Download Ecology of Vertebrate Animals in Relation to Chaparral Fire in the

Ecology of Vertebrate Animals in Relation to Chaparral Fire in the Sierra Nevada Foothills
Author(s): George E. Lawrence
Source: Ecology, Vol. 47, No. 2 (Mar., 1966), pp. 278-291
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1933775
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ECOLOGY OF VERTEBRATE ANIMALS IN RELATION TO
CHAPARRAL FIRE IN THE SIERRA NEVADA FOOTHILLS
GEORGE E. LAWRENCE
Museum of Vertebrate Zoology, University of California, Berkeley, Californial
Abstract. Chaparral fire brings decided changes in the species composition and density of
both plant and animal populations in the Sierra Nevada foothills.
Some species decrease
whereas others increase following a burn, but no species is totally eliminated, nor is there any
apparent diminution of total life on a burn after plant growth resumes.
These conclusions were reached in the course of a 4-year study of adjoining burned
and unburned areas near Glennville, Kern County, California. Field work began in 1953 at
which time study plots were selected and plant and vertebrate population were censused. A
year later part of the study area was burned, and ensuing investigation compared populations
on the burned and check areas for a period of 3 years, terminating in 1957.
At the time of the fire, temperatures were recorded in sites both above and below ground,
and the actions of animals were observed. There was very little evidence of direct mortality
among any of the vertebrates, most of them escaping the heat in one way or another. The
woodrat was perhaps the most vulnerable species because of its dependence on houses made of
dry twigs. However, in the bare ash after the fire many species were severely exposed to
predation, and populations of most small mammals and some brush-dwelling birds decreased
rapidly. Predatory birds and mammals increased, as did some seed-eating birds that found
good foraging on the exposed earth.
When the rains stimulated new plant growth, a very different habitat developed in the area
of burned chaparral. Most of the original trees sustained little damage, although the pitchy
digger pines were largely eliminated. However, the extensive brush stands were reduced by
almost 90%, and there was a corresponding increase in invading grasses and forbs.
Birds and mammals that normally exhibit a strong preference for chaparral habitat were
substantially reduced in numbers in the years following the burn. Conversely, some of the
birds that normally prefer grassland or oak woodland increased in number. The fire resulted
in an overall increase in densities of nesting birds. None of the small mammals increased in
numbers but some of the larger predators, such as the coyote and badger, moved into the burn
during the months following the fire.
County, California, constitutes the bulk of this
report. A private land owner, Henry Bowen,
initiated experiments with chaparral burning as a
means of improving forage production on an area
of more than 1,200 acres. The present study began
in 1953 with census and study of the resident plant
and animal species. On July 28, 1954, the area
was cleared of the dominant shrub species by the
use of controlled fire. Subsequent field observations to analyze the shifts in the flora and fauna
caused by the fire then followed for 3 years,
through September 1957.
Elevation of the study area ranges from 2,650 ft
to 3,206 ft above sea level. The area is bounded
on two sides by streams, Cedar Creek on the east
and Poso Creek on the west. The geographical
location is 3 miles south of Glennville and 33
miles northeast of Bakersfield, California. Ecologically the plant associations typify the Upper
Sotloran life zone, and the dominant woody forms
include blue oak (Quercus douglasii), digger pine
STUDY AREA
(Pinus sabiniana), interior live oak (Quercus
A series of "before" and "after" field observa- wislizenii), and buckbrush (Ceanothus cuneatus).
The terrain in this area of the Sierra Nevada
tions on an area in the vicinity of Glennville, Kern
1 Present address: Bakersfield College, Bakersfield, foothills slopes shelflike toward the west. Major
stream drainage is generally westerly in direction,
California.
INTRODUCTION
Chaparral covers over 7 per cent of the total
area of California; it is distributed in the arid foothills of the coast ranges and the interior mountains. These lands are becoming increasingly
important in watershed protection, livestock production, and wildlife management. Because chaparral itself gives little nourishment to domestic
animals, and crowds out valuable grasses, it is often
removed by range managers. The method most
frequently employed is controlled brush burning.
For instance, in 1950 ranchers in 30 counties
cleared 97,000 acres of chaparral (Love and Jones
1952; Wieslander and Gleason 1954). Little is
known, however, about the impact of chaparral
fires on the total biota. The present study was
undertaken to determine the effect of controlled
brush burning on the native animal populationsboth by direct mortality and by indirect effects
occasioned by changes in the plant cover.
Early Spring 1966
VERTEBRATES
AND
CHAPARRAL
279
FIRE
) w
CLIMATE
1~~~~~~~~~~~~~
k110J
iJA (,-s
Buckbrush-oak
U
chaparral
Pine-oakwoodland
Grassland
d.. ^s * .-
t5aZ3
I1>
ta
i,
<,s\
Birdcensus areas
9/////fim4'
Mammalcensus lines ........ff
Vegetationtransects-- '
FIRE
Sti
Meter quadrant plots
The study area is characterized by winters which
are short and comparatively mild with occasional
snow, and summers which are long, warm, and
uninterrupted by rainfall. More than 80% of the
annual precipitation falls during the winter and
early spring of the year. During March and
April, rapid plant growth occurs. Optimum conditions of soil moisture, air temperature, and sunshine permit the maximum growth and flowering
for species of both chaparral shrubs and the annual grasses and forbs. By June most of the fruit
and seed production is complete, and vegetation
dies or becomes dormant.
Over the 4-year period, the average rainfall
in the area was 15.64 in., with a range of 20.47 to
12.19 in. (Fig. 2). Snow fell each winter, but
often the fallen snow melted immediately. Less
frequently, a thin blanket of snow remained on the
ground for several days. Mean monthly temperatures and precipitation totals were obtained from
a U. S. weather station at Glennville, California,
which was 3 miles north of the area under investigation, and at a similar elevation.
.
How hot is fire in chaparral? The method used
to answer this question was to install a series of
temperature-sensitive furnace thermocouples in a
1954
FIG. 1. Major plant associations and specific experimental sites, within the 1,200 acre experimental burn area,
Glennville, Kern County, California.
the Kern River
and- adjacent
Cedar
although
Creek flow almost due south at the higher elevations and then veer off toward the west as they
approach the central San Joaquin Valley (Fig. 1).
The rolling foothill country is dissected by numerous small gullies and intermittent
stream courses.
Those slopes which are tilted toward the direct
sun support a xeric
rays of the late afternoon
cover
thin
and
have
soil with a minivegetational
mum of surface humus.
By contrast, the far sides
of these same ridges are in the shadow, and the
of the plant cover, the topsoil, and
composition
the animal populations
all reflect this difference in
The woody tree species occur almost exslope.
on the shaded northeast-facing
clusively
slopes.
Also characteristic
of the foothill area are wide,
level stream terraces
with open, parklike
oakTall valley oaks (Quwercus
grassland vegetation.
dislob-ata) and denser blue oaks are sparsely
tributed throughout
continuous
grassy meadows.
Major vegetation
types are indicated on Figure 1.
1955
~~~~~~8
100.
100
-~~~~~7
97
90
8o.
80.
C~~~~~~~~~~~~~~~~L
4141~~~~3
J
FIG.
A S6NA
AM
AM
MONTH
2.
MA
~8
5
W~~~~~~~~~II
70
4
I
tation indicated
LIr i
and240miim
FD
1957
W9
80
t
170-
TJJAS
MONTH
along
the
broken
I'~~~~~~~~~~
9ntebrgah
1~~~~~~~~~~~~0
10
20
MONTH
I
inr
n
j
,i
4
temnhypeii
2
MONTH
FIG. 2. Climatic data showing the temperaturemaxima
and minima on the bar graphs and the monthly precipitation indicatedalong the broken line for Glennville, California.
280
Ecology, Vol.. 47, No. 2
GEORGE E. LAWRENCE
variety of locations during a chaparral fire. Bimetal units which conduct an increasing amount
of electrical current when exposed to higher temperatures were first calibrated in an electric furnace. Readings were made by noting the current
flow on a microvoltmeter in the field, and these
readings were then converted into 'F. Two distinct types of data are available from this thermocouple technique: fire temperature and duration
of fire heat.
Fire temperatures were taken in five situations
which represented possible animal habitats during
fires. Thermocouples were placed on the soil
surface-in dry surface humus, on a grassy surface, and under a dead oak log. Two subsurface
sets of readings were made, one from a site buried
in the soil at a depth of 2 inches, and another in
an open burrow 6 inches below the soil surface.
The surface temperature readings in the humus
layer of dry twigs and leaves rose quickly to 6700F
after only 12 min of burning and proceeded to cool
after the surface tinder was consumed. Thirty
minutes after the fire began in the area, the surface litter site had cooled to 3600; it continued to
cool gradually during the subsequent 3 hours.
Highest recorded temperatures, with a maximum
reading of 1,0400F, were recorded under the burning oak log (Fig. 3). The temperature was sufficient to produce a white ash residue. This is a
common habitat for such invertebrates as scorpion
(Vejovis sp.), ground beetle (Eleodes sp.), and
millipede (Narceus sp.). Vertebrates often occuF
Thermocouple Positions
1. suribce humus
2. surface grass
a beneath log
4. in soil 2 inches
5. in buryw 6 inches--//
10oo
1000
900/
800
/
/
700
3/
600
/
A
I
500:
VEGETATION
400!
Y
300
200
/-
100
0
20
pying burrow systems beneath logs in this region
include the deer mouse (Peromyscus maniculatus),
the slender salamander (Batrachoseps attenuatus),
and the legless lizard (Anniella pulchra).
Subsurface temperatures measured at a depth
of 2 inches revealed a remarkable heat insulation
quality of the dry soil above the thermocouple.
The temperature was found to rise moderately to
a maximum level of 1560F during the first 50 min.
of the fire. An erratic rise in the readings later
was produced by the falling oak branch (Fig. 3).
A significant feature of this 2-inch depth record is
the fact that this thin soil layer held the temperature at a level 5440F lower than the surface humus
temperature during the first 30 min. Rodent burrows in this region are formed by pocket gophers,
and such burrows are dug to depths ranging from
9 inches on the level to 36 inches on steep slopes
(Scheffer 1931). The readings of the 2-inch
depth thermocouple are higher than the temperatures obtained in the undisturbed compact soil
because of the loosening of the soil structure in
installing the unit. Projecting the insulation effect
of 2 inches of soil which lowered the temperature
544 degrees below the surface heat during the peak
of the chaparral fire might give a 4-inch soil layer
the insulation capacity of more than 1,0000F, and
would provide safe temperature conditions for
organisms which seek underground positions during fire conditions.
The fifth thermocouple was situated in an open
burrow at a depth of 6 inches below the surface
of the ground. Although the majority of thermal
currents carried the heat upward away from the
burrow thermocouple, the temperature increased
to 1620F after 20 min of radiation of heat from
surface flames. Had the unit been installed in a
burrow that was curved to prevent the direct
exposure to the flames, this same depth reading
would probably have been considerably cooler.
This unit measured the air temperature following
the passage of the peak of the fire, and after about
110 min of fire the air temperature in the 6-inclh
burrow had returned to the preburn level.
40
60
80
100
MINUTES
120 140
160
180 200
FIG. 3. Soil temperatures during fire, determined by
thermocouple readings during the Springville, Tulare
County fire, August 3, 1957.
Sampson (1944) has reported on the effects of
controlled burns on vegetation in five major bruslhland areas in California. Recent studies by
Sweeney (1956) on the chaparral areas of Lake
County, and by Horton and Kraebel (1955) on
chamise chaparral development after brush fires
in southern California, have added greatly to the
present level of understanding regarding successional responses to fire.
The aim of this investigation was to trace the
changes in plant cover as they relate to survival
Early Spring 1966
VERTEBRATES AND CHAPARRAL
FIRE
281
of the resident animal population. Vegetational associations. Although the shift in chaparral can
changes were evaluated by strip transects, m2 be clearly seen on the ground records, change in
quadrat studies, and direct photographic records. the cover of woody trees and larger shrubs is alThe preburn dominant chaparral plant cover con- most indistinguishable from the air. The contrast
sisted of overaged buckbrush and gooseberry in flora on opposite slope exposures is well shown
(Ribes quercetorumr) which had achieved a growth in the aerial photographs.
form of dense clumps with their centers composed
Transect analysis of vegetation
largely of dead and dying old wood. New growth
An objective measure of shift in dominant plant
in such clumps was found at an ever-increasing
distance from the original central stalks. Photo- species following controlled fire was obtained from
graphs taken in June 1953 show this accumulation the comparison of 2,000 m2 of individual plant
of dead wood in the Ceanothus chaparral, and coverage along two linear transects. The transect
later photo records taken from the same points lines, marked out with metal markers, extended in
serve to demonstrate the shift from chaparral an east-to-west direction, and included both the
toward an open oak-grassland association. The shaded and sunny slopes. The sampling technique
gentle slopes on the western section of the region consisted of walking along the line of the transect
under investigation are shown in Figure 4, where with a meter stick while recording the dominant
the Poso Creek drainage has been largely cleared plant species found in each m2 of the transect. The
of the dense chaparral, and grasses (Az'ena bar- transects were recorded in June during the time
bata and Brownms tectorumn) now form the domi- when the maximum plant cover was developed,
nant ground cover beneath the blue oaks.
and fruiting structures were sufficiently mature
Aerial photographs also were taken of the study to assure accurate identification of the grasses and
area both before and after the fire. There is a herbaceous forms. The technique was used to
remarkable constancy in the major vegetational compare per cent species composition during a
4-year period. Of course, the plant species as
recorded in June might be reported quite differently in September when such late-growing forms
as vinegar weed (Trichostena lanceolatuni) or
the composite (Hemizonia corymbosa) reach their
peak of growth. Nonetheless, the transect sampling does permit an accurate year-to-year estimate
of the shifts in vegetation.
One year's transect record, taken in June 1954,
forms the single sample of the undisturbed chaparral vegetation before controlled burn. Regrowth
and change in plant cover was then recorded in
the following 3 years of transect data. Plant
species were recorded after pacing 5 m distance
along the 1-m wide transect, noting all species that
covered at least 1 m2.
Plant species with ability to reoccupy exposed
areas are here designated as invader species. The
tree species are those which normally reach heights
above 15 ft, and shrubs reported are those which
grow to heights of less than 12 ft. The surface
area which each plant species occupied during the
4-year study is reported in numbers of m2 along
the total 2,000 m linear transects.
4. Chaparral succession on the east slope of Hill
FIG.
3206, showing the growth of annual and perennial grasses
following the removal of the chaparral cover. The upper
preburn view was taken June 12, 1953, and the postburn
lower view July 2, 1956.
Results
Trees.-Trees were least influenced by the controlled burning of the surface vegetation as indicated by a small decline in coverage from 278 to
251 m2 during the first postburn year; later insect
damage further lowered the tree coverage to 233 in2
at the end of the period of study. The shrublike
young blue oaks and the inflammable, pitchy dig-
282
GEORGE E. LAWRENCE
ger pines were the species most affected. The
deciduous species-blue oak, elderberry, and buckeye, were all found to be heat tolerant to a large
degree. Their leaves were drying and the corkforming abscission cells were active in this latesummer period of the fire. The largest trees,
including valley oaks and digger pines, had grown
to such size that thickness of bark, as well as
elevation of lower branches above the chaparral,
prevented serious scorching. The evergreen interior live oak revealed a remarkable response to
the fire. This tree forms a thick rounded canopy
which is virtually in contact with the ground surface; the fire failed to affect this species because
of the absence of inflammable material near or
under its branches and leathery leaves. The species most severely reduced in numbers during the
controlled burn was the digger pine. That only
about 17% of the original stand of digger pines
survived the fire was indicated by the reduction of
83 m2 of transect surface to only 13 m2 of surface
in the year after the burn. The pitch in the bark,
in the cones, and at the base of the needle bundles
was a unique physiological hazard in the burn.
The cones themselves were a further hazard, as
they frequently fell from the tree during the fire
and carried the fire across the firebreak in the
areas of steeper slopes.
Shrubs.-Chaparral was the most widespread
plant association in the Glennville region before
plant cover modification by man. Of the total
2,000 linear m of transect, 452 m were originally
chaparral. This coverage was reduced to 41 m
by controlled fire. The shrub species which increased following the fire was poison oak (Rhus
diversiloba). The occurrence of this plant in
granitic outcrops protected it from the blaze. Total
coverage of two shrubs valuable as browse for the
deer, mountain mahogany (Cercocarpus betuloides) and redberry (Rhantnus crocea), was
reduced only slightly, as the moist shaded slopes
where they were most prevalent were least affected
by burning.
Grasses.-Two different types of fire responses
were noted in grass species. The perennial purple
needlegrass (Stipa pulchra) was able to survive
the heat beneath the soil surface. The first delicate shoots of green found emerging through the
ash layer proved to be needlegrass leaves. The
fact that the perennial rootstocks were in a viable
state is further indication of the minimal depth
of heat penetration. The ability of the postburn
turf to resist erosion under the impact of late fall
precipitation is enhanced by the presence of living
root systems of such perennial plant species. The
second type of grass response to the fire disturbance was found in oats (Avena barbata) and
Ecology, Vol. 47, No. 2
downy chess grass (Bromus tectorum). These
species reseed more rapidly under conditions of
exposed mineral soils than in the static habitat
situation. The transect coverage of the wild oat
nearly doubled in the 3 years of postburn regrowth, increasing from 231 to 411 m2 of the
area sampled. Significant to resistance of sheet
erosion in this region of the southern Sierra Nevada foothills is the extent of soil coverage by
continuous plant cover. The chaparral species
Ceanothus cuneatus and Ribes quercetorum give
minimal protection to the ground surface beneath
the spread of their branches. The leaf -form in
this xeric habitat is too small to intercept many
rain droplets. In addition, the surface of the soil
beneath these shrub species is essentially bare
except for a thin accumulation of litter from deciduous leaf fall. As a consequence of the increase
in total grass turf coverage following the removal
of dominant chaparral shrubs, the soil surface is
somewhat better covered and more insulated from
impact of rain droplets. The grass species which
grows most actively in the winter and early spring,
ripgut brome (Broamus rigidus), was a turfforming soil binder in the rainy season. During
the 4 years of sampling ripgut brome increased
in total transect coverage from 76 to 164 m2 of
surface area and was a significant part of the postburn plant succession.
Postburn plant invader species.-The most
important early invader species which reseeded
naturally in denuded areas were redstem filaree
(Erodium cicutarium), vinegar weed (Trichostems lanceolktum), popcorn flower (Plagiobothrys nothofulvus), blue phacelia (Phacelia purpusii), miner's lettuce (Montia perfoliata), and
golden brodiaea (Brodiaea ixoides). Denuded
areas, particularly in the areas of hottest fire
where white ash was deposited, were reseeded with
a hardy annual grass, Lolium multiflorum. Some
native species of plants more quickly occupied the
patches of exposed soil than the artificially spread
annual ryegrass.
The high seed-producing potential and the tolerance of variable temperature conditions by seeds
help these species reoccupy patches of exposed
soil early. Seed production by miner's lettuce
on northeast facing slopes not only accelerated
regrowth of the general plant cover in these moist
areas but also contributed to the increased food
supply for flocking bird species such as Mourning
Dove (Zenaidura macroura) and the Western
Meadowlark (Sturnella neglecta). The earlier
work of Biswell, Taber, Hedrick and Schultz
(1952) reports the presence of Montia perfoliata
in crops of both Mourning Doves and California
Quail (Lophortyx californicus) examined from
Early Spring 1966
VERTEBRATES AND CHAPARRAL
postburn chaparral areas. Along the sampling
strip the transect coverage for this seed plant increased from a preburn level of 14 m2 to the postburn coverage of 69 M2. Redstem filaree is another
important seed-producing species which not only
is physiologically adapted to reseeding in postburn
denuded areas, but also forms a rosettelike ecotype which provides maximum ground coverage.
Moreover, the high protein content of the seeds
and stems of this plant, as reported by Hutchinson
and Kotok (1942), contributes to its value as a
forage plant. Filaree rosettes as large as 16 in.
in diameter occurred in white ash areas.
Invader species that have been identified as
toxic to some degree are: milkweed (Asclepias
californica), wild tobacco (Nicotiana bigelovii),
and white larkspur (Delphinium parishii). These
plants constituted a small part of the total surface
coverage and diminished after the first postburn
year of regrowth.
Herbaceous plant species.-Disturbance of the
annual plant cover led to an increase of broadleaved herbs. Thus goosegrass (Galium aparine),
which occupied only 11 m2 of transect coverage
before the burn, was found to dominate 32 m2
of the equivalent area during the first postburn
census year. The golden poppy (Eschscholtzia
californica) tended to spread into the black ash
areas and increased from 11 to 34 m2 of transect
surface. Other broad-leaved forms, such as coyote
mint (Monardella leucocephala), maintained a
nearly constant level during the 4 years of transect investigation. The dense epidermal hairs of
the yellow tarweed (Hemizonia corymbosa) and
of the common mullein ( Verbascum thapsus) make
these herbs largely avoided as forage plants when
they have reached maturity. The mullein is frequently one of the earliest plants to occupy a denuded area of deep white ash, particularly on
shaded northeast-facing slopes.
Annual plants normally fluctuate from year to
year in relative abundance. In view of this variation, precise correlation of vegetational shifts with
the influence of burning may be made only with
caution. During the first postburn year the majority of the herbaceous species consistently increased in m2 coverage. In the following 3 years
they were replaced by grass. Diametrically opposite trends were obtained for the postburn
grassland and the invasive herbaceous plants following the fire (Fig. 5). The grass species declined from 542 to 383 m2 of transect area in the
first postburn census and then recovered by reseeding to become the most widespread of the plant
associations in the area, occupying a total of
856 m2 of the transect terrain. The invader plants
were able to make a quick coverage of much of the
283
FIRE
I
IL
Grassland
Invader plants--Denuded soil ................
Chaparral
Trees ..............
Herbs
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284
Ecology, Vol. 47, No. 2
GEORGE E. LAWRENCE
the same area. The blue oaks, which are on more
level ground, reveal a more columnar tree form
after burning of the chaparral understory, since
the fire tended to kill the tips of the branches which
were farthest from the resistant central trunk parts
of the oaks. The widely studied Tillamook burn
in Oregon produced the same columnar effect on
the postburn woody vegetation, and created a tree
population consisting of older-age groupings (Neiland 1958).
THE
MAMMAL
POPULATIONS
Each morning the traps are checked, and collected
animals are weighed, measured, ear-marked, and
released.
A total of four such Calhoun lines was set each
3-month period. Trapping results were reported
in trap-night units. The location of the four
trap lines is indicated in Figure 1. The lines
were originally laid out to sample both the chaparral area to be burned and terrain beyond the
proposed fire. Sixty traps set per line for three
consecutive nights totals 180 trap-nights per line;
and four lines set gives 720 trap-night per 3-month
period. Preburn censusing of the small mammal
population was posisble for 1 year, and the postburn census continued for 3 years. The total
sampling between September 1953 and June 1957
included 11,520 trap-nights.
Live-trapping census methods
The impact of the chaparral fire on the populations of the resident birds and mammals was investigated. Although field records were made of
the local amphibian and reptile species, no organized attempt was made to census these groups.
Habitat preference of resident mammals
The larger mammals observed in the course of the
Preference of the resident mammal species for
fieldwork were not restricted to the specific study
specific microhabitats was apparent from the traparea, but moved in and out of the region in their
ping data (Fig. 6). Within an area as small as a
foraging activity. Consequently, black-tailed deer
50-ft circle, it was found that several rodent species
(Odocoileus hemionus columbianus), coyote
(Canis latrans), bobcat (Lynx rufux), badger
(Taxvidea taxus), raccoon (Procyon lotor), and
MAMMALHABITATSELECTION
170
the California jackrabbit (Lepus californicus) are
Peromyscus truei
Peromyscus maniculatus
not considered here. Research in the brushland
Peromyscus californicus
areas of northern California by Biswell et al.
1
Neotoma fuscipes
(1952) has carefully documented the changes in
Ei Perognathus
160
californicus
density of deer on postburn chaparral lands. SubMicrotus californicus
agilis
Dipodomys
stantial sampling of the fawn/doe ratio indicated
111DReithrodontomys megalotis
increased ovulation and fawn production on those
Thomomys bottae
areas opened up by controlled brush fires. The
MM Sylv i lagus bachmani
50present report contributes information on the reMus musculus
sponse of nongame mammal populations.
Grid markers were established in 1953, and
these served as permanent points of reference for
40the movements of mammals and wide-ranging
0)
bird species during the 4-year study. Seventy30two metal tags were attached to trees and rock
a
outcroppings to delineate the grid. All predator
activity and animal movements were noted in the
field records in relation to the grid markers.
2-o
Relative density of small mammals was measured by line-trapping. This method allows the
comparison of relative numbers of preburn and
20
postburn animals, but does not give an absolute
measure of density. The Calhoun method of small
mammal trapping has been widely adopted, and
the results of many different investigations are
published annually in the North American census
of small mammals (Calhoun 1950). The type B
Ceanothus Ribes BuckeyeOak Grnite Grass Ash
line was adopted. This consists of a total of 60
live-traps set in groups of three at each site; trapFIG. 6. Frequency of occurrence of the live-trapped
sites are spaced along the line at 50-ft intervals, mammals within specific habitat types in the Glennville
and traps are set for three consecutive nights. study area during the 4-year study period.
-
Early Spring 1966
VERTEBRATES
AND
might be found living in ecologically distinct segments of the habitat. The differentiation of grassdominated, chaparral-covered, and granitic rocky
terrain as mammal habitats has been noted in the
central California foothills region by Fitch ( 1954)
and Quast (1954).
The most frequently trapped species was the
chaparral mouse (Peromyscus truei, with a total
of 289 of the total 405 live-trapping records. The
dominant shrub species, Ceanothus cuneatits, was
the preferred habitat of the chaparral mouse; 174
of the 289 individuals were caught in live-traps set
under the overhanging branches of Ceanothus
shrubs (Fig. 6). At the same time, the chaparral
mouse appeared to be the most eurytopic of the
trapped mammals for it was found in all six of the
habitat types sampled.
Other small mammals found in the wedgeleaf
ceanothus chaparral were house mouse (Mus musculus), deer mouse (Peromyscus maniculatus),
California mouse (Peromyscus californicus), and
pocket mouse (Perognathus californicus).
Woodrats were most frequently trapped in the
fringes of gooseberry patches, because they often
built their mounded house of twigs in the center
of a thorny stand of Ribes. Gooseberry thickets,
which frequently encircle rocky outcrops or dead
tree trunks, form an almost impregnable barrier
to the approach of large predators. The advantage of the nest location in resisting predation is
offset, however, by the vulnerability of the nest to
fire.
Under the canopy of such trees as buckeye and
blue oak, the mammal species most frequently
trapped were the brush rabbit (Sylvilagus bachmani) and the large California mouse. Chaparral
mice, pocket mice, field mice and deer mice were
all live-trapped in the ash-covered postburn region
(Fig. 6). The field mouse (Microtus californicus) was trapped most frequently under the shade
of blue oaks where the ripgut brome grass was
more than 16 in. in height, providing coverage
for the surface runways. However, Microtus is
scarce in this xeric habitat; it prefers denser grass
and continual summer growth of the grass shoots
upon which it feeds (Pearson 1959).
The most unaffected mammal habitats were outcrops of fractured granite. Cracks and crannies
in the rock are utilized by the deer mouse and the
pocket mouse.
Several workers have successfully conducted
live-trapping on burned-over forest land, showing
that many individual animals survive the burning
(Horn 1938; Tevis 1956; Gashwiler 1959; and
Pruitt 1953). Much less is known about the animals' responses to the postburn conditions of
habitat in terms of the altered cover, food, and
CHAPARRAL
285
FIRE
nesting materials which must be available to sustain the animals. Williams (1955) carried out
trapping studies on a burn in Colorado during the
summer months 13 years following the fire, and
reported that the food supply was substantial, and
that the cover provided by the shrub and herbaceous growth sustained even greater animal populations on the burned-over lands than were present
elsewhere.
Preburn and postburn mammal populations
The actual numbers of live-trapped animals
during a 5-year period before and after the controlled fire are summarized in Figure 7. Two
30.
Grassland Area
Experim-ental
Control
t10A
burn~t
V
u,30
1
Chaparral Area
ExperimentalI
E
10
-
\-
-
SEP DEC MAR. JUN SEP DEM MAR JUN SEP DEC MAR. JLM SEP DM MAR. JUL.
1956
1955
1954
1953
FIG.
7. Quarterly totals of live-trapped mammals on
the four Calhoun lines, indicating the preburn and postburn mammal populations.
controlled trap lines were located on undisturbed
areas adjacent to the experimental burned-over
grassland and chaparral lines. Although the control lines were not exact duplicates of the experimental trap lines, they were reasonably equivalent
in respect to slope, elevation, and vegetative cover.
Animal numbers were reduced immediately after
the fire. The 3-day September 1954 trapping
period which followed the controlled brush fire
by 32 days produced no trapped animals on either
experimental area. At the same time the adjacent
control trap lines reflected no emigration of earclipped small mammals from the burn area. Aboveground movements of the resident mammals on
the experimental postburn region were limited
for many weeks following the fire, owing to the
exposure of surface runways and burrow openings. Evidence that the resident small mammals
did survive the effects of the fire is based on the
live-trapping findings of December 1954. The
presence of small mammals previously ear-marked
in the burned area 4 months after the fire indi-
286
Ecology, Vol. 47, No. 2
GEORGE E. LAWRENCE
cates survival of a resident population. These
findings are confirmed by results after a small wildfire on the chaparral control area. Ear-marked,
small mammals were recovered in the black ash
area 3 days after this fire was extinguished
September 26, 1956. The insulating effect of
granite boulder outcroppings and blue oaks was
sufficient to protect such dense shrubs as yellow
gooseberry, Ribes quercetorum. Surface trapping
of small mammals was most successful in such
insulated habitats, following the disturbance of
fire.
Movements of resident mammals
Of the 289 chaparral mice (Peromyscus truei)
caught in the course of this study, 26% of the
records were those of retrapped animals. Eleven
per cent or 32 animals were dead when the traps
were checked in the early morning. Home range
activity was within a 50-ft radius, or within the
interval of one Calhoun trap-line site. One earmarked individual was taken in a trap five stations
removed (250 ft) from the original trap site.
Although most recatches occurred within 3 months
of the time of original marking, one individual was
recaptured as late as 9 months after marking.
The small mammal species which demonstrated
the greatest tendency to be repeatedly trapped was
the large Peromyscus californicus; 38% of the total
trap occurrences were retrapped individuals. This
trap hardiness was also more pronounced in the
woodrat (Neotoma fuscipes) than in the smaller
species of mammals caught.
Shift in weight of the Peromyscus
truei population
Since this small mammal species was the dominant form taken in the traps, it was studied in
somewhat more detail than the other species. The
change in vegetation resulting from controlled
burning exposed the normal foraging area of Peromyscus true, and a reduction in mean weight in
the population followed (Fig. 8).
Exposure of the small animal population to the
attack of predators is an immediate result of the
removal of vegetative cover by fire. This exposure
also limits the success of foraging movements of
the small mammals, both in the distance the rodents are able to move above ground and in the
availability of food. Such food supplies as seeds
and bark of the dominant Ceanothus cuneatus
shrub were more than 90% eliminated by the controlled burning.
A higher percentage of young occurred in the
experimental population immediately after the burn
owing to the reduction in the numbers of the resident adults, which served to stimulate the repro-
RATIO
2.0
EXPERIMENTALANIMALS?---?
CONTROL
1.8
ANIMALS
*
0\
1.4
1.21.0
burn
D. M. J. S D. M. J. S. D M. J. S. D. M. J
1956
1955
1957
1954
body weight
body condition ratio
body length
FIG. 8. The ratio of the body condition or relative fatness of the resident populations of Peromyscus truei on
both the experimental and control areas.
ductive rate. This shift in the age ratio of the
population was a temporary factor in the total
weight loss of the experimental animals.
The impact of the lack of food was investigated
by evaluating the body condition, which distinguishes the normal young animals from adult animals of similar weight which are undernourished.
This body condition ratio or fatness ratio is obtained by dividing body length in cm by body
weight in g. Martin Murie (1963) found that
the accumulation of scapular deposits of fat varies
seasonally with the changes in the metabolism of
three different species of Peromyscus. Consequently the comparative fatness ratios of the experimental group of chaparral mice and the control group were computed for each quarterly
trapping period. Although the preburn ratios
were nearly comparable, the ratios for the March
1955 period after the burn revealed a decline of
the experimental group to 1.28, whereas the control group was 1.46 (Fig. 8). During the second
and third year after the postburn recovery period,
ratios of the two populations approached the preburn level.
Weight loss of the postburn experimental population is apparently caused by reduction of body
fat rather than by lowering of the age ratio in the
population.
Exposed and protected habitat sites
During a fire an individual animal may encounter habitat conditions which range from the highest lethal temperatures to moderate temperatures
in nearby protected sites. Some of these contrasting situations within chaparral during fire are
Early
Spring
1966
VERTEBRATES
AND
I. Comparison of the habitat areas which provided
maximum survival conditions, with the areas of minimal
survival conditions
TABLE
Exposedhabitatsites
1. Southwest-facing
slopeswith
dry vegetation,low soil moisture,
and exposureto winds
Protectedhabitatsites
1. Northeast-facing
slopes
whichareshaded,moist,
and protectedfromprevailing
winds
2. Uppersurfacesof graniteoutcroppingssurrounded
by chaparral
2. Deep crevicesin graniteoutcroppingssurrounded
by
little inflammable
material
3. Withinhollowlogs on the ground
or standing,with muchexposed
woodunprotectedby livingbark
3. Underlogs or tree trunks
insulatedby livingbark
4. Shallowburrowsless than5
inchesin depthand lackingcross
ventilation
4. Burrowsystemswellbelow
the 5-inchdepthwith several
surfaceopenings
5. Lowerbranchesof the woodytrees
whichare abovedensedry grass
or dry chaparral
5. Lowerbranchesof woodytrees
whichtouchthe groundand
have no inflammable
material
below
6. Underground
nestingareaswhich
are poorlyinsulated
6. Underground
nestingsites
insulatedby dry mineralsoil
enumerated in Table I. Cleavage of the rock in
this foothill region forms deep crevices which
serve as refuges for the resident animals. Rodents,
lizards, and even bats of the genus Pipistrella have
been found to occupy such crevices. In investigations on the San Joaquin Experimental Range by
Howard, Fenner, and Childs (1959), caged animals placed in rocky crevices and among large
living roots during chaparral fire were found to
survive.
Survival of mammals under high temperatures
Fire does not produce a uniform heat throughout the chaparral habitat. Entire islands of vegetation are left unburned during the course of controlled burning, as shown in Table I, as well as in
the Madera County report of Howard et al.
(1959). Within such unburned islands the survival of resident animals is determined by insulation from direct heat and by the availability of
fresh air. The actual heat which results from
burning of grass and humus reaches moderate
temperatures of 5500F and less. The residue from
moderate fire heat is characteristically a black ash.
Dry chaparral and woody vegetation burns at
temperatures above 9000 F as measured by thermocouples (Fig. 3). Such high fire temperatures
leave a deep residue of white ash, and the former
dimensions of fallen logs can be detected by the
nature of the ash deposit following the fire.
Temperature measurements below the ground
surface indicate that animals probably experience
CHAPARRAL
FIRE
287
no ill effects of the fire heat if they occupy a burrow system of at least 3 inches in depth. The
theoretical insulation of the dry soil was investigated more realistically by testing live animals
under simulated fire conditions. Much of the
literature implies that temperature alone is responsible for the death of animals under fire conditions. Actually, a more critical expression of
the suitability of the air is vapor pressure, as
emphasized by Thornthwaite (1940). The evaporation of water vapor from a unit area of lung
surface into the air is a function of the difference
in water vapor pressure of the lung surface and
the atmospheric air. Consequently, subsurface
temperature and relative humidity were both measured in experiments involving live mice so that
vapor pressure could be determined.
Since vapor pressure is proportional to air temperature, it is evident that fire temperatures raise
the vapor pressure of water in a burrow system.
To lower body temperature, mammals evaporate
water vapor from the surface of the lungs and
accomplish evaporative cooling and maintenance
of a constant temperature in a heated environment.
An experiment carried out with chaparral mice
in Contra Costa County demonstrated the necessity for air movement in the underground burrow
system. All five experimental animals were found
suffocated following a fire built on the surface of
the soil above the burrow with only one opening.
A subsequent experiment on June 14, 1959 was
designed to allow underground air movement. A
burrow was made with a soil sample borer which
extended to a depth of 12 inches along a 10-foot
passage. Five meadow mice were placed in nonconductive fiberboardand screen cages and located
in the burrow at depths of 6, 9, and 12 inches.
The relative humidity reached 82% in the burrow
at the 12-inch depth, 75%oat the 9-inch depth, and
nearly 60%oat the 6-inch burrow depth. At the
surface of the soil the relative humidity was 48%o
and the temperature was 81'F. All five animals
survived the fire which warmed the soil to 1390F
at a depth of 6 inches. In the deepest point in the
burrow the temperature was raised only 2 degrees
above the normal soil temperature.
The lethal limits of vapor pressure, at which
point small mammals are no longer able to regulate temperature and release body heat to the
atmosphere, are not adequately published in the
literature. At the lower limits of 22% relative
humidity, the lethal temperatures range from 138
to 1450F. The vapor pressures in these conditions
are from approximately 30 to 38 mm Hg. The
lethal vapor pressure is reached at much higher
temperatures under conditions of low relative
humidity, and at lower temperatures when the
288
Ecology, Vol. 47, No. 2
GEORGE E. LAWRENCE
160I 140/
102
E 1201
A.,g.
X 34
233
S
Grassland Mammals.....::=
,W100
4
80.
.,.*Rocky- outcrop
0
60.
I~ 0
40
20
Mammals
\
We
~~~~~~~Chaparral
Ma2mm~als
Li)
0
N
E5
12
60O
040
1600
14cy 800
1600
Temperature 0F
FIG. 9 The relationship of vapor pressure to temperature in air of 82%, 50%, and 22% relative humidity. The
higher tolerance of small mammals to air conditions of
low relative humidity in high temperature is indicated.
C
E 5)
- ?.
5L
.
2
3
{,
X. eloOak
*;; - woodland
c-
Mammals
!3<O
Predator Mammals
relative humidity is high. Although animals sur1955
1956
1954 *
1957
vived temperatures above 1420F at 22% relative
BURN
humidity (Howard et al. 1959), the lethal temFIG. 10.
Shifts in relative abundance of the resident
perature tolerance of the chaparral mice drops to mammal populations before and after the influence of. the
120'F when the relative humidity is above 60% July 28, 1954 Glennville area fire. The ordinate represents
(Fig. 9). The small amount of rise in vapor animals live-trapped per 240 trap-nights in each category.
pressure at the lower 22% humidity level is much
less significant to the difference between the vapor soon filled by the grassland mammal types. Inpressure of the air and the animal's lung surface creased numbers of predators were noted during
than the rise which occurs at the higher 82% the second and third year of postburn succession
relative humidity. Lethal limits in the hyperthermy (Fig. 10). My results agree with the findings of
test were found to be at temperatures of 121'F Cook (1955) who reported a species shift from
and relative humidity of just over 50%. The brush-dwelling species to grass-dwelling species
resulting vapor pressure of 44 mm Hg was above following fire in Tilden Park, Alameda County,
the range of physiological tolerance of the tested California.
animals.
BIRD POPULATIONS
The results of these experiments suggest that
Grid
mapping
of bird activity areas
animals survive as long as the burrow conditions
Data on breeding bird populations consist of
allow vapor pressures below 40 mm Hg. The
survival observed here occurred at high tempera- the recorded activity of the resident bird species
on burned and unburned 20-acre plots of chaparral
tures with low relative humidity.
and grassland, respectively. The route of the
General effects on mammals
observer through the plot included two U-turns
The significant change in composition of the so that each point in the area came within 33
postburn mammal population was a shift from yards of the view or hearing of the observer.
Duplicated overlay pages were prepared in adchaparral-adaptedforms to grassland forms. The
secondary effect of reduced cover had a greater vance of field work, and activities of each bird
impact on the resident small mammals than did species were recorded on a separate map. Although
killing by fire. Chaparral rodents such as Pero- both sexes of birds were recorded, the census was
myscus truei and P. californicus were the species based primarily on observations of males. The
least able to maintain their preburn numbers in the hours of censusing were early in the day from
altered environment. Such grassland forms as 7:30 am to 11:00 am. Nest-building and terriPerognathus californicus, Peromyscus maniculatus, torial behavior were apparent from late March
and Reithrodontomys megalotis were able to oc- through June. Each gridded 20-acre plot was
cupy the newly established grassland areas of the traversed five or six times during this period, and
postburn habitat. Expanded grassland areas were the composite of all activity movements was then
Early Spring 1966
VERTEBRATES AND CHAPARRAL
compiled for the year. Where the activity records
for a given species revealed that a pair of birds
was repeatedly present in an individual clump of
trees, the area was circled with an approximate
boundary and recorded as a resident pair.
Brewer Blackbirds, Goldfinches, and House
Finches, all of which are nonterritorial birds are
reported with less accuracy than those species
which establish distinct breeding territories. Wideranging large forms such as hawks and ravens
were reported as resident pairs if their foraging
areas were found to occur repeatedly within the
grid plot.
The populations of birds on the control plots
(unburned) were considerably lower than on the
equivalent burned areas. During the 4 -year
period the burned chaparral plot sustained a density of more than 340 pairs of resident birds per
100 acres. The chaparral control- plot averaged
only 254 pairs per 100 acres during the same
period. A similar difference occurred in the grassland type with an average of 389 pairs of birds per
100 acres on the experimental plot and 283 pairs
on the equivalent control area of grassland.
289
FIRE
II. Occurrence of pairs of breeding birds within
the 20-acre experimental plot, recorded during the
spring months both before and after the controlled
chaparral fire
TABLE
Number of pairs in
Category
GRASSLAND BIRDS
Mourning Dove
Western Kingbird
Western Meadowlark
Willow Goldfinch
Lark Sparrow
Total
CHAPARRAL BIRDS
California Valley Quail
Bewick's Wren
California Thrasher
California Scrub Jay
Brown Towhee
Spotted Towhee
Total
OAK WOODLAND BIRDS
Acorn Woodpecker
Nuttal's Woodpecker
Red-shafted Flicker
Ash-throated Flycatcher
Western Flycatcher
Violet-Green Swallow
Plain Titmouse
Common Bush-tit
White-breasted Nuthatch
Brewer's Blackbird
Western Bluebird
Hutton's Vireo
Pileolated Warbler
Bullock's Oriole
Black-headed Grosbeak
House Finch
Western Chipping Sparrow
1954
1955
1956
1957
4
1
0
2
0
7
2
4
0
4
8
2
4.6
2
3
10
2
5
1
2
7
17
19.6
20
7.8
2.3
1
4
9
0
4.5
1
0
4
2
4
3.9
2
1
4
1.6
3
4
2
0
3.5
1
1
24.1
15.5
15.5
11.5
4
0
2
4.2
0
4
7
0
1
2
1.7
0
1
3
1
3
1.6
4
1
2
3.6
2
4
6
0
2
1
1
0
1
5
1
1.5
0
4
0
1
5
1
3
6
0
1
2
2.4
0
0
4
1
1
0
2.7
1
1
5.5
3
2
4.7
1
1
2
3.7
1
0
4
0
3
0
Shifts in the resident bird numbers
The postburn bird population was higher on
both chaparral and grassland experimental plots
as early as the spring of 1955, the year following
the burn. The chaparral plot density increased
from 347 to 364 pairs per 100 acres during the
first spring census following the chaparral fire.
Numbers on the grassland plot increased even
more, from a preburn density of 386 to a postburn
count of 441 pairs per 100 acres. The previously
reported climatic conditions were sufficiently uni35.6
35.5
35.1 32.4
Total
form during this period to discount weather as the
factor responsible for such a population change. PREDATOR BIRDS
0
1
0
1
Cooper Hawk
Increased availability of food seems the more logi0
0
1
1
Sharp-shinned Hawk
2
1
1
0.3
Red-tailed Hawk
cal explanation.
1
1
1
0
Desert Sparrow Hawk
Fire in chaparral concentrates the seeds of the
1
0
1
3
Common Raven
shrub and grass vegetation on the soil surface.
1
0
1
1
Great Horned Owl
Such important avian seed foods as miner's lettuce
4
51.3
9
Total
(Montia perfoliate) are readily dispersed from
capsules only 8 inches above the ground. Both
Mourning Doves and Valley Quail have been ob- (Howard 1950). The exposure which caused
served to feed on miner's lettuce following chap- resident rodents to be vulnerable to predation by
arral fire. Seed availability on burned ground hawks and owls similarly caused surface insects
has been well demonstrated by Sweeney (1956),
to be vulnerable to the feeding activity of Meadowand the resulting concentration of food appears larks, Valley Quail, and Brown Towhees.
directly correlated with the postburn numbers of
Invasion and evacuation of bird groups
birds on the two experimental plots.
The impact of the controlled fire on the populaInsect numbers on the soil surface are not only
high following chaparral fires, but insects are also tion of resident birds caused little change in the
more exposed to bird predation. Harvester ants total population numbers. In the grassland the
have been reported in large numbers after a bird density per 100 acres after its initial increase
brushland burning in Madera County, California then declined to 356 pairs at the end of the four-
290
Ecology, Vol. 47, No. 2
GEORGE E. LAWRENCE
year period. Similarly in the chaparral, birds
leveled off at 343 pairs per 100 acres. Although
total numbers did not change materially, there was
a shift in species composition which is shown in
Table II.
Chaparral birds.-Analysis of the chaparraladapted species from year to year shows that the
original 26.6 pairs of nesting birds were largely
displaced by the fire succession, and numbers
dropped to 11.7 pairs per 20 acres during the postburn spring. After 3 years of postburn census
these chaparral birds were reduced to 10 pairs
per 20 acres. Brown Towhees, California Thrashers, California Quail, California Jays, and Bewick
Wrens represented the most typical chaparral
adapted forms. Field observations during the several chaparral fires indicated that the shrub association birds were not fire-killed, but rather survived the flames to relocate in an appropriate
adjacent habitat. The critical limiting factor in
the habitat appeared to be the removal of the shrub
cover which was required for these particular
birds.
Grassland birds.-The original seven pairs of
resident grassland bird species increased to 17
pairs in the first postburn year, then to 19.6 pairs
and 20 pairs in the subsequent 2 years. The
Western Meadowlark, Mourning Dove, Kingbird,
Willow Goldfinch and the Lark Sparrow constituted the principal species in the grassland association of birds. The influx of grassland species is
more marked when the temporary feeding groups
of grassland species are included. Only the resident spring breeding numbers are reported in the
bird population figures. The movement of the
grassland-adapted bird species into the postburn
area may be termed an invasive movement, but in
the light of the permanent nature of the population, these birds might better be described as expanded resident population.
Oak-woodland birds.-The majority of the species of birds censused in this region were residents
of the upper levels of the several species of oak
trees. Eighteen of the reported species are included in this habitat association. The Plain Titmouse, Western Bluebird, California Acorn
Woodpecker, Western Flycatcher, Bullock Oriole,
and House Finch were the most abundant forms
representative of the oak-woodland area. This
population remained essentially stable during the
chaparral succession over the 4-year period. In
1954, 33.5 pairs per 20 acres constituted the population. Following the fire, the oak-woodland birds
were counted at 35.1 pairs in 1955, 32.4 pairs in
1956, and then to the approximate preburn level of
35.6 pairs in 1957 within the limits of the 20-acre
grid plot No. 1. Survival of the stands of oak
trees is indicated by the stability of numbers of
resident pairs of birds which used the oaks for
foraging and nesting sites over the 4-year period.
Increased predation following fire
One of the immediate effects of the chaparral
fire was to attract a large number of predators to
the area. Chaparral removal increased the availability of prey to the hawks and owls. Those
predators which increased in number following the
controlled burning of the vegetative cover in the
Glennville area were the Red-tailed Hawk, Cooper
Hawk, Sharp-shinned Hawk, Sparrow Hawk,
Great Horned Owl, and Raven. The spring following the fire, nine pairs of predatory birds were
present in the 20-acre grid plot for foraging purposes. The peak population then declined to five
pairs in 1956 and four pairs in 1957. Postburn
predation probably was a more restrictive factor
on the small animal and bird population than direct
killing by the fire itself. The lack of cover produces a more exposed environment during this
immediate postburn period, and the surviving
prey species are forced to forage in undesirable
places lacking safe roosting sites, nesting sites,
and escape cover. The degree of habitat recovery
during the first spring following the chaparral fire
was sufficient to support an accelerated reproductive rate in some prey species including mice of the
genus Peromyscus and such grassland birds as
Meadowlarks and Mourning Doves.
ACKNOWLEDGMENTS
I should like to express my thanks to A. Starker Leopold for guidance of this study and for his help in the
organization and revision of the manuscript; to 0. P.
Pearson for his aid in the measurements of environmental
factors in fire; to H. H. Biswell for his assistance in
vegetation analysis; and to A. M. Schultz and R. D. Taber
for their kind assistance in the field measurement of animal and plant populations.
LITERATURE
CITED
Biswell, H. H., R. D. Taber, D. W. Hedrick, and A. M.
Schultz. 1952. Management of chamise brushlands
for game in the north coast region of California.
Calif. Fish and Game 38: 453-484.
Calhoun, J. B. 1951. North American census of small
mammals, release No. 4. Roscoe B. Jackson Mem.
Lab. Rept. 4: 1-136.
Cook, S. F., Jr. 1959. The effects of fire on a population of small rodents. Ecology 40: 102-108.
Fitch, H. S. 1954. Seasonal acceptance of bait by
small animals. J. Mammal. 35: 39-49.
Gashwiler, J. S. 1959. Small mammal study in westcentral Oregon. J. Mamm. 40: 128-138.
Horn, E. E. 1938. Some wildlife forest relationships.
Third No. Amer. Wildlife Conf. Trans., 376-380.
Early Spring 1966
COMMUNITY
EVOLUTION
AND THE ORIGIN OF MAMMALS
291
Horton, J. S., and C. J. Kraebel. 1955. Development
of vegetation after fire. Ecology 36: 244-260.
Howard, W. E., R. L. Fenner, and H. E. Childs, Jr.
1959. Wildlife survival on brush burns. J. Range
Mgmt. 12: 230-234.
Hutchinson, C. B., and E. I. Kotok. 1942. The San
Joaquin experimental range. Agric. Exp. St. Bull.
663: 3-145.
Love, R. M., and B. J. Jones. 1952. Improving California brush ranges. Calif. Agric. Exp. Sta. Circ.
Quast, J. C. 1954. Rodent habitat preferences on foothill pastures in California. J. Mamm. 35: 515-521.
Sampson, A. W. 1944. Plant succession on burned
chaparral lands. Univ. Calif. Agric. Exper. Sta. Bull.
No. 685: 1-144.
Scheffer, T. H. 1931. Habits and economic status of
pocket gopher. U.S.D.A. Tech. Bull. No. 224: 1-9.
Sweeney, J. R. 1956. Responses of vegetation to fire;
a study of the herbaceous vegetation following chaparral fires. Univ. Calif. Publ. Bot. 28: 143-250.
371: 4-38.
Tevis, L., Jr. 1956. Effect of slash burn on forest mice.
Murie, M. 1963. Homing and orientation of deermice.
J. Wildl. Mgmt. 20: 405-409.
J. Mamm. 44: 338-349.
Thornthwaite, C. W. 1940. Atmospheric moisture in
Neiland, B. J. 1958. Forest and adjacent burn in the
relation to ecological problems. Ecology 21: 17-28.
Tillamook burn area of northeastern Oregon. EcolWieslander, E. E., and Clark H. Gleason. 1954. Major
ogy 39: 660-671.
brushland areas of the coast ranges and Sierra CasPearson, 0. P. 1959. A traffic survey of Microtuscade foothills in California. Misc. paper No. 15.
Reithrodontomys runways. J. Mammal. 40: 169-180.
Southeast Forest and Range Exp. Sta., Berkeley,
Pruitt, W. O., Jr. 1953. An analysis of some physical
California.
factors affecting the local distribution of the shorttail
shrew Blarina brevicauda in the northern part of the Williams, 0. 1955. Distribution of mice and shrews
in a Colorado montane forest. J. Mammal. 36: 221lower peninsula of Michigan. Univ. Mich. Misc. Publ.
231.
Zool. No. 79, 39 p.
COMMUNITY EVOLUTION
AND THE ORIGIN OF MAMMALS'
EVERETTC. OLSON
Department of Geophysical Sciences, University of Chicago, Chicago, Illinois
Abstract. The evolutionary course from primitive pelycosaurianreptiles through therapsids
to mammals can be profitably studied in relationship to modifications of the structure of the
communities in which these reptiles existed. For this purpose the community is defined in
very broad terms. Three types of communities are recognized upon the basis of the nature
of the food chain. Each has an importanttetrapod component.
Early phases of the evolution that culminated in mammals took place in communities that
were strongly tied to water by the structure of the food chain. The physiological bases of
the developmentof mammals appear to have been related to this environmentalrestriction. In
successive pulses, however, the pelycosaur-therapsidcommunitiesdeveloped terrestrial reptilian
herbivores and thereby broke with the water-based food chain. More strictly terrestrial communities developed concurrently, with the insects, which were a food source for the reptiles,
as the principal herbivores. From this sort of community came the terrestrial lepidosaurianarchosaurianreptilian radiation.
The terrestrial communities so developed came into competition. In this competition the
therapsid lines were temporarilyunsuccessful,leaving only small, but very mammal-like, representatives in the late Triassic. After a long period with relatively little adaptive radiation,
these remnantsprovided the basis for the radiationsof mammals that led to the great successes
of the Cenozoic era.
INTRODUCTORY
EXPLANATIONS
Studies of vertebrate evolution centered around
the concept of faunal modifications have constituted one of the major fields of interest of the
writer over the last decade and a half. A number
of publications which have resulted from this interest, as cited specifically in the following text,
have stressed the changes of communities with
the passage of geological time. The present paper
represents a continuation and extension of this
kind of work. Most of the data upon which it is
based have been included in the earlier studies,
1
The research leading to this paper was supported by
NSF grants 19093 and 2543.
but the synthesis is somewhat more general than
any attempted previously, and the interpretations
are more directly ecological.
Studies, such as this one, which involve broad
areas of paleoecology are necessarily cast at rather
different levels from those of most neoecological
investigations. Naturally, as well, a strong element of speculation must enter in, for assumptions
of a rather sweeping nature are necessary and
conclusions often must be based on complexly
interwoven threads of evidence. Yet the resulting
insights into the relationships of ecology and evolutionary processes are such that, even though
crude, the interpretations are stimulating in them-