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Amphibians and Reptiles in Southwest Riparian Ecosystems1' 2
Charles H. Lowe 3
Abstract.--Obligate riparian amphibians and reptiles in
Arizona and Sonora, Mexico are discussed. Local population
extinctions in Arizona are examined. Special status for obligate riparian species is proposed.
Among the obligate riparian species that occur
both in southern Arizona and adjacent Sonora, Mexico
there are two native turtles and four native snakes;
these riparian reptiles also have limited distributions in adjacent southern New Mexico. All but one
have wider distributions in the Republic of Mexico
than in the United States.
Obligate Riparian Species
Kinosternon flavescens
Kinosternon sonoriense
Yellow Mud Turtle
Sonoran Mud Turtle
Thamnophis
Thannophis
Thannophis
Thamnophis
Mexican Garter Snake
Black-necked Garter Snake
Checkered Garter Snake
Narrow-headed Garter Snake
eques
cyrtopsis
marcianus
rufipunctatus
In spite of what may appear to be wide geographic distributions, these species are riparian
obligates that have narrowly limited ecological
distributions and low total species-population densities, however dense may be some of the local populations that remain. While strictly speaking they
are not yet endangered as species, all~ clearly
threatened species throughout their southwestern
distributions. Perhaps oddly, the two turtles have
more robust populations in Arizona than do the
riparian snakes. Only one of four snakes (I.
cyrtopsis) is dependably found when searched for
in its old known localities and habitats in Arizona.
While it remains among those obligate species somewhat less affected by riparian alterations in the
Southwest, during the 1980's it is also on the brink
of elimination from some riparian communities in
Arizona where it is now seriously reduced in population size.
1Paper presented at the first North American
Riparian Conference, Tucson, Arizona, April 16-18,
1985.
2 For discussions on amphibians and reptiles in
Southwest riparian ecosystems I thank William A.
Calder, Stephen F. Hale, Terry B. Johnson, Cecil R.
Sch~Jalbe, and William H. Woodin.
3Department of Ecology and Evolutionary Biology,
University of Arizona, Tucson, Arizona, 85721.
339
Local extinctions (extirpations) of populations
of obligate riparian reptile species has been in
progress in Arizona for about 20 years but are
essentially unrecorded. One of these extinctions is
shown in Figs. 1 and 2 for T. marcianus on the
former Santa Cruz River at Tucson, Arizona. The
data are for a once robust population located on
the western floodplain of the river between Grant
Road and Sweetwater Drive. The density graphed is
mean density per five-year period for transect observations on the floodplain using Silverbell Road.
The last permanent water in the Santa Cruz River at
Tucson was in 1941. Th~ last checkered garter snake
was seen in the population in 1976. The survivorship curve in Fig. 2 is an extinction curve for
this population. The final cause of extinction was
encroachment--agricultural followed by urbanization.
15
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Fig. 1.--Thamnophis marcianus on the floodplain of
the Santa Cruz River, northwest Tucson, Arizona.
Local population extinction due to man-made
encroachment.
Native animal species that are obligate riparian species are usually the first to disappear from
the riparian community as the result of significant
alterations to the environment. Important man-made
perturbations causing riparian habitat alteration
and destruction in the Southwest &re given in Table
1. Documented cases of local population extinctions
of obligate riparian reptiles (3) and amphibians (1)
in Arizona are given in Table 2.
Table 1.--Han-made perturbations causing
riparian habitat alterations and
destruction in the North American
Southwest, United States and Mexico.
80
Thomnophis
morcionus
P6 =15.5078-0.1637 t
50
r = -0.998
Pump Down, Damming
p <.001
Encroachment
Agr ic ul tur al
Reclamation
Urbanization
et al.
>- 20
.....
:::i
ID
<(
ID
0
a::
5
Pollution
Acid Rain
Pesticides
Trace Metals
et al.
0..
0.1
Grazing, Woodcutting
Exotic Species
Fig. 2.--Probability on time for survivorship of
Thamnophis marcianus, after 1960 to extinction; see Fig. 1.
These extirpation events took place in the
1960's and 1970's. The local extinction of riparian
amphibian and reptilian populations in Arizona is
clearly in process having been underway for a time
equal to about one human generation. This is not
unexpected. Arizona is participating in the current
man-made global faunal mass extinction that was
accelerated earlier during this century.
AMPHIBIANS
Our vertebrate amphibian fauna is far more
diverse than many suspect. There are 24 species in
Arizona and 31 in Sonora. Host of the amphibians
in Arizona-Sonora are anurans (frogs and toads);
three are salamanders. All are riparian species
and all but two are obligately so, for they must
use surface water--permanent or temporary, running
or standing--for reproduction. While we assume
that all of these species may not be equally
threatened in 1985, all are riparian species and
thus are clearly threatened as we near the close
of the century. They are threatened principally
by encroachment, pump down, and by pollution
including acid rain and pesticides.
On what basis should we finally judge the
threatened status of Arizona's amphibian species?
It is self-evident that the ranid frog (genus Rana)
populations in Arizona are threatened and 3 species
are properly so listed (AGFD 1982). There is (was)
a total of 5 native Rana species in Arizona, and
in the longer run to the end of the century and
Table 2.--Local extinctions of obligate riparian populations of amphibians and reptiles due to
man-made alteration and destruction of riparian habitats in Arizona.
Species
Locality
Population
Status
Decade
Riparian
Alteration
Riparian
Type
Thamnophis eques
Rillito floodplain
vicinity Tucson
Extinction
1960
Encroachment
Urbanization
xeror i par ian
Thamno_ehis marcianus
Santa Cruz floodplain
vicinity Tucson
Extinction
1970
Encroachment
Urbanization
xeroriparian
Thamnophis rufi,eunctatus
Fort Valley
vicinity Flagstaff
Extinction
1960
Pump Down
hydroriparian
Ran a tarahumarae 1
Santa Cruz County ( 6)
Extinction
1970
Pollution
mesoriparian
1
Species apparently eliminated from the fauna of Arizona; work in progress (S. F. Hale).
340
beyond they all are unquestionably in subequal
jeopardy with Rana tarahumarae the ~1adrean Tarahumara Frog which already may be completely eliminated
from Arizona's fauna by habitat pollution (Hale and
May 1983).
tara frogs to succumb first, and the semiaquatic pip
frogs (that are periodically terrestrial) to be
eliminated at some intermediate time point. I.Je will,
of course, have to wait and see. It should not be a
long w'ait, particularly if the price for Arizona
copper goes up substantially and/or the Nacozari
smelter goes into operation on schedule.
What is the time frame that we have in mind?
What is our time perspective for threatened and
endangered species? Is it this decade (?), next
decade (?), or the next century that is nmv less
than one human generation away? In talking to
variously concerned people interested in the subject
of threatened and endangered species in the South\llest, both in Hexico and the United States, I find
that their time perspective on the subject is often
hazy or even unconsidered.
Whatever the lethal mechanism and wherever it
is mediated--in the water, soil, food, shelter, all
or other--the obligate riparian species we have
left are threatened now (Table 1). In general,
species are "threatened" in the "now" and
"endangered" in the "future;" they are finally
doomed however when the time perspective is
inadequate in the now.
It is, of course, much later than we think.
For example, in Arizona during 1970-1980 the
Tarahumara Frog was eliminated from 5 of the 6, or
6 of the 6, of its historically known populations
(see Hale and May 1983). Most or all of the other
Rana species in Arizona, all four of which are
RECOMMENDATIONS
With regard to Arizona's riparian species,
several recommendations are made at this time for
the current listing of Threatened Native Wildlife
in Arizona (AGFD); see Table 3. The listing of
Table 3.--Specific recommendations on obligate riparian species of amphibians
and reptiles for Threatened Native Wildlife in Arizona by the Arizona
Game and Fish Commission (see AGFD 1982).
&~HIBIANS,
Retain:
Add:
Sources of Alteration and Destruction
Obligate Riparian
All 7 species listed
Rana chiricahuensis
Rana yavapaiensis
Encroachment, Pollution
Pump Down, Encroachment, Pollution
TURTLES, Obligate Riparian
Retain:
Add:
Encroachment, Pollution
Encroachment, Pollution, Pump Down
Kinosternon flavescens
Kinosternon sonoriense
SNAKES, Obligate Riparian
Retain:
Add:
Thamnophis
Thamnophis
Thamnophis
Thamnophis
eques
rufipunctatus
cyrtopsis
marcianus
Encr;achment,
Encroachment,
Encroachment,
Encroachment,
"pips" in the Rana J?.!.Eiens complex of leopard frogs,
are surely to follow the "taras" if, indeed, pollution related to acid rain and/or trace metals is
substantiated beyond reasonable doubt as a causal
factor in the extirpation of !· tarahumarae.
The case is particularly instructive. In
freshwater populations where the taras have been
eliminated (e.g., Sycamore Canyon, Santa Cruz Co.,
Arizona), the sympatric pips remain as do the native
fish. If lethal pollutant toxin(s) is mediated
partly or wholly through the stream water, we
could be observing the beginning of a riparian
community physiological extinction pattern in which
the aquatic-semiaquatic species with the highest
physiological flushing rate is eliminated first;
fluid flushing processes are but one of the possible
skin-related avenues for toxic poisoning in frogs.
Because fishes are integumentally waterproofed
(Hith lover urine flows and lower glomerular
filtration rates), while frogs are integumentally
highly permeable to water (with higher urine flows
and higher glomerular filtration rates), the fishes
would be expected to succumb last, the highly aquatic
341
Grazing
Pump Down
Grazing
Pump Down
still additional threatened riparian species is
appropriate and soon should be considered further.
Regarding the field (status) investigations of
Special Status Species, they should include for each
known population and locality (i) the current agespecific population density, and (ii) an historic
resume on presence, density, habitat condition, and
man-made perturbations affecting habitat alteration
and quality at least since the turn of the half
century.
LITERATURE CITED
Arizona Game and Fish Department. 1982. Threatened
native wildlife in Arizona. Arizona Game and
Fish Commission, Phoenix, Arizona.
Hale, S. F. and C. J. May. 1983. Status report for
Rana tarahumarae Boulenger. Report from Arizona
Natural Heritage Program, Tucson, to U. S. Fish
and \Vildlife Service, Albuquerque, New Mexico.