Download Using Montane Mammals to Model Extinctions Due to Global Change

Using Montane Mammals to
Model Extinctions Due to
Global Change
KELLYA. MCDONALD*
JAMES H. BROWN
Department of Biology
University of New Mexico
Albuquerque, NM 87131-1091, U.S.A.
Abstract: We use data on the species-area relationship and
the nested subset structure o f the boreal m a m m a l f a u n a s
inhabiting isolated mountaintops in the Great Basin to develop a simple quantitative model that predicts the number
and identity o f species that w o u l d go extinct under an assumed scenario o f changing climate and vegetatior~ Global
warming o f 3 ° C is predicted to cause the loss o f 9-62% o f the
species inhabiting each m o u n t a i n range and the extinction
o f three o f fourteen species throughout the regior~ These resuits suggest (1) that it is possible to make highly plausible
predictions about the susceptibility o f species to extinction
without detailed information about their population biology, and (2) that global and regional environmental changes
seriously threaten the survival o f species that are restricted in
distribution to both natural "'habitat islands" and biological
reserves.
Introduction
Human activities threaten the maintenance of biological
diversity. Perhaps the most pervasive threat posed by
m o d e m humans is alteration of the environment on a
global scale through climate change, land use practices,
resource depletion, and pollution. These changes jeopardize the survival of so many species and populations of
*K McDonald'spresent address is Department of Biology, New Mexico State University, Box 30001/Dep£ 3AF,, Las Cruceg NM 88003OO01, U.SA
Reprint requests should be addressed to J. Browrt
Paper submitted May 3, 1991; revised manuscript accepted November 6, 199L
R e s u m e n : Usando datos acerca de la relact6n especie-area
y la estructura en subgrupos anidados que caracteriza a la
f a u n a de mamiferos boreales que habitan los picos mon.
taftosos aislados en "Great Basirg " desarrollamos un modelo
quantttativo que predice el m~mero e identidad de los especies que se extinguirfan bajo un supuesto escenario de cambios en clima y vegetaci6rL Se predice que un calentamiento
global de 3 ° C causaria la pdrdida de un 9 - 6 2 % de las especies que habitan carla cadena montaf~osa y la extinci6n de
tres de las 14 especies a trav&s de la regi6rL Estos resultados
sugieren que: (1) es posible hacer predicciones con un alto
grado de seguridad acerca de la suceptibilidad de las e.species
a la extinci6n, sin contar con una detallada informaci6n
acerca de sus biologlas poblacionale~ y (2) que cambios
ambientales tanto globales como regionale~ a m e n a z a n seriamente la supervivencia de las especie~ cuya distribucidn
se encuentra restringida tanto a "'habitat insulares'" de caracter natural como a reservas biol6gicag
organisms over such a large area that traditional conservation approaches focusing on specific organisms or local regions will be inadequate. A general predictive
framework is needed to identify endangered populations and vulnerable habitats. These predictions, in turn,
could be integrated into programs developed to mitigate human impact and to manage threatened organisms
and environments.
Of particular concern is the global modification of
climate apparently caused by anthropogenic greenhouse gases and the effects of this change on geographically restricted plant and animal populations. Current
predictions, based on empirical trends in greenhouse
gas concentrations and air temperatures (Pastor 1988;
4o9
ConservationBiology
Volume6, No. 3, September1992
410
PredictingExtinctions
Lester 1989, 1990; Grover 1990) as well as on general
climate models ( GCMs ) (Schneider 1990 ), suggest that
northern temperate latitudes will warm 2 - 6 ° C in the
next century. Changes of this magnitude threaten the
survival of many organisms, especially those restricted
to fragmented habitats or small biological reserves (Peters & Darling 1985). The magnitude of predicted climate change is sufficient to alter the habitat of these
isolated patches, causing the extinction of populations
with narrow requirements (Peters & Darling 1985; Peters & Lovejoy 1992; Murphy & Weiss 1992).
We use the theory of insular biogeography (MacArthur & Wilson 1967; Brown 1986) and available data on
the distribution of mammals on isolated mountain
ranges of the Great Basin in western North America
(Brown 1971, 1978; Brown & Gibson 1983; Grayson
1987) to predict the extinctions that would be caused
by a specific scenario of climatic and global change. The
facts that the number of mammal species is closely correlated with the area of the inhabiting mountain range
and that the c o m p o s i t i o n of these faunas exhibits
"nested subset structure" (Patterson & Atmar 1986;
Patterson 1987; Cutler 1992) provide the basis for predicting the number and identity of species that will go
extinct on each mountain range. The kind of model that
w e use has broad applicability because (a) species-area
relationships and nested subset structures are characteristic of many fragmented habitats, and ( b ) extinctions
can be predicted by assuming a particular global warming scenario and using existing presence-absence data
on the distribution of species----detailed information on
the life history and ecology of each species is not required.
McDonad & Brown
sin during the Pleistocene. With the onset of the Holocene about ten thousand years ago, the boreal habitats
retreated to higher elevations in response to a w a r m e r
and drier climate. In the absence of immigration, some
of the mammal species inhabiting each isolated mountain range have gone extinct. Thus, the present fauna of
each mountaintop consists of those species that have
survived the last major episode of climate change. (For
more complete accounts of the present and historical
distributions of these mammals and of the climatic history of the Great Basin see Brown 1971, 1978; Brown &
Gibson 1983; Wells 1983; Grayson 1987.)
The composition of the isolated mammal faunas, and
thus the pattern of extinction since their isolation, are
highly predictable in at least two respects. First, there is
a high, positive correlation b e t w e e n number of species
and area of the mountaintop above 2280 meters elevation, which is the approximate lower limit of woodland
habitat (Fig. 1, see also MacArthur & Wilson 1967;
Brown 1971, 1978; Brown & Gibson 1983). Second, the
faunas of the different mountain ranges comprise nearly
perfect nested subsets with respect to species composition: each fauna of successively higher species richness
tends to contain virtually all of the species in more species-poor faunas plus one or more additional species
(Table 1, see also Patterson & Atmar 1986; Patterson
1987; Cutler in press). Taken together, these two fea-
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5
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.
The System
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Volume 6, No. 3, September 1992
•
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The boreal mammals inhabiting the isolated mountain
ranges in the Great Basin provide an excellent system
for predicting the extinctions that might be caused by
different scenarios of global change. Fourteen species of
small mammals are currently restricted to coniferous
forest, meadow, and streamside habitats that occur only
at higher elevations on these mountains. There appears
to be virtually no contemporary migration of these boreal species across the desert valleys separating mountain ranges. Nineteen mountain ranges have been well
surveyed for mammals, have peaks over 2990 meters
elevation, and offer generally similar environments.
These are separated by valleys lower than 2280 meters
and barriers of hot, dry climate and desert shrub vegetation. These montane islands represent the relictual
fragments of woodland, forest, and associated mesic
habitats (such as meadows and streams) that w e r e
widely and contiguously distributed over the Great Ba-
e15
01
e19
~I4IS
e17
20010
E
Z
.
.
.
.
.
.
.
.
.
.
10
'
'100
•
,
,
,
,
, r ,
1000
Area (krn2)
Figure 1. Relationship, on logarithmic axeg between
n u m b e r o f species o f s m a l l boreal m a m m a l s a n d
area above 2280 m elevation f o r nineteen isolated
m o u n t a i n ranges in the Great Basin ( f r o m data in
B r o w n & Gibson 1983). The p o w e r f u n c t i o n f i t t e d to
these data (regression ling w h e n transforwaed to logarithmic axes) is S = 1.188 A 0"326. The m a g n i t u d e o f
residual values a r o u n d this regression were preserved
to predict the n u m b e r o f species r e m a i n i n g after extinctions d u e to climate change a n d retreat o f boreal
vegetation to the area above 2745 m elevatiott N u m bers identify the m o u n t a i n rangeeg see Table 2.
McDonald & Brown
Predic'ting E~l~ct]o~
411
Table 1. Distribution of fourteen small boreal mammal species among nineteen isolated mountain ranges in the Great I ~ i n at present
(from Brown a GHmon IM3) and after predicted extlactlom due to effects of global ~m'ming.
M o u n t a i n ranges
Species
Eutamias u m b r i n u s
Neotoma cinema
Eutamias d o r s a l i s
Spe~nopbilus
lateralis
Microtus
longicaudus
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
E
E
E
X
X
X
X
X
X
X
X
X
X
X
X
X
X
E
X
X
E
E
E
E
X
X
X
E
E
E
E
E
E
E
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
E
X
X
X
E
E
E
X
X
X
E
X
X
E
X
X
E
X
X
17
17
17
17
17
14
E
14
10
13
I0
12
5
11
10
8
7
7
5
6
1
5
5
1
0
2
0
2
0
X
SytviU~us
nuttalii
Marmota
flaviventris
Sorex vagrans
Sorex palustrls
Mustella
ermtnea
Ochotona
princeps
Zapus princeps
Spermophilus
beldtngi
Lepus
townsendii
Present number
of species
Predicted number
of species
X
X
E
E
E
E
E
E
E
E
X
X
E
E
E
E
E
E
E
E
E
E
E
E
E
E
13
12
11
11
10
10
9
8
8
8
7
6
5
4
4
4
3
3
3
11
8
9
10
8
7
5
5
3
6
4
4
3
2
2
2
2
2
2
E = Species predicted to go extinct under assumed scenario o f climate a n d vegetation change
X = Species expected to persist
tures of the faunas suggest that the number of extinctions can be predicted from area, and the identity of the
species that have gone extinct can be predicted from
the nested subset structure of the species-by-mountaintop matrix.
The Model
The orderly species-area and nested subset patterns not
only indicate the predictability of the e x t i n c t i o n s
caused by the climate and vegetation change since the
end of the Pleistocene, but they can also be used to
predict additional extinctions that should be caused by
global warming (Murphy & Weiss 1992). We use the
existing distributions of mammals on the mountaintops
and one scenario of global warming to develop an empirically-based model to predict the number and identity of species on each mountain range that would be
expected to go extinct. The goal of this exercise is not
to make definitive predictions of future extinctions, but
to present one kind of model that can be used to make
such predictions given certain assumptions about climate and vegetation change.
Several scenarios of climate change are being derived
from general circulation models (GCMs). These differ in
their spatial resolution, the magnitude and seasonal pattern of temperature change predicted for different regions, and the extent to which associated changes in
cloud cover, precipitation, and other important variables can be inferred. Given these differences among
GCMs, there is much uncertainty in the patterns of
change in climate and vegetation that can be e x p e c t e d
to occur in the next century (see Ramanathan 1988;
Lester & Myers 1989, 1990; Schneider 1990; Grover
1990). To illustrate our method, w e follow Murphy and
Weiss (in press) and assume the following scenario of
climate change and associated events:
1.
The climate of :~he Great Basin will warm by 3°C
a ~d the relative amount of precipitation will remain
unchanged. The figure of 3°C is the intermediate
Conservation Biology
Volume 6, No. 3, September 1992
412
2.
3.
4.
Pred/ctmgExtinctions
McDonald & Brova2
feet ( 2 2 8 0 m e t e r s ) t o 9 0 0 0 f e e t ( 2 7 4 5 m e t e r s ) .
P l a n i m e t r y o f t h e areas o f e a c h m o u n t a i n r a n g e
a b o v e t h e s e c o n t o u r s o n U.S. G e o l o g i c a l S u r v e y
m a p s o f states ( s c a l e 1 : 5 0 0 , 0 0 0 ) w a s u s e d to estim a t e t h e h a b i t a b l e a r e a at p r e s e n t a n d after t h e ass u m e d c l i m a t e c h a n g e ( T a b l e 2). T h e p o w e r funct i o n S = 1.188 A °'326, f i t t e d to t h e s p e c i e s - a r e a
r e l a t i o n s h i p for t h e c o n t e m p o r a r y b o r e a l m a m m a l
faunas o f t h e m o u n t a i n s , w a s t h e n u s e d to e s t i m a t e
t h e r e d u c t i o n in s p e c i e s r i c h n e s s o n e a c h m o u n t a i n
f o l l o w i n g t h e shift in v e g e t a t i o n a n d t h e d e c r e a s e in
h a b i t a b l e area. This w a s d o n e in s u c h a w a y as t o
preserve the relative magnitude of the deviation (of
t h e residual o n a l o g a r i t h m i c s c a l e ) for e a c h v a l u e
w i t h r e s p e c t to t h e r e g r e s s i o n line (Fig. 2). This
a s s u m e s that t h e residuals a r o u n d t h e r e g r e s s i o n
line for t h e c o n t e m p o r a r y faunas r e f l e c t p r i m a r i l y
deterministic characteristics of each mountain
r a n g e ( s u c h as v a r i a t i o n in latitude, l o n g i t u d e , a n d
g e o l o g y ) that will n o t b e a l t e r e d b y a s c e n a r i o o f
global warming.
. T h e p r e d i c t e d s p e c i e s r i c h n e s s for e a c h m o u n t a i n
range w a s t h e n r o u n d e d off t o t h e n e a r e s t integer,
a n d this value w a s used, t o g e t h e r w i t h t h e h i g h l y
o r d e r e d s t r u c t u r e o f t h e s p e c i e s - b y - m o u n t a i n mat r i x ( T a b l e 1 ), to p r e d i c t t h e i d e n t i t y o f t h e s p e c i e s
that are m o s t likely to g o e x t i n c t .
v a l u e for g l o b a l w a r m i n g p r e d i c t e d t o o c c u r b y t h e
m i d d l e o f t h e n e x t c e n t u r y at t e m p e r a t e l a t i t u d e s in
t h e n o r t h e r n h e m i s p h e r e ( P e t e r s 1987; J a e g e r
1988). B e c a u s e o f u n c e r t a i n t i e s a b o u t effects o f
g r e e n h o u s e gases a n d g l o b a l w a r m i n g o n p r e c i p i t a tion, w e a s s u m e that t h e rainfall r e g i m e w i l l r e m a i n
essentially u n c h a n g e d .
A s s u m i n g that t h e l a p s e r a t e w i l l b e unaffected b y
g l o b a l w a r m i n g , this m a g n i t u d e o f c l i m a t e c h a n g e
will c a u s e t h e z o n e s o f v e g e t a t i o n o n t h e G r e a t Basin m o u n t a i n s t o b e d i s p l a c e d u p w a r d b y 5 0 0
m e t e r s in elevation, a n d t o s h r i n k in a r e a a c c o r d ingly. This a s s u m p t i o n f o l l o w s H o p k i n s ' s "bioclim a t i c l a w " r e l a t i n g air t e m p e r a t u r e to e l e v a t i o n
( M a c A r t h u r 1972; P e t e r s & Darling 1985). It f u r t h e r
a s s u m e s t h a t t h e r e has b e e n sufficient t i m e for t h e
v e g e t a t i o n to shift its d i s t r i b u t i o n t o e q u i l i b r a t e
w i t h c l i m a t i c change.
T h e r e s p o n s e o f e a c h b o r e a l m a m m a l s p e c i e s to climate and vegetation change can be predicted from
its p r e s e n t e c o l o g i c a l a s s o c i a t i o n s a n d g e o g r a p h i c
d i s t r i b u t i o n s . As t h e a r e a o f suitable h a b i t a t shrinks
in r e s p o n s e to c l i m a t i c change, t h e d i s t r i b u t i o n a n d
abundance of each mammal species will decrease
p r o p o r t i o n a t e l y until e x t i n c t i o n occurs. W e f o l l o w
B r o w n ( 1971; s e e also L o m o l i n o 1 9 8 6 ) in a s s u m i n g
that t h e r e is n o c o n t e m p o r a r y d i s p e r s a l o f t h e s e boreal m a m m a l s a c r o s s t h e d e s e r t valleys to c o l o n i z e
u n i n h a b i t e d m o u n t a i n s . Such c o l o n i z a t i o n w o u l d b e
even more unlikely under our proposed scenario of
c l i m a t e change.
T h e first t w o a s s u m p t i o n s c a n b e u s e d to shift t h e
lower border of pinon-juniper woodland upward
a p p r o x i m a t e l y 500 m e t e r s , f r o m t h e p r e s e n t 7 5 0 0
Application of the Model
T h e o p e r a t i o n o f o u r m o d e l c a n b e s t b e i l l u s t r a t e d b y an
e x a m p l e . T h e T o q u i m a - M o n i t o r Range w a s p r e d i c t e d to
have its b o r e a l h a b i t a t s r e d u c e d b y 35%, f r o m 4 5 5 to
Table 2. Effects of the assumed scenario of 3°(] warming on the area of boreal habitat and the number of small mammal species on
nineteen isolated mountain ranges in the Great Basin.
Mountain
range
Number
Toquima-Monitor
Schell Creek-Egan
White-Inyo
Toiyabe-Shoshone
Snake
Ruby
White Pine
Deep Creek
Diamond
Grant-Quinn Canyon
Spring
Desatoya
Oquirrh
Stansbury
Sheep
Roberts Creek
Spruce-South Pequop
Panamint
Pilot
5
13
1
4
15
8
12
16
7
11
9
3
19
18
10
6
14
2
17
Area > 2 2 8 0 m
( k i n 2)
455
394
185
264
161
141
101
86
61
58
48
32
32
22
21
20
19
18
5
Area > 2 7 4 5 m
( k m 2)
230
106
121
137
65
42
19
17
3
16
14
2
9
4
3
2
3
3
1.2
Present
percent
reduction
Predicted
number
o f species
Number
o f species
Percent
reduction
35%
84%
94%
49%
50%
90%
96%
71%
71%
86%
73%
82%
74%
85%
60%
81%
76%
82%
72%
11
8
11
13
10
12
7
9
4
5
6
8
10
10
3
4
4
3
3
9
5
10
11
8
8
4
5
2
3
4
3
7
6
2
2
2
2
2
18%
37%
9%
15%
20%
33%
43%
44%
50%
40%
33%
62%
30%
40%
33%
50%
50%
33%
33%
° N u m b e r s assigned to the m o u n t a i n ranges are those used to identify them in Figures 1 a n d 2.
ConservationBiology
Volume 6, No. 3, September 1992
McDonald& Brown
PtedictingEkltnciions
10
o~
o~°
~
~
o
¢D
"
E
Z
1
.
.
.
.
10
.
.
100
i
1000
Area (kin2)
Figure 2. Predicted changes in the number o f species
of small mammals inhabiting nineteen isolated
mountain ranges in the Great Basin after extinctions
owing to the assumed scenario of climate and vegetation change For each mountain range (identified
by number, see Table 2), the unshaded circle represents the present number of specie~ the shaded circle
indicates the predicted number after extinctions, and
the arrow connecting the two points shows the magnitude of change.
230 square kilometers (Table 2). The predicted reduction in species richness was 18%, from 11 to 9 (Fig. 2").
F r o m t h e n e s t e d s u b s e t n a t u r e of the species-bym o u n t a i n m a t r i x ( T a b l e 1), the t w o s p e c i e s n o w
present in the Toquima-Monitor Range that are most
likely to go extinct are Ochotona princeps and Zapus
princepg so these two species have b e e n designated by
an E in Table 1 to indicate the e x p e c t e d change in the
faunal list after climate and vegetation change.
Repeating this p r o c e d u r e for all the mountain ranges
allows us to predict the n u m b e r and identity of species
that will go extinct on each mountaintop (Tables 1 and
2). Under the assumptions of o u r model, individual
mountain ranges are predicted to lose 35-96% of their
area of boreal habitat and 9 - 6 2 % of their present boreal
mammal species. Individual species are predicted to disappear from 0 - 1 0 0 % of the mountain ranges w h e r e
they presently occur. Three species, Zapus princepg
Spermophilus belding~ and Lepus townsendi~ are predicted to go extinct in the entire Great Basin, and only
two species, Eutamias umbrinus and Neotoma cinereg
are predicted to survive on all mountains w h e r e they
presently occur.
Discussion
The approach that w e use here to predict the effects of
global climate change on biological diversity has b o t h
limitations and advantages. Perhaps the greatest limitation is o u r assumption o f a particular quantitative sce-
413
nario for b o t h climate change and resulting shifts in the
elevational zonation of vegetation. Obviously o u r specific predictions d e p e n d on the accuracy of these assumptions. As GCMs b e c o m e m o r e refined and as the
trajectory of actual climate change b e c o m e s clearer, the
predicted magnitude of change in t e m p e r a t u r e and precipitation may b e revised. The advantage of our approach, however, is that it is very general. Our model
could easily be modified to incorporate different assumptions about changes in climate and vegetation, and
then it would predict different patterns of species extinctions and diversity.
Our model also assumes that the present boreal mammal faunas are in a p p r o x i m a t e equilibrium w i t h the
present regime of climate and vegetation. Then it predicts species composition and richness on the mountain
r a n g e s a f t e r t h e e s t a b l i s h m e n t o f a n e w quasiequilibrium. We k n o w that the faunas are not in exact
equilibrium--in the absence of ongoing colonization,
the equilibrial species richness of each mountain range
would be zero species, even under current conditions
(Brown 1971). However, there is good reason to assume that the nature of extinction ( o f relaxation curves,
see Diamond 1971 ) in fragmented habitats w h e r e there
is no recolonization is such that there tend to be t w o
classes of species: ( 1 ) those that are highly vulnerable
and go extinct soon after isolation, and ( 2 ) those that
are relatively resistant and tend to survive for long periods. We assume that both the present and predicted
quasi-equilibrial faunas are comprised solely of the second class of species.
A potential p r o b l e m even with this assumption is evidence suggesting that climate was w a r m e r and vegetation zones w e r e shifted to higher elevations in this region during an "altithermal" period about six thousand
years ago (Wells 1983; Grayson 1987; Betancourt et al.
1990). Such a w a r m period might already have caused
the extinction of the most susceptible species on each
mountain range. This would suggest that the e x p e c t e d
n u m b e r of extinctions w o u l d b e less than predicted by
our model. However, two lines of evidence suggest that
the present species would be highly susceptible to any
future habitat restriction resulting from climatic and
vegetative change: ( 1 ) the populations of several species on certain ranges are very small and localized
(Brown's personal observations); and ( 2 ) o n e species,
Eutamias umbrinus, appears to have gone extinct from
the Sheep Range within the last forty years (W. L. Gannon and T.E. Lawlor, personal communication); the
cause of its disappearance is unknown. All of the mountain ranges have probably b e e n affected to s o m e degree
by hunting and gathering by aboriginal people during
the last few thousand years, and by livestock grazing,
timber cutting~ mining, w a t e r diversion, fire suppression, and recreational activities by m o d e r n humans during the last century.
Conservation Biology
V o l u m e 6, No. 3, S e p t e m b e r
1992
414
PredictingExtinctions
Another p r o b l e m is that our m o d e l is completely deterministic. The probabilities of survival of individual
species on particular mountain ranges are not precisely
predictable, as is obvious from the deviations in the
present faunas from perfect nestedness (Table 1). It is at
least theoretically possible to make a m o r e complex,
m o r e realistic m o d e l based on the observed variance in
the present distributions. Factors that could be incorporated in such a model include a probabilistic treatm e n t of extinction, different global w a r m i n g trends
( 2 ° C and 4°C scenarios), precipitation change scenarios ( - 1 0 % ), different scenarios of vegetational migrations, and direct physiological effects of climate change
on mammals (for example, see studies of birds by Root
1988agb). Addition of such complexity might increase
the accuracy of the model, but it is beyond the scope of
this simple heuristic exercise. Given the uncertainties in
the e x p e c t e d magnitude and kind of climatic change,
there is little justification for going to the effort of constructing a m o r e elaborate model at present.
A major advantage of our approach is that we can
develop a quantitative model and make specific predictions based solely on the present geographic distributions of the species. Most c o n t e m p o r a r y approaches to
conservation biology attempt to assess vulnerability to
extinction one species at a time based on detailed considerations of life history, demography, and genetics.
Our approach (see also Patterson 1987; Cutler 1992)
contrasts with this tradition. It uses only presenceabsence data on the present distributions of each species a m o n g habitat fragments to estimate susceptibility
to extinction. It does not require detailed knowledge of
the population biology of each species. Of course, it is
these details of biology that determine the nested subset
structure of the faunas, the probability of extinction of
each species on each mountain, and the deviations from
the regression line in Figure 1. But w e do not have to
k n o w all of these details to make highly plausible predictions. Sometimes it will be necessary or desirable to
use detailed, species-specific models to predict biotic
responses to certain kinds of environmental changes.
Often, however, the kind of "macroscopic," multispecies approach developed in this paper offers a simpler,
less costly, and m o r e practical way to address a variety
of global and regional conservation problems. Such alternative a p p r o a c h e s will be especially valuable for
guiding data collection and making management recommendations w h e n knowledge of the biota is limited and
there are insufficient resources or time for m o r e detailed studies (see also Arita et al. 1990).
T w o general conclusions can be derived from this
m o d e l i n g exercise. First, the magnitude of climatic
change predicted to o c c u r within the next century is
likely to cause substantial reductions in biological diversity within many geographic regions and habitat types.
"Habitat islands" and the species that inhabit them will
ConservationBiology
Volume6, No. 3, September1992
Mcl~nald & Brown
be especially vulnerable. Such habitat islands include
not only naturally insular patches of unique environment, such as m o u n t a i n t o p s , wetlands, and d e s e r t
springs, but also fragmented habitats, such as forests and
prairies, that have b e c o m e isolated as a result of h u m a n
activity.
Second, w e caution that any conservation strategy
that is based solely or entirely on isolated reserves is
v e r y susceptible to any kind of global or regional
change. While others have voiced similar concerns (see
Peters & Darling 1985, Peters & Lovejoy 1992), the
magnitude of extinctions predicted b y our quantitative
model is sobering. Considering that ( 1 ) the Great Basin
mountain ranges are m u c h larger than m a n y present and
planned reserves, ( 2 ) climate and vegetative change will
likely reduce but not eliminate the habitable area on
each range, and ( 3 ) even so, four of nineteen mountain
ranges are predicted to lose at least 50% of their species
and three of fourteen species are predicted to go extinct
throughout the region, what is the prognosis for smaller
reserves of m o r e h o m o g e n e o u s habitat?
Acknowledgments
We thank S. Mistry and P. Nicoletto for technical assistance, H. D. Grover, J. R. Gosz, and M. L. Taper for helpful discussions, and two anonymous reviewers for valuable suggestions. The research was supported in part by
NSF Grant BSR-8807792 to J.H.B.
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