Historic and Current Composition of Lizard Communities in Urban Preserves of Central Arizona, USA
Brian K. Sullivan1,*, David Vardukyan2, and Keith O. Sullivan3
1PO Box 37100, School of Mathematical and Natural Sciences, Arizona State University, AZ 85069-7100, USA. 2School of Life Sciences, Arizona State University, PO Box 874501, Tempe, AZ 85287-4501, USA. 3Contracts Branch, Arizona Game and Fish Department, Phoenix, AZ 85086, USA. *Corresponding author.
Urban Naturalist, No. 2 (2014)
Abstract
Urbanization is rapidly enveloping isolated remnants of Sonoran Desert habitat in southern Arizona. Understanding the means by which herpetofaunal elements can persist in these habitats in the face of multiple impacts is vital to conservation efforts to retain intact biotic communities, especially those with a high diversity of reptile species. We surveyed twelve preserves in the Phoenix Metropolitan region, five of which had been surveyed de- cades earlier, and obtained estimates of the diversity (species richness) and relative abun- dance of lizards. In comparison to surveys of the same preserves 20 to 40 years prior, one lizard species is absent from one large preserve where it was once present, but communities are otherwise largely similar in diversity and abundance over the past few decades. Larger preserves have higher diversity but not higher abundance of lizards. The range in diversity indices (i.e., species richness and evenness) across preserves we documented encompasses the range in diversity indices derived from other studies of urban lizard communities in the Southwest. Individual variation in distribution and abundance of component species must be investigated to adequately assess declines at the community level. We lack historically detailed data on distribution of many lizards and snakes, preventing an accurate analysis of species loss over the past 50 years.
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Urban Naturalist
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B.K. Sullivan, D. Vardukyan, and K.O. Sullivan
22001144 URBAN NATURALIST No. 2:N1–o1. 82
Historic and Current Composition of Lizard Communities in
Urban Preserves of Central Arizona, USA
Brian K. Sullivan1,*, David Vardukyan2, and Keith O. Sullivan3
Abstract - Urbanization is rapidly enveloping isolated remnants of Sonoran Desert habitat
in southern Arizona. Understanding the means by which herpetofaunal elements can persist
in these habitats in the face of multiple impacts is vital to conservation efforts to retain intact
biotic communities, especially those with a high diversity of reptile species. We surveyed
twelve preserves in the Phoenix Metropolitan region, five of which had been surveyed decades
earlier, and obtained estimates of the diversity (species richness) and relative abundance
of lizards. In comparison to surveys of the same preserves 20 to 40 years prior, one
lizard species is absent from one large preserve where it was once present, but communities
are otherwise largely similar in diversity and abundance over the past few decades. Larger
preserves have higher diversity but not higher abundance of lizards. The range in diversity
indices (i.e., species richness and evenness) across preserves we documented encompasses
the range in diversity indices derived from other studies of urban lizard communities in the
Southwest. Individual variation in distribution and abundance of component species must
be investigated to adequately assess declines at the community level. We lack historically
detailed data on distribution of many lizards and snakes, preventing an accurate analysis of
species loss over the past 50 years.
Introduction
The Sonoran Desert of the southwestern United States is under increasing
pressure from expanding metropolitan areas, most notably Phoenix—an urban
landscape stretching across more than 20,000 km2 of arid lands in central Arizona
(Kane et al. 2014). Impacts associated with this seemingly ever-expanding metropolitan
area have been varied, but one outcome has been the isolation of a variety
of preserves, patches of remnant habitat enveloped by this urban sprawl (Esbah et
al. 2009, Sullivan et al. 2014). These preserves literally represent islands of desert
habitat in a sea of urbanization, varying in both age since separation from any surrounding
natural habitat and in size, as well as a host of other factors. These sites
contrast with the adjacent areas that have been directly converted to some form of
anthropogenic habitat, primarily housing or agricultural lands. Some areas were
converted to agricultural lands over 100 years prior and were only recently transitioned
to high-density housing. Others, especially on the edge, were more recently
turned from desert directly to high-density housing without any patches of remnant
habitat. Last, some regions were converted to complex patches of low-density hous-
1PO Box 37100, School of Mathematical and Natural Sciences, Arizona State University,
AZ 85069-7100, USA. 2School of Life Sciences, Arizona State University, PO Box 874501,
Tempe, AZ 85287-4501, USA. 3Contracts Branch, Arizona Game and Fish Department,
Phoenix, AZ 85086, USA. *Corresponding author - bsullivan@asu.edu.
Manuscript Editor: David Krauss
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ing and remnant habitat patches, generally in affluent neighborhoods (see reviews
in Ackley et al., in press; Esbah et al. 2009; Litteral and Wu 2012).
Knowledge of relationships between key environmental variables and diversity
and abundance of vertebrates, especially in natural areas isolated within urban landscapes,
is increasingly important to conservation (reviewed in Davis et al. 2013,
MacNally and Brown 2001, Sullivan and Williams 2010). Trubl et al. (2011) argued
that understanding ecological factors associated with decline of many native organisms,
often in stark contrast to expansion of some native and especially non-native
forms, is vital to advancing our management of remnant preserves within urban areas,
and to the mitigation of anthropogenic impacts in outlying areas while native forms
are still present. Our understanding of how reptiles respond to anthropogenic impacts
on habitat structure within remnant communities is increasing rapidly, at both the
community level (e.g., Garden et al. 2007, Koenig et al. 2001) and species level (e.g.,
Barrows et al. 2008, Fisher et al. 2002, French et al. 2008, Sullivan et al. 2004). Nonetheless,
we still lack basic historical information, such as presence and absence of
various species, for the vast majority of reptiles impacted by urbanization.
There is growing evidence that identification of critical ecological factors is
vital to conservation of squamate reptiles (Buckley and Jetz 2010, Fischer et al.
2005). For example, Sullivan and Williams (2010) documented that plant diversity
accounted for 75% of the variation in abundance of the lizard Sauromalus ater
(Dumeril) (Common Chuckwalla) across a series of Sonoran Desert preserves.
Similarly, Sullivan et al. (2014) documented the presence of Phrynosoma solare
Gray (Regal Horned Lizard) in 50% of the Phoenix area preserves they surveyed
in spite of the fact that these lizards have been collected for the pet-trade for well
over 100 years and the fact that close relatives have suffered significant declines in
some areas (Fisher et al. 2002, Jennings 1987). Horned lizards are dietary specialists
feeding almost exclusively on seed-harvester ants, depending on the species
(reviewed in Sherbrooke 1981, 2003), and it appears these lizards may persist if
their primary prey persist (Sullivan et al. 2014). Such investigations indicate that
conservation of some reptiles can be enhanced by consideration of a relatively small
number of ecological attributes across preserves (Kitchner et al. 1980, Martin and
Lopez 2002, Santos et al. 2008, Sarre et al. 1995).
Study sites and hypothesis under test
Sullivan and Flowers (1998) hypothesized that Phoenix Mountain Preserves, established
over the past six decades, provide a non-random array of habitats that favor
herpetofauna species which prefer rocky microhabitats. These preserves are primarily
upland slope habitat (steep, rocky formations) with relatively few washes or
expansive flats (sandy soils with little elevational gradient; Fig. 1). They documented
that five interior preserves, isolated islands of rocky habitat, continue to support
populations of saxicolous squamates (e.g., Common Chuckwalla) but appear to lack
those taxa typically restricted to alluvial soil flats such as Phyrnosoma spp. (horned
lizards), Dipsosaurus dorsalis (Baird and Girard) (Desert Iguana), and Gambelia
wislizenii Baird and Girard (Long-nosed Leopard Lizard). They concluded that the
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lack of habitat heterogeneity present in the primarily upland slopes of the preserves
in large measure accounted for the lack of lizard diversity they observed—as few
as three species in some preserves and only nine in the largest preserve—relative to
approximately a dozen species known from just outside the Phoenix Metropolitan
region (Table 1, Appendix 1). Sullivan and Flowers (1998) documented that one
preserve, the largest interior preserve in the metropolitan area and containing ~2 km2
of flats, still retained horned lizards in the 1990s (1995–1998).
In a continuing effort to more adequately evaluate the factors influencing the efficacy
of the current preserve system in providing habitat for historic herpetofauna
Figure 1. Lookout Mountain Preserve, one of the smallest preserves; note the absence of
Creosote-Bursage flats and xeric washes.
Table 1. Diversity (number of species detected) and abundance (average number of lizards observed
each 60-min survey) for all sites surveyed. Diversity did not vary across time periods (Wilcoxon
Matched-pairs, signed ranks: Z = 1.73, P = 0.083, n = 8 sites), nor did abundance (Wilcoxon Matchedpairs,
signed ranks: Z = 0.75, P = 0.462, n = 6 sites).
Diversity Abundance
Site 1990s 2010s 1990s 2010s
Adobe Dam (AD) 7 6 - 17.8
Cave Buttes (CB) 7 6 - 22.2
Hedgpeth (HH) 3 3 4.5 5.8
Lookout (LO) 3 3 10.1 17.6
North (NM) 4 4 8.4 11.1
Piestewa Peak (PP) 6 5 12.0 10.0
Shadow (ShM) 4 4 12.1 8.2
South (SM) 9 9 18.3 13.4
Papago Park (PPk) - 3 - -
Deem Hills (DH) - 6 - 25.5
Rose Garden Lane (RGl)- 6 - 36.2
New River (NR) - 6 - -
Estrella Park - 11 - -
Pinnacle Peak Park - 10 - -
San Tan Park - 10 - -
White Tank Park - 13 - -
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in the Phoenix region, we selected a larger array of sites than those originally
sampled by Sullivan and Flowers (1998), and added two sites that represent flood
plain recreational areas with abundant alluvial soil patches rather than exclusively
upland, mountain preserves. Adobe Dam (AD) and Cave Buttes (CB) are large
sites on the northern metropolitan edge that comprise all major local habitat types,
including large areas of Larrea tridentata (D.C.) Coville (Creosote Bush) - Ambrosia
deltoidea (Torr.) Payne (Triangle Bursage) flats dominated by fine textured
alluvial soils (see Jones et al. [2011] and Sullivan et al. [2014] for discussions of
this habitat in the Sonoran Desert biotic community). AD and CB were surveyed intensively
from 1990 to 1995, primarily with respect to the amphibian communities,
but incidental observations were gathered on reptiles encountered while conducting
amphibian surveys (Sullivan and Fernandez 1999). We returned to these sites
in 2010 and conducted surveys over the next 48 months to document the reptile
community; we also revisited the interior sites surveyed by Sullivan and Flowers
(1998), and added additional sites that had been established as preserves over the
last decade to bring the total surveyed to 12 preserves of varying sizes and habitat
constitution (Table 1). Herein we present the results of the 2010–2014 surveys
from these 12 metropolitan preserves, and compare those to data from Sullivan and
Flowers (1998) and Parker (1967). We specifically evaluate the changes in diversity
(= species richness) and abundance of lizards from the preserves surveyed by Sullivan
and Flowers (1998) in the mid-1990s and the one preserve (South Mountain)
surveyed intensively by Parker (1967) in the mid-1960s.
Methods
A total of 12 sites in the Phoenix Metropolitan area were surveyed for lizards:
Adobe Dam (AD), Cave Buttes Dam (CB), Deem Hills (DH), Hedgpeth Hills (HH),
Lookout Mountain (LO), New River Dam (NR), North Mountain (NM), Papago
Park (PPk), Piestewa Peak (PP), Rose Garden Lane (RGL), Shadow Mountain
(ShM), and South Mountain (SM) (Fig. 2). Additionally, four sites on the edge of
the Phoenix metropolitan region—Estrella, San Tan, and White Tank Mountain
regional parks, and Pinnacle Peak Park—were selected for “historical baseline”
reference comparisons of lizard diversity in similar Sonoran Desert habitats independent
of the surveys presented herein. These four sites were used because
complete inventories of the squamate communities were available as a consequence
of detailed studies conducted in the vicinity during the 1990s and early 2000s (inventoried
over the course of studies on a variety of squamate taxa; see citations
in Sullivan and Kwiatkowski 2007). We used the four outlying sites and a fifth
non-preserve metropolitan area site with documented shifts in lizard diversity and
abundance over the years, the Desert Botanical Garden adjacent to one of the interior
preserves (PPk), to compare historic diversity of lizard communities with extant
preserves. Because our intention was to specifically test the hypothesis of Sullivan
and Flowers (1998) that creosote-bursage flats are critical to site occupancy by various
lizards, we focused on those habitats within preserves (as opposed to washes or
rocky slopes).
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Two approaches were used to survey for lizards. To obtain standardized estimates
of relative abundance in the more recent surveys (2010–2014), three to five visual
encounter surveys (VES), each for 60 min, were conducted in each of the five primary
sites (LO, NM, PP, ShM, SM) surveyed by Sullivan and Flowers (1998), and a sixth
site, HH, surveyed as part of a Common Chuckwalla survey (Sullivan and Sullivan
2008). Data from three 60-minute surveys conducted 1994–1996 were available for
six sites (HH, LO, NM, PP, ShM, SM; Sullivan and Flowers 1998), but only diversity
Figure 2. Mountain preserves of the Phoenix Metropolitan area. Major roadways are shown
in red and major cities are labeled, and survey sites indicated by abbreviations defined in
the text.
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estimates were gathered for other sites visited in the course of other studies described
above. In those surveys, species were recorded in daily field notes, but no data were
gathered concerning relative abundances of lizards observed.
To maximize detection of lizards, surveys were conducted during warm weather
(air temperature = 18–30 °C) from spring (March) through the fall (October), after
which lizard activity drops off dramatically, and were opportunistically undertaken
when weather conditions had been optimal for lizard activity over the prior two
days to avoid variation in abundance associated with shifts in activity due to inclement
weather. These same practices were used by Sullivan and Flowers (1998).
The entire area of open habitat (“flats”) was surveyed by walking in parallel lines
15 m apart in smaller preserves (≥2 ha); in larger preserves, areas adjacent to trails
were selected based on historical records for horned lizards (1990–2000 obtained in
other work; Sullivan et al. 2014), to maximize the opportunity that these and other
open-habitat lizards could be detected. Because trails leading to areas of flats in
preserves always passed through rocky slopes and washes, these latter habitats were
sampled incidentally to the focus on flats. In preserves with more than 2 ha of flats,
VES surveys were terminated after 60 min. Repeat surveys covered the same area in
each preserve. Direct observations of each species were required of all forms except
Regal Horned Lizards, the only species in the Phoenix area reliably identifiable by
its unique fecal pellets (Sullivan et al. 2014); thus, indirect sign was used for this
one species at some sites (2 of 12). At the conclusion of five surveys, a preserve
received a “presence” score if a lizard of a given species had been observed and a
total abundance score by summing all lizards seen of each species.
A second means of gathering information on the presence or absence of lizards
at each site was through work on other organisms, including monitoring of toads
and tortoises (e.g., Gopherus morafkai Murphy, Berry, Edwards, Leviton, Lathrop
and Riedle [Sonoran Desert Tortoise]). During these other field activities, incidental
observations were obtained on any lizards observed. At CB, all lizards observed
along a 4-km stretch of roadway (paved and unpaved), including a small number
of surveys in the early evening, were counted and recorded on 20 days each year
to provide a rough estimate of relative abundance of the species present at that
site, for comparative purposes. Road-riding is an established method for assessing
the diversity and abundance of squamate communities (Sullivan 2000, 2012);
evening surveys for amphibians at two sites, CB and SM, allowed observation of
Coleonyx variegatus (Baird) (Western Banded Gecko) and Regal Horned Lizards,
active just after sunset in the Sonoran Desert. Surveys for snakes, using an array of
coverboards under which they take refuge, were also used in lizard tallies for the
CB site because Banded Geckos and some other species sought shelter under the
coverboards well. Efforts at these four sites—AD (20 surveys), CB (158 surveys),
LO (32 surveys), and SM (151 surveys)—allowed large samples for a more accurate
assessment of variation in abundances of each species across preserves relative to
the more limited surveys across all preserves.
We calculated Shannon-Weaver and adjusted Simpson (i.e., reciprocal) diversity
indices, following the equations provided by Cross et al. (2012), to compare species
richness (i.e., diversity) and evenness for sites with large numbers of observations
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across all taxa (i.e., >200 observations across species: AD, CB, LO, and SM). These
same values were derived from data provided in prior studies of the Phoenix (Banville
and Bateman 2012, Parker 1967) and Tucson (Germaine and Wakeling 2001)
areas. For these analyses, we used survey results for diurnal forms alone, excluding
the small number of Western Banded Geckos detected at two of our sites by use of
road-riding at night (i.e., CB and SM), and those found by Parker (1967) using pitfall
traps.
Because of small sample sizes, especially for historical comparisons (i.e.,
the five primary sites surveyed by Sullivan and Flowers [1998]), we used nonparametric
statistical tests exclusively (following Hollander and Wolfe 1973, Zar
1999). We used Spearman’s rho (nonparametric correlation) to assess relationships
among independent (e.g., size of preserve) and dependent (e.g., species
counts) variables, and Wilcoxon signed-ranks, matched-pairs tests to assess
shifts in both diversity and abundance at sites across sampling time periods. Tests
were conducted using SPSS (version 20.0, SPSS for Windows, Rel. 20.0.0, 2011;
SPSS, Inc., Chicago, IL).
Results
Lizard diversity within the 12 interior preserves ranged from three to nine species
during both the 1990s and the 2010s (Table 1). The four outlying parks selected
for baseline comparisons exhibited significantly higher species richness, ranging
from 10 to 13 species (Mann-Whitney U, Z = 2.98, P = 0.003, n = 16).
The five preserves surveyed in the mid-1990s (LO, NM, PP, ShM, and SM) by
Sullivan and Flowers (1998), and the one (HH) by Sullivan and Williams (2010),
retained the same species richness and abundance of lizards in more recent surveys
during the 2010s (Table 1), except for the loss of a single species at one site. The
only species historically present but undetected during our surveys in the 2010s
was the Regal Horned Lizard in the PP site (evidence presented elsewhere indicates
it has been absent from this site since 1998; see Sullivan et al. 2014). Using data
from those five sites in addition to data from the two flood-control sites (AD, CB)
and a third site (HH), species richness did not vary across time periods (Wilcoxon
matched-pairs, signed ranks: Z = 1.73, P = 0.083, n = 8 sites), nor did abundance
(Wilcoxon matched-pairs, signed ranks: Z = 0.75, P = 0.462, n = 6 sites).
Though there was not a significant shift in species richness overall, one species
detected at the two flood-control sites, the Desert Iguana (AD = one individual
in 1994, CB = one individual in 1990), has not been detected there since (neither
during incidental work through the 1990s, nor during thousands of hours of fieldwork
since 2010). Desert Iguanas were observed at the NR site and the edge of
the SM site in the more recent surveys (2011–2014 for NR, 2010–2011 for SM;
Appendix 1). Only a single Desert Iguana was observed at the SM site in the mid-
1990s, and the 2010s.
There was a significant positive relationship for the current species counts and
preserve size (rs = 0.72, n = 12, P < 0.02). Thus, larger preserves exhibited more
speciose lizard communities; there was no relationship with abundances of lizard
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counts overall (r = 0.23, n = 10, P > 0.20). Uta stansburiana Baird and Girard
(Common Side-blotched Lizard) were numerically dominant at three of the four
intensively surveyed sites (Table 2), and Aspidoscelis tigris (Baird and Girard)
(Tiger Whiptail) were ranked first or second in relative abundance across all four
intensively surveyed sites (Table 2). Five species accounted for the vast majority of
lizard observations overall (Table 2, Fig. 3). The average number of lizards (all species,
summed) observed during each of the 60-minute VES surveys varied widely
across preserves (from 5–36 lizards; Table 1).
For sites with extensive survey results across all species, exclusive of the nocturnal
geckos, Shannon-Weaver diversity indices varied from 0.55 for the low
diversity, highly uneven LO site dominated by Common Side-blotched Lizards
(~90% of all lizards observed), to a high of 2.45 for the relatively more even (no
species accounted for >30% of lizards observed) and diverse SM site (Tables 2, 3).
Similarly, the Simpson’s index varied from a low of 0.61 (LO) to a high of 2.61
(SM), precisely parallel to the Shannon-Weaver values (Table 3).
Table 2. Abundance of lizards (see Appendix 1 for abbreviations) observed at four sites (AD = 20
surveys, CB = 158 surveys, LO = 32 surveys, SM = 151 surveys), 2010–2014, from all survey methods
combined.
Adobe Dam Cave Buttes Lookout South Mtn
Species n % n % n % n %
UTST 239 69% 3352 65% 675 90% 324 14%
ASTI 69 20% 829 16% 51 7% 618 30%
CADR 8 2% 632 12% 0 - 530 23%
SCMA 6 2% 279 5% 0 - 107 5%
SAAT 21 6% 5 <1% 23 3% 471 20%
PHSO 5 1% 14 <1% 0 - 1* <1%
COVA 0 - 23 <1% 0 - 31 1%
UROR 0 - 0 - 0 324 14%
Totals 348 5134 749 2351
*Based on fecal counts
Table 3. Shannon-Weaver and Simpson (reciprocal) diversity indices for three prior studies and four
sites (AD = 20 surveys; CB = 158 surveys; LO = 32 surveys; SM = 151 surveys) from the present
study. All values exclusive of the nocturnal Western Banded Gecko (COVA). “T” = Tucson; “P-R”
= Phoenix, riparian habitats; AD = Adobe Dam, CB = Cave Buttes, LO = Lookout, and SM = South
Mountain Preserve.
Diversity Total observ. Shannon-Weaver Simpson Site and Study
6 348 1.39 0.97 AD: this study
6 5111 1.47 1.06 CB: this study
3 749 0.55 0.61 LO: this study
8 2320 2.45 2.56 SM: this study
9 349 1.98 1.41 T: Germaine and Wakeling 2001
7 83 2.43 2.42 P-R: Banville and Bateman 2012
8 472 1.84 1.30 SM: Parker 1967
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Discussion
Historical diversity and abundance
One of the sites we surveyed, SM, was sampled extensively in the 1960s (Parker
1967). Species richness and evenness remains unchanged 50 years later (Table 3).
Parker (1967) established five sampling sites at the eastern edge of SM using pitfall
traps, which capture lizards that take refuge under covers over open traps. We
observed all nine of the species he trapped at his five pit-fall sites in the 1960s
(Table 2, Appendix 1). However, Parker observed a tenth species, Long-nosed
Leopard Lizard, in the vicinity of his eastern-most traps, an area which since has
been replaced by a golf course and housing. His photographs reveal the habitat in
the vicinity of his eastern-most sites (1 and 2; see Parker 1967) was sandy, open
Creosote flats not unlike the Salt River floodplain less than 2 km to the north. All of
the sites sampled by Parker that remain intact (i.e., those to the west, his sites 3–5)
within the boundaries of the preserve retain the same level of diversity documented
in our more recent surveys (Table 1, Appendix 1). Interestingly, though species
richness was equal, the diversity indices derived from Parker’s pit-fall trap surveys
were somewhat lower than those calculated from our VES surveys (Table 3). This
difference was a result of the relative unevenness of his data (i.e., variation in
abundance values), due in part to his methods failing to document higher numbers
Figure 3. The five most abundant lizards of mountain preserves in the Phoenix metropolitan
region: Tiger Whiptail (ASTI; upper left), Zebra-tailed Lizard (CADR; upper right), Desert
Spiny Lizard (SCMA; lower left), and Side-blotched Lizard (UTST; lower center) and
Common Chuckwalla (SAAT; lower right).
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of rock-dwelling forms (Common Chuckwalla and Urosaurus ornatus Baird and
Girard [Ornate Tree Lizard]) we observed regularly, as well as his surveys having
exceptionally large numbers of nocturnal, ground-dwelling geckos (Western
Banded Gecko), to be expected with pit-fall traps.
The results from other preserves surveyed by Sullivan and Flowers (1998) in the
mid-1990s are largely similar in diversity and abundance of lizards present to those
from the 2010s, save for the loss of one species of horned lizard from one preserve
surveyed historically. Given that horned lizards occupied a very small area of the
PP preserve (<2 ha), it is perhaps unsurprising that the apparent change in availability
of their prey that Sullivan and Flowers (1998) noted could have significant
consequences. Regal Horned Lizards are still present at the SM site, as are their
prey, seed harvester ants (Sullivan et al. 2014). Loss of one species may be viewed
as inconsequential, but it does represent a 17% decrease in the original lizard diversity
of the PP site in only twenty years (see review in Sullivan et al. 2014). Those
intervening twenty years comprise impacts by recreational users, reptile collectors,
and heat-island effects, to list only three (see review of these factors in Sullivan et
al. 2013). The smallest preserves, LO and ShM, have not lost any additional species,
but neither have most other preserves. Last, no preserves have been recently
occupied by either native or non-native taxa, as apparently occurred at the Desert
Botanical Garden in southeast Phoenix following the introduction of Sceloporus
magister Hallowell (Desert Spiny Lizard) in the 1960s (Barnes 1992, Feldman
1978), and in southern Mesa on the eastern edge of the Phoenix metropolitan area
with the introduction of Chalcides ocellatus Forsskål (Ocellated Skink) in the
1990s (Gunn et al. 2012).
The absence of Desert Iguanas since the mid-1990s at the AD and CB sites is
potentially troubling. Surveys for Desert Iguanas over the past ten years allow us
to offer a more nuanced explanation for variation in the distribution of species in
the Phoenix region and its absence from the CB area and the newly established
Sonoran Preserve to the immediate west (see below). Detection of Desert Iguanas
at the AD and CB sites were based on single individuals observed at each site in
the early 1990s: no additional specimens were ever observed, either in the 1990s,
or during hundreds of hours of observation and surveys from 2009–2014. Surprisingly,
Desert Iguanas have been consistently observed in highly disturbed riparian
corridors along the New River 6 km west of the AD site (Fig. 4) and the Salt
River along the boundary of the SM site. These observations reveal a pattern of
occurrence in which Desert Iguanas are found in the sandy soils associated with
floodplains of the Salt and Gila rivers and their tributaries (e.g., Agua Fria, New
River, Skunk Creek, Cave Creek; see Banville and Bateman 2012 and Sullivan
and Vernon, in press). As one progresses northward along these riparian corridors,
in even highly disturbed habitats, one can still observe iguanas in some locations,
though they have undoubtedly declined in numbers due to some development.
The single individuals observed in the early 1990s, at AD (Skunk Creek) and CB
(Cave Creek) may represent the naturally occurring local northern distributional
limit of this lizard in the Phoenix region. Thus, this species may not have been
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lost from these northern sites over the subsequent 20 years; rather, it may have
never been established this far to the north. This perspective is supported by the
ease with which this species can be observed at sites further to the south along the
Agua Fria and New rivers in western Phoenix (Fig. 4). In the absence of a distributional
“limit” to explain the absence of this lizard from the expansive Creosote
flats of the CB site (>10 km2), their loss from this relatively undisturbed habitat is
inexplicable at present.
Our observations of relatively low-diversity communities at the smallest
preserves are consistent with the notion that habitat patch size influences
community-level species richness and diversity deterministically, as suggested
for reptile communities on mountain tops in the Southwest (Jones et al. 1985) and
Figure 4. Desert Iguana (DIDO) in urbanized landscape, adjacent to the New River, in
northwestern Phoenix. The roadside lot is shown to the top left panel (note lizard on top of
gate, leftside), a closeup of the lizard is shown on the upper right panel, and the lower panel.
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birds of desert (Litteral and Wu 2012) and urban (Davis et al. 2013) areas. Under
this view, the original habitat variation encompassed by the urban preserves
determined the diversity of lizards present. Additionally, it may be that our resurvey
interval was too short, and that additional species will be lost over time,
especially if they are more long-lived than is widely appreciated. For example,
Common Chuckwallas persist at the LO site even though population density is
the lowest of all populations studied to date (Sullivan and Sullivan 2012). Given
these lizards live to over thirty years of age, our survey intervals may have been
too short to detect the loss of this species.
Current diversity and abundance
The urban preserve sites we surveyed ranged in diversity from three to nine
species, which is low compared with the known diversity of the adjacent Sonoran
Desert (Table 1; Jones et al. 1985, Parker 1967). The observation that larger
preserves have higher diversity is consistent with the traditional species-area relationship
that larger islands of habitat with (presumably) higher habitat diversity
should support more diverse communities (see reviews in Buckley and Jetz 2010,
Davis et al. 2013, Farnsworth et al. 2014). This hypothesis was corroborated by
work on mountain top (i.e., higher elevation) communities of squamate reptiles
across the Sonoran Desert by Jones et al. (1985). They found that the diversity of
species found in non-desert habitats (e.g., chaparral and desert grassland biomes)
was strongly predicted by the size of those habitats present at higher elevation on
mountain tops in a “sea” of desert habitat. A positive influence of area on diversity
is widely established for a number of taxa, including birds of the Sonoran Desert
(Litteral and Wu 2012) and Western Australia (Davis et al. 2013).
Banville and Bateman (2012) found that for three riparian sites along the Salt
River running east–west across the southern Phoenix Metropolitan region, Tiger
Whiptails (25%) and Side-blotched Lizards (33%) were numerically dominant
across the lizard community of seven species, which included both the Desert Iguana
and a riparian-corridor specialist, Urosaurus graciosus (Hallowell) (Long-tailed
Brush Lizard), that was otherwise absent from the entire Phoenix area (Brennan and
Holycross 2005). They suggested that increased vegetational complexity explained
variation in the diversity of lizards in the riparian sites they sampled, consistent
with the overall notion that habitat complexity in part explains the variation in
preserve diversity we documented. Shannon-Weaver and Simpson diversity indices
for their study (SW = 2.43, S = 2.42, n = 83 lizards total; Table 3) were relatively
high but within the range of values we obtained across our four most extensively
surveyed sites; similar results were obtained for Parker’s study of SM from the
1960s and Germaine and Wakeling’s (2001) study of the Tucson area in the late
1990s (Table 3). Considering the results of the aforementioned studies together, we
infer that the Phoenix area preserves are somewhat lower in species diversity and
evenness relative to outlying areas of the Sonoran Desert, but within the range of
urban riparian sites in the Phoenix area, and the entire urban region of the much
smaller Tucson metropolitan area.
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2014 No. 2
The relative abundance of taxa across the range of size of preserves, from one
of the smallest, LO, to one of the largest, CB, reveals that one lizard virtually absent
from most of the developed metropolitan region, the Side-blotched Lizard, is
the predominate species among the lizard communities of preserves, representing
a minimum of 65% of all lizards seen and as much as 90% of lizards in the lowerdiversity
sites (i.e., LO; Table 2). The relative ranking in abundance across the four
most intensively surveyed sites reveals that Side-botched Lizard and Tiger Whiptail
are the most abundant lizard species in the preserve communities, together accounting
for 80–95% of lizards observed. Interestingly, across the vast majority of the
metropolitan area apart from remnant habitat patches, Side-blotched Lizards have
been lost, presumably as a result of their ground-dwelling habits and vulnerability
to predators (e.g., Felis catus L. [Domestic Cat]) and anthropogenic habitat change
favoring arboreal species (e.g., OrnateTree Lizard) now found throughout the urban
landscape (Ackley et al. 2015, Germaine and Wakling 2001). The riparian communities
surveyed by Banville and Bateman (2012) were also dominated by this same
pair of lizard species (58% of all lizards they observed).
The exception to numerical dominance by Side-blotched Lizards was the SM
site. This preserve was relatively more even, as indicated by the highest values of
diversity indices (also associated with the higher species richness observe therein),
with higher numbers of saxicolous lizards, both Common Chuckwallas, rockcrevice–
dwelling specialists, and Tree Lizards, rock- or tree-dwelling specialists.
At this site, Side-blotched Lizards composed only 15% of the lizards observed, and
Tree Lizards, entirely absent from the other three extensively surveyed sites, together
with Common Chuckwallas, accounted for over 34% of all lizards observed.
Tiger Whiptails, the second most abundant lizard in most preserves, dominated
the SM site (Table 2); along with Tree Lizards, Tiger Whiptails are one of the two
lizards occupying highly impacted urbanized landscapes over much of Phoenix
(Ackley et al. 2015). Whiptails dominated the urban surveys (55% of all lizards
seen) in the Tucson area in a study of the lizard community there in the late 1990s,
while Side-blotched Lizards were exceedingly rare (less than 1%) (Germaine and Wakeling
2001). The somewhat contrasting indices of our surveys at SM and those of Parker
in the 1960s highlight the importance of sampling methods, all of which have inherent
biases depending on the behavioral ecology of the target organisms.
Conclusions
In the early 1990s, one of us (B.K. Sullivan) served as a consultant to the
Phoenix Parks Department regarding planning for a large preserve on the northern
edge of the Phoenix Metropolitan area (Burke and Ewan 1999). One of the primary
recommendations was that future preserves should include both open flats
(Creosote-Bursage valleys) and xeric washes (Palo Verde-Ironwood-Acacia) to
increase the habitat heterogeneity relative to that of the interior mountain preserves
established over the previous five decades. This recommendation was based in part
on the presumption that the addition of these habitats would result in the presence
of a variety of reptiles lacking from the interior preserves (i.e., Desert Iguanas,
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Long-nosed Leopard Lizards, Regal Horned Lizards, and Desert Horned Lizards),
subsequently reinforced by the work of Sullivan and Flowers (1998). Sullivan and
Flowers (1998) may have wrongly assumed that the community of Sonoran Desert
flats, dominated by Creosote and Bursage, was more uniform across the southern
portion of the state than was advisable. The prediction of a more diverse lizard
community was not to be: the area that was eventually preserved (Sonoran Preserve
Park in 2009, adjacent to the CB site) is roughly similar in diversity to other
mid-sized preserves (6 species) rather than to the outlying sites (10–13 species) or
even the similarly sized SM site (9 species). Surprisingly, inclusion of a large area
of flats did not increase the diversity of reptiles, most likely because of historical
contingencies in the respective distributions of these lizards. These results suggest
that there is no substitute for detailed natural history information on the local distribution
of taxa in conservation planning for preserves.
Acknowledgments
We thank Leah Kapa and Daniel, Justin, and Elizabeth Sullivan for assistance in the
field. Observations were conducted under authority of permits provided by the Arizona
Game and Fish Department (AGFD). Collecting methods were approved as part of an
IACUC protocol (B.K. Sullivan) for surveying reptiles and amphibians. Maricopa County
Flood Control personnel, especially Dennis Duffy, Dianna Cunningham, and Diana Stuart,
provided considerable assistance at the CB site, as did Rob Patterson and Andy Long of the
Phoenix Parks Department, and Roger Moncayo provided assistance with site protection. A
number of individuals have helped clarify our thinking about urban impacts and desert lizards:
Jeff Ackley, John Alcock, Dale DeNardo, Tom Jones, Matt Kwiatkowski, and Jianguo
Wu were helpful in the development of our study, while Bob Bezy, Tom Gatz, and Gillian
Rice assisted during the preparation of our manuscript.
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Appendix 1. Lizards observed in Phoenix Metropolitan area during all survey periods,
1990–2014. Abbreviations in parentheses represent species names used elsewhere in the
manuscript.
Species Sites
Colenoyx variegatus (Banded Gecko; COVA) AD, CB, SM
Sauromalus ater (Chuckwalla; SAAT) AD, CB, DH, HH, LO, NM,
NR, PPk, RGL, ShM, SM
Crotaphytus spp. (Collared lizards) -
Phrynosoma platyrhinos (Desert Horned Lizard) -
Dipsosaurus dorsalis (Desert Iguana; DIDO) NR, SM
Sceloporus magister (Desert Spiny Lizard; SCMA) AD, CB, DH, NM, NR, PP,
RGL, ShM, SM
Heloderma suspectum Cope (Gila Monster) -
Gambelia wislizenii (Leopard Lizard) -
Phrynosoma solare (Regal Horned Lizard; PHSO) AD, CB, DH, PP, RGL, SM
Uta stansburiana (Common Side-blotched Lizard; UTST) AD, CB, DH, HH, LO, NM,
NR, PP, PPk, RGL, ShM,
SM
Aspidoscelis tigris (Tiger Whiptail; ASTI) AD, CB, DH, HH, LO, NM,
NR, PP, PPk, RGL, ShM,
SM
Urosaurus ornatus (Tree Lizard; UROR) PP, SM
Callisaurus draconoides (Zebra-tailed Lizard; CADR) AD, CB, DH, NR, PP*, RGL,
SM*
*Cophosaurus texanus Troschel (Greater Earless Lizard) was erroneously listed for these
two preserves (n = 1 in each preserve) in Sullivan and Flowers (1998), but subsequently
were determined to represent this taxon, C. draconoides.