Great Basin Naturalist Memoirs
Volume 2 Intermountain Biogeography: A Symposium
Article 9
3-1-1978
Patterns of avian geography and speciation in the
Intermountain Region
Ned K. Johnson
Museum of Vertebrate Zoology and Department of Zoology, University of California, Berkeley, California 94720
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Johnson, Ned K. (1978) "Patterns of avian geography and speciation in the Intermountain Region," Great Basin Naturalist Memoirs:
Vol. 2, Article 9.
Available at: http://scholarsarchive.byu.edu/gbnm/vol2/iss1/9
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PATTERNS OF AVIAN GEOGRAPHY AND SPECIATION
IN THE INTERMOUNTAIN REGION
Ned
K. Johnson
1
Abstract— The Intermountain Region comprises a huge arena where major avifaunas more highly developed
beyond the Great Basin come into contact. For this reason the distribution and composition of avifaunas in riparian and pinyon-juniper woodlands and in coniferous forests in this region are understood most clearly if considered as part of a broader system of patterns evident in western North America. Riparian woodland samples
point to a complex meeting ground in the Intermountain Region of two major avifaunas of equivalent size, but
from opposite distributional backgrounds. These northern and southern avifaunas do not mix among the samples
represented except in western Nevada where two species of ultimate southern origin have penetrated a basically
northern avifauna. Approaching the Great Basin, from the two centers of abundance of riparian species in the
Snake and Colorado River drainages, species richness drops. Habitat depletion and, to a lesser extent, insularity
play roles in this impoverishment. In the most depauperate riparian avifaunas, six species commonly coexist, each
in a different family. Comparison of the pinyon zone avifaunas of two groups of mountain ranges, 90 km apart
along the California-Nevada border, demonstrates a striking trade off
biogeographic histories. The northern or Boreal forms,
had easy access
to the favorable, cool,
and
relatively moist
the species of southern or Austral derivation prefer the
the south.
Few
many
of
among
species of northern
which are numerous
pinyon
warm and
in the Sierra
and southern
Nevada, have
forests in the adjacent spur ranges. In contrast,
very arid pinyon woodland a short distance to
species are confined to either northern or southern
sites,
overlap in species composition
is
great,
and equivalent species richness is achieved. However, despite these similarities, strong geographic differences in
abundance of most species in each pinyon avifauna and the occurrence of at least 12 specific and subspecific
range boundaries suggest the interposition between northern and southern pinyon areas of a substantial, but as
yet poorly characterized, climatic barrier. Boreal species richness declines,abmptly from high values in the south-
ern Cascades and Sierra
From near 116°
W
Nevada
Longitude
low
to
zone of impoverishment across western and central Nevada.
Nevada, coincident with the appearance of fir and/or bristlecone pine
levels in a
in eastern
species numbers climb gradually until the main Rocky Mountains are reached. There, species richness
compares favorably with that in the Sierra Nevada. The proportion of species favoring riparian woodlands over
coniferous forests is higher on the island mountaintops of the Great Basin than in the Boreal "continents" of the
Sierra Nevada-Cascades and Rocky Mountains. The western edge of the Great Basin richly demonstrates examples of stages in avian speciation. A full range of interactions is represented, from intergradation of poorly char-
forests,
acterized races, through abrupt zones of hybridization between strongly
marked subspecies
of different racial
complexes or of semispecies, to sympatry and infrequent interbreeding of closely-related, full species in recent
secondary contact. These various zones of population interaction coincide strikingly with sharp floristic and climatic gradients.
A
major avifaunal break occurs between coastal or Sierra Nevadan forms that inhabit oak-
chaparral and/or coniferous forest and closely-related interior forms that prefer pinyon-juniper or aspen-willow
associations. In keeping with the special requirements of each species, contact zones and areas of disjunction
show
general, rather than precise, coincidence in the western Great Basin.
There
is
no Great Basin Boreal Avi-
fauna; the most distinctive interior forms occur across the entire span from eastern California to Colorado. Thelow desert trough along the east side of the Sierra Nevada that divides major mountain systems in the western
portion of the Intermountain Region
is
not the principal barrier dividing coastal Sierra
Nevada from
interior avi-
faunas. Instead, the major avifaunal transition occurs in a belt of variable width just east of the crest of the Cas-
cade Mountains and Sierra Nevada, where the precipitation shadow and continental climate begin to exert crucial influence.
Although much neglected
in the past be-
cause of difficulty of access and false im-
pressions
'Museum
of
biotic
of Vertebrate Zoology and
sterility,
Department
the
Inter-
mountain Region of western North America
is emerging as a fascinating natural laboratory
requiring
the
of Zoology, University of California, Berkeley, California 94720.
137
close
attention
of
the
GREAT BASIN NATURALIST MEMOIRS
138
biogeographer-ecologist.
Avifaunal Transects in the
Intermountain Region
conifers
of
Islands
or aspen, cool mountain meadows, and at-
tendant biotas on scores of mountaintops
by
isolated
in the Sierra
lie
Nevada and Rocky Mountains.
The unique physical
plicity of more or less
multi-
a
of
setting
parallel ranges, each
differing in size, geology,
and configuration
from adjacent ones, cannot be duplicated
among
elsewhere
continental mountain sys-
human
tems. And, because of low
many
of the
montane
density,
habitats are relatively
undisturbed, thus permitting a good look at
man-unmodified
original,
insular
of
distributions
In the valleys as well, oases offer
species.
Riparian woodlands
from source stocks
seas of desert
No. 2
environments for a variety of organ-
isms whose interrupted ranges intrigue the
yons,
valleys
and can-
and aspen on mounmajor habitats for
the region between the axis of the
Nevada-Cascade Range and the
and coniferous
taintops
birds in
Sierra
forests
comprise
the
Rocky Mountains. Analyses of the richness
and composition of selected avifaunas
breeding in sample areas of these woodlands
and forests expose several patterns that
elucidate
fundamental organization of
the
avian distribution in the region. Moreover,
these
samples along transects demonstrate
the complex
pects
biogeographer.
in
pinyon woodlands on mountainsides,
modes by which
avifaunal
of
historical as-
distribution
influence
present-day patterns of species richness and
With increased
field efforts
by a number
of workers in recent decades, the bird species
many
breeding on
of the major basin
ranges have been tallied,
community
of
theories
pha" exploration
With
enlarged
more diverse and deenough
tailed kinds of analyses. But, already
distributional
to
information has accumulated
several
suggest
patterns
challenge
that
the avian geographer. In this paper
fy
and discuss these patterns
common
more,
I
biogeographic
specify
among
I
identi-
in a
search for
themes.
Further-
the birds of the In-
termountain Region distributional situations
relevant
range
to
hypotheses on
boundaries.
Finally
the
I
control
describe
prominent zone of active speciation
in
Woodland Avifaunas
Bird species requiring riparian vegetation
for
breeding occur discontinuously
willows,
species of land birds breeding in such
habitats are
compared (Table
in the
in
and
1957, 1973).
checklist
sequence
of
North
of
11
south,
prominent
some unexpected
numbers,
species
differences
similarities
emerge
composition
of
and
in total
the
avi-
faunas according to evolutionary derivation,
the
and structure of the avifaunas by ecologic
roles. That these three topics are inter-
the
dis-
species
American
birds except where superceded by the 32nd
supplement (American Ornithologists' Union
the
among
north to the Colorado River drainage
the
will
become evident
Total species numbers.—
terest
Nomenclature
1),
of
continuities.
follow
the
sample areas from the Snake River drainage
related
floral
Great Basin region.
throughout
When
cussion to follow.
and
cot-
ashes,
oases
interpret this zone in relation to coincident
climatic,
in
and associated
thickets along watercourses and at valley
tonwoods,
western part of the region and attempt to
physiographic,
lo-
Latitudinal Variation in
Riparian
tests
additional "al-
in the future, the
data base will permit
shows the
avifaunal relationships.
biogeog-
insular
raphy (Johnson 1975). However, these
are just the beginning.
1
and information
has reached a level allowing tests of certain
fundamental
structure. Figure
cation of the sample areas and their basic
is
in
the
Of immediate
dis-
in-
the striking similarity of the geo-
graphically extreme stations, Central Idaho
and Colorado River, in species richness, 28
versus 25, from the pool of 43 species. But,
upon entering the Great Basin from the
Snake River drainage, species totals drop, to
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
21 in the Elko sample, 17 in Humboldt, 15
in
Toiyabe, and to 12 in Kawich. Similarly,
moving north from the Colorado River, total species numbers fall to 13 in Pahranagat,
12 in Meadow Valley Wash, and 11 at Ash
Meadows-Pahrump. Figure 2 demonstrates
the strong and abrupt avifaunal attenuation
Fig.
1.
Sample areas
of riparian
woodland (squares and
that
the
occurs with
lush
increasing
in
and southern riparian woodland avifaunas
is
to
the
north
and south. The smallest tallies, for the Kawich area and Ash Meadows-Pahrump, probably reflect habitat impoverishment in view
of
the
limited
extent
and
growth of the riparian clumps
circles)
and pinyon woodland
interrupted
there.
For
(triangles) avifaunas in
Numbers
in
squares or circles
Arrows indicate closest avifaunal relationships. Northern and
and open triangles, respectively. Approximate boundary of northern
shown by the dashed line between samples 7 and 8.
Table
southern pinyon areas are denoted by solid
from
distance
woodlands
riparian
the Intermountain Region. Shading grossly defines regions of forest and woodland.
correspond to those of sample areas
139
1.
Table
No. 2
GREAT BASIN NATURALIST MEMOIRS
140
1.
123456789
Species breeding in riparian woodlands at selected localities in the Intermountain Region'.
10
c3
s£
Northern Element
11
a-
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
Meadows-Pahrump and Pahranagat
we also may be seeing the effects
Ash
samples,
of insularity. Several Colorado River drain-
age species, such as the Abert's Towhee
permanently
probably have had great
highly
sedentary,
form),
(a
resident
difficulty
background or derivation into (1) a northern
element consisting of species with northern
centers of distribution and affinities, (2) a
southern element comprised of species with
southern distributional relationships, and
curring
However, one could also argue that
this and other potential colonists are missing
from these remote areas because the proper
States or beyond.
kinds
of riparian
habitats,
the Colorado River and
(22
element
of
the
southern element
species)
reflects
basis
position
its
evolutionary
of
ED northern
the
of
is
samples
species
found
Valley
any
in
of
the
any samples
in
Wash
or Pahrana-
two major avifaunas approach, but do not contact and intermix, in
gat. Therefore, the
the southern Intermountain Region.
Although basically of northern derivation,
the riparian avifauna of the Truckee-Carson
tains
£/] southern
Meadow
north of
occurs
none
below),
sample
D widespread
ele-
Kawich and, with one exception
Origin of riparian woodland avifaunas.—
entire riparian woodland avifauna can
the
three
south of
The
on
these
northern
well-watered eastern base of the Car-
divided
according to
1
United
are
son Range.
be
western
species are grouped
ments. Note that none of the species of the
(see
size
Truckee-Carson sample
at the
Table
in
The
found along
lacking.
The moderate
throughout the
tributaries,
as
its
(3)
a widespread element including species oc-
colonizing the isolated riparian areas to the
north.
141
of special interest because
is
a
group
distinctive
of
four
it
con-
species
with affinities to populations of northeastern
39%
Central Idaho
I.
50
4 Truckee-Carson
43
2
Elko
42
6
Ely
47
5 Toiyabe
3 Humboldt
58
7 Kawich
45
10
42
into the western Great Basin via the
Ash Meadows-Pahrump
8 Pahranagat
44%
II.
Density and origin of riparian woodland
bird
of
avifaunas
in
according to geographic
11
sample areas. These are arranged
riparian
The widespread component comprises between 39 and 62 percent of each sample
2).
decline
No
clear evidence
northern
of
(or
of differential
southern)
shown
versus
widespread elements
is
of impoverishment
approached. Thus, the
diminution
in
order of decreas-
ary of northern and southern avifaunas, between Ka-
wich and Ash Meadows-Pahrump. For example, the
Colorado River sample has 25 species, 11 (= 44 percent of which are widespread and 14
(
=
jor
is
is
as the
zone
general and involves approx-
56 percent)
components of each avifauna.
Community
woodland
ing avifaunal size (species density) toward the bound-
of southern derivation.
Modoc
imately equivalent shrinkage of the two ma-
species
Fig. 2. Percent composition,
derivation,
Downy
Plateau.
(Fig.
Colorado River
25
15
of these species, the
probably originated from stocks in the CenValley of California, stocks that spread
6?
5
One
Woodpecker, is of northern affinity; two
species, the Western Bluebird and Bewick's
Wren, are of southern derivation. The Song
Sparrow is widely distributed. The western
Nevadan populations of these four species
tral
9 Meadow Valley Wash
:
California.
A
structure
surprising finding
is
and
ecologic roles.—
that the widespread
species fraction seems to have stabilized, at
either
seven
or
five
species,
for
the
five
sample areas with the smallest numbers of
species.
Thus, the Humboldt, Toiyabe, and
Kawich samples have seven widespread
spe-
GREAT BASIN NATURALIST MEMOIRS
142
And, the
mountain Begion, and these woodlands
The Ash
therefore represent an important habitat for
cies in addition to northern ones.
are
species
of
lists
identical!
No. 2
Meadows-Pahrump and Meadow Valley
Wash samples include southern species and
breeding birds. Typically
However, in these
two samples the coincidence in identity of
or clumps of small trees with
widespread forms
sites,
five that are widespread.
is
incomplete; each shares
three of their five species.
of the
Six
widespread species, Falco
11
cals,
Tyrannus
verti-
and
galbula,
Icterus
cirens,
Icteria
vening
scat-
is
brush.
much
on
Occasionally,
inter-
favorable
growth can be luxuriant, and true forachieved. Here trees grow to heights
of 10 m or more, and the closed canopy
est
sparverius, Colaptes auratus,
pine
this
tered over mountain slopes in open stands
is
shades
needle-strewn
a
site-to-site variability in
forest
This
floor.
growth form of
pi-
Melospiza melodia, occur in nine or more
nyon
sample areas. Because these species com-
has long been known, birds clearly respond
monly
coexist, they
form a group of great
view them as a "standard riparian
woodland species" group analogous to the
"standard Boreal species" group I identified
interest. I
montane
in
habitats of the western United
States (Johnson 1975).
species
six
fore,
is
among
examined
body
wide
elevational range often occupied on a single
mountainside. In the White Mountains, Cal-
example, pinyon occurs from ap-
ifornia, for
development between 2450 and 2600 m (St.
Andre et al. 1965).
In the Grapevine Mountains of southern
Nye County, Nevada, these variable charac-
predominates
here
different
designs.
Included
second
There-
family.
fundamentally
of
A
the
is
proximately 2000 to 2900 m, with the best
in a different
species
to physical features of vegetation.
noteworthy aspect of pinyon
Note that each of the
coexistence in the riparian woodland
avifaunas
intrigues the ornithologist because, as
Table
are five
1
pinyon are seen clearly (Miller
of
teristics
in the tallies of
scrubby trees grow in scat-
Small,
1946).
pairs of closely related species, three pairs
tered stands, exceptionally fine tracts of old
which are congeneric. Furthermore,
two feet in diameter form
on the high northeast slopes,
and the pinyon zone is wide, extending to
of
within each pair, each species
ferent distributional element.
is
from a
The
dif-
pairs are
trees with trunks
forests locally
m
Coccyzus erythropthalmus, and C. american-
2650
Dendrocopos pubescens and D. scalaris,
Tyrannus tyrannus and T. verticalis, Troglo-
breeding avifauna has responded to
us,
dytes aedon and
Thryomunes bewickii, and
and Vennivora luciae.
differences.
normally
Dendroica petechia
spacing
Not
that
all
of the species of each pair contact
locally,
but the interactions of those that do
meet would merit
field
fertile
awaits
phologist
willing
structure,
foraging
adaptations
close
among
to
the
analyze
roles,
Indeed,
study.
a
ecomorcommunity
and morphologic
the coexisting species of
at the
The complement
lives in
is
present,
typically
breed
dently
top of the highest peak. The
in
the
the
pinyon of usual form and
and several kinds of birds
avoid
this
most
latter
these
of species that
species
formation
plant
luxuriant
Evi-
stands.
attracted
are
to
physiognomic features of the pinyons that
resemble those of the coniferous forest they
ordinarily
inhabit.
Furthermore,
least three additional species that
at
otherwise
birds in any of the riparian woodlands dis-
would be absent occur
cussed.
reaches of the zone, apparently responding
in
the
there to preferred or required
Latitudinal Variation in
perature regimes.
Pinyon Woodland Avifaunas
Among
leaf
the
(Pinus
monophylla)
most extensive formation
in
comprise
the
also
occurrences of birds in
arboreal habitats, groves of single-
pinyon
I
Inter-
pinyon woodland
of central
in
the
cool
upper
summer tem-
have noted similar
unusually dominant
Kawich Mountains
Nye County, 100 km
north-
northeast of the Grapevines (Johnson 1956).
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
In recent years
I
have explored additional
143
cottonwoods and willow
Ne-
in west-central
areas of heavy pinyon that invite avifaunal
vada. Although both species use pinyon at
the Grape-
more northerly sites, the wren is uncommon, and the owl is rare in this association. Empidonax wrightii nests in tall
comparison with the groves
vines.
in
These stands of old trees grow
local-
Nevada near the California line, in
= Mount MaPalmetto Mountains
gruder plus the Silver Peak Mountains), Esmeralda County; the Wassuk Range, Mineral
County; the Pine Grove Hills, Lyon
County; and in the Sweetwater Mountains,
in
ly,
the
(
and Lyon counties. In each of
these regions, open woodlands of small trees
and stands of intermediate height and density predominate on most slopes; the forests
are more local in distribution.
For purposes of comparison of the aviDouglas
faunas,
is
it
useful to consider these several
pinyon areas as comprising
(1)
a northern
the
sagebrush,
and juniper
bitterbrush,
lacking but
tation in southern Nevada. In other species
the habitat change
more
is
nyon
in
ities,
but
both northern and southern
numbers
their
warmer
Some
striking
differences in avifaunal composition and nu-
merical
(Table
relationships
2).
species
of
emerge
Although the northern and south-
ern localities are only 90
km
apart, several
diminish
tween north and south coincides with a
shift
3 species,
in subspecific status and, in
How
2).
we
are
to
interpret these contrasts
pinyon association of
in the avifaunas of the
geographic
adjacent
closely
in habitat prefer-
regions?
interrelated issues, (1) variation in the
ment and
Table
histories of the bird
Species composition and numerical
2.
woodland avifaunas
tionships of pinyon
of the northern
Sierra
Nevada
tane islands to the southeast 2
1
.
versus those of
(A) species are distinguished.
Species present only in north:
3
mountain systems and thus
breed in only one of the two regions. Unexpectedly, 19 species change in average population density along the gradient. For some
Empidonax ivrightii), the change is
(e.g.,
Cyarwcitta
others
(e.g.,
Vireo
solitarius)
ent
between northern and southern moun-
mon-
Boreal (B) and Austral
Certhia familiaris
For
rela-
of spur ranges
Myadestes townsendi
population densities are dramatically differ-
cli-
temperature-moisture prefer-
(2)
enda and distributional
Sphyraupius ruber3
subtle.
Two
matic aspects of the pinyon zone environ-
Glaucidium gnomu 3 (B)
these
the
portions of the pinyon belt. In 5 of
species reach their distributional limits be-
tween
localin
19 species, the change in density be-
the
ence (Table
west to southeast gradient.
in-
north where they prefer the more arid and
Nevada, which
Wassuk and Sweetwater mountains and the Pine Grove Hills, and (2) a
southern subset of wooded islands lying to
the southeast, including the Palmetto and
Grapevine mountains. The massive White
Mountains lie between the two groups. The
northern and southern subsets (Fig. 1) permit comparison of avifaunas along a north-
Parus
subtle.
ornatus and Dendroica nigrescens inhabit pi-
with a concurrent change
includes the
is
scarce or absent in such vege-
is
subset occupying the spur ranges connected
to the east side of the Sierra
the
in
northwestern Great Basin where pinyon
(B)
Species present in both areas but
more numerous
Dendrocopos
villosus
4
Piranga ludoviciana (B)
(B)
Tachycineta thalassina (B)
Parus gambei 45 (B)
Carpodacus
Sirto carolinensis (B)
Junco huemalis
5
Empidonax
(A)
wrightii
5
Psaltripurus
minimus 4
(B)
more numerous in south:
Thyromanes bewickii 45
Polioptila caerulea
(A)
Aphelocoma coerulescens
cassinii (B)
Chlorura chlorura (B)
Species present in both areas but
asio*
(B)
in north:
Sialia currucoides (B)
Aegolius acadicus (B)
Otus
(B)
stelleri (B)
5
(A)
(A)
Vireo soliarius (B)
(A)
(A)
Species present only in south:
tains.
Regulus calendula 3
completely
switch habitats. For example, Thryomanes
bewickii and Otus asio, common and unSeveral
common
species
residents,
partly
or
respectively,
in
the
pi-
nyon woodlands of the southern Great Basin
and northern Mojave Desert, prefer riparian
Icterus parisorum (A)
(B)
Junco caniceps (B)
Vireo vicinior (A)
'Wassuk Range, Sweetwater Mountains, and Pine Grove
2
3
em
Hills.
Grapevine Mountains and Palmetto Mountains (= Mt. Magruder +
Silver
Peak Mountains).
Breeds onlv verv locally
in
pinyon woodland
in the
northern or south-
mountains.
4
Subspecies
5
Partial or
in
north different from that
complete habitat
shift
in
south
from north to south.
GREAT BASIN NATURALIST MEMOIRS
144
species
northern and southern stations,
at
No. 2
principally to the
warm and
relatively arid
may offer partial explanation.
From rainfall data for the nearby White
portions of the zone, and northern species,
Mountains and several other pinyon sites in
the southwest, St. Andre et al. (1965) con-
moist portions of the zone.
cluded that overall aridity characterizes
Wassuks, Sweetwaters, and Pine Grove Hills
this
plant formation. Unfortunately, no moisture
data
exist
for
the
pinyon zones
the
in
adapted chiefly
The
is
origin
the relatively cool and
to
of
bird
the
species
of
the
of particular interest because the pinyon
woods
these ranges directly contact the
in
ranges of special interest here. Nonetheless,
coniferous forests of the Sierra Nevada. For
pinyon certainly occupies a range of
this reason,
rainfall
and the more northern sites, although still relatively arid, presumably have
regimes,
greater average
than the southern
rainfall
bird populations of the pine-fir
zone of the east side of the
latter
have had easy access
forests lying
to the east.
why
cies are
not
the lower part, a fact readily apparent to
tion.
is
researcher working at
the
eleva-
different
The unusual upper extension of the pinyon zone in the White
Mountains (St. Andre et al. 1965) and in the
Grapevine Mountains (Miller 1946) would
tions in this region.
enhance temperature differences that normally exist between the different elevations.
Finally,
because of the difference
tude,
is
it
in
lati-
probable that the northern
sites
average cooler than the southern ones.
The
and
climatic preferences
distribution-
backgrounds of the bird species agree
al
closely with the groupings of Table 2.
5 species confined to the north and
forms that are more numerous
The
all
in the
10
north
Cyanocitta
familiaris
pinyon
It
the
present
species
only
reaching their greatest
in
the
south
numbers
in
all
or
the
al histories
woodlands.
gradient (Table
community
I
conclude that
this
major trend
composition and density of pinyon zone
in
avifaunas along the Nevada-California bor-
der
apparently
along
a
results
from the exchange
temperature-moisture
gradient
of
southern pinyon woodland species, adapted
2).
Instead, the evolution of
structure at various localities in
the pinyon formation probably has involved
subtle adjustments
among
the species
foraging zones, food quality, and /or habi-
Those species of birds repreby different geographic races at
northern and southern localities would be
especially worthy of close study in this retat preference.
sented
gard.
Gradients in Boreal Avifaunas
in their distribution-
with southwestern pinyon or oak
in the
range from source populations.
Note that from the pool of 27 pinyon
zone species, nearly identical total numbers
of breeding forms, 23 and 24, respectively,
are recorded for northern and southern stations. Such equivalence is significant. Importantly, no neat system of replacement or
of switches in abundance by species with
similar ecologic roles occurs along the
southern pinyon areas are of Austral origin,
having been associated
nest in this forma-
to
sumably because of the great distance of
in
forms of Boreal derivation (Miller 1951),
known
noteworthy that none of these
this
grounds associated with northern coniferous
In contrast, except for Regulus cal-
and, locally, Certhia
Desatoya Mountains of central Nevada, pre-
more
forests.
stelleri
and Sphyrapicus ruber breed in
these areas. Otherwise these spe-
forms inhabits the old pinyon groves
are Boreal species with distributional back-
endula, Vireo solitarius, and Junco caniceps,
in
range
Ready access may be one reason
Temperature data also are scarce, except for the White Mountains. However, I
can conclude at least that the upper part of
the zone averages considerably cooler than
sites.
pinyon
to
recently discussed controls of richness of
I
Boreal species on
11
sample areas
in
the
and Rocky Mountains and on 20 mountaintop islands in the
Intermountain Region (Johnson 1975). Here
I wish to expand on portions of that analySierra Nevada, Cascades,
sis
through examination of both longitudinal
and
latitudinal trends in
numbers of species
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
occupying particular Boreal habitats alongfour transects across the Great Basin. These
transects
(Fig.
3,
and pass through 20
equidistant latitudinally
of the sample areas described in
vious
paper.
Species
roughly
are
I-IV)
lines
my
pre-
now have
numbers
been determined for eight additional areas,
and these new data are included. Transect
HI fails to cross the entire Intermountain
Region because no species lists are available
for restricted portions of the main Rocky
Mountains directly east of the Snake Range
in eastern Nevada.
Total species
richness.—
demonstrate the abrupt drop
All
transects
in total
num-
bers of species in the north-south-oriented
swath that extends across
and central Nevada
species totals of 56
Fig. 3.
ness
declines sharply along transect
along transect
III,
between Carson and Pine
mite and Glass Mountain. Species numbers
then climb toward the east to totals of 56
and 55
Uinta and Aquarius-Boulder,
at
spectively,
at
the
ends
impoverishment along each
mountain range moderately rich
eral
re-
tran-
transect, a
in
Boreal
species interrupts the general smoothness of
each species density curve
bidge;
Shasta,
the
Highest totals for the Sierra NevadaCascades always equal or exceed those for
the Rocky Mountains. In the region of gen-
of western
Mt.
of
sects.
From high
at
be-
Nut; and along transect IV, between Yose-
4A).
64
I,
tween Warner and Pine Forest; along transect II, between Lassen and West Humboldt;
II,
Ruby;
III,
(Fig. 4A):
I,
Jar-
Toiyabe-Shoshone; and
IV, White-Invo.
Four transects across the Intermountain Region, passing through 28 sample areas of Boreal avifaunas.
in Fig. 4. No native firs are known to occur between the two vertical dashed
These sample areas are named
lines.
Lassen, Tahoe, and Yosemite, species rich-
much
(Fig.
to
145
Species richness according to habitat.— In
Figure 4B
I
portray the percentage of Bo-
real species in
each of four major habitats
549), i.e., Aquatic, Wet
(Johnson
1975:
Meadow,
Riparian
Forest.
iferous
sible
of
No. 2
GREAT BASIN NATURALIST MEMOIRS
146
Woodland,
Con-
and
This permits study of pos-
changes along gradients of proportions
species
preferring
these
habitats.
For
have calculated the average
in each of the four
habitats for Sierra Nevada-Cascades samples, Great Basin Islands samples, and
Rocky Mountains samples (Table 3).
each transect
I
percentage of species
Between-transect comparisons of proporof species in continental samples in-
tions
dicate
striking
similarities.
" a> S>i2
o £ .£=
oIq.q.1
For
example,
2s o
o -c
t- en
-Aquatic
Wet Meadow
•
"^—Riparian
Woodland
.*^ Coniferous
Forest
II
II
I
If
rz
A.
Fig. 4.
Boreal bird species richness
Numbers
of Boreal bird species
B.
(left)
Composition of boreal avifaunas
and percent avifaunal composition by four major habitats
(right)
along four transects across the Intermountain Region. Continental sample areas are underlined; island sample
areas are not.
added are
Twenty sample
as follows: A,
areas are described
more
hilly in
Mt. Shasta (Grinnell and Miller 1944);
Johnson (1975). Species
Wasatch (Behle and Perry 1975); M, Tahoe (Orr and Moffitt
White Pine Mtns. (author's unpubl. data); S, Schell Creek-Egan Ranges
Mesa (Hayward et al. 1963).
L.
R,
totals for eight
West Humboldt Range
1971); O, Pine Nut Mtns.
I,
new
ones
(author's unpubl. data);
(author's unpubl. data);
(author's unpubl. data);
and
Y,
Pahute
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
number
of species in coniferous forests var-
birds
in
ies
only from 62.5 to 69.6 percent of the to-
birds
in
tal
Boreal avifauna in the Sierra Nevada-
island
147
samples resemble those of
Rocky
Mountains
Cascades, and those of riparian woodland
Cascades.
vary only from 23.2 to 24.6 percent
not follow the pattern of the
in the
Rocky Mountains.
several samples from the
Other habitats on continents
sim-
al
more
of
complex and show differences among
trans-
uniformity.
Island
the
Specifically,
ects.
illustrate
samples
island
samples
of
and II have proportionately
more aquatic and riparian woodland spetransects
cies,
I
fewer
but
coniferous
than those of transects
III
forest
species,
and IV.
and
species
in
III)
point to
more
and fewer coniferous
first
three
do
ir.
major habitats are similar among samples
the Sierra Nevada, Great Basin, and
Rocky Mountains.
The composition of coniferous forest avifaunas.— The fraction of coniferous forest
bird species deserves special
comment
be-
cause some of the trends in total species
I,
ly
by the distribution of
The following group
woodland
cies.
forest species
cies,
riparian
of transect IV
richness discussed earlier are explained part-
Within-transect comparisons (transects
II,
The avifaunas
that the proportions of species in the sever-
are
ilar
more
samples
than those of forms in the Sierra Nevada-
samples for the Great Basin Islands ver-
Nevada or the Rocky MounHowever, the ecologic preferences of
distributed
suitable tree spe-
of forest bird spe-
discontinuously
Intermountain Region,
in
the
illustrates this point:
sus the Sierra
Sphyrapicus varius, Sphyrapicus thyroideus,
tains.
Empidonax hatnmondii, Empidonax
Table
3.
Average percent composition, according
ects.
Sierra
Transect
I
Aquatic-
Wet Meadow
Riparian
Woodland
Coniferous Forest
Transect
II
Aquatic
Wet Meadow
Riparian
Woodland
Coniferous Forest
Transect HI
Aquatic
Wet Meadow
Riparian
Woodland
Coniferous Forest
Transect IV
Aquatic-
Wet Meadow
Riparian
Woodland
Coniferous Forest
to habitat
difficilis,
preferences, of species in avifaunas along trans-
Sitta canadensis, Certhia familiaris,
Regains
Breeding Range Boundaries and
Interspecific Distributional
Relationships
Regulus calendula, Hesperiphona
vespertina, and Spinas pinus. Although most
of these forms occasionally breed in other
satrapa,
kinds of conifers or in deciduous trees, they
prefer
concolor or Pseadotsaga
(Abies
firs
and avoid or nest sparingly
menziesii)
in
forests lacking these trees. Thus, the distri-
bution of
is
No. 2
GREAT BASIN NATURALIST MEMOIRS
148
in
firs
the Intermountain Region
of special significance
these species.
for
Broad trends in avian geography are best
through comparison of major
identified
components of avifaunas. Other, more detailed issues can be seen most clearly by inspection of the distributions of single species
and
contrasting
the various sample areas. Note the repeated
long
list
numbers of species, of
all kinds of birds and of coniferous forest
birds, in relation to the appearance of firs.
particular
indicates the presence of
firs
in
instances of increased
Here
their close relatives.
four
4A
Figure
of
possible
cases,
species-level
I
discuss
chosen from a
examples,
that
illustrate
phe-
distributional
nomena occurring among
birds of the Inter-
mountain Region. Three of these examples
support the view offered earlier that
also
Firs are absent from the zone of avifaunal
impoverishment across western and central
Nevada (Fig. 3). Although skimpy clumps
grow in the Sweetwater Mountains, no firs
occur in any of the other ranges along the
California-Nevada border southeast of the
Carson Range. Firs reappear
vada between 115° and 116°
Among
W
considered
ranges
the
Ne-
in eastern
Longitude.
sub-
here,
tains,
MounWhite Pine Mountains, and on Mount
Irish.
Critchfield
stantial stands
port white
tains,
occur
in the Jarbidge
and Allenbaugh (1969)
locally
fir
the
in
but the avifauna
at
re-
Ruby Moun-
that site has not
been explored.
Rristlecone
pine
(Pinus
eastern Nevada. This tree
longaeva) domi-
is
important for
coniferous forest birds in part because
firs.
physiognomic
Apparently
oc-
it
well-shaded stands
casionally forms closed,
strong
resemblance
to
in response to this feature,
several kinds of birds from the above
list
of-
ten occupy well-developed bristlecone pine
forests
when
region presently serves as an important
arena
of
and
contact,
disjunction,
inter-
mixture of species representing avifaunas of
differing geographic derivation.
Grace's Warbler
southeastern
Rocky Mountains
bler
and ponderosa pine.—
Nevada,
as
in
generally,
in forests of
Warcommonly
Grace's
(Dendroica graciae) breeds
and exclusively
In
southern
the
ponderosa pine
But near the northwestern edge of the occurrence of the
Rocky Mountain form of the ponderosa
ponderosa).
(Pinus
pine
(P.
ample
ponderosa var. scopulorum), for exin
the
Quinn
Canyon Range, the
scattered and stunted trees occupy marginal
nates the upper slopes of several ranges in
with
this
firs
are scarce or lacking.
The
two species of Sphyrapicus, Empidonax difSitta canadensis, and Regains calen-
ficilis,
dula exemplify
this
point.
In
Nevada, the
sites,
and the bird
is
rare. It
is
absent from
grow locally in the
Highland Range, on Mount Wilson, and in
the southern White Pine Mountains, and,
the few ponderosas that
farther north, from the extensive ponderosa
pine habitat in the Snake Range (Fig.
This case
cause
prior
is
to
was not known
particularly
be-
Grace's
Warbler
breed anywhere
in south-
1963
to
5).
interesting
the
ern Nevada. Since then, numerous records
have been obtained, several from places
that
previously had been
well-explored
(Johnson 1965, 1973, 1974). Therefore, the
east of the
evidence points to an active northwestward
on Figure 3 that
marks the boundary of white fir distribution, and in many places the two species
occur in mixed stands.
range expansion in the past decade or two.
best stands of this conifer
right vertical
dashed
line
grow
Although
further
extension
into
presently
unoccupied ponderosa pine habitat
sible,
total
distributional
is
pos-
evidence suggests
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
by a multiplicity of interacting
that the approximate northwestern limit in
trol
the Intermountain Region already has been
that
However, given continued range
reached.
149
themselves fluctuating.
are
or
ponderosa pine forests of southern Califor-
competes with D. graciae and,
seems probable, for the species
is
al-
ready vagrant to that area, and a number of
other pine-foliage
son, has adjusted
dance
with
other species of the interior Southwest have
action
northwestern
there
in
recent
years
and Garrett 1974). Insofar as
known, Grace's Warbler has never nested
(Johnson
is
in
the vast ponderosa pine forests of the central
and northern Rockies, or
in
the Sierra
its
insectivore
that
obviously
for this rea-
distribution
and abun-
population inter-
as a result of recent
become
established
factors
wish to
emphasize that no other species of warbler
expansion, eventual colonization of the arid
nia
I
species.
the
Instead,
perimeter
distributional
of
Grace's Warbler seems to be set by other
this
environment. For example,
the
of
aspects
warbler
may be dependent upon popu-
lations of pine arthropods that fluctuate in
near
Nevada.
annual
The warbler thus illustrates two biogeographic phenomena typically seen in
iphery of their ranges. Or,
ature and moisture barriers dictated by the
species with well-documented distributions:
metabolic
the actual range
(1)
graphic
habitat
extent
and
(2)
of
falls
short of the geo-
seemingly
appropriate
the distributional boundaries
are unstable because of their
complex con-
especially
density,
per-
the
summer temper-
requirements and preferenda of
the warbler, the arthropods
upon which
control the range border.
I
it
may
pines they both inhabit,
feeds, or the
have no data
to
support a convincing explanation.
Downy Woodpecker and Ladder-backed
Woodpecker in disjunct allopatry.— These
two congeners fail to contact in the Great
The Downy
Basin.
and
northern
the
form D.
p.
inhabits aspen groves in
eastern
leucurus),
mountains
(the
or cottonwoods
and
willows along rivers in west-central Nevada
(the
lives
in
form D.
turati),
p.
the
in
and the Ladder-back
cottonwood-willow association
woodland
valleys or Joshua tree
Mojave
Unexpectedly,
Desert.
in
the
substantial
areas of apparently suitable habitat occur in
the wide
zone
now unoccupied by
species. Thus, the
either
Downy Woodpecker
has
not been found in the fine aspen woodland
in
the
Wilson,
Creek Range or on Mount
Schell
among
a
number
of seemingly ap-
propriate places. Similarly, the Ladder-back
avoids considerable Joshua tree growth in
Esmeralda and Nye Counties, Nevada.
A
Grace's Warbler
Ponderosa Pine
v
nally, neither species
riparian
"•
woods
in
Fi-
breeds in the excellent
Owens
Valley, although
the Ladder-back has been recorded at
Lone
and
Pine, at the foot of the valley (Grinnell
Fig. 5.
Warbler
(stippled
Approximate breeding distribution
(triangles)
areas)
in
of Grace's
and occurrence of ponderosa pine
southeastern Nevada and south-
Miller 1944).
I
conclude that the respective
southern and northern limits of these two
within
western Utah. Lower elevational perimeters of wood-
species,
lands in mountains also are indicated.
tated primarily
suitable
habitats,
are
dic-
by temperature and mois-
GREAT BASIN NATURALIST MEMOIRS
150
Considering the
ture.
wide unoccupied
swath between their ranges
the central
in
Intermountain Region, distributional limits
sharp
in
shift
temperatures.
No. 2
average spring and summer
The approximate coincidence
of the zone of parapatry with the northern
Mojave Desert supports
this
and Western
Fly-
there certainly are not controlled by inter-
limits
specific competition.
suggestion.
in
Mountain Bluebird and Western Bluebird
parapatry.— These two congenericthrushes (Sialia currucoides and S. mexicana,
catcher in broad sympatry.— As in the prealso
concerns a
respectively) breed in contiguous allopatry,
species of northern derivation,
Hammond's
much
of the
Hammond's
Flycatcher
examples,
vious
this
case
of the southern Inter-
Flycatcher (Empidonax hammondii), and a
mountain Region. One might conclude that
congener of southern derivation, the West-
or parapatry, in
such a distributional relationship
competition
sult of interspecific
at least three general areas
cies
is
the re-
(Fig. 6). In
where both spe-
have been recorded (Panamint Range,
Delamar Range, Zion National
relationships
spatial
cause
of
habitat
their
well-known
able,
I
results
A
Until
Fig. 6.
how
(E.
difficilis).
the
These
two
impressive
particularly
ex-
complex interplay
of
phylogenetic and distributional history has
influenced present-day habitat selection and
in
probably
tributional
Hammond's Flycatcher
is
details are avail-
and numerical
relationships
of
these species in western North America.
is
commonest
cool, moist Boreal forests of the Sierra
vada,
Cascades, and Rocky Mountains;
in
Neits
conclude that the striking parapatry
principal range therefore surrounds the In-
from the geographic position of a
termountain Region on three
sides.
In con-
Mountain Bluebird
A Western
(S.
more
of
a
ecologic interaction. Figure 7 shows the dis-
infrequent or absent, given a situation of local sympatry.
ample
offer
be-
But,
differences
interaction
Flycatcher
Park), their
unclear.
preference (Grinnell and Miller
competitive
1944),
are
ern
forms
Bluebird
Breeding distributional relationship of the Mountain Bluebird
mexicana
bairdi) in southern
Nevada and adjacent
states.
{Sialia currucoides)
and Western Bluebird
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
the
trast,
greatest
forests
arid
Western Flycatcher reaches
densities
(small
in
form
warm
E.
d.
Pacific
difficilis)
interior highland forests of the
mountain
Region
and
southern
its
Coastal
and
in
Inter-
Rocky
151
New Mexico
where E. hammondii is either absent or rare and local. In
areas of contact between Hammond's Flycatcher and the similarly sized E. d. diffiMountains
(large
form
of
Arizona
and
E. d. hellmayri),
Empidonax
difficilis
£. d. difficilis
E.
Fig. 7.
d.
hellmayri
Breeding distributional relationship of Hammond's Flycatcher (principal range outlined) and Western
Flycatcher in the contiguous western United States. Relative abundance of three forms of Western Flycatcher is
shown by patterns as follows: E. d. difficilis, heavy slanted lines indicate very common to abundant populations,
and light slanted lines indicate common to uncommon populations; E. d. hellmayri, black indicates very common
to abundant populations, and dotted pattern indicates common to uncommon populations. E. d. insulicola is
common to abundant on the California Channel Islands, indicated in black. Scattered localities for F. d. hellmayri are shown by circles; note that many of them occur within the main range of E. hammondii. A few localities for
the latter species, located south of the main range, are omitted.
GREAT BASIN NATURALIST MEMOIRS
152
cilis
Northwest and on the
Pacific
the
in
west side of the Cascades, interspecific
ter-
defended (Johnson 1966 and un-
ritories are
publ. data). In the Rockies of southern Col-
orado the larger E.
overlaps
hellmayri evidently
d.
hammondii
with E.
territories
in
(Beaver and Baldwin 1975). The important
point
two species are largely
that these
is
numbers,
bulk
the
considering
allopatric;
of
their
from selection
this results directly
though not identical, habitats
of similar,
Where
different major climatic zones.
overlap, one species
in
they
nearly always com-
is
mon, the other uncommon or
rare.
Despite
the geographic shifts in relative abundance
and subtle differences in habitat selection,
which presumably are consequences of millenia of competitive interaction, the two
occur
do
species
sympatrically
in
one species has not
places. Obviously
Grinnell
manifold
aries of
enda
by
a
forms of closer relationship.
series of other
The Hammond's Flycatcher
Least Flycatcher
allied to the
is
minimus) and
(E.
to other
nated. In the
the
re-
its
was
relevance was subordi-
more recent
literature of orni-
thological ecology, the limitation of geogra-
phic
through
range
become
1972, Cody
competitive
exclusion
theme (MacArthur
1974). However, the invocation
of competition in this context has been pre-
has
a prominent
mature.
agreement
In
with
Salt
and climate
whose
(1952),
paper clearly describes
how metabo-
interrelate to control the
distributions of three species of finches (CarI
see
reason
little
to
postulate
competition as an effective determinant of
range borders of species of birds
termountain
Region.
in the In-
The question
is
not
whether competition occurs among the
many overlapping
species of Boreal derivation (Johnson 1963).
what form does
The Western Flycatcher, in contrast, is closest to Neotropical montane stocks such as
as such,
and
along
They favored niche
not mentioned or
podacas),
species are separated phylogenetically
changing
factors
as the crucial dictates of species distri-
lism
Within the genus Empidonax, these two
bound-
distributional
butions; interspecific competition either
limit-
7).
of
quirements and physiologic-climatic prefer-
classic
(Fig.
controls
perimeter.
many
ed the geographic range of the other
others,
animal species, with the importance
particular
range
and Bartholomew
have emphasized
1917)
(1914,
among
(1958),
of
No. 2
From
can
all
it
species here; rather,
this
is
it
competition take, and,
influence range boundaries?
the evidence
I
have seen, includ-
of southern
ing that from the four examples discussed,
derivation (N. K. Johnson, unpubl.). Appar-
competition probably has had little influence in the determination of range
E. flavescens
ently
is
certainly
such evolutionary backgrounds have
permitted extensive convergence
in habitat
preference and foraging ecology, for in the
hammondii and
commonest Empidonaces
western United States,
are the
difficilis
dense coniferous
E.
boundaries of birds. Instead, alterations in
density, such as
of
of
of competitive
gest
forest.
shown
that
it
interactions.
is
unwise
boundaries of tropical
Controls of Boundaries
of Species'
The
barest
previous
introduction
to
the
only
cause
it
comment
lies
vast
subject
on
I
conclude
species
are
Weske
set
sugthat
by
1975), in
rence of those environmental requisites cru-
of
cially related to the distribution of the bird
it
is
species in question.
appro-
this topic
Patterns of Avian Speciation
in the intermounta1n region
be-
the heart of biogeography
at
and because of
briefly
Finally,
the
the determinants of borders of breeding dis-
priate to
two species
outcomes
likely
the virtual absence of data on the occur-
offer
tributions of birds. Nonetheless,
to
competition (Terborgh and
Ranges
accounts
for the
Empidonax, are the more
E.
significant trends in the per-
tinent literature.
In
this
section
I
concentrate on what
I
consider to be active zones of speciation in
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
153
the Great Basin region. These are illustrated
on each
by currently interacting populations of
Habitats in the contact areas themselves are
related
closely
well-differentiated
taxa,
at
above that of the incipient
subspecies. For those concerned with more
subtle examples of racial differentiation in
various
levels
typically intermediate
interior
trated
zone (Table
side of the contact
In
situations.
by
sentative
between
and
coastal
major trend
a
5).
illus-
six pairs
of taxa, the coastal repre-
inhabits
coniferous forest or oak
region, Behle (1963) provides a useful
this
review. Information on speciation zones
is
of significance to the data previously pre-
sented on avifaunal gradients because of the
same environmenclimate and flora ulti-
Table
interior
Examples
4.
taxa
that
Nevadan and
of coastal or Sierra
phenomena
speciation
illustrate
along the interface of the Cascades-Sierra Nevada and
Great Basin.
great probability that the
tal discontinuities
in
weak
mately are responsible for both.
primary
Gradual
demonstrate
range
of
phenomena along
speciation
western edge of the Great Basin,
in a
extending south-southeastward from
2.
Empidonax difficilis
Western Flycatcher.
3.
Eremophila alpestris sierrae and
the
zone
4.
delineated
part
actually
is
Psaltriparus
eus or
6.
P.
7.
megarhynchus and
Dendrocopos pubescens
divergent racial complexes:
zones
of
portrays
9.
10.
for nine of these pairs of taxa.
Several
patterns
first
concerns
the striking, although imprecise, geographic
coincidence
that
link
divide)
(or
populations.
several
the
of
a
zones
contact
coastal
Secondly,
environmental
b.
P.
i.
fidva or
P.
and D,
turati
p.
leucurus.
P.
i.
of strongly
kernensis and
P.
zaleptus. Plain Titmouse.
Vireo solitarius cassinii
large
and
and
V.
plumbeus.
s.
of
gradients
or
dis-
continuities in the western Great Basin per-
and
lutescens
Orange-crowned Warbler.
Narrow zone or sympatry and
Soli-
V.
c.
orestera.
interspecific hybridiza-
tion:
12.
Sphyrapicus ruber daggetti. Red-breasted Sapsucker
13.
Junco hyemalis thurberi. Dark-eyed (Oregon) Junco
and /. caniceps caniceps. Gray-headed Junco.
interior
number
Vermivora celata
11.
and
species are represented, suggesting that significant
nevadensis.
tary Vireo.
emerge from study of
wealth of material. The
this
m. plumb-
P.
between representatives
Parus inornatus inornatus or
i.
and hybridization
intergradation
ne-
Downy Woodpecker.
gion that illustrate stages in the speciation
schematically
c.
monoensis. Fox Sparrow.
Disjunct allopatry
8
a.
and A.
superciliosa
minimus californicus and
Passerella iliaca
i.
wish to discuss 16 selected pairs of taxa
Figure
.
lampro-
m. providentialis. Bush-tit.
(Table 4) in the western Intermountain Re-
process.
E.
Sage Sparrow.
8.
I
E. d. hellmayri 2
Amphispiza belli canescens and A.
Columbia and beyond. Here
to British
racial
vadae. Scrub Jay.
of a
larger biotic separation that extends north-
ward
and
difficilis
Aphelocoma coendescens
5.
just
na-
chroma. Horned Lark.
south-
from the general distribution and speciation
of birds in western North America that the
zone
t.
complexes:
evidence
is
S.
between representatives of strongly divergent
ern Oregon to southeastern California and
southern Nevada. Indeed, there
and
thyroideus
Narrow zone(s) of primary or secondary intergradation
wide
particularly
a
thyroideus
Williamson's Sapsucker.
taliae.
Birds
boundaries
racial
:
Sphyrapicus
1.
Zones of Intergradation, Disjunction,
and Secondary Sympatry
integradation;
1
S.
varius nuchalis. Red-naped Sapsucker.
Disjunct allopatry between species:
14.
Pica
Yellow-billed
nuttalli.
Magpie and
P.
pica
hudsonia. Black-billed Magpie.
vasively
influenced
the
differentiation
of
15.
few other regions in North
America can claim a comparable diversity
birds.
of
Indeed,
speciation
stricted
phenomena within such
geographic
Furthermore,
interacting
V. virginiae. Virginia's
Leucosticte
16.
tephrocotis
Rosy Finch and L.
re-
limits.
the
Vermivora ruficapilla ridgwayi. Nashville Warbler
and
'Many additional examples
taxa
of
Gray-crowned
Black Rosy Finch.
of this category could
be cited
(see
Behle
1963).
!
nearly every pair occupy different habitats
atrata.
Warbler.
dawsoni.
In each
form.
pair of taxa
the coastal-Sierran form
precedes the interior
154
GREAT BASIN NATURALIST MEMOIRS
3
No. 2
I
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
1978
"
£
o
M
oS
i Z
o 2
5 u
3 o
J
2<u S S
<
"2
a- o
^
o
r
Zb,
r>ozt,>u
I a 5 J
s
s
155
GREAT BASIN NATURALIST MEMOIRS
156
No. 2
woodland-chaparral, and the interior race or
clearly suggests that population densities are
species lives in pinyon-juniper woodland. In
reduced
four additional pairs of taxa, the coastal or
zones of intergradation. Thus, the environ-
Sierran
niferous
such
form
occupies
and the
forest,
to riparian
oak-chaparral,
brush
or
forest,
interior
co-
with
associated
form switches
aspen or willows. Earlier
I
dis-
cussed the latter trend (Johnson 1970).
In 5 of the 16 pairs, the coastal and interior representatives are disjunct,
with ranges
separated by gaps that overlap or straddle
many
of the zones of intergradation of the
11 pairs of forms that
do contact. Evidently
selection
the
Mountains
significant
Seven of the 11 pairs that meet and
interact occur less
zone
than
in
commonly
adjacent
the
Fig. 8. Left, zones of
in the contact
regions.
Jay); 6,
4,
(1973),
Nevada
Sierra
Munz and Keck
meet
the
(1968), Bald-
and Miller (1951) describe the
floristic, and cli-
physiographic,
matic changes ultimately responsible for the
evolution of contrasting avifaunas in this region.
primary intergradation and hybridization (secondary contact),
the Great Basin, in seven species of birds.
Horned Lark;
This
and
Great Basin.
win
species.
on bird populations in
zone where the Cascade
pressures
restricted
the intervening habitat separating the dis-
unsuitable for these particular
their
ments at the western border of the Intermountain Region are unsuitable for certain
species and marginal for others. In any case,
the variety of distributional and speciation
phenomena represented underscores the
steep environmental gradients that influence
junct forms
is
and near
for several species in
Numbers correspond
to those in
Table
4:
at the
2,
western border of
Western Flycatcher;
3,
Scrub Jay (two zones); 5, Bush-tit (two zones, one of which is identical with a zone of the Scrub
7, Fox Sparrow (two zones); and 8, Downy Woodpecker. Bight, localities of sympatry and
Sage Sparrow;
hybridization between Bed-breasted and Bed-naped Sapsuckers (triangles) and between Dark-eyed (Oregon) and
Gray-headed Juncos
(dots).
1978
INTERMOUNTAIN BIOGEOGRAPHY: A SYMPOSIUM
Physiographic Breaks and
lations
Avifaunal Boundaries
ealities
157
on the west side of the trough at lowell-removed from their principal
centers of distribution from central
A
Nevada
major gap in the mountain systems of
the western United States results from the
eastward. These occurrences provide clear
low trough that begins in southeastern Oregon, courses southward through western Ne-
some
vada, veers southeastward between
Walker
evidence
the
for
suitability,
interior species, of those
for
at
least
montane en-
vironments located directly adjacent to the
southern Cascades and east side of the
Sierra Nevada. But these same ranges also
Lake and the Paradise Range, then continthrough the Death Valley area and
beyond into the Mojave and Colorado
host many Cascadian and Sierran forms that
have colonized from the west. Thus, pecu-
With such a geographic
would be easy to assume that
mountain systems (Table
ues
deserts.
position,
this
it
trough
Harly intermixed avifaunas breed in
6).
One
all
four
of the four
separates the Boreal avifaunas of the Cas-
ranges, the Sweetwaters, has only a
cades and Sierra Nevada from those of the
representation of interior forms; the others
interior.
However,
this
is
not the case, for a
and species of birds
occur westward to the Warner, Wassuk,
Sweetwater, and White mountains. These
interior forms
therefore maintain popuhost of interior races
Table
6.
minor
have numerous Great Basin-Rocky Mountain
All of the other mountain systems
species.
along
the
Nevada-California
Carson Range,
Grove
Hills,
Pine
border,
and Glass Mountain, lack
Intermixture of Cascade-Sierra Nevadan and interior components
1
in
the
Nut Mountains, Pine
inte-
Boreal avifaunas of mountain
ranges 2 along the western edge of the Great Basin.
Cascade Mountain-Sierra Nevadan
Forms
Dendragapus obscurus
sierrae
Warner
Mountains
Wassuk
Range
Sweetwater
Mountains
White
Mountains
158
GREAT BASIN NATURALIST MEMOIRS
Boreal avifaunas are entire-
rior forms; their
comprised of Sierra Nevadan and wide-
ly
spread species. Because the most distinctive
and
interior species
Selasphorus
as
racial
Vermivora
vir-
and the ivoodhonseii Group of Aphelocoma coerulescens, extend all the way
from eastern California to Colorado, there
is no
Boreal avifauna typical only of the
Great Basin section of the interior.
giniae,
The
aforementioned
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