Vascular Plant and Vertebrate Inventory of Saguaro ... - USGS

elevation range of the district allows it to contain
many of the reptiles and amphibians associated
with these very different ecological provinces.
As a result, many representatives of each of the
four major regions are present, including a large
number of species not present in the Tucson
Mountain District of the park. An interesting
note is that a few low-desert Sonoran Desert
species found in the Tucson Mountain District,
such as the sidewinder (Crotalus cerastes) and
desert iguana (Dipsosaurus dorsalis) reach
the eastern edge of their ranges in the Tucson
Mountains. Thus, the Rincon Mountain District’s
herpetofauna contains classic Sonoran Desert
species (e.g., desert tortoise), Rocky Mountain
species (e.g., ring-necked snake), Chihuahuan
Desert species (e.g., greater earless lizard), and
Madrean species (e.g., Madrean alligator lizard).
Many of these species are on the edge of their
range in the district. A few taxa, including the
southern plateau subspecies of the eastern fence
lizard, occur in the Rincon and nearby Santa
Catalina Mountains as disjunct populations
(Stebbins 2003).
There are also a large number of species
that occur close to the Rincon Mountains but
that have not been observed in the district. Our
inventory confirms regional distribution patterns
of herpetofauna first described by Lowe (1994)
who noted that many Madrean species reach
their northern limits along what he referred to as
the “Madrean Line” that corresponds roughly to
Interstate-10, which runs just to the south of the
district (See Fig. 2.1). Lowe (1994) focused on
several Madrean rattlesnakes that are found in
the Santa Rita Mountains but not in the Rincon
or Santa Catalina mountains, including the twinspotted
rattlesnake (Crotalus pricei), banded
rock rattlesnake (C. lepidus), and Arizona ridgenosed
rattlesnake (C. willardi). By contrast, the
western rattlesnake, a “Rocky Mountain” species,
is found in the Rincon Mountains but not in the
Santa Rita Mountains. Lowe’s observation has
been confirmed by biogeographical analyses of
recent inventories (Swann et al. 2005), including
ours. Rumors have long persisted that some of
these Madrean species (especially banded rock
rattlesnakes) occur in the Rincon Mountains, but
this inventory provides further evidence that they
do not.
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Other species found near Tucson that
do not occur in the district include many mesic
riparian species, including the Mexican garter
snake (Thamnophis eques) and Woodhouse toad
(Bufo woodhousii). The Texas horned lizard
(Phyrnosoma cornutum), a Chihuahuan Desert
species, has been found in Mescal (20 km SE
of the district; Roger Repp, pers. comm.) but is
unlikely to occur in the district.
Abundance and Distribution
The Rincon Mountain District has a well-studied
herpetofauna compared to other areas, due
mainly to its proximity to Tucson. In particular,
recent field studies of individual species have
facilitated incidental observations of reptiles and
amphibians that are not often seen. In addition,
the size of the staff at Saguaro in comparison with
smaller NPS units in the Sonoran Desert Network
has resulted in better documentation of sightings,
including collection of roadkill. On the other
hand, the district is large, mostly roadless, and
topographically complex, which makes studies
there difficult.
Our study is the first to quantify relative
abundance and distribution of amphibians and
reptiles in the district and to evaluate patterns of
these parameters in space and time. Many of the
patterns that we documented confirm patterns
observed in previous studies. However, the
greater rate of detections per hour on extensive
(4.1 detections/hr) vs. intensive (3.6 detections/
hr) surveys was dramatically different than in
the Tucson Mountain District, where extensive
surveys (4.5 detections/hr) produced far fewer
detections than intensive surveys (6.3 detections/
hr) (Flesch et al. 2006). Tables 4.5 and 4.8
suggest that this may be due to the effect of
greater numbers of intensive surveys at higher
elevations, where detection rates were lower than
on low-elevation plots.
In general, both abundance and
distribution of reptiles and amphibians decreased
with increasing elevation in the district. This
pattern is well-known and certainly corresponds
to declining species richness of reptiles (but
not amphibians) across an increasing latitudinal