EFFECTS OF BROWSING ON
GROWTH AND REPRODUCTIVE
OUTPUTOFENGORDACABRA
IN NORTH-CENTRAL MEXICO
A. Romero-Manzanares
E. Garcia-Moya
ABSTRACT
Extensive grazing of rangelands in central and northern
Mexico began after the Spanish conquest with the introduction of livestock and the establishment oflarge land
holdings. The Agrarian Reform, which arose in 1910-17,
brought about increased range deterioration, mainly due
to lack of experience in range management (HernandezXolocotzi and Ramos-Sanchez 1987; Trevino-Fernandez
1980; Jameson and others 1984; LaBaume and Dahl 1986).
At the present time, the rangelands are overgrazed to the
point that their capacity to produce goods and services
needed by an increasing population is at stake. Few species are as important as a forage resource under these conditions as the shrubby legume engordacabra (Dalea bicolor
Humb. & Bonpl. ex Willd.).
Engordacabra ("goat-fattener") was selected for study
because of its wide geographic distribution (Barneby 1977)
(fig. 1). Herbarium specimens show that the species is
present in 21 Mexican states. It is found in grasslands,
microphyll shrublands, mesquite shrublands, pinyon pine
woodlands, oak woodlands, submontane shrublands, pine
forests, and in low deciduous rain forests. It also occurs in
areas highly modified by human activities, such as fencerows, abandoned farmlands, and forest sites disturbed by
logging and grazing (Barneby 1977; Romero-Manzanares
1982).
Engordacabra is very palatable for ungulates, which
roam freely on rangelands. Mean consumption rate per
plant varies from 17 to 38 percent on ranges in good to
poor condition. Winter and spring represent the periods
of maximum foraging (50 to 75 percent use), while summer
and autumn foraging rates are reduced (0-25 percent use)
due to availability of other forage during these periods
(Romero-Manzanares 1987). These results contradict
Luna and others ( 1988), who used fistula ted animals and
concluded that engordacabra was consumed heavily in
summer-fall and only minimally during the winter-spring
period.
Engordacabra is highly nutritious, with mean nutrient
levels on a par with alfalfa, though it does contain slightly
more crude fiber (table 1).
In the present investigation, we examined the effects
of browsing intensity on the vegetative growth of engordacabra at different seasons and on its reproductive output.
We hypothesized that browsing would have an effect on
resource allocation and growth that would vary seasonally
and as a function of intensity. We hoped to be able to use
the information obtained to define a management system
for this plant which would permit its utilization on a sustained basis.
Engordacabra (Dalea bicolor: Fabaceae) is a shrubby
legume browsed by livestock in central and northern
Mexico. This study of the effect of variation in browsing
intensity on population structure and on growth and reproductive variables showed that the species tolerates moderate browsing by shifting its resources from reproduction
to vegetative growth, but that it cannot tolerate excessive
browsing. The species behaves as an increaser on native
ranges. Recruitment from seed was limited even under
protection from browsing. The plants provide forage year
round.
INTRODUCTION
Shrublands are plant communities characterized by a
shrub canopy, often over a grass understory. The shrublands arose after the grasslands as a consequence of climatic changes during the Pliocene and Pleistocene, at
least in some areas of the Chihuahuan Desert (Martin
and Harrell 1957). Other causes of their origin include
tectonic events, erosion, and human activities (Axelrod
1978; Rzedowski 1978).
The shrublands of Mexico are one ofits most extensive
plant resources and occupy approximately a million square
kilometers, over 50 percent of its area. The Sonoran and
Chihuahuan Deserts account for about 650,000 square
kilometers, while the remaining area is made up of other
types of temperate and tropical shrublands. Flores-Mata
and others (1971) classify Mexican shrublands into seven
types differing in areal extent: microphyll shrublands
(20. 7 percent), succulent shrublands (6.6 percent), rosette
plant shrublands (1.3 percent), mesquite shrublands (6.2
percent), submontane shrublands (0.9 percent), chaparral
(1.8 percent), and low deciduous rain forest (16.0 percent).
Shrublands are characterized by low productivity and
consequently by a low carrying capacity, even for areas
in good condition (COTECOCA 1973, 1974). In most cases
this is due to limiting environmental conditions such as
precipitation, temperature, salinity, and soil fertility.
Paper presented at the Symposiwn on Cheatgrass Invasion, Shrub DieOff, and Other Aspects of Shrub Ecology and Management, Las Vegas, NV,
April 5-7, 1989.
A. Romero-Manzanares is Investigador Docente and E. Garcfa-Moya is
Profesor lnvestigador Titular, Centro de Botanica, Colegio de Postgraduados, Chapingo, Mexico 56230.
317
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Figure 1-Geographic distribution of engordacabra (Dalea bicolor)
in Mexico.
The three study sites have similar semiarid climates
(fig. 3). Mean annual temperatures average approximately 16 °C with little month-to-month variation. Most precipitation occurs in summer. The year of study was
slightly above average in precipitation.
The principal class of browsing livestock in the area is
goats. Horses and cattle also use the area but their numbers are limited, as is their use of shrubs for forage.
The three site conditions corresponded to three levels
of browsing intensity. Browsed and unbrowsed plants
were used to calculate browsing intensity according to
a modified McNaughton's equation (McNaughton 1979).
According to this equation, BI = 1 - glng, where BI =
browsing intensity, g =biomass produced with browsing,
and ng =biomass produced without browsing (in an exclosure at each site). We used forage volume as an estimate
of biomass. Using this equation with unbrowsed plants
at the average site (El Palmar) as a reference point
(0 percent use, no browsing), browsing intensities were
calculated at 17 percent (light browsing) at El Palmar,
71 percent (moderate browsing) at Palma Pegada, and
91 percent (heavy browsing) at Trancoso.
Sampling was carried out each season at each study
site (browsing intensity). Vegetative growth variables
were evaluated four times during the year at each locality, while reproductive output variables were evaluated
once, at the appropriate season. Data from unbrowsed
treatments (exclosures) were collected at the El Palmar
and Palma Pegada sites. At El Palmar the exclosure
had been in place for 5 years at study initiation, while
the exclosure at Palma Pegada had been in place for
6 months. No exclosure treatment was possible at
Trancoso.
Vegetative growth variables included shape and size
(height in centimeters) of 1,000 randomly chosen individuals, forage volume and number of limbs at the base
Table 1-A comparison of forage composition (percent)
of engordacabra and alfalfa 1
Component
Engordacabra
Alfalfa
Crude protein (N x 6.25)
Crude fiber
Fat
(NFE) carbohydrates
Ash
Calcium
Phosphorus
14.56
40.71
4.14
32.95
7.64
15.44
27.21
2.94
41.95
1.81
1.73
.29
.63
8.82
1
Sources: Betancourt-Galan (1967), DeAlba (1971).
STUDY AREAS AND METHODS
Three field sites were selected for this study (fig. 2).
The first, El Palmar (Villa de Arriaga Municipality, San
Luis Potosf), was chosen to represent a site in average
condition (56 percent desirable species). Palma Pegada
(Salinas Municipality, San Luis Potosf) represented a
site in poor condition (23 percent desirable species), while
Trancoso (Guadalupe Municipality, Zacatecas) was selected to represent a site in very poor condition (0 percent desirable species). Each study site is located at an elevation
of2 150m on semiarid alluvial plains of igneous origin.
The' soils are Aridisols, and the vegetation consists of a
mixed shrubland of thorny and succulent species interspersed with patches of yucca. Characteristic species include Opuntia streptacantha, 0. leucotricha, 0. robusta,
0. cochinera, 0. rastrera, Yucca decipiens, Acacia schaffneri, Mimosa biuncifera, Dalea bicolor, Agave salmiana
ssp. crassispina, Bouteloua gracilis, B. curtipendula, and
Leptochloa dubia.
318
of the shoots of 200 adult plants, and relative growth rate
(using length of current growth for each season) for five
apical branches of each of 100 individuals. Relative growth
rate was calculated as [log (final length) -log (initial
length)]ldays between measurements (Grimes and Hunt
1975).
Reproductive output components included number of
inflorescences/plant for 200 individuals, number of flowers/
inflorescence for 200 inflorescences of each of 10 individuals, number of filled and aborted podslinfructescence
for 200 infructescences on each of 10 individuals, and
germination capacity of 50 scarified seeds. Reproductive
potential/plant was obtained through the following equation: mean filled podslinfructescence x mean inflorescences/
plant x germinative capacity (percent viable seeds). Each
so
0
100
EL PALMAR
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PALMA PEGADA
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TRANCOSO
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Figure 3-Ciimagrams for weather stations at or
near the study sites: El Palmar (Villa de Arriaga Station, 24-003), Palma Pegada (La Tepocata Station,
24-036), Trancoso (Trancoso Station, 32-036).
319
LLJ
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Figure 2-Photographs of the study sites:
A, El Palmar, Villa de Arriaga, San luis
Potosf; B, Palma Pegada, Salinas, San luis
Potosf; C, Trancoso, Guadalupe, Zacatecas.
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engordacabra pod produces a maximum of a single seed, so
that the equation yields an estimate of the number of seeds
produced per plant.
Seedling emergence and survival both within and outside
exclosures at each site were followed from August through
February on ten 1-m2 plots. Population size distributions
for each treatment were graphed from the plant height
data.
Data on height distribution for each study site, browsing
treatment, and season were examined using univariate
analysis to characterize the shape of the frequency distributions, which were also examined graphically. Departures from normality (skewness and kurtosis) were evaluated using the statistical procedures described by Naylor
(1976). The height data were also subjected to analysis
of variance and to linear regression analysis. Seed-yield
data were subjected to nonparametric analysis (U MannWhitney Test).
HERBIVORY RATE
There was a positive relationship between stocking
rate for each area (as determined by discussion with local
users) and calculated browsing intensity. For the year
of study, stocking rates of 0.00 AU/ha/yr (exclusion),
0.26 AU/ha/yr (El Palmar), 0.63 AU/ha/yr (Palma Pegada),
and 0.82 AU/ha/yr (Trancoso) corresponded to browsing
intensities of 0, 17, 71, and 91 percent, respectively. Forage production expressed as forage volume showed a corresponding tenfold decrease with an increase in animal numbers. The areas studied, even the one with relatively light
browsing, are overutilized, if we consider that COTE COCA
(1973, 1980) recommends a stocking rate of 10 halAU/yr
for ranges in good condition in this geographic area.
PLANT SIZE AND POPULATION SIZE
STRUCTURE
A significant amount of the variation in plant size
(height in em) was accounted for by the effects of range
condition (browsing intensity) and season of sampling.
This was true both for unbrowsed (exclosure) plots (F2,7997 =
251.65, p < 0.0001) and for browsed plots (F211 997 = 549.18,
p < 0.0001). Mean plant height with browsirig'at light
and medium intensity was similar, while heavy browsing
(Trancoso) reduced height to a mean value less than half
of values at light to medium intensity (table 2). Height
recovery in exclosures was more marked at light intensity
(El Palmar) than at medium intensity (Palma Pegada), possibly because of the longer period of exclusion.
Plant height also varied as a function of season of sampling (table 2, fig. 4). At the light-intensity (El Palmar)
Table 2-Univariate analysis for height (em); n = 1,000 for each site, season, and browsing treatment. Deviations from normal distribution are significant (p < 0.0001) in each case
Treatment
Mean
Variance
Minimum
Maximum
Skewness
Kurtosis
El Palmar
Browsed
Spring
Summer
Autumn
Winter
Exclosure
Spring
Summer
Autumn
Winter
63.6
47.7
70.5
84.6
39.8
38.4
42.2
38.6
62.6
80.6
59.8
45.7
2.0
.5
.8
8.2
315.0
200.0
260.0
250.0
1.06
.72
1.03
.72
1.87
.21
1.31
.58
92.4
76.3
90.4
101.9
36.2
43.8
39.4
33.7
39.2
57.4
43.6
33.1
8.6
.8
1.7
.8
213.0
300.0
242.0
240.0
.41
.16
.32
.60
-.13
.09
.50
.67
70.2
59.8
81.3
75.6
28.5
37.9
28.7
28.2
40.6
63.4
35.3
37.3
2.5
1.1
21.1
4.7
210.0
206.7
227.0
245.3
1.34
.23
.99
1.26
3.89
-.16
1.76
3.10
70.8
54.5
94.1
94.9
48.8
44.1
33.1
40.2
69.0
80.9
35.1
42.4
2.0
.9
16.6
26.7
228.1
244.0
221.4
255.0
1.80
.79
1.02
1.33
.10
.47
1.23
1.78
24.6
32.6
37.9
30.9
20.1
21.7
22.4
19.3
81.8
66.5
59.1
62.5
.1
.2
.1
1.2
218.0
173.3
206.5
111.5
2.35
1.67
1.45
.97
11.31
4.98
5.62
.84
Palma Pegada
Browsed
Spring
·Summer
Autumn
Winter
Ex closure
Spring
Summer
Autumn
Winter
Trancoso
Browsed
Spring
Summer
Autumn
Winter
320
30
(/)
~
El PALMAR
SPRING
30
(/)
~
EL PAL.UAR
SUMMER
30
(/)
::J.
20
::i.
20
LL..
15
LL..
15
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::J. 20
::J. 20
LL..
0
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15
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0
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1>.1
1>.1
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~
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~ 10
0
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25
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§ 25
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15
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b
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5
20
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HEIGHT CLASS (10 CU lllfTERVAL)
PALMA PEGADA
AUTUMN
25
30
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§ 25
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15
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5
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30
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15
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25
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WINTER
25
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20
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1>.1
0
a 10
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AUTUMN
a..
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0
a::
0
HEIGHT CLASS {1 0 CM INTERVAL)
30
1>.1
a..
a..
(/)
a..
15
5
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5
0
SUMMER
20
0
a..
0
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5
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~ 10
~
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1>.1
a..
a..
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PALMA PEGADA
WINTER
~
1>.1
0
...b
0
5
HEIGHT ClASS {10 CM INTERVAL)
~ 10
~ 10
a..
a..
5
ffi
a..
30
SUMMER
a..
a..
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10
0
30
SPRING
0
1>.1
a..
0
30
(/)
0
a::
13
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1>.1
a..
0
LL..
0
5
1>.1
1>.1
~
~
EL PALMAR
WINlER
25
a..
g
15
30
(/)
~
~
...b
AUTUMN
25
25
25
EL PALMAR
0
0
Figure 4-Height-class frequency distributions for each study site on spring, summer, autumn, and winter sampiing dates. Crosshatched bars = browsed treatment; open bars = exclosure treatment. N = 1,000 individuals for
each site, season, and treatment.
and medium-intensity (Palma Pegada) sites, there was
a decrease in mean plant height from spring to summer
sampling dates, followed by a recovery from summer to fall
and a leveling-off from fall to winter. This pattern was observed both in browsed and exclosure treatments and was
largely due to an influx of seedlings into the population in
response to summer rains (fig. 4). At the heavily browsed
site (Trancoso), mean plant height increased from spring
through fall and decreased from fall to winter.
Population structure for engordacabra presented positive
skewness (asymmetry with tail toward right) and leptakurtosis (peakedness) with or without browsing at any
season, but the size frequency distribution in exclosed
areas tended more toward a normal distribution (table 2,
fig. 4). This response to herbivory is similar to that of the
grass Schizachyrium scoparium (Butler and Briske 1988).
Plant height-frequency distributions were more strongly
right-skewed in browsed than in exclosure treatments especially at El Palmar, where plants had had 5 years torespond to exclosure (table 2, fig. 4). In browsed treatments,
both right-skewness and the tendency to leptokurtic distributions increased with an increase in browsing intensity.
Right-skewed size distributions may be caused by differential growth responses of individuals in different age
classes, or as a consequence of competitive interactions
resulting in large numbers of suppressed individuals
(Weiner and Solbrig 1984; Schmidt and others 1987).
In the best scenario, such distributions suggest continual
regeneration through recruitment of individuals into seedling and juvenile size classes. In the present case, overrepresentation of small size classes may be as much a consequence of excessive browsing of adult plants as it is of
the age distribution ofthe population.
PLANT SHAPE
Browsing intensity affected the shape of plants. Plants
acquired a conical shape when protected from browsing in
exclosures, while heavily browsed plants adopted irregular
shapes as well as suffering a decrease in forage volume and
height. The number of limbs emerging at the soil surface
was greater in plants subjected to heavy browsing. A geometric progression of one, two, four, and eight limbs observed at the base of the shoot corresponded to 0, 17, 71,
321
and 91 percent browsing rates. This is probably due to
loss of apical dominance, a direct effect of loss of the apical
meristem, or possibly due to indirect photomorphogenic effects that are a consequence of reduced cover (Butler and
Briske 1988).
Meijden and others 1988). The quick response to protection further supports the idea that engordacabra is browsing tolerant and is an increaser species on these ranges.
RELATIVE GROWTH RATE
Both exclosure and browsing intensity had a significant
effect on reproductive output of engordacabra (table 4).
Number of flowers/inflorescence and number of inflorescences/plant were significantly affected by browsing treatment (U Mann-Whitney approximate to normal with correction for ties in seven combinations, p > 0.0001). Mean
number of inflorescences/plant was dramatically reduced
with increased browsing intensity in the browse treatments, and browsed plants had fewer inflorescences than
exclosure plants at both light and medium intensities.
Number of flowers/inflorescence was less sensitive to treatment and showed a notable reduction only at the highest
browsing intensity.
Seed-set percentages were directly related to browsing
intensity, with the light-intensity treatment showing the
lowest fill (19 percent) and the high browsing intensity
treatment showing the highest fill (89 percent) (table 4).
Seed set was similar for exclosure and browsed plants at
both light- and medium-browsing intensity. Seed quality
measured as germinative capacity of scarified seeds was
also directly related to browsing intensity, varying from
75 percent for the light-intensity browsing treatment to
100 percent for the heavy-intensity treatment. Exclosure
had little effect on seed quality.
Reproductive potential expressed as viable seeds/plant
is the product of the reproductive components discussed
above. It was higher by a factor of almost 200 for exclosure
plants at the light-intensity site than for browsed plants
at the heavy-intensity site (table 4). This is true in spite
of the fact that individual flowers at the heavily browsed
site were much more likely to produce a viable seed. The
tremendous increase in number of inflorescences/plant
with each decrease in browsing intensity was able to compensate for lower seed set and lower seed quality many
times over.
It appears that the ability of engordacabra to show a
compensatory growth response when browsed depends
on its ability to shift limited resources from reproductive
activity to vegetative growth. Heavily browsed populations
sacrifice the ability to increase population size through
seedling recruitment in order to tolerate excessive browsing. Rodriguez and Welch (1989) have reported a drastic
reduction in seed output as a response to heavy browsing
in big sagebrush. They postulate that this could be one
factor responsible for lack of recruitment on the native
site. Theory on resource allocation predicts an inverse relationship between productivity and reserves for reproductive structures, which may explain why more browsing is
conducive to a smaller reproductive potential, and why the
option of resistance for survival at high browsing intensity
must be bought at the expense of reproduction (Watson
1984; Watson and Casper 1984).
REPRODUCTIVE POTENTIAL
Relative growth rate (RGR) of browsed engordacabra is
maximum and positive in spring-summer and negative in
summer-fall and fall-winter, whereas unbrowsed plants
maintain a positive RGR year-round (table 3). A net positive mean annual relative growth rate was maintained
at light and medium browsing intensities, but growth during critical periods hardly compensated for browsing. At
heavy browsing intensity an annual net negative growth
rate or loss was observed. The mean annual RGR was
much higher for plants in the exclosure treatment than
for browsed plants at both light and medium browsing
intensities, and exclosure values were similar at the two
sites. In the browsing treatment, mean annual RGR was
somewhat higher at medium than at light intensity.
Grime and Hunt (1975) describe the range ofRGR for
legumes as between 0.041 and 0.214/day, which is similar
to values for engordacabra but low in comparison with
other angiosperms. They characterize species with low
potential RGR as classic restriction-tolerant species, tolerant of external limiting factors that consistently limit their
productivity. A similar relationship between resource
availability and RGR has been suggested by Coley (1983)
and Coley and others (1985), who postulate that habitat
quality is the most important selective force in the evolution of plant antiherbivore defense systems, because ofits
influence on growth rate.
In the present study, we have seen that reduction in
browsing intensity favors the growth of engordacabra.
Exclusion of browsing animals allows regeneration of individual plant biomass. However, extended periods of
protection from browsing may not be necessary for sustained productivity. At moderate browsing intensity, compensatory growth mechanisms resulted in a positive net
annual RGR. Such compensatory growth is often observed
in response to herbivory (MeN aughton 1979, 1983, 1986;
Table 3-Relative growth rates (per day) for apical limbs of engordacabra, by study site, browsing treatment, and season
Study
site/treatment
Spring·
summer
Summer·
autumn
Autumn·
winter
0.5924
.6849
0.1681
-.6201
0.0989
-.0046
0.2865
.0201
.5118
.5272
.2452
-.0050
.0639
-.3143
.2736
.0693
.1100
-.1993
-.1594
-.0083
Mean
El Palmar
Ex closure
Browsed
Palma Pegada
Exclosure
Browsed
Trance so
Browsed
322
Table 4-Mean values for seed yield components for engordacabra by study site and browsing
treatment. Pods contain a maximum of one seed
Component
Inflorescences/plant
Flowers/inflorescence
Flowers/plant
Filled podslinfructescence
Filled pods (seeds)/plant
Percent filled pods (seed set)
Mean seed size (mg)
Germinative capacity (percent)
Reproductive potential
(viable seeds/plant)
Studl site/browsing treatment
Palma Pegada
El Palmar
Exclosure Browsed
Exclosure Browsed
Trancoso
Browsed
874.8
44.0
38,447
8.4
7,305
19
1.50
83.2
285.3
46.7
13,315
10.7
3,061
23
1.30
75.0
46.8
48.8
2,285
14.2
663
29
1.37
90.9
26.5
41.6
1,104
13.3
353
32
1.25
90.3
0.4
21.4
7.7
19.0
6.9
89
1.11
100
1,155
520
175
102
6.1
SEEDLING RECRUITMENT
ACKNOWLEDGMENTS
Most of the engordacabra plants (90-95 percent) on
open ranges are adult individuals. Regeneration from
seed is limited, in spite of high reproductive potentials
observed with light or no browsing. Causes oflow establishment could include seedcoat impermeability, somatic
heterochrony, microhabitat quality (safe site availability),
competitive interactions, and the effects of environmental
uncertainty (for example, variation in weather patterns)
on recruitment of seedlings and survival of juveniles.
Engordacabra emergence and survival was greater in
areas subjected to browsing than in exclosures (p < 0.001
at the moderate-intensity site and p < 0.05 at the lightintensity site), probably due to the presence of more open
spaces generated by both grazing and browsing.
This work was supported in part by the Consejo N acional de Ciencia y Tecnologfa Project No. PCCECCNA050778. We also wish to thank Susan E. Meyer of the
Shrub Sciences Laboratory for her critical review and extensive editing of the manuscript.
REFERENCES
Axelrod, D. I. 1978. The origin of coastal sage vegetation,
Alta and Baja California. American Journal of Botany.
65: 1117-1131.
Barneby, R. C. 1977. Dalea imagines. Memoirs New York
Botanical Garden. 27: 1-891.
Betancourt-Galan, M. A. Amilisis y valoracion de los principios nutritivos de laDalea tuberculata (engordacabra). San Luis Potosi, Mexico: Escuela de Ciencias
Qufmicas, Universidad Aut6noma de San Luis Potosi.
Tesis profesional.
Butler, J. L.; Briske, D. D. 1988. Population structure and
tiller demography of the bunchgrass Schizachyrium
scoparium in response to herbivory. Oikos. 51: 306-312.
Coley, P. D. 1983. Herbivory and defensive characteristics
of tree species in a lowland tropical forest. Ecological
Monographs. 53: 209-233.
Coley, P. D.; Bryant, J.P.; Chapin, F. S. 1985. Resource
availability and plant antiherbivore defense. Science.
230: 895-899.
COTECOCA. 1973. Coeficientes de agostedero de la
Republica Mexicana, estado de San Luis Potosf. Mexico,
Distrito Federal: Comision Tecnico Consultiva para la
Determinacion Regional de los Coeficientes de Agostadero. Secretarfa de Agricultura y Ganaderfa.
COTE COCA. 1974. Coeficientes de agostedero de la
Republica Mexicana, estado de Baja California Norte.
Mexico, Distrito Federal: Comision Tecnico Consultiva
para la Determinacion Regional de los Coeficientes de
Agostadero. Secretarfa de Agricultura y Ganaderfa.
COTECOCA. 1980. Coeficientes de agostedero de la
Republica Mexicana, estados de Zacatecas y Aguascalientes. Mexico, Distrito Federal: Comision Tecnico
SUMMARY
The individual and population growth responses of
engordacabra to browsing support the idea that it is an increaser species on rangelands, typical of a native disclimax
in response to use by nonnative ungulates. According to
McNaughton (1979, 1983, 1986), moderate levels of herbivory can result in compensatory growth so that productivity is sustained in spite of increased consumption. Low
levels of herbivory favor an increase in fitness (greater
RGR), increased reproductive potential, and good establishment of seedlings and juveniles with resultant improvement of population size structure. High rates ofherbivory (utilization beyond the optimum point for maximum
productivity) produce detrimental effects such as a precipitous drop in repoductive potential, problems in the establishment of prereproductive individuals, and negative RGR
resulting in a downward spiral in productivity and capacity for response.
Engordacabra responded favorably to moderate levels
of browsing in this study, although it showed increased
growth when browsing was excluded. It has the potential
for growth all year round. Size frequency distribution was
improved and reproductive potential was restored with
browsing exclusion, but yearly recruitment of progeny
was low at any rate ofherbivory.
323
Consultiva para la Determinacion Regional de los
Coeficientes de Agostadero. Secretar!a de Agricultura y
Ganaderia.
DeAlba, J. 1971. Alimentaci6n del ganado en America
Latina. Mexico, Distrito Federal: La Prensa Medica
Mexicana.
Flores-Mata, G.; Jimenez-L6pez, J.; Madrigal-Sanchez, X.;
Moncayo-Ruiz, F., Takaki-Takaki, F. 1971. Mapa de tipos
de vegetaci6n de la Republica Mexicana. Escala 1:200
000. Mexico, Distrito Federal: Secretar!a de Agricultura
y Recursos Hidraulicos.
Grime, J. P.; Hunt, R. 1975. Relative growth rate: its range
and adaptive significance in a local flora. Journal of Ecology. 63: 393-422.
Hernandez-Xolocotzi, E.; Ramos-Sanchez, A. Mejoramiento
de las plantas forrajeras en Mexico. In: Xolocotzia II.
Revista de Geograffa Agricola. Chapingo, Mexico:
Universidad Aut6noma de Chapingo: 533-549.
Jameson, D. A.; Armijo, J. R.; Galo, M. T.; Nava, R. C.
1984. Marginal benefits of grazing and agricultural
practices on a Mexican ejido. Journal of Range Management. 37: 195-199.
LaBaume, J. T.; Dahl, B. E. 1986. Communal grazing:
the case of the Mexican ejido. Journal of Soil and Water
Conservation. January-February: 24-27.
Luna, L. M.; Chavez, R. M. G.; Aguado, S. G. A.; Barretero,
H. R. 1988. Importancia de la engordacabra en la dieta
de caprinos en pastoreo en un matorral micr6filo del
noreste de Jalisco. In: Memorias XV. Congreso Nacional
de Buiatr!a, 1988 August 19; Queretero, Qro., Mexico.
Martin, P. S.; Harrell, B. E. 1957. The Pleistocene history
of temperate biotas in Mexico and the eastern United
States. Ecology. 38: 368-380.
McNaughton, S. J. 1979. Grazing as an optimization
process: grass-ungulate relationships in the Serengeti.
American Naturalist. 113: 692-703.
McNaughton, S. J. 1983. Compensatory plant growth as
a response to herbivory. Oikos. 40: 329-336.
McNaughton, S. J. 1984. Grazing lawns: animals and
herds, plant form, and coevolution. American Naturalist.
124: 863-887.
McNaughton, S. J. 1986. On plants and herbivores. American Naturalist. 128: 765-770.
324
Meijden, E. Vander; Wijn, M.; Verkaar, H. J. 1988. Defense and regrowth alternative plant strategies in the
struggle against herbivores. Oikos .. 51: 355-366.
Naylor, R. E. L. 1976. Changes in the structure of plant
populations. Journal of Applied Ecology. 13: 513-521.
Rodriguez, R. L.; Welch, B. L. 1989. Effects of heavy grazing by mule deer on 'Hobble Creek' mountain big sagebrush seed stalk production. In: Wallace, A.; McArthur,
E. D.; Haferkamp, M. R., compilers. Proceedingssymposium on shrub ecophysiology and biotechnology.
Gen. Tech. Rep. INT-256. Ogden, UT: U.S. Department
of Agriculture, Forest Service, Intermountain Research
Station: 141-143.
Romero-Manzanares, A. 1982. Estudio de tres leguminosas forrajeras arbustivas del Altiplano PotosinoZacetecano. Cuernavaca, Morella: Escuela de Ciencias
Biol6gicas, Universidad Aut6noma del Estado de Morelia. 254 p. Tesis profesional.
Romero-Manzanares, A. 1987. Reproducci6n y crecimiento
del arbusto forrajero Dalea bicolor (Leguminosae) en
diferentes condiciones de sitio de agostadero. Chapingo,
Mexico: Centro de Botanica, Colegio de Postgraduados.
193 p. Tesis de Maestr!a en Ciencias.
Rzedowski, J. 1978. Vegetaci6n de Mexico. Mexico, Distrito Federal: Limusa.
Schmidt, J. J.; Eccleston, D. W.; Ehrhardt, D. W. 1987.
Dominance and suppression, size dependent growth,
and self-thinning in a natural stand of Impatiens capensis. Journal of Ecology. 75: 651-665.
Trevino Fernandez, J. C. 1980. Importancia de la fauna
silvestre como recurso natural en relaci6n con la ganaderia en el noroeste de Mexico. Rangelands. 2(1): 33-34.
Watson, M. A 1984. Development constraints: effects on
population growth and patterns of resource allocation
in a clonal plant. American Naturalist. 123: 411-426.
Watson, M. A.; Casper, B. B. 1984. Morphogenetic constraints on patterns of carbon distribution in plants.
Annual Review of Ecology and Systematics. 15:
233-258.
Weiner, J.; Solbrig, 0. T. 1984. The meaning and measurement of size hierarchies in plant populations.
Oecologia. 61: 334-336.