Ecology, 83(9), 2002, pp. 2477–2488
q 2002 by the Ecological Society of America
THE JOINT EFFECTS OF GRAZING, COMPETITION, AND TOPOGRAPHIC
POSITION ON SIX SAVANNA GRASSES
NORMA L. FOWLER
Section of Integrative Biology, University of Texas, Austin, Texas 78712 USA
Abstract. I investigated the separate and joint effects of herbivory (grazing) and competition on six grass species and on their distributions across the landscape. In a factorial
field experiment, cattle were excluded from half the subplots, and neighboring plants were
removed from half the subplots. Transplants of each of the species were planted in plots
on hillsides and in flat areas, and the entire experiment was replicated in two pastures.
The effects of grazing were negative and were proportional to the ungrazed height of
each species. In the absence of grazing and competition, the flat plots were more favorable
than the hillside plots. Grazing reversed the relative favorableness of flat and hillside plots,
most likely because grazing was more intense in the flat plots due to cattle preference for
flatter ground. The removal of neighboring plants always had a positive effect, indicating
competition from neighboring plants. The relative impact of competition from pre-existing
plants was generally greater in the flat plots than in the hillside plots.
The relative impacts of grazing were generally greater in the absence of competing
plants, and the relative impacts of competition were generally greater in the absence of
grazing. The most likely explanation for this antagonistic interaction between the effects
of herbivory and competition is an indirect positive effect of grazing that arose from a
reduction in competition experienced by a target plant due to the defoliation of its neighbors.
The results of this study, when combined with the known distributions of the six grass
species in this region, indicate that herbivory, rather than competition or the physical
environment, controls the distribution of at least five of the six species across the landscape.
For example, Schizachyrium scoparium grew well in ungrazed flat subplots but very poorly
in grazed flat subplots; grazing, not the physical environment or competition, accounts for
the absence of S. scoparium from most flat sites in this region. Competition modulates the
effects of herbivory and edaphic factors on landscape distributions but does not control
the distribution of any of the six species.
Key words: Bothriochloa ischaemum; Bouteloua spp.; Buchloe dactyloides; community composition; competition; defoliation; grassland; herbivory; savanna; Schizachyrium scoparium; Texas;
topography.
INTRODUCTION
It is often obvious that herbivores, especially grazing
ungulates, greatly affect plants. Nevertheless, much remains to be understood about the effects of herbivory
on plant populations and communities, despite a number of excellent studies (e.g., of the effects of specialist
insects [Doak 1992, McEvoy et al. 1993, Louda and
Potvin 1995, Callaway et al. 1999], and of the effects
of other herbivores [Polley and Detling 1989, Bergelson and Crawley 1992, Dyer and Rice 1997, Edwards
et al. 2000, van der Wal et al. 2000]; see also reviews
by Louda et al. [1990] and Crawley [1997]).
In particular, the ways in which herbivory and plant
competition interact to affect individual plants, plant
populations, and plant communities are still not well
understood. Few studies of herbivory have manipulated
plant competition; of those that have, not all have tested
for a statistical interaction between the effects of herbivory and competition (see Discussion). The present
Manuscript received 17 August 2000; revised 10 July 2001;
accepted 25 July 2001; final version received 1 February 2002.
study examines the effects of grazing on individual
plant performance and on competitive interactions
among plants. The directions and magnitudes of the
interaction between the effects of herbivory and competition were measured, to see whether and how the
joint effects of herbivory and competition differ from
predictions made from their separate effects.
Range scientists have identified plant species that
increase and decrease under various levels of grazing
(e.g., Ellison 1960, Stoddart et al. 1975, Noy-Meir et
al. 1989), but the mechanisms causing these changes
in plant community composition have generally not
been investigated. For example, we do not know whether ‘‘decreasers’’ decrease in abundance when grazed
because they are preferred by grazing ungulates or because they are more sensitive to equivalent levels of
defoliation than are ‘‘increasers,’’ and whether plant
competition reinforces or weakens the effects of grazing. These two questions are both addressed in this
study.
Another goal of this study was to determine the relative importance of grazing, competition, and edaphic
2477
NORMA L. FOWLER
2478
and topographic factors, separately and in interaction
with each other, in determining the distribution of plant
species across a landscape. The study was conducted
in a part of central Texas where the composition of the
herbaceous communities of grasslands and savannas is
known to be correlated with slope, which is in turn
closely related to soil type (Fowler and Dunlap 1986).
The six grass species studied were selected to represent
a range of species’ distributions (see Methods).
The study species were also chosen to include a range
of probable responses to herbivory. Schizachyrium scoparium (formerly Andropogon scoparius) and Bouteloua curtipendula are considered to be decreasers (i.e.,
they are less abundant or absent from more heavily
grazed sites; Buechner 1944, Dyksterhuis 1946,
Launchbaugh 1955, Smeins et al. 1976, Thurow et al.
1988, O’Connor 1991). Bouteloua rigidiseta and Buchloe dactyloides are considered to be increasers (i.e.,
they are more abundant in more heavily grazed sites;
Dyksterhuis 1946, Launchbaugh 1955). Bothriochloa
ischaemum (formerly Andropogon ischaemum), the
only non-native species of the six, can increase in abundance under heavy grazing (N. Fowler, personal observation) and is particularly abundant in disturbed areas (B. Gabbard, personal communication). Because
transplants of each of the six species received each
treatment combination, the experimental design permits comparisons among these species in their responses to herbivory, to topographic location, and to competition. The factorial design used in this study also
provided an opportunity to measure the relative impact
of herbivory across the landscape and similarly to measure the relative intensity of competition across the
landscape.
In general, the most accurate and sensitive way to
measure of the effects that grazing or other types of
herbivory have upon plants is to measure the plants
themselves, as was done in this experiment. This comparison of grazed and ungrazed plants integrates the
amount and type of tissue eaten by grazing animals at
different dates, the indirect effects of grazing (e.g.,
trampling) and plant responses to grazing (e.g., compensatory regrowth) to produce a measure of the net
effect of grazing upon a plant. The effects of grazing
upon the herbivore (cattle) were beyond the scope of
this study; for such a study measures of cattle behavior
or diet might be more appropriate.
METHODS
On the eastern Edwards Plateau of central Texas
where this study was conducted, plant communities
dominated by herbaceous plants differ not only in their
degree of woody cover (from grasslands to savannas
containing clusters of woody plants to glades located
within woodlands) but also in the relative abundances
of their herbaceous species. The composition of the
herbaceous plant community is correlated with slope
(Fowler and Dunlap 1986): on steeper slopes, the taller
Ecology, Vol. 83, No. 9
grass species, including Bouteloua pectinata and Schizachyrium scoparium, are more abundant and the shorter grass species, including Bouteloua rigidiseta and
Buchloe dactyloides, are less abundant. The topography
of this part of the Plateau is due to erosion of horizontal
layers of its limestone and marl bedrock. Flat areas are
found where particularly erosion-resistant limestone
layers occur, sometimes as flat hilltops and sometimes
as steps on hillsides. Elsewhere the terrain varies from
gentle slopes to vertical cliffs.
The experiment was conducted at Shield Ranch in
western Travis County, Texas. Two pastures (‘‘Middle’’
and ‘‘Rockhouse,’’ referred to henceforth as pastures
A and B, respectively) were selected because they each
provided both hillsides and flat areas close to, and equidistant from, a watering trough. Conducting the experiment close to water sources guaranteed relatively
heavy grazing. Cattle were present in each pasture yearround. All plots were located in open areas away from
all woody plants, large cacti, and vehicle tracks. Two
hillside plots and two plots in flat areas were located
in each pasture. The flat plots in pasture A were on a
step and the flat plots in pasture B on a hilltop. In both
pastures Bouteloua pectinata and Schizachyrium scoparium were abundant in the hillside plots and Bothriochloa ischaemum, Buchloe dactyloides, and Bouteloua rigidiseta were abundant in the flat plots.
In this region, slope is closely associated with soil
type and soil depth. All of the hillside plots had the
Brackett soil that is typical of such locations (U.S.
Department of Agriculture 1974). This soil is technically a gravelly clay loam with interbedded soft limestone. The soil was very thin to entirely absent in these
plots, as is also typical; the underlying parent material
was alternate flat layers of limestone and marl; the very
irregular slopes were 58 to 108. The pasture A flat plots
were located on a Volente series soil, an alluvial dark
silty clay loam over clay; the slope was ,58. Volente
soils develop at the foot of Brackett soils, as in this
site. The pasture B flat plots were located on Tarrant
soil, a shallow gray-brown clay soil; the slope was 0 8.
The two topographic treatments thus represented differences in a whole set of edaphic factors, all of which
probably affect water and nutrient availability. Soil water availability was obviously a limiting factor everywhere during much of the year in all of these sites and
throughout the region, as shown by grass die-back, forb
wilting and leaf-drop, plant deaths, and dry soils.
Each plot was divided into 16 2 3 2 m subplots.
With one exception, each plot was laid out as two sets
of eight contiguous subplots (two rows of four subplots
each) with a 2-m walkway between the two sets of
contiguous subplots, so that each unfenced subplot was
accessible to the cattle. In pasture A, neither of the two
flat plots could be positioned as two sets of eight contiguous subplots without impinging upon windmill access (and on the associated vehicle tracks). Instead,
each flat plot in this pasture was laid out as a long,
September 2002
GRAZING, COMPETITION, AND TOPOGRAPHY
two-subplot-deep row, with some gaps to avoid large
cacti. The 16 subplots within a plot were each assigned
to one of four treatment combinations in a latin-square
design. The four treatment combinations were (1)
fenced or unfenced, crossed with (2) pre-existing vegetation removed or not. There were four replicate subplots per treatment combination per plot.
To exclude cattle from the fenced subplots, standard
ranch fencing (three strands of barbed wire) was augmented by wire mesh (ground level to 60 cm) and
chicken wire (ground level to 25 cm) around each
fenced subplot. Regular visual inspection confirmed
that all of the unfenced subplots, even those with fenced
subplots on three of their four sides, all walkways within plots, and all areas around the plots were grazed,
and that no grazing occurred within any fenced subplot.
Cattle were the only domestic ungulates present. The
fences probably also excluded white-tailed deer ( Odocoileus virginianus) because of their unwillingness to
enter small enclosures, but white-tailed deer eat little
or no grass (McShea et al. 1997). The fences might
have excluded jackrabbits (Lepus californicus), but
there was no evidence of jackrabbit grazing in any
plots. The effects of the fencing treatment can therefore
be ascribed to cattle grazing.
Vegetation was removed from half the subplots with
a combination of glyphosate (Round-up, Monsanto, St.
Louis, Missouri, USA) applied with a sponge applicator
and hand weeding (by cutting plants at soil level, rather
than digging, to minimize soil disruption). Vegetation
was removed initially (February 1989) and in April and
October of each year. The initial vegetation removal,
the construction of the fences, and the digging of holes
in the ground for the transplants were all completed
before the transplants were planted. A hammer drill
was used to make the holes for the fence posts and the
holes into which the transplants were planted.
Seed of Bouteloua curtipendula (grown by George
Werner Seed, Hereford, Texas, USA, labeled ‘‘variety:
Haskell’’), Buchloe dactyloides (grown by Frontier
Seed, Abernathy, Texas, USA, near Lovington, New
Mexico, USA), and Schizachyrium scoparium (from
the 7W ranch near Hillsboro, Texas, USA) was purchased in 1988. Seed of Bothriochloa ischaemum (from
D. W. King, San Antonio, Texas, USA, no grower given) was purchased in 1989. Seed of Bouteloua rigidiseta was collected at Pedernales Falls State Park in
May 1988. This seed (except B. ischaemum) was planted in the summer of 1988 in styrofoam cups (15 cm
tall; 10 cm upper diameter) with a hole at the bottom
for drainage and filled with a 50% clay loam : 50% river
silt mixture. Each cup contained one plant. B. ischaemum was planted in the summer of 1989. Seed of Bouteloua pectinata could not be purchased, nor could sufficient viable seed be collected. Small individuals of
this species were collected at Pedernales Falls State
Park on 31 May 1988 and 14 June 1988 and transplanted into styrofoam cups, one plant per cup. The
2479
cups were slightly sunk (to hold them upright) in the
ground in an outdoor garden in Austin, Texas, USA.
This garden had been plowed and then fumigated with
methyl bromide in April 1988 to control weeds. Transplants were watered and fertilized as needed until transplanting.
A grid of 4 3 4 points 30 cm apart was laid out in
each subplot, leaving a buffer strip ;0.5 m wide around
the grid. Initially, three plants of each of five species
(all but Bothriochloa ischaemum) were transplanted
into each subplot at randomly selected grid points (randomly leaving one of the 16 grid points unused), on
2–16 March 1989. The styrofoam cups, but not the soil
in them, were removed just before the transplants were
planted in the field. Temporary fencing was put up
around unfenced quadrats before transplanting and taken down a month later to allow transplants to become
established before grazing began. In the second year
of the experiment Bothriochloa ischaemum was added
to the design. Transplants of this species were planted
on 12–14 March 1990 at the unused grid point in each
quadrat, and into grid points where a Bouteloua pectinata transplant had died, for a maximum of three
Bothriochloa ischaemum transplants per subplot.
Buchloe dactyloides transplants were harvested 24
May and 2–12 June 1990 because individuals of this
stoloniferous species would have competed with other
transplants in their subplots and would have grown out
of their subplots had they been allowed to grow for
another season. The other five species were harvested
18 June–9 July 1991. In all instances, plants were harvested at the soil surface (i.e., roots left in the ground)
and the aboveground tissue of each plant dried and
weighed separately.
Because aboveground dry mass at harvest was the
most sensitive measure of plant performance, the results of its analysis are reported in this paper. Other
measures of plant performance (number of tillers, basal
area, plant height, proportion of plants surviving, proportion of plants setting seed, and number of reproductive culms, measured during the course of the experiment and at its end) tended to be highly correlated
with aboveground dry mass at harvest. Values of these
variables and the results of their statistical analyses are
in the appendices.
Plant height (used in the correlation of grazing impact and height) was measured 18–27 June 1990 and
22–31 May 1991. Plant height was measured as the
greatest distance above the soil surface reached by any
vegetative tissue (i.e., excluding flowering culms)
while the measurer was not touching the plant. In other
words, the leaves were allowed to bend naturally during
measurement. Buchloe dactyloides transplants were too
short to measure their height accurately. Therefore, for
purposes of the correlation of grazing impact and
height, mean Buchloe dactyloides height was estimated
to be 5 cm in 1990.
This experimental design had four levels of repli-
2480
NORMA L. FOWLER
cation: pasture, plot within pasture, subplot within plot,
and plant within subplot. The entire experiment was
repeated in two pastures. Topographic location was replicated at the level of plot, with two plots per topographic location per pasture. The grazing and competition treatments were replicated at the level of subplot,
each combination of the grazing and vegetation removal (i.e., competition) treatments having four subplots per plot. Species was replicated at the level of
plant within subplot. The three plants of each species
in each subplot represent variation among plants within
a subplot.
For statistical analysis, each species was analyzed
separately, because the species differed so greatly in
size and morphology. Dry mass of each species except
Bouteloua pectinata was analyzed with analysis of variance (ANOVA). Pasture was treated as a fixed effect,
as were topographic location, grazing (i.e., fencing),
and competition (i.e., vegetation removal). Plot and
subplot were each considered to be random effects.
Therefore the F values of pasture, topographic location,
and their interaction (P, T, and P 3 T in Table 1) were
constructed with plot as their denominators; the F values of grazing and its interactions (G, P 3 G, T 3 G,
and P 3 T 3 G in Table 1) were constructed with plot
3 grazing as their denominators; the F values of competition and its interactions (C, P 3 C, and T 3 C, and
P 3 T 3 C in Table 1) were constructed with plot 3
competition as their denominators; the rest of the terms
had the residual error as their denominators.
There was some mortality of every species (Appendix B). Therefore subplot means of dry mass were calculated after log transformation of the individual measurements. These subplot means were used in the ANOVAs of Bouteloua curtipendula and Buchloe dactyloides dry mass. There were 128 subplot means for
each species (two pastures 3 two topographic locations
3 two plots per pasture–topographic location combination 3 two levels of grazing 3 two levels of competition 3 four replicate subplots per grazing–competition combination 5 128).
Because Bothriochloa ischaemum, Bouteloua rigidiseta, and Schizachyrium scoparium each had at least
one subplot with no surviving plant by the end of the
experiment, a further reduction of the data was done
for their ANOVAs. For each species separately, the four
means of each group of four replicate subplots in the
same plot and with the same grazing and competition
treatment were averaged. These means of replicate subplot means formed the new data set. This data set had
32 observations for each species (two pastures 3 two
topographic locations 3 two plots per pasture–topographic location combination 3 two levels of grazing
3 two levels of competition 5 32). These 32 values
were used in ANOVA of dry mass of each of these
three species. SAS (GLM procedure, SAS 1985) was
used to do all the ANOVAs.
The survival of Bouteloua pectinata was too poor to
Ecology, Vol. 83, No. 9
justify an ANOVA of dry mass. Instead, this species’
survival during the first 15 mo after transplanting
(March 1989–June 1990) was analyzed using a categorical model of the sort suggested by Grizzle, Starmer,
and Koch (GSK model; Kleinbaum and Kupper 1978:
447–485) with a response vector of (1,0), using the
CATMOD procedure of SAS (SAS 1985) and dropping
plot and subplot from the analysis to avoid empty cells.
The absolute and relative intensities of competition
(CIABS, CIREL) were calculated as
CIABS 5 (dry mass without neighbors)
2 (dry mass with neighbors)
CIREL 5 ([dry mass without neighbors]
2 [dry mass with neighbors])
4 (dry mass without neighbors).
In a parallel manner the absolute and relative impacts (not
intensity; GIABS, GIREL) of grazing were calculated as
GIABS 5 (dry mass ungrazed)
2 (dry mass grazed)
GIREL 5 ([dry mass ungrazed]
2 [dry mass grazed])
4 (dry mass ungrazed).
These four indices were calculated from the subplot
means of final dry aboveground biomass of each species. Each set of 16 subplot means from the same topographic location and receiving the same grazing and
competition treatments (two pastures 3 two plots per
pasture–topographic location combination 3 four replicate subplots per grazing–competition combination 5
16) was averaged to give a single mean of subplot
means. There were therefore eight such means of means
per species (two topographic locations 3 two levels of
grazing 3 two levels of competition 5 8). Finally, these
means of means were back transformed. These backtransformed means of means were used with the formulas given above to obtain CIABS, GIABS, CIREL, and
GIREL.
Since the analyses of Table 1 were done with logtransformed variables, they tested the null hypotheses
that grazing and competition had multiplicative effects
on each species. Because the analyses of Table 1 tested
multiplicative effects, they directly correspond to the
relative, not absolute, measures of competition intensity and grazing impact in Tables 2 and 3. A significant
grazing 3 competition term in Table 1 is equivalent to
a statistical test comparing CIREL between grazing treatments and GIREL between competition treatments.
GRAZING, COMPETITION, AND TOPOGRAPHY
September 2002
Nonparametric Spearman correlation coefficients
(rS) were calculated between GIREL and ungrazed vegetative height. Only the values of GIREL calculated from
subplots without competition (i.e., neighbors removed;
first half of Table 2) were used in this analysis. Two
values of rS were calculated, one using height in 1990
(year 1, Fig. 2) and the other height in 1991 (year 2,
Fig. 2), but the same values of GIREL were used for
each year. This analysis used only vegetative heights
measured in ungrazed subplots from which neighbors
had been removed. For all but Buchloe dactyloides,
individual plant heights were log transformed and then
subplot means were calculated for the height of each
species. Each set of 16 subplot means from the same
topographic location and receiving no grazing and no
competition (two pastures 3 two plots per pasture–
topographic location combination 3 four replicate subplots per plot 5 16) was averaged to give a single mean
of subplot means. There were therefore two such means
of means per species per year, one for each topographic
location. These means of means were then back transformed. Buchloe dactyloides was too short to measure
accurately, so the mean height of this species in 1990
in each topographic location was estimated, as 5 cm.
This species was omitted from the 1991 analysis because it was harvested in 1990. The calculated or estimated mean height of each species in each topographic location was paired with the corresponding value of
GIREL to calculated rS. Hence N 5 12 in year 1 (1990
heights; six species 3 two topographic locations 5 12)
and N 5 10 in year 2 (1991 heights; Buchloe dactyloides omitted; five species 3 two topographic locations 5 10).
The significant interaction between the effects of
grazing and competition on Bouteloua curtipendula dry
mass (see Results) and on other measures of size of
two other species (reported in Appendix C), together
with inspection of Fig. 1, suggested that there was a
grazing 3 competition effect that was almost too weak
to be detected statistically by the single-species ANOVAs of Table 1. Wilcoxon signed-ranks test for two
groups arranged as paired comparisons (Sokal and
Rohlf 1995:443) was therefore used to compare CI REL
between grazing treatments and to compare GIREL between competition treatments on all six species simultaneously.
RESULTS
Direct effects of topography
The hillside plots provided a less favorable physical
environment than did the flat plots for five of the six
species. In the absence of grazing and competition (the
treatment combination of fencing and neighbor removal), transplants of five of the six species were larger in
flat plots than in plots on hillsides (Fig. 1). The effect
of topography was always statistically significant, ei-
2481
ther as a significant main effect or as a significant interaction term (Table 1).
Bouteloua pectinata was the only species for which
the flat plots were not clearly more favorable in the
absence of grazing and competition. While transplants
of the other five species had high rates of survival
everywhere, transplants of Bouteloua pectinata had
significantly poorer survivorship in flat plots than in
hillside plots (Table 1, Fig. 1), even in the absence of
grazing and competition. However, whatever factors
reduced survival in flat plots apparently did not also
reduce growth rate: surviving transplants of this species, like those of the other five species, were larger
in flat plots than in hillside plots in the absence of
competition and grazing (Appendix A).
Direct effects of grazing
As expected, transplants in ungrazed subplots grew
larger than grazed transplants did (Fig. 1). The magnitude of the grazing effect depended, however, upon
topography and, sometimes, competition, and differed
among species. The negative effect of grazing was
greatest on Schizachyrium scoparium, the tallest species; grazing reduced the size of this species up to 95%
(in flat subplots with neighbors removed). Grazing also
had significant negative effects upon Bothriochloa ischaemum and Bouteloua curtipendula dry mass (reductions up to 79% and 88%, respectively), but had
much weaker, nonsignificant effects on the two shortest
species, Bouteloua rigidiseta and Buchloe dactyloides
(Table 1, Fig. 1).
The relationship between the relative impact of grazing (GIREL) and ungrazed plant height was significant
(P , 0.01), positive, and close to linear (Table 2, Fig.
2). This relationship is not an artifact of the positive
relationship between mass and height, because there is
no mathematical reason for GIREL, a unitless ratio of
masses, to be correlated with mass. The correlation
therefore must be ascribed to biological reasons (see
Discussion).
Direct effects of competition
As expected, the direct effects of the removal of
neighboring plants on plant size were significantly positive, indicating that neighboring plants competed with
transplants (Table 1, Fig. 1). The magnitude of the negative effect of competition sometimes depended upon
topography and grazing. Averaged across all species
and both topographic locations, the presence of neighboring plants reduced plant dry mass by 65%.
There was no apparent relationship between plant
size and relative competition intensity (CIREL, which is
relative to size when grown alone), although as expected the absolute effect of competition (CIABS) was
positively related to plant size (Table 3). The effect of
competition upon relative plant size did not differ consistently between the shorter, small species and the
larger, taller species, even in the ungrazed subplots.
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NORMA L. FOWLER
Ecology, Vol. 83, No. 9
FIG. 1. Effects of grazing (fenced or unfenced), topography (hillside or flat), and competition on final aboveground dry
mass (Bothriochloa ischaemum, Bouteloua curtipendula, Bouteloua rigidiseta, Buchloe dactyloides, Schizachyrium scoparium)
or upon the proportion surviving the first year (more exactly, 15 mo) after transplanting (Bouteloua pectinata). Squares
represent flat plots; triangles, hillside plots; filled symbols, neighbors present; open symbols, neighbors absent; solid lines,
pasture A; dashed lines, pasture B. Note that the y-axes are on a log scale.
GRAZING, COMPETITION, AND TOPOGRAPHY
September 2002
TABLE 1.
2483
Summary of the results of statistical analyses of the responses of each species.
Grazing 3 competition
Grazing
Pasture, topography
Species
P
T
P3T Plot
Final aboveground dry mass
Bothriochloa
**
*
ischaemum
NS
Bouteloua
*
curtipendula
NS
NS
Bouteloua
rigidiseta
**
*
Buchloe
dactyloides
NS
Schizachyrium NS
scoparium
G
Competition
P3T
P3G T3G 3G
C
P3T
P3G T3G
P3C T3C 3C G3C 3C
3C
P3T
3G
3C
NS
NS
****
**
****
***
****
NS
NS
†
NS
NS
NS
NS
NS
NS
***
*
**
†
***
†
NS
NS
NS
NS
*
NS
†
NS
NS
NS
NS
NS
***
NS
*
*
NS
†
NS
NS
NS
NS
NS
NS
NS
*
***
NS
*
NS
NS
NS
NS
NS
NS
NS
***
NS
**
NS
***
NS
NS
†
†
NS
NS
†
*
NS
NS
NS
NS
NS
NS
NS
NS
NS
Proportion of plants surviving 15 mo after transplanting
Bouteloua
*** ****
*
2
NS
NS
pectinata
Notes: Dry mass and plant height were analyzed with univariate ANOVA models. A categorical model was used to analyze
the proportion of plants surviving (see Methods). Abbreviations and symbols: P, pasture; T, topographic location (hillside or
flat); G, grazing treatment (fenced or unfenced); C, competition treatment (neighboring plants removed or not). Plot was
nested within P 3 T.
†, P , 0.10; * P , 0.05; ** P , 0.01; *** P , 0.001; **** P , 0.0001; 2, term not included in the model; NS, not
significant.
Evidently height did not confer a competitive advantage, perhaps indicating that competition was not primarily for light.
Interaction of grazing and topography
The effects of grazing differed significantly between
hillside plots and flat plots (Table 1). Whether an absolute or a relative measure of grazing impact is used,
grazing reduced plant size more in flat plots than in
hillside plots, regardless of transplant species, whether
or not it had neighbors, with a single exception: Buchloe dactyloides in subplots without neighbors was little
affected by grazing in either topographic position (Table 2, Fig. 1). The greatest contrast is Schizachyrium
TABLE 2.
scoparium dry mass, reduced 95%, on average, in the
flat plots, but only 30%, on average, in the hillside
plots. Averaged across all species and both competition
treatments, grazing reduced individual dry mass 54%
in the flat plots and 5% in the hillside plots.
The difference in the magnitude of the impact of
grazing in the two topographic positions was so great
that it significantly altered the relative favorableness
of flat and hillside plots for Bouteloua curtipendula
and Schizachyrium scoparium, in both pastures, in subplots with and without competitors (Table 1, Fig. 1).
Grazing also reversed the relative favorableness of hillside and flat plots for Bothriochloa ischaemum in pasture A and reduced the relative favorableness of flat
Impact of grazing on transplants.
Flat
Hillside
Species
GIABS
GIREL
GIABS
GIREL
Without competition
Bothriochloa ischaemum
Bouteloua curtipendula
Bouteloua pectinata
Bouteloua rigidiseta
Buchloe dactyloides
Schizachyrium scoparium
8.65
33.88
1.85
1.35
20.19
90.41
0.79
0.85
0.55
0.47
20.04
0.95
20.10
8.14
0.12
0.17
20.13
11.47
20.03
0.49
0.05
0.13
20.06
0.57
With competition
Bothriochloa ischaemum
Bouteloua curtipendula
Bouteloua pectinata
Bouteloua rigidiseta
Buchloe dactyloides
Schizachyrium scoparium
0.80
18.08
0.05
0.09
0.43
22.34
0.48
0.88
0.13
0.20
0.28
0.94
0.04
0.36
20.42
20.02
20.10
0.09
0.05
0.08
20.57
20.03
20.09
0.02
Notes: GIABS, absolute impact of grazing on aboveground dry mass, in grams; GIREL, relative
impact of grazing on aboveground dry mass (i.e., the proportion by which grazing reduced
mass).
2484
NORMA L. FOWLER
FIG. 2. The relationship between ungrazed vegetative
height and the impact of grazing. Abbreviations are: Bc, Bouteloua curtipendula; Bd, Buchloe dactyloides; Bi, Bothriochloa ischaemum; Bp, Bouteloua pectinata; Br, Bouteloua rigidiseta; Ss, Schizachyrium scoparium. Squares represent flat
plots, year 1 height; diamonds represent flat plots, year 2
height; triangles with points up represent hillside plots, year
1 height; and triangles with points down represent hillside
plots, year 2 height. Both values of rS are significant at P ,
0.01.
plots for this species in pasture B (Fig. 1); these effects
were also significant (Table 1). Grazing and topographic location also had a significant interaction effect upon
the survival of Bouteloua pectinata, which was particularly low in grazed flat subplots (Table 1, Fig. 1).
Ecology, Vol. 83, No. 9
of dry mass (Table 1). There was, however, a significant
topography 3 grazing 3 competition (T 3 G 3 C)
term in the analysis of Bouteloua curtipendula dry
mass (Table 1). In the flat plots, grazing and competition had independent effects on this species: grazing
alone reduced size to 15% of ungrazed size (40 g to 6
g), competition alone reduced it to 51% (40 g to 21 g),
and grazing and competition together reduced it to 6%
of ungrazed, no-competition size (from 40 g to 2 g),
the approximate product of the two (0.15 3 0.51 5
0.08). However, in the hillside plots, grazing and competition interacted in their effects: separately, the reductions were 51% and 26%, but jointly the reduction
was only 24%, not 13% (0.51 3 0.26 5 0.13). The
result was a significant T 3 G 3 C term.
There were also some significant grazing 3 competition terms among the analyses of other measures
of size (Appendix C): final tiller number of Bouteloua
ischaemum (T 3 G 3 C, P , 0.05), final tiller number
and final basal area of Bouteloua curtipendula (T 3 G
3 C, P , 0.01), and final basal area (G 3 C, P , 0.05;
P 3 T 3 G 3 C, P , 0.05) and final tiller number (P
3 T 3 G 3 C, P , 0.01) of Schizachyrium scoparium.
To test whether there was an overall grazing 3 competition interaction effect not strong enough to reach
significance in most of the statistical analyses of separate species, the relative grazing impacts (GIREL, Table
2) of all six species were analyzed together in an analysis that compared the two competition treatments. The
relative grazing impact was significantly greater in the
absence of competition (Wilcoxon signed-ranks test, n
5 12, TS 5 13, P 5 0.02). When the relative competition intensities (CIREL, Table 3) of all six species were
used to compare the two grazing treatments, the relative
intensity of competition was significantly greater in the
Interaction of competition and topography
Topography altered the effects of competition on
Bouteloua rigidiseta and Buchloe dactyloides, as
shown by significant topographic location 3 competition (Table 1). Whether grazed or ungrazed, the relative intensity of the competition (CIREL) experienced
by these two species was significantly greater in flat
subplots than in hillside subplots (Table 3). Although
the strength of the competition 3 topography interaction did not reach significance in the other species,
the relative intensity of competition was greater in flat
subplots than in hillside subplots in 10 of 12 comparisons (six species 3 two grazing treatments; Table 3).
Averaged across all species and both grazing treatments, competition reduced individual dry mass 72%
in the flat plots and 58% in the hillside plots.
Interaction of grazing and competition
In general, grazing and competition had statistically
independent effects, as shown by the mostly nonsignificant grazing 3 competition terms (G 3 C, P 3 G
3 C, T 3 G 3 C, and P 3 T 3 G 3 C) in the analyses
TABLE 3. Intensity of competition, that is, the impact of
naturally present neighbors upon transplants.
Flat
Hillside
CIABS CIREL
CIABS CIREL
Ungrazed
Bothriochloa ischaemum
Bouteloua curtipendula
Bouteloua pectinata
Bouteloua rigidiseta
Buchloe dactyloides
Schizachyrium scoparium
9.31
19.41
3.00
2.41
3.24
70.95
0.85
0.49
0.88
0.84
0.68
0.75
2.19
12.11
1.75
0.65
1.01
14.79
0.74
0.74
0.71
0.50
0.47
0.74
Grazed
Bothriochloa ischaemum
Bouteloua curtipendula
Bouteloua pectinata
Bouteloua rigidiseta
Buchloe dactyloides
Schizachyrium scoparium
1.47
3.60
1.20
1.16
3.86
2.87
0.63
0.59
0.78
0.75
0.77
0.67
2.33
4.33
1.21
0.47
1.04
3.41
0.76
0.52
0.51
0.41
0.45
0.40
Species
Notes: CIABS, absolute intensity of competition on aboveground dry mass, in grams; CIREL, relative intensity of competition on aboveground dry mass (i.e., the proportion by
which competition reduced mass).
September 2002
GRAZING, COMPETITION, AND TOPOGRAPHY
2485
absence of grazing (Wilcoxon signed-ranks test, n 5
12, TS 5 13, P 5 0.02).
exclosures, especially in flat areas, did appear to be of
uniform height (N. Fowler, personal observation).
DISCUSSION
Interactions between the effects of grazing
and competition
Plant height and grazing impact
There was a strong linear relationship between the
relative impact of grazing on each of the six grass
species and its ungrazed height (Fig. 2): the taller the
species, the greater the relative reduction in mass that
grazing caused. It has been recognized for decades that
taller grass species are more negatively affected by
grazing than are shorter species (Vallentine 1990), although I am not aware of any previous quantification
of the relationship of the sort presented here. Some of
the available information about the individual species
used in this study also supports this relationship. Schizachyrium scoparium, the tallest of the species studied
and the one found to be most affected by grazing, is
known to be a ‘‘decreaser’’ and Buchloe dactyloides,
the shortest and least-affected, is known to be an ‘‘increaser’’ (see Introduction).
Unexpectedly, there was no indication that any of
these six grass species was affected by grazing more
than its ungrazed height would predict (Fig. 2). Grazing
had been expected to be particularly deleterious to Bouteloua curtipendula, because it is locally reputed to be
an ‘‘ice-cream plant,’’ i.e., a species highly preferred
by cattle. Bothriochloa ischaemum is distinguished by
an unusual degree of plasticity in height, becoming
completely prostrate when heavily grazed (N. Fowler,
personal observation), and therefore was expected to
be particularly tolerant of grazing. But neither the preference of cattle for Bouteloua curtipendula nor the ability of Bothriochloa ischaemum to become prostrate
changed the impact of grazing on them from that predicted by ungrazed plant height.
There are many studies comparing the nutritional
value, palatability, regrowth rates, and other traits of
grass species. Much of this literature tacitly or explicitly assumes that differences in such traits cause animals to graze selectively within a site and cause grazed
species to differ in their responses to grazing (Vallentine 1990). However, the present study provides no
experimental support for any differences among grass
species in the effects of grazing on them, whether due
to grazer selection or to plant response, that are not
directly related to grass height.
The simplest hypothesis to account for the results of
this study is to posit that cattle, at least in these study
sites during this experiment, acted rather like lawn
mowers, cropping all the grass species down to the
same height. If grasses are defoliated to a uniform
height, the taller species lose a greater proportion of
their biomass, which would be expected to result in a
relatively greater impact of grazing on them, all else
being equal. The vegetation outside the experimental
Overall, competition between transplants and the
pre-existing vegetation was more intense in ungrazed
subplots, and the impact of grazing was greater in plots
from which the pre-existing vegetation had been removed: competition was greater in the absence of grazing, and grazing had more effect in the absence of
competition. The effects of competition and grazing
thus to some extent weakened each other, rather than
being independent or strengthening each other.
A likely explanation for this finding is that the transplants benefited from a grazing-caused reduction in the
biomass of competing neighbors, and that this positive
indirect effect of grazing partially counteracted the direct negative effect of tissue removal from the transplants by the cattle. In other words, defoliated neighbors outside the exclosures probably had less competitive impact upon the transplants than did their undefoliated counterparts inside the exclosures. In the
absence of competing neighbors, grazing would have
had no such positive indirect effect to partially counteract its negative effect. The occasional weak positive
effects of grazing (Table 2), if real, likely also arose
from the same source. There is no reason to postulate
a direct beneficial effect of herbivory (Belsky 1986)
upon these transplants to account for the results.
This study was designed to detect and measure interactions between the effects of grazing and competition. Although many authors have implicitly assumed
that herbivory and competition interact in their effects
on plants, fewer have tested whether such an interaction
has indeed occurred. Field experiments on the joint
effects of herbivory (or predation, in animal studies)
and competition have recently been reviewed by Gurevitch et al. (2000). Their meta-analysis found that, in
general, removing competitors had a greater effect
when predators (herbivores, in plant studies) were absent, just as was found in the present study and by van
der Wal et al. (2000) in a study of the salt marsh graminoid Triglochin maritima (geese and lagomorph herbivory). In other words, the joint effects of herbivory
and competition tend to be less than the product of
their separate effects (i.e., antagonistic). Sometimes,
however, the joint effects of competition and herbivory
are greater than their separate effects (i.e., synergistic;
e.g., Parker and Salzman 1985, McEvoy et al. 1993 [in
one of two years, by my calculations from cover values
estimated from their Fig. 8], Bonser and Reader 1995,
Dyer and Rice 1997, Rachich and Reader 1999). The
direction of an effect may even be reversed; for example, Norris (1997) found that the outcome of competition between sugar beet and purslane was reversed
by a leaf miner. Finally, there may be no interaction
in the statistical sense. For example, Rees and Brown
2486
NORMA L. FOWLER
(1992) and Reader and Bonser (1998) found no significant interactions between the effects of insect herbivory and competition (with a multiplicative model).
Clearly, all three possible types of joint effects do occur
in nature: synergistic, antagonistic, and independent.
Significant interactions between the effects of simulated herbivory of a target species and competition have
also been detected (e.g., Bentley and Whittaker 1979,
Lee and Bazzaz 1980, Kennett et al. 1992, Ramsell et
al. 1993) but not in all experiments that looked for
them (e.g., Fowler and Rausher 1985, Augner et al.
1997). If the mechanism postulated above, that cattle
grazing reduced the intensity of competition in this
study by reducing the biomass of neighboring plants,
is common, one would expect antagonistic effects of
herbivory and competition to be particularly common
when the herbivore is a generalist and therefore likely
to eat a target plant’s neighbors as well as the target
plant. There are not yet enough comparable data to
determine whether this is so.
Significant interactions between the effects of herbivory and neighboring plants can also arise from
mechanisms other than competition. Should a neighboring plant not only compete, but also provide some
protection from herbivory (e.g., Louda and Rodman
1995), or should the neighbor both compete and harbor
herbivores (e.g., Ellison 1987, Bergelson 1990, Reader
1992), the net effect of the neighboring plant will depend on whether or not the herbivore is present. There
is no evidence for such effects occurring during the
present study, although the persistent flowering culms
of Schizachyrium scoparium could perhaps deter cattle
under certain conditions.
Effects of grazing and competition upon species’
distributions across the landscape
The results indicate that grazing is the dominant factor controlling the distribution of these species in the
landscape. Grazing had a much greater effect upon
transplants in the flat plots than in the hillside plots,
actually reversing the effects of edaphic factors on four
of the six species. In the absence of grazing, all six
species grew larger in the flat plots than in the hillside
plots, but, in the presence of grazing, four of the species
grew larger in the hillside plots, significantly so for
three of them (Fig. 1). These four species (Bothriochloa
ischaemum, Bouteloua curtipendula, Bouteloua pectinata, and Schizachyrium scoparium) were also the taller
species, and therefore more affected by grazing than
the other two species (Bouteloua rigidiseta and Buchloe
dactyloides).
It therefore appears that, in this region, hillsides are
intrinsically less favorable for the growth of these grass
species, and by extension other native grass species,
than are flatter sites. This is contrary to the conclusion
that Fowler and Dunlap (1986) drew from the observed
distributions of these and other species across the landscape. Instead of being due to edaphic factors like water
Ecology, Vol. 83, No. 9
availability, the greater abundance of Schizachyrium
scoparium and other taller grasses where slopes are
greater (Fowler and Dunlap 1986) is apparently due to
past or present grazing pressure.
The effects of grazing on the two shorter species,
Bouteloua rigidiseta and especially Buchloe dactyloides, were smaller. Nor did grazing make hillsides more
favorable than flat areas for these species. One would
expect them to be more common, therefore, in flat areas. In fact, they are nearly restricted to such areas,
which they often dominate (Fowler and Dunlap 1986).
The most likely cause of the greater impact of grazing in flat plots was that cattle spent more time in those
plots and therefore removed more tissue from those
plants. Cattle prefer flatter areas and tend to avoid
rocky hillsides (see Vallentine 1990 and references
therein). Casual observations of cattle, cattle trails, and
cowpats in and near the study sites and elsewhere in
the region were consistent with this. (Cattle also spend
more time in areas nearer to water sources; for this
reason in each of the sites the hillside and flat plots
were located at equal distances from the watering
trough.) However, the present experiment was not designed to measure cattle behavior, but the effects of
this herbivore on plants, and did not separate the effects
of different amounts of defoliation in the two topographic positions from potentially different plant responses to being grazed in the two topographic positions. The experimental design also did not separate
the effects of defoliation from the effects of trampling
and dung and urine deposition. All of these may be
involved in determining the effects of grazing.
The results do not support an important role for competition among herbaceous species in determining species distributions across this landscape, although competition with pre-existing vegetation reduced transplant
dry mass by a mean of 65%. There were no obvious
relationships between the intensity of competition and
species’ distributions, or even between the intensity of
competition and morphology. However this study involved only herbaceous species, and competition between woody and herbaceous plants probably does
plant an important role in this region.
If we assume that ungrazed plant size reflects site
productivity, and therefore that the flat sites were the
more productive, perhaps the greater intensity of competition in flat plots was due to their greater productivity (Goldberg and Barton 1992, Goldberg and Novoplansky 1997). However, there are other possible explanations for greater competitive intensity in flat plots.
For example, the roots of many plant species growing
on hillsides extend into the marl layers that lie between
the layers of hard limestone (N. Fowler, personal observation), while, in at least one pasture (pasture B),
the flat plots had thin soil over a flat layer of hard
limestone that grass roots probably could not penetrate.
Thus the rooting zone in some or all of the flat plots
September 2002
GRAZING, COMPETITION, AND TOPOGRAPHY
may have been more constrained, which might have
increased the intensity of competition there.
The story that this study reveals is surprisingly simple. Cattle grazing, not soil properties, other aspects of
the physical environment, or competition with other
herbaceous species, is the primary determinant of the
distribution of the dominant grass species in this region. The effects of grazing upon community composition arise from the interaction of two rather simple
biological phenomena: the preference of cattle for flatter ground and the greater impact of this herbivore upon
taller species. More complex factors, such as herbivore
preference or differences in plant responses to competition, are probably also involved, but the first-order
story is a simple one.
It seems likely that this relatively simple story may
often characterize cattle-grazed grasslands and savannahs, as there is no reason to suppose that the system
studied here is unusual. Some of the other results of
this study, especially the antagonistic interaction between the effects of competition and herbivory, may
extend to other large grazers and the plants eaten by
them. Extension to invertebrate herbivores would be
much more problematic, since those herbivores are usually much more selective in their diets.
ACKNOWLEDGMENTS
I am very grateful for the support of the National Science
Foundation, which funded this study. The Ayres family, especially Mr. Robert Ayres, graciously allowed me to use their
ranch to conduct this experiment. Without their generosity
and support this study could not have been done. I also thank
the Shield Ranch staff, who facilitated the study in many
ways. I thank my research assistants, Douglas Brown, Don
Campanella, Jennifer Fritz, Richard Miller, Kevin Rhodes,
Mike Scioli, and Kim Warren, without whom the project could
not have been done. The transplants were initially grown at
the Brackenridge Field Laboratory of the University of Texas.
Finally, I thank Scott Wilson and two anonymous reviewers
for comments upon an earlier version of this manuscript.
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APPENDIX A
Tables showing the means of subplot means (before back transformation), are available in ESA’s Electronic Data Archive:
Ecological Archives E083-051-A1.
APPENDIX B
Tables showing the percentages of transplants that reproduced (i.e., set seed), are available in ESA’s Electronic Data
Archive: Ecological Archives E083-051-A2.
APPENDIX C
Tables showing summaries of the results of statistical analyses of the responses of each species are available in ESA’s
Electronic Data Archive: Ecological Archives E083-051-A3.