitability (y axis) for three scorpion species
occurring in middle of the BCP. Grey diamonds represent individual grid cells from Maxent species distribution
models, and the black line is a moving average trendline based on the average of every 200 grid cells.
Appendix S9. Correlation between latitude (x axis) and habitat suitability (y axis) for three scorpion species
occurring in southern Baja California. Grey diamonds represent individual grid cells from Maxent species
distribution models, and the black line is a moving average trendline based on the average of every 200 grid cells.
Appendix S10. Correlation between latitude (x axis) and habitat suitability (y axis) for three scorpion species
endemic to the Baja California Cape Region. Grey diamonds represent individual grid cells from Maxent species
distribution models, and the black line is a moving average trendline based on the average of every 200 grid
cells.
© 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 450–461