BIOG EOGR APHY OF GREA T BASI N MAM MALS :
PARA DIGM LOST ?
TIMOTH Y
E. LAWLO R
Arcata, CA 95521
Departm ent of Biologi cal Science s, Humbo ldt State Univers ity,
of the Great Basin of
Biogeo graphic relatio nships of mamm als occurri ng on mounta intops
acquire d inform ation. Conwester n North Americ a are fe-exam ined on the basis of newly
d betwee n species richness
trary. to pre".ious indicat ions, a relativ ely weak pattern is reveale
varies only by a factor
and Island Size. The range of species numbe rs per mounta in range
species found on any
a
that
of h~O, wi~ n:ost islands having betwee n 9 and 13 species , so
species occurrences
of
es
Analys
one Island IS lIkely to be found on many, if not all, others.
of area. Consistent
effects
strong
(nested ness, presen ce-abse nce pattern s) also do not reveal
intops from one
mounta
of
n
isolatio
of
with prior studies , there is no discern ible effect
te number of
ortiona
disprop
the
from
ble
anothe r or from putativ e mainla nds; this is explica
faunas. Ale
montan
among
forms
ine
mobile woodla nd species and few sedenta ry subalp
late Pleisthe
since
intops
mounta
from
eared
though species and popula tions have disapp
es shapprocess
graphic
Biogeo
.
species
ine
subalp
tocene , they involve mostly cold-ad apted
a
involve
tly
eviden
Basin
Great
the
in
faunas
ing charact eristics of modem mounta intop
of
ce
resistan
ion
extinct
and
al
dispers
untain
intenno
ve
dynam ic interpl ay betwee n extensi
that montan e mammals
remain ing species . Brown 's (1971, 1978) hypoth esis, which posits
by extinct ions and no
in the Great Basin compri se rerrman t faunas formed exclus ively
colOni zations , is not suppor ted.
Key words:
biogeo graphy , mamm al faunas, mounta intops, Great Basin
For 2 decade s mamm al faunas on mountaintop s in the Great Basin of wester n North
Ameri ca (Fig. 1) have been portray ed as a
model of effects of habitat fragme ntation on
species distribu tions and extinct ion patterns. Brown (1971, 1978) argued that these
monta ne assemb lages demon strated a pattern of faunal collapse resultin g from widespread local extinct ions withou t replace ments. At the time, this hypoth esis was contrary to the prevail ing view (MacA rthur and
Wilson , 1963, 1967) that species numbe rs
on islands resulte d from a dynam ic equilib rium betwee n opposi ng rates of coloniz ation and extinct ion.
Severa l compel ling pieces of eviden ce
suppor ted Brown 's view (Brown , 1971,
1978; Brown and Gibson, 1983; Grayso n,
1993; Lomol ino, 1986; Patters on, 1987), as
follow s: he detecte d no effect of relative
isolatio n of mounta in ranges (an index to
contem porary invasio n rates) on species diJou/'I1a/o fMammal ogy. 79(4):111 1-1J30, 1998
versity ; he found a strong relation ship between island area (an index of extinction
probab ility) and species richnes s; montane
species commo n in the nearby Sierra Nevada and Rocky Mount ains were missing or
patchil y distribu ted in mounta ins of the
Great Basin; current species occurre nces
were highly nonran dom with respect to island size, faunal richnes s, and faunal compositio n; species missin g or discont inuous ly
distrib uted in Great Basin mounta ins were
found as late-Pl eistoce ne or early-H olocen e
fossils in mounta in ranges that they no longer occupy ; and montan e species also were
docum ented from fossil eviden ce in desert
valleys .
This eviden ce is consist ent with a nonequilib rium (relaxa tion) model in which
species numbe rs in isolate d faunas decline
as a result of local extinct ions in the absence of coloniz ations, as on true landbridge islands separat ed from mainla nd ar1111
1112
JOURNAL OF MAMMALOGY
Vol. 79, No.4
100 km
I
_.:._._._._._ ....1._._._._._.,._._.
I,
t
i,I
I
i
AU
;
I'
PI
SS
rI
1i
I
i
~DC
t:
SN
I
I
I
~.-.-.-.-
-.-.-. -,~ -
I
110
FIG. I.-Map of mountaintops in the Great Basin examined in this study. For each northern mountain range, the outermost line circumscribes elevations >2,135 m (7,000 feet); blackened areas denote
elevations >2,300 m (7,500 feet). Abbreviations are: CA, Clan Alpine Range; DC, Deep Creek
Mountains; DE, Desatoya Mountains; DI, Diamond Range; GQ, Grant-Quinn Canyon ranges; MT,
Monitor-Toquima-Hot Creek ranges; QQ, Oquirrh Mountains; PA, Panamint Range; PI, Pilot Peak;
RC, Roberts Creek Range; RU, Ruby Mountains; SC, Schell Creek-Egan ranges; SH, Sheep Range;
SN, Snake Range; SP, Spring Range; SS, Spruce-South Pequop mountains; ST, Stansbury Mountains;
TS, Toiyabe-Shoshone ranges; WH, White Mountains; Wp, White Pine Range.
eas by water barriers (Lawlor, 1986, 1996).
According to Brown (1971, 1978), disappearances of species in Great Basin mountains probably occurred during glacial retreats at the end of the Pleistocene, when
widespread coniferous forests shrank and
became fragmented as they moved upslope
in response to post-glacial warming. Over
time, the combination of local extinctions
of conifer-associated species in these isolated patches and the absence of recruitment
across newly formed arid valleys caused
complete disappearance of some montane
mammal species and spotty relictual occurrences of others. A strong relationship between species numbers and area also should
prevail because extinction rates should be
higher on small mountaintops than on large
ones. Predictably, the pattern was more
striking for mammals than it was for birds
because in vagile species periodic extirpations should be offset by repeated immigrations. Indeed, Brown (1978) showed that
patterns of species richness for birds of the
November 1998
ALS
LAWLO R-BIOG EOGRA PHY OF GREAT BASIN MAMM
Great Basin, unlike those of mamm als,
were best explain ed by combi ned effects of
high immig ration rates and availab ility of
suitabl e habitat s.
Becaus e of its widesp read accept ance,
Brown 's non-eq uilibriu m model has attained the status of an ecolog ical paradig m
(Belov sky, 1987; Brown and Gibson , 1983;
Colinv aux, 1993; Conno r and McCo y,
1979; Diamo nd, 1984; Murph y and Wilcox ,
1986; Shafer, 1990; Wens, 1983; William son, 1981; Wright , 1981; but see Rosenzweig, 1995). It also has been used as the
basis for modeli ng future impact s of global
warmi ng (Brown , 1995; McDon ald and
Brown , 1992; Murph y and Weiss, 1992)
and as a counte rpoint for evalua ting distribution al pattern s of more vagile taxa in the
Great Basin (Wilco x et aI., 1986).
Recent ly, howeve r, the genera lity and robustne ss of the model have been questio ned
(Grays on, 1993; Grayso n and Living ston,
1993; Skaggs and Boeckl in, 1996). For example, species though t to be missin g from
some mounta in ranges of the Great Basin
have been reporte d, and the recalcu lated
slope (z-valu e) of the log species -log area
curve has been reduce d from Brown 's
(1971, 1978) earlier ones (Grays on and
Living ston, 1993). Many species considered by Brown (1971, 1978) to be dependent on conifer ous forest are recorde d from
woodl and or scrub habitat s at low elevations (Grays on and Living ston, 1993; Grayson et aI., 1996; Skaggs and Boeckl in,
1996). Also, Quater nary deposi ts from
Home stead Cave, Lakesi de Mount ains,
wester n Ut~ reveal an interm ittent presence of one species (the bushy- tailed woodrat, Neotom a cinerea ) during the Holoce ne;
eviden tly the specie s recolo nized these
mount ains recentl y (Grays on, pers. comm. ).
These studies sugges t that biogeo graphic
relatio nships of mounta intop mamm als in
the Great Basin may be more dynam ic than
origina lly envisio ned. Weak relatio nships
betwee n species numbe rs and montan e area
and isolatio n would imply that these island
faunas are charact erized by greater inter-
1113
connec tednes s among species populations.
Compl ete isolatio n may occur only for a
small subset of the compo nent species.
I provid e a compre hensive re-eval uation
of the relaxat ion hypoth esis on the basis of
an expand ed databas e and a reanaly sis of
all availab le data. To provid e more robust
tests and reinfor ce conclu sions drawn from
traditio nal species -area and species -isolation relatio nships, I also explore other biogeogra phic indicat ors of isolatio n and area
effects , includi ng pattern s of species occurrences and faunal compo sitions . I questio n
if existin g data for montan e mamm als of
the Great Basin suppor t a relaxat ion model
(Brow n, 1971, 1978), in which isolatio n has
been so comple te and pervas ive that species
presen ces relate only to the likeliho od of
past extinct ion episode s (areas of mounta intops), or if availab le data better fit a dynamic model involvi ng an interpl ay among factors such as ecologi cal opport unity (habita t
divers ity), coloni zation probab ilities
(moun taintop separat ions), and extinct ion
potenti al.
MATER IALS AND METHO DS
Data assemb ly and analys is.-Val ues for
most biogeographic variables (island areas and
distances) were obtained from previously published sources (e.g., Brown, 1978) or were discerned from United States Geological Survey
(USGS) maps (Table 1). Because extent of
mountaintop isolation was fundamental to discriminating between relaxation and dynamic
models, I measured it in three ways: 1) minimum straightline distance from the nearest
"mainl and" source (Sierra Nevada or Rocky
mountains); 2) minimum distance from the nearest neighboring mountaintop; and 3) minimum
distance from the mainland or nearest larger
mountain range, whichever is least. I determined
habitat-diversity scores for mountain islands according to the method of Johnson (1975), aided
by personal observations and recent infonnation
on presences of conifer species (Charlet, 1996;
J. Hamrick, pers. conun.). Those scores took
into account diversity of conifer species and extent of riparian. meadow, and aquatic habitats.
Species lists (Table 2) were compiled from published sources (notably Brown. 1978, as updated
JOURNAL OF MAMMALOGY
1114
TABLE
Vol. 79, No.4
I.-Biogeographic characteristics of Great Basin mountain ranges.
Distance to potential source (km)h
Maximum
Number
of
Mountain range'
Toiyabe-Shoshone (TS)
White (WH)
Monitor-Toquima (MT)
Ruby (RU)
White Pine (WP)
Oquirrh (OQ)
Snake (5N)
Deep Creek (DC)
Desatoya (DE)
Stansbury (ST)
Roberts Creek (RC)
Schell Creek-Egan (SC)
Diamond (m)
Spruce-South Pequop (S5)
Grant-Quinn Canyon (GQ)
Clan Alpine (CA)
species
13
13
12
12
II
II
10
9
9
9
9
9
8
8
6
6
Mountaintop area
(km2)b
1,772
1,911
3,051
943
679
212
1,080
578
215
145
135
2,642
(2,620)
(6,133)
(982)
(1,11 J)
(494)
(1,360)
(737)
(308)
(287)
(297)
(4,047)
412
127 (237)
388 (874)
135 (205)
elevation
(m)
3,593
4,341
3,642
3,471
3,410
3,239
3,982
3,688
3,040
3,362
3,089
3,622
3,235
3,128
3,444
3,038
Habitat
diversity
score<
7
13
9
9
II
10
14
II
5
13
4
12
7
4
9
4
To
mainLand
176 (175.0)
16
182
237
240
30
142
166
133
62
346
182
(179.0)
(236.0)
(236.0)
(28.0)
(137.0)
(165.0)
(133.0)
(61.0)
(339.0)
(181.0)
304
250 (249.0)
221 (220.0)
118 (116.0)
To nearest
To nearest
larger
larger
neighbor
neighbor or mainland
15 (11.0)
15(11.0)
16
16
15
15
14
21
17
25
14
21
32
17
(11.0)
(14.0)
(7.0)
(18.0)
(4.5)
(4.5)
(13.0)
(18.0)
(28.0)
(7.0)
8
26 (24.0)
42 (23.0)
10 (7.5)
182
14
14
30
17
25
14
21
32
105
(182.0)
(82.0)
(7.0)
(18.0)
(4.5)
(4.5)
(13.0)
(18.0)
(28.0)
(7.0)
8
26 (24.0)
47 (23.0)
10 (7.5)
'Abbreviations (in parentheses) identify mountain ranges in Fig. I.
t> Values f,x areas and distances are provided for islands delimited by 2,300-m (unenclosed) and 2,135-m (enclosed in parentheses)
map contO'Jrs (see text).
'The method for determining habitat diversity scores is provided in text.
by Grayson and Livingston, 1993) and from my
own surveys. Except for obvious species such
as yellow-bellied marmots (Marmota flavivenIris), visual observations were verified by collection of voucher specimens which were deposited in the Vertebrate Museum, Department
of Biological Sciences, Humboldt State University.
Although lists of species constituted cumulative totals of survey data dating to the nineteenth
century, I assumed that they accurately repre~
sented CUrrent species occurrences. For a variety
of reasons, major sources of error in species
counts caused by recent disappearances, colonizations, or species turnover could be ruled out.
I confirmed the presence of virtually all relevant
species in recent surveys of most mountaintops.
Only naturally uncommon species (e.g., ermines, Mustela enninea) occasionally went undetected. Additionally, observations that extirpations in the Great Basin have spanned 10,000
years and that they probably were highest during
the hypsithennal period several thousand years
ago suggest that recent losses of species, if they
have occurred at all, have had inconsequential
effects on overall distributional patterns. Finally,
"missing" species recently detected on Great
Basin mountaintops (Grayson and Livingston,
1993) evidently represented unreported occurrences, not recent invasions, because new records overwhelmingly came from mountain
ranges that previously were poorly surveyed. To
the extent that sources of bias in numbers of
species remain in the present analysis, they also
must be found in Brown's (1971, 1978) original
datasets.
For the most part, new records of species occurrenCeS reported in this paper were gathered
incidental to a separate study of small-mammal
populations in the Great Basin, so sampling effort was not equal for all species or for all mountain rangeS. Equivalent sampling regimes for all
species would be impossible to design in any
case, because species such as water shrews (Sorex palustris), pikas (Ochotona princeps), cliff
chipmunks (Tamias dorsalis), and ennines, have
different habitat requirements and other natural
history features. Nevertheless, I surveyed mammals in virtually all Great Basin ranges discussed in this paper for periods of 2:3 days and
sampled many habitats in each. Interestingly,
some of the newly reported "missing" mam-
X
X
OCPR
LETO
TAAM
MUER
ZAPR
13
X
X
X
X
X
SOPA
SPBE
X
X
TADO
SOYA
13
L
X
X
I
X
X
X
X
X
12
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
MT
WH
TS
SPLA
MAFL
TAUM
MILO
SYNU
NECI
abbreviation
12
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
II
K
D
D
D
X
X
X
X
WP
RU
II
X
10
X
X
X
X
X
E
X
X
X
X
X
X
X
SN
X
X
X
X
X
G
X
OQ
9
X
K
X
X
X
X
X
X
X
DE
9
X
X
X
X
X
X
X
X
X
DC
Mountain rangeb
9
X
X
F
X
X
X
X
E
X
ST
9
X
A
X
9
A
X
X
X
X
X
X
X
X
C
B
X
X
B
B
SC
RC
2.-Species occurrences in Great Basin mountain ranges,"
8
H
X
X
X
A
J
X
X
DI
8
A
X
A
A
X
X
A
X
SS
6
X
X
A
X
X
X
GQ
6
C
C
C
C
C
C
CA
5
6
6
6
6
10
9
16
16
16
16
15
14
13
of ranges
occupied
Total
number
'x denotes records summarized in Brown (1971, 1978) and Brown :lnd Gibson (1983); additional records, from neW collection data and other sources. are: A, this study; B, this study
(see also Grayson and Livingston, 1993); C, this study (sec also Hail, 1946); D, Blair (pers. Comm.); E. Egoscue (196!); F, Egoscue (1988); G, Egoscue (peTS. comm.); H, Elliot (peTS.
comm.); I, Grayson (1987); J, Grayson and Livingston (1993); K, Hall (\946); L, Sutton and Nadler (1969).
b Abbreviations for mountain ranges arc identified in Fig. 1 and Table I.
, Includes E. PCIIlWllilllilZlIS (White Mountains).
d Includes S. 1<!lIe/llls (White Mountains).
Neotoma cinerea
Sylvilagus nuttalli
Tamias umbrinus
Microtus /ongicQudus
Marmota jlaviventris
Spermophilus lateralis
Tamias dorsalis<
Sore:.: vagrans d
Sorex palustris
Spermophilus beldingi
ZaPIIS princeps
Mustela enninea
Ochotona princeps
Lepus townsendii
Tamias amoenus
Total species per range
Species
Species
TABLE
u;
0>
r'
,.
;(
;(
,.s:
~
..;
Cl
."
0
"'
,.m"
,.'"
:t
~
0
Cl
m
"I
'Cl(5"
0
r'
"
r'
,.
'"~
~
•
,<~
,•""
1116
JOURNAL OF MAMMALOGY
mals are common at locations where they occur,
underscoring suspicions of Grayson and Livingston (1993) that mammal species in mountain
ranges of the Great Basin have been underreported.
To calculate species-area relationships, I used
log-transfonned measures of variables to produce the linear equation log S =: c + z(log)A,
in which S and A represent species number and
mountaintop area, respectively, and c and z are
fitted constants. In the power model, c and z
both contribute to the slope of the species-area
curve (Lomolino, 1989), so their values must be
interpreted with caution. However, because of
their general utility (Rosenzweig, 1995), I emphasized z-values in comparisons. If faunas in
the Great Basin are not fully isolated and selective extinctions do not detennine the character
of these faunas, a log species-log area relationship should be weak. or absent.
Patterns of species occurrences were examined using several methods. When distances
from potential source populations are plotted
against area for mountaintops occupied by each
species, predictable patterns should emerge
(Lomotino, 1986). If occurrences are driven by
extinction, a minimum-area effect should be observed; populations of each species should be
found mainly on islands above some minimum
size because extinctions should predominate on
small islands. By contrast, if occurrences are
driven by colonization, a maximum-isolation effect, in which species are confined largely to islands (irrespective of size) near putative source
faunas, should prevail. Random or ubiquitous
occurrences of species should characterize
mountaintop faunas not structured by island area
or isolation.
Because survival on islands is contingent on
maintenance of viable population sizes, species
occurrences also should be non-randomly ordered on islands of relatively large size. I explored this in two ways. First, I used the Wilcoxon rank-sum test (Mann-Whitney U-tcstKadmon, 1995; Patterson, 1984; Simberloff and
Martin, 1991) to test for randomization of species occupying mountain ranges ranked by species richness and montane area. Because omnipresent species do not contribute to nestedness,
they were omitted from consideration. The resulting statistic (Wilcoxon score) for each species gave a probability that its observed presences and absences deviated from a random as-
Vol. 79. No.4
sortment of occurrences on islands of different
size or species diversity. I determined the probability that assemblages of species as a whole
were ordered using Fisher's test of combined
probabilities (Sokal and Rohlf, 1981). This statistic, which is distributed as a chi-square, was
calculated according to the fonnula, X2 =
-Z(ln)P, where P was the probability of nonrandomness for Wilcoxon scores (2) from individual species. The advantage of this procedure
was that deviations computed for individual species could be used to identify the relative contribution of each species to patterns of nestedness while simultaneously enabling comparisons
of whole assemblages (Simberioff and Martin,
1991).
Second, I employed the RANDOMO and
RANDOM 1 statistical designs of Patterson and
Atmar (1986), as modified by Cutler (1991), to
examine further stratification of species assemblages sorted by area and isolation. In any set
of islands arranged according to species richness, a nested ensemble ~ of species is one in
which depauperate faunas constitute subsets of
richer faunas. Although initially thought to be a
consequence of the deterministic effects of extinctions, nestedness may instead be a general
property of communities established and structured in a variety of ways (Cutler, 1991, 1994;
Kadmon, 1995; Lomolino, 1996; Patterson,
1987, 1990; Patterson and Atmar, 1986; Simberloff and Martin, 1991; Skaggs and Boecklen,
1996; Wright and Reeves, 1992). Nevertheless,
nestedness appears more prevalent and better developed in extinction-driven systems (Patterson,
1990; Wright and Reeves, 1992). A high degree
of nestedness and non-random distribution of
species would be consistent with the relaxation
hypothesis, as demonstrated for montane faunas
in the American Southwest (Patterson, 1984;
Patterson and Atmar, 1986) and the Great Basin
(Kodric-Brown and Brown, 1993) using
Brown's (1978) original data set. By contrast,
species in a dynamic model should be more randomly distributed (non-nested). Finally, using
Brown's (1971) ecological characterizations of
each species, I investigated the extent to which
individual mountaintop faunas are composed of
species ordered by body size, trophic level, or
habitat specialization.
lslands.-Following Brown (1978) and subsequent authors, islands (Fig. 1, Table 1) were
identified initially as mountain ranges having at
November 1998
LAWLOR-BIOGEOGRAPHY OF GREAT BASIN MAMMALS
least one peak >2,990 m (9,800 feet) that were
separated from nearby ranges by elevations
<2,300 m (7,500 feet) at 2:"8 km (5 miles) across
(Brown, 1978).
With a few modifications, mountain ranges
were the same as those used originally by Brown
(1971, 1978). The Pilot Range in northeastern
Nevada (Fig. 1) was omitted because it was surrounded by the waters of Lake Bonneville during its maximum in the late Pleistocene. Consequently, its flora and fauna were not contiguous with those elsewhere in the Great Basin at
that time. Thus, from a historical standpoint, the
mammalian fauna now occupying the Pilot
Range is not equivalent to faunas on other
mountaintops in the Great Basin. The absence in
the Pilot Range of the Uinta chipmunk (Tamias
umbrinus), a species that is otherwise ubiquitous
in montane habitats of the Great Basin, together
with the unusual occurrence of the woodlandadapted cliff chipmunk (T. dorsalis) at all elevations, gives credence to this scenario. The
Clan Alpine range in west-central Nevada (Fig.
1) was added. This mountaintop satisfies the criteria used by Brown (1978).
Unless otherwise indicated, I also excluded
three southern ranges (Panamint, Spring, and
Sheep ranges; Fig. 1) used by Brown (1971,
1978). These mountains contain relatively few
species for their size. In addition to being among
the smallest ranges, these mountaintops are located in a region marked by hotter summer temperatures and milder winters. Each is surrounded
by creosote (Larrea) scrub instead of the sagebrush-saltbush (Artemisia-Atriplex) flats typical
of low-elevation desert adjacent to northern
ranges. Lowland valleys isolating these southern
ranges are much lower «1,300 m) than are
those separating northern ones (> 1,660 m).
Also, habitat diversity of these ranges is poor
(Johnson, 1975). These factors combine to create relatively impoverished habitat patches surrounded by formidable barriers. A plausible explanation for the paucity of species in these
ranges is that the relatively inhospitable habitats
of mountains and valleys at these latitudes produced correspondingly higher rates of extinction
and lower rates of colonization than has been the
case for northern mountain ranges. Paleozoologkal data demonstrate that post-Pleistocene extinctions have not been synchronous across
mountain ranges in the intermountain region but
instead proceeded sooner and more rapidly in
llJ7
southern than in northern parts (Grayson, 1993;
Grayson and Livingston, 1993). Thus, the depauperate character of these southern faunas
may reflect greater current and former isolation.
Southern mountains are located only marginally in the Great Basin. From a phytogeographic
perspective, many investigators (Brown and
Lowe, 1980; Cronquist, 1978; Cronquist et aI.,
1972; Thompson, 1990) have treated them as islands in the Mojave Desert. Of course, any distinction between Great Basin and Mojave deserts is somewhat arbitrary (Grayson, 1993), but
discrimination between Great Basin (northern)
mountains and the three southern ranges can be
justified from their climates, compositions of floras and faunas, and habitat characteristics. This
distinction is also consistent with Brown's
(1971, 1978) use of similar criteria to exclude
outlying northern Great Basin mountains (e.g.,
the Jarbidge Range) from his original analyses.
Proper definition of mountaintops and their
biogeographic characteristics is crucial for distinguishing between biogeographic signal and
noise. Overall, criteria used by Brown (1971,
1978) to define islands are somewhat arbitrary
and may not be biologically meaningful. For example, the maximum-height limitation (2,990
m), employed by Brown (1971, 1978) to ensure
adequate habitat complexity, preferentially excluded small mountain ranges with potentially
impoverished faunas. Brown (1971. 1978) used
the lower elevational limit to provide a simple
approximation of the low-elevation extent of
pinyon-juniper woodland, but occurrence ofpinyon-juniper in the Great Basin is highly variable
in both altitude and latitude (West et aI., 1978).
Furthermore, the 8-km restriction may compromise detection of mountaintop isolation. For instance, if movements of species across desert
valleys are possible only at distances of <8 tan
and this filtering effect goes unmeasured, isolation effects will not be detected.
I conducted a second analysis to address these
problems. To better estimate availability of suitable habitat, I attempted to measure area by using lower limits of forest depicted on 1:500,000
USGS maps. However, the mapped vegetation
was discontinuous and patchy for many mountain ranges, so that procedure proved infeasible.
Instead, I used the 2,135-m (7,000 foot) contour,
which more reasonably approximated lower forest boundaries on mountaintops than did
Brown's (1971, 1978) higher elevation. I aban-
JOURNAL OF MAMMALOGY
1118
doned the 8-km distance requirement and the
maximum-elevation limitation. Unfortunately,
these modifications did not facilitate inclusion of
additional mountaintop faunas into the analysis
because faunas of other mountain ranges were
too poorly known. One unexpected consequence
of measuring island area by 2,13S-m contours
was a redefinition of islands in the Great Basin:
the White Mountains lost their insular separation
from the Sierra Nevada and the Diamond Range
coalesced with the Monitor and Toquima ranges.
Remaining islands totaled 14 (Table 1).
Species,-Species treated in this study confonned to those used by Brown and his co-workers (Brown, 1978; Brown and Gibson, 1983;
Kodric-Brown and Brown, 1993; McDonald and
Brown, 1992), although they differ slightly from
Brown's (1971) original study (Brown, 1978).
All were montane species found mainly in
woodland (pinyon-juniper), subalpine (pine-firspruce), or associated meadow and riparian habitats, Many of these species have been reported
from lowland habitats (Grayson and Livingston,
1993; Grayson et al., 1996; Hall, 1946; Skaggs
and Boecklin, 1996), although these records
were often from riparian corridors extending
from mountain ranges or from peculiar low-elevation pockets of suitable habitat. Large mobile
species and bats were excluded, Arguably, a few
other species also could have been included,
such as porcupines (Erethizon dorsatum), longtailed weasels (Mustela jrenata), and northern
pocket gophers (Thomomys talpoides), each of
which is widespread in mountains of the Great
Basin (Hall, 1946), and wolverines (Gulo gulo),
known from only the White Mountains (Kovach,
1981) and Snake Range (Barker and Best, 1976;
Muir, 1918), The single occurrence of the yellow-pine chipmunk (Tamias amoenus) in the
White Mountains is new to the data matrix (Table 2); it was reported earlier (Hall, 1981) and
subsequently overlooked,
I considered subalpine species to be those inhabiting relatively mesic coniferous forests (firs,
spruce, and limber and bristlecone pines) at
higher elevations, and woodland species to be
those frequenting the more xeric pinyon-juniper
and sagebrush at lower elevations. I use montane
species as a collective term for both.
RESULTS
Species richness.-Analyses using area
variables calculated at 2,300-m and at
Vol. 79, No, 4
1001r------------------------
10
100
1,000
10,000
ISLAND AREA (km2)
FIG. 2.-Log species-log area slopes resulting
from studies beginning with Brown (1971) and
culminating with the present one: a) 0.428
(Brown, 1971); b) 0.326 (Brown, 1978); c)
0.309 (Brown and Gibson, 1983; Cutler, 1991);
d) 0,289 (Kodric-Brown and Brown, 1993; a
similar value, 0.288, was reported by Grayson,
1993); e) 0.205 (present study, with 19 ranges
included and Pilot Peak excluded; see text); and
f) 0.128 (present study, with 16 northern Great
Basin ranges included and Pilot, Panamint,
Spring, and Sheep ranges excluded; see text).
2,135-m contours reveal that correlations
between species number and island area
were relatively weak in both cases, although the slopes (z-values) of the speciesarea curves were significant. The relevant
equations were, respectively: log S = 4.243
+ 0.128 log A, r ~ 0.605, P < 0.05 and
log S ~ 4.280 + 0.117 log A, r ~ 0.538,
P < 0.05. Correlations of log-transformed
values of species richness and mountaintop
areas from the first analysis (areas >2,300
m) were depicted (Fig. 2) for the series of
studies beginning with Brown (1971; Fig.
2A) and culminating in the present one
(Fig. 2F). For comparison, I also calculated
the curve (Fig. 2E) using updated species
numbers and Brown's (1971, 1978) expanded set of islands (a total of 19, including
Panamint, Spring, and Sheep ranges, but
without the Clan Alpine Range); it was
somewhat steeper and there was a stronger
correlation between species number and
area (log S
0.648, P
~
2.395 + 0.207 log A, r
< 0.01).
~
ALS
LAWLO R-BIOG EOGRA PHY OF GREAT BASIN MAMM
Novemb er 1998
A
•
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ISOLATION (km)
in the Great Basin, using min3.-Hyp othetic al species -occurr ence patterns on mounta intops
Presenc es are indicate d by
1).
(Table
n
isolatio
of
e
measur
imum island-m ainland distanc e as the
m-area distribu tion pattern; B) hy.
closed circles, absence s by open circles. A) hypothe tical minimu
pattern; D) hypothe tical random
pothetic al maximu m-isola tion pattern; C) hypothe tical compen satory
pattern.
FIG.
Irrespe ctive of differe nces in targete d
mounta intop areas, the log species -log area
curve has flattene d marked ly with increas ingly more compl ete survey s of small
mamm als in Great Basin mount ain ranges.
Overal l, seven of the 15 total specie s occurred on ~14 of the 16 total mounta intops,
and four species were ubiqui tous. Becaus e
numbe rs in large mounta in ranges have
change d little, shifts in the species -area
curve were due mostly to infillin g of missing data from poorly survey ed faunas located on small mounta intop islands . Specie s
contin ue to accumu late for these faunas
(Grays on, 1987; Grayso n and Living ston,
1993; Kodric -Brown and Brown , 1993).
Twenty -five records (Table 2), exclud ing
those from the Clan Alpine Range, were additions to Brown 's (1978) data matrix ; they
constit uted 15% of total species occurrences. Thirtee n of the 15 total species were
reporte d for mounta in ranges from which
they were unknow n previou sly; these ad·
ditiona l records were spread over most
mounta intops. To illustra te, during several
days of collect ing in summe rs of 1992 and
1993 in the Roberts Creek and SpruceSouth Pequop ranges, both of which are
small in size and not well survey ed, I dou·
bled the known montan e species from each;
Grayso n and Living ston (1993) also observed some of the same species from the
Robert s Creek Range.
Specie s richness correla ted negatively
but insigni ficantly (P > 0.05) with all three
measu res of island isolatio n for both sets of
targete d montan e areas. There also was a
positiv e but insignificant relation between
specie s numbe rs and habitat -divers ity
scores (S = 0.244 HDS + 7.523; r = 0.380;
P > 0.05).
Specie s occurr ences and nested ness.Presen ce-abse nce data plotted by mountaintop area and isolation reveale d that neither
montan e area nor isolatio n heavily influenced occurre nces of the majorit y of species. Irrespe ctive of which of the three distance measur es was used to index spatial
isolatio n, 13 of 15 species analyze d are either rare (one species), occurre d every-
JOURNAL OF MAMMALOGY
1120
A
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,
•
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•
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ISOLATION (km)
G
•
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,
~
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ISOLATION (km)
0
••
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lEPUS
TOWNSENOI
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ZAPUS
PRINCEPS
•
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,~
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SOREX
VAGRANS
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!o!U!3TE(.A
ER!o!INA
B
Vol. 79, No.4
•
0
0
0
'00
~
~
ISOLATION (km)
-
FIG. 4.-Species-occurrence patterns for small-mammal species on mountaintops in the Great Basin
using minimum island-mainland distance as the measure of isolation (Table 1). Rare species (those
with three or fewer presences) and ubiquitous species (those with three or fewer absences) are not
included; there are eight such species (Table 2). A) maximum-isolation pattern in ermines; B) compensatory pattern in vagrant shrews; and C-G) other (largely random) patterns in five additional
species.
where (or virtually so; seven species), or
showed no meaningful pattern (five species;
Table 2). Hypothetical and actual examples
of species-occurrences were plotted (Figs. 3
and 4) using one distance measure (minimum distance to nearest mainland). Only
two species (Sorex vagrans and M. erminea) gave a meaningful pattern. Overall,
these data did not support a prevalence of
minimum-area effects as found by Lomolino (1986) using Brown's (1978) earlier
dataset.
A significant nested pattern (P < 0.01)
was revealed by the RANDOMO and RANDoM! statistical designs (Patterson and Atmar, 1986; Cutler, 1991) for species pres-
X
X
TAAM
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(2)
SC
X
X
X
X
X
X
X
X
X
(8)
DC
X
X
X
X
X
X
X
X
X
(11)
DE
X
X
X
X
X
X
X
X
X
(13)
ST
X
X
X
X
X
X
X
X
X
X
X
X
(!O)
OQ
X
X
X
X
X
X
(16)
CA
0.151
0.681
0.003
0.011
0.006
0.020
0.097
0.581
0.065
-DAll
-2.929
-2.535
2.763
2.321
1.658
0.553
1.847
1.437
0.151
0.872
p
-1.437
0.162
z
Wilcoxon tests
b Abbreviations for mountain ranges are identified in Fig. I and Table I; parenthetical numbers denote the sile ranking of individual mountaintops (largest'" I; see Table 4); actual areas
of mountain ranges are provided in Table I.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
SS
(15)
DI
(9)
(14)
RC
• Species are Qrdered from most (top) to fewest (bottom) occurrences; abbreviations for species names are identified in Table 2.
X
SOPA
OCPR
ZAPR
MUER
SPBE
LETO
X
X
X
X
X
X
X
X
X
X
X
X
TADO
SOYA
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
NECI
MILO
MAFL
SPLA
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
TAUM
SYNU
(5)
(12)
(7)
(6)
(1 )
(4)
(3)
Species'
X
X
SN
OQ
WP
RU
MT
TS
WH
Mountain range b
TABLE 3.-Species richness matrix and tests of nonrandomness (Wilcoxon scores, Z) for small mammals in Great Basin mountain ranges sorted
by largest (left) to fewest (right) number of species:
"
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1122
JOURNAL OF MAMMALOGY
ences at sites ranked by species richness
(Table 3). Cutler's (1991) values of Ua and
Up were 13 and 5, respectively, yielding 18
departures from perfect nestedness. Those
values indicated that the species-richness
matrix was "hole-rich" rather than "outlier-rich." In Cutler's (1991) tenns, that
meant that there were more unexpected absences (holes) than unexpected presences
(outliers). Overall, however, the updated
data resulted in somewhat less nestedness
than that reported by Grayson and Livingston (1993) in their analysis of Brown's
(1978) larger set of montane islands.
Tests for non-randomness continned that
the entire matrix (Table 3) was ordered
(Fisher's test of combined probabilities: X2
~ -2 InP ~ -2( -29.258) ~ 58.516; df.
= 22; P < 0.005) with respect to species
richness. As might be expected, all four of
the species that were highly ordered individually (each with Wilcoxon scoreS of P
< 0.05) were cool-adapted forest species;
by contrast, all woodland species were
unordered (each with P > 0.05).
By contrast, faunas did not sort neatly by
mountaintop area (Table 3). The largest
mountaintops did not necessarily contain
the richest faunas, or the smallest the poorest. If faunas were rearranged by island size
and re-analyzed, a different result was obtained (Table 4). Combined probability
analysis revealed that the new matrix was
not ordered (X' ~ -2( -12.460) ~ 24.920;
dj = 22; P > 0.10), and only one species
(Sorex vagrans) was individually distributed in a non-random fashion, verifying again
the modest impact of montane area on species distributions.
The nestedness pattern for mammals also
differed from the pattern for birds (Table 5).
Unlike the significantly nested pattern revealed by birds on mountaintops ranked by
species richness (X 2 = -2(-14.480) =
28.960; df. ~ 16; P < 0.025), species of
mammals arranged according to the same
sequence of mountaintops assorted randomly(X' ~ -2(-14.099) ~ 28.198;d.f ~ 22;
P > 0.10).
Vol. 79, No.4
Faunal composition.~Consistent with
Brown's (1971, 1978) analyses, common or
omnipresent species on Great Basin mountaintops consisted entirely of generalist herbivores having catholic habitat tolerances.
Species occupying peculiar habitats (e.g.,
water shrews, pikas) or those with carnivorous (ermines) or insectivorous (shrews)
diets occurred on fewer islands.
Montane faunas of the Great Basin contained few subalpine species associated
with cool mesic habitats. Of the 15 studied
species, only four (27.3%) could be classified- as such (M. erminea, O. princeps, Zapus princeps, Lepus townsendii); the remainder were woodland species frequenting
more xeric habitats at lower elevations.
Subalpine species constituted only 23 of
155 total occurrences on mountaintops (Table 2)~a value significantly (P < 0.005)
lower than their expected representation
(42). By contrast, subalpine species constituted about one-half of all species in montane faunas in the desert Southwest (Lomolino et al., 1989).
DISCUSSION
The effect of new additions to the data
set from the Great Basin is dramatic, and
the resulting pattern of species occurrences
requires an explanation different from the
one now generally accepted (Brown, 1971,
1978; Kodric-Brown and Brown, 1993).
The combination of 1) a relatively weak:
species-area relationship, 2) lack of a relationship between species richness and measures of isolation and habitat diversity, 3)
absence of maximum-isolation or minimum-area effects on patterns of species
presences and absences, 4) poor nestedness
of individual species (except for a few subalpine species) and absence of nestedness
for the entire assemblage in relation to
mountaintop area, and 5) prevalence on
mountaintops of generalist woodland species, indicates that processes shaping distributional patterns in Great Basin mountains
are dynamic. Evidently, current faunal compositions and numbers of species are not
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(5)
SN
X
X
X
X
X
X
X
X
X
X
X
X
(6)
RU
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(8)
DC
X
X
X
X
(7)
WP
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
DE
(Il)
X
GR
(10)
DI
(9)
OQ
X
X
X
X
X
X
X
X
X
X
X
(12)
ST
X
X
X
X
X
X
X
X
X
(13)
"Species are ordered from most (top) to fewest (bottom) occurrences; abbreviations for species names are identified in Table 2.
~ Abbreviations fOf mountain ranges ure identified in Fig. I and Table !; actual areas of mountaintops are provided in Table I.
X
LETO
X
X
X
X
X
X
X
X
X
X
X
(4)
TS
(3)
WH
TAAM
SPBE
QCPR
ZAPR
MUER
X
X
X
X
X
X
X
X
X
X
X
TADO
SOYA
SOPA
SPLA
X
X
MAFL
X
X
X
X
(2)
SC
X
X
X
X
(1)
MT
TAUM
SYNU
NECI
MILO
Species
a
Mountain rangeb
X
X
X
X
X
X
X
X
X
(14)
RC
X
X
X
X
X
X
X
X
(15)
SS
X
X
X
X
X
X
(16)
CA
0.597
0.163
0.340
1.085
-2.549
-1.694
0.158
0.705
-1.345
-1.l91
-1.116
z
0.481
0.551
0.87l
0.734
0.295
0.234
0.179
0.011
0.090
0.874
0.908
p
Wilcoxon tests
TABLE 4.-Species richness matrix and tests of nonrandomness (Wilcoxon scores, Z) for small mammals in Great Basin mountain ranges
arranged according to size of mountaintop, largest (1, left) to smallest (16, right).
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Vol. 79. No.4
JOURNAL OF MAMMALOGY
1124
TABLE S.-Species richness matrices and tests of nonrandom ness (Wilcoxon scores, Z)for resident
birds (upper) and small mammals (lower) in Great Basin mountain ranges. Each matrix is arranged
identically according to mountaintops with most (left) to ones with fewest (right) number of bird
species.
Mountain range"
Taxon
Resident
birds~
SN
WH
DC
(3)
TS
(4)
Species
(5)
PAGA
prVI
OEOB
SICA
CIME
CYST
PIPU
SIPY
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
ORPI
Small mammals"
PITR
TAUM
MILO
SYNU
NECI
TADO
MAFL
SPLA
SOYA
MUER
SOPA
OCPR
ZAPR
LETO
SPBE
TAAM
X
X
X
X
X
X
X
X
ST
RU
GQ
DE
SS
(13)
(6)
(10)
(11)
(15)
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(8)
OQ
(l2)
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Z
P
-1.241
-1.330
-2.322
-0.464
-1.412
1.995
0.133
1.419
0.215
0.894
0.020
0.642
0.158
0.046
0.184
0.156
0.000
-0.709
-1.333
-2.322
-2.280
-1.809
0.109
0.116
1.064
0.266
1.064
0.999
0.478
0.894
0.020
0.023
0.071
0.914
0.908
0.287
0.790
0.287
X
X
X
X
X
X
X
X
X
Wilcoxon tests
X
'The number of mountain ranges is limited to those for which bird-species information is available (Brown, 1978; Dobkin and
Wilcox, 1986; Johnson, 1975); parenthetical numbers refer to the nmk order of mountaintop sizes shown in Thble 4.
"Identities of abbreviations for bini species are: CIME, Cine/us mex;cmws; CYST, Cyanositta sfelleri; DEOB, Dendrogapus
obscurus; ORP£, OrcO/1yx pic/a; PAGA, Purus gambeli; PIPU, Picoides pubescens; PITR, Picoides rridacrylus; PIYI, Picoides
villoslIs; SICA, Silta carolinensis; SIPY, Sitra pygmaea.
'Abbreviations for mammal species are identified in Table 2.
maintained solely by extinctions in the absence of replacements but instead are due
to incomplete isolation and extinction resistance, or both.
The recalculated log species-log area
curve for mountaintops >2,300 m (Z =
0.128; SE ~ 0.045) differs significantly
from Preston's (1962) canonical estimate
(0.26) for insular faunas (t-test; P < 0.05),
but it closely resembles relationships described by MacArthur and Wilson (1967)
for interconnected populations on continents. Indeed, the slope closely approximates "mainland" samples reported by
Brown (1971) for mammals occupying areas of different size in the Sierra Nevada (Z
~ 0.121) and by Patterson (1984) and Patterson and Atmar (1986) for sites in the
Rocky Mountains (Z = 0.070); these z-values are n9t significantly different (P >
0.05). Surprisingly, the slope more closely
resembles ones calculated for highly vagile
taxa such as butterflies (z = 0.147-Murphy and Wilcox, 1986) and resident birds
(Z = 0.165- Brown, 1978) occurring on
most of the same islands than it does to the
much higher values previously reported for
mammals (~0.282; Fig. 2). These obser-
November 1998
LAWLOR-BIOGEOGRAPHY OF GREAT BASIN MAMMALS
vations are explicable from the disproportionate representation of woodland species
among mammals in montane habitats. Because intervening desert barriers are more
permeable to movements of these species,
their prevalence diminishes effects of forest
fragmentation and results in a more constant number of species on all mountaintops
of the Great Ba.<;in irrespective of size.
Further evidence that montane faunas are
incompletely isolated comes from altitudinal distributions of component species. All
15 species in the foregoing analysis have
known contemporary distributions below
the somewhat arbitrary lower elevational
limit (2,300 m) established by Brown
(1971) for defining mountaintop islands,
and some woodland species (e.g., cliff chipmunks, T. dorsalis; Nuttall's cottontail, Sylvilagus nuttalli) frequent habitats mostly
below this elevation. Further, movements
along stream courses or across arid valleys
during cooler times of the year, as suggested by Hall (1946), seem highly likely, especially for woodland species at lower elevations. This has been demonstrated convincingly for such species in the American
Southwest (Davis and Dunford, 1987; Davis et aI., 1988; Lomolino et al., 1989).
For woodland species, potential dispersal
among mountaintops has been enhanced by
the recent northward expansion of pinyonjuniper forests. Singleleaf pinyons and associated habitats now carpeting the flanks
of Great Basin mountains were missing
there until after the last glaciopluvial period, and may not have reached their present
extent until very recently (Grayson, 1993;
Thompson, 1990)-a fact not known to
Brown (1971) when he developed his ideas.
These and other associated plant species are
indicators of relatively xeric conditions. Although no mammals in the Great Basin are
pinyon-forest obligates, some xeric-adapted
(i.e., woodland) species (e.g., T. dorsalis)
that Brown (1971, 1978) originally considered to be post-glacial relicts may be relatively recent invaders of many mountains in
the Great Basin.
1125
Brown's (i971, 1978) alternative explanation for the absence of isolation effects
on species numbers, in which desert valleys
act as absolute barriers to dispersal of montane species, is not supported by the present
analysis. Although both Brown (1971,
1978) and Lomolino (1986) concluded that
there was no contemporary dispersal of
montane mammals to and from mountain
ranges in the Great Basin, their analyses
were based on Brown's (1971, 1978) incomplete database for species occurrences.
Montane faunas of the Great Basin satisfy expectations of the land bridge relaxation model in one important respect. Overall, mammalian faunas are markedly impoverished, just as are those of butterflies
and birds (Austin and Murphy, 1987; Behle, 1978; Johnson, 1975, 1978; Murphy and
Wilcox, 1986). For all three taxa, missing
species are largely subalpine forms that otherwise are found in cool, mesic, and highelevation habitats in adjacent larger mountain masses to the east and west (Brown,
1978; Johnson, 1978; Murphy and Wilcox,
1986). That subalpine species have fared
poorly relative to woodland species in
mountains of the Great Basin is supported
not only by differences in their modem distributions but also by the complete disappearance of several trophic or habitat specialists once present but now limited to
high-elevation forests of the nearby Sierra
Nevada and Rocky mountains (Grayson
1981, 1987; Heaton, 1985; Mead and
Mead, 1989; Mead et al., 1992), such as
martens (Martes americana), bog lemmings
(Synaptomys borealis), and heather voles
(Phenacomys intermedius). Presence of
these species as late Pleistocene or early
Holocene fossils in mountains of the intermountain region is contrary to Brown's
(1978) proposition that woodland but not
<'boreal" (synonymous with subalpine in
this study) forms were able to invade these
islands during the late Pleistocene. MoreOVer; some high-elevation species (e.g., pikas) occupy only a portion of mountaintops
they formerly frequented, and many mon-
1126
JOURNAL OF MAMMALOGY
tane species are known from Pleistocene
deposits in valleys where they no longer occur (Grayson, 1981, 1982, 1983, 1987,
1993; Grayson and Livingston, 1993;
Mead, 1987; Thompson and Mead, 1982).
Consistent with Brown's (1971) relaxation
model, montane species evidently were distributed more continuously across the Great
Basin during and immediately succeeding
the last glacial episode. These observations,
together with other biogeographic characteristics of extant species populations reported here, suggest that once diverse mammalian faunas in the intermountain region
collapsed with the post-glacial retreat of conifers to upland locations. However, cooladapted subalpine species evidently declined at higher rates than woodland forms,
resulting in the current impoverished mix of
species in which widespread, dispersal-capable, and extinction-resistant woodland
forms predominate.
In contrast to the Great Basin, species
richness of montane faunas in the 'desert
Southwest (Arizona, New Mexico) is correlated strongly with both size of island and
isolation from the main body of the southern Rocky Mountains, irrespective of differences in the reference species pool, although area effects dominate (Lomolino et
al., 1989; Patterson, 1984, 1995; Patterson
and Atmar, 1986). Species also are ordered
in a highly nonrandom and nested fashion
by island size. In an analysis of boreal (i.e.,
northern-derived) species only, Patterson
(1984) found that no species reported from
small mountain ranges were missing from
larger ones; species were nested perfectly
with respect to size of mountaintop. These
results were repeated in a more recent study
of boreal species in the Southwest using an
expanded set of mountain ranges (Patterson, 1995). The implication is clear: when
cool-adapted species constitute a substantial
proportion of mountaintop faunas, attrition
proceeds in ways predicted from an extinction-based relaxation model. Differences
between montane faunas in the Great Basin
and Southwest are to be found in the oc-
Vol. 79, No.4
cupational histories and ecological peculiarities of the component species.
Mammals remaining in Great Basin
mountains also may be extinction-proof
survivors of the relaxation process. In addition to the fact that woodland species
have broad habitat tolerances and good dispersal abilities, there is a strong likelihood
that most vulnerable species or populations
already have gone extinct for two reasons.
First, as faunas relax after isolation in insular habitats, susceptible species should
disappear relatively rapidly, leaving only
resistant ones (Diamond, 1972). Second,
the hypsithermal warming that occurred in
the Great Basin several thousand years ago
(Grayson, 1993) must have culled vulnerable montane species. That recorded disappearances from mountains in the Great Basin are confined wholly to high~elevation
species lends support to these observations.
Consequently, mammalian faunas in these
mountains already may be reduced to levels
that will be affected only modestly by predicted global warming scenarios. On the
other hand, warming sufficient to dramatically increase aridity in desert valleys might
isolate species now enjoying some degree
of dispersal across desert valleys, setting off
a round of extinctions involving woodland
forms.
The role that habitat diversity plays in
maintaining the relative constancy of mammal species numbers on islands in the Great
Basin is elusive. As Murphy and Wilcox
(1986) noted, taxa in which vulnerability to
extinction relative to area is low and vagil- .
ity in relation to isolation is high should
effectively saturate available habitat, so
species numbers for such groups should be
controlled largely by habitat diversity. Yet
species richness of montane mammals in
the Great Basin is uncorrelated with habitat
diversity indices, at least as measured here.
Absence of a relationship may be due to a
failure of Johnson's (1975) habitat-diversity
index, which was developed for birds, to
properly capture ecological variables appropriate to mammals. Also, Brown's (1971)
November 1998
LAWLOR-BIOGEOGRAPHY OF GREAT BASIN MAMMALS
criterion that all studied islands have at
least one peak exceeding 2,990 m may inadvertantly create a set of islands with relatively uniform ecological characteristics.
As a result, habitats on even the smallest
mountain ranges may be diverse enough to
support small but viable populations of virtually all species of mammals, especially
generalist woodland ones, still occurring in
the Great Basin. Effects of habitat diversity
may be detectable only after islands with a
broader range of elevational and associated
habitat differences have been examined.
The extent to which species turnover has
affected distributional patterns in Great Basin mountains remains to be determined.
The intermittent occurrence of N. cinerea
in western Utah during the Holocene suggests turnover related to changes in paleoclimates (D. Grayson, pers. comm.). I have
recorded at least one possible recent extinction in the intermountain region. Fieldwork
suggests that Uinta chipmunks (Tamias umbrinus) have disappeared from the Sheep
Range in southern Nevada, a range outside
the immediate scope of this study but part
of Brown's (1971, 1978) original analyses.
Cliff chipmunks (T. dorsalis) now occupy
all conifer habitats and elevations there.
However, earlier surveys (Burt, 1934) recorded Uinta chipmunks in subalpine forests above 2,435 m (8,000 feet), cliff chipmunks in pinyon-juniper woodland at low
elevations, and a mix of the two in ponderosa pine forests at intennediate elevations.
Although I agree with Grayson and Livingston (1993) that better knowledge explains
most of the shift in our understanding of
species occurrences in the Great Basin, the
discoveries of missing species on mountaintops may be attributable in part to recent
dispersal events. In any case, evidence of
turnover would furnish added support for a
dynamic interpretation of biogeographic
patterns.
My analysis diminishes the value of the
species assemblage in the Great Basin for
assessing effects of global warming
(Brown, 1995; McDonald and Brown,
1127
1992; Murphy and Weiss, 1992). In McDonald and Brown's (1992) analysis of extinctions owing to climate change, predictability of the model depends on assumptions that small-mammal faunas of the intermountain region are extinction driven, so
that there is a strong species-area correlation, and that there is no contemporary immigration. In light of available data, neither
assumption appears correct. In fact, contrary to prior indications (McDonald and
Brown, 1992; Murphy and Weiss, 1992),
virtually no extinctions can be expected
from a projected 3°e rise in temperature
with global warming.
Historically, faunas in the Great Basin
evidently consisted of a sequence of three
different assemblages of species. The first
was a diverse group of widespread subalpine and woodland species that occupied
mountains and intervening valleys in the
late Pleistocene. The second contained an
assemblage from the early Holocene that
collapsed as montane species became increasingly isolated in upland forest fragments following the end of the last glaciopluvial, causing a disproportionate loss of
high-elevation species. The third was a
modern impoverished fauna composed
mainly of ubiquitous woodland species and
a few discontinuously distributed subalpine
ones whose aggregate numbers are currently sustained by high population survivorship and cross-valley dispersal.
In his seminal paper, Brown (1971) drew
three main conclusions regarding distributional patterns of montane mammals in the
Great Basin: 1) the species-area slope is extremely steep--steeper than those demonstrated for comparable true islands; 2) islands are completely isolated-intervening
valleys are absolute barriers to dispersal;
and 3) montane faunas are true relicts,
shaped by extinctions and no colonizations,
and therefore species numbers do not represent an equilibrium between opposing
rates of immigration and extinction. On
closer scrutiny, and in light of recent paleontological and modem distributional data,
JOURNAL OF MAMMALOGY
Il28
it is clear that these conclusions are incorrect, Evidence overwhelmingly points to a
modern fauna composed of extinction-resistant woodland species capable of considerable movement among mountain ranges, resulting at best in a modest relationship between species numbers and mountaintop
area. Contrary to Kodric-Brown and Brown
(1993), new records substantially alter
quantitative and qualitative aspects of biogeographic relationships of mammals of the
Great Basin.
ACKNOWLEDGMENTS
Pennutations of this paper were reviewed by
W, Boecklen, J, Brown, A Cutler, D, Grayson,
L Heaney, N, Johnson, S, Livingston, M, Lomolino, and B, Patterson, whose suggestions are
much appreciated. For discussions and improvements to early drafts of this paper I am grateful
to the graduate student discussion group at
Humboldt State. J. Hamrick graciously provided
unpublished data on occurrences of conifers in
Great Basin mountains. I extend thanks also to
S, Casassa, B. Foley, S, Miller, and J. Turner for
help with the preparation of tables and figures,
For financial support, I am indebted to the Humboldt State University Foundation, which provided institutional funds, and to the National
Science Foundation, which provided research
support during the period I was employed there
as a program officer, Lastly, I am grateful to my
colleague, R, Sullivan, and many current and
fonner students who aided fieldwork in the
Great Basin, as follows: J. Eliason, B, Foley, W,
Gannon, P. Guthrie, M, Hedin. D, Kain, V. Krula, M, Mandt, M. Murray, J. Ososky, G. Perlmutter, D. Silber, S. Smith, W. Stanley, and P.
Stapp.
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Submitted 11 March 1997. Accepted 20 December
1997.
Associate editor was Janet K. Braun.