Effects of Cattle Grazing on Diversity in Ephemeral Wetlands

Abstract

Cattle are usually thought of as a threat to biodiversity. In regions threatened by exotic species invasion and lacking native wild grazers, however, cattle may produce the type of disturbance that helps maintain diverse communities. Across 72 vernal pools, I examined the effect of different grazing treatments (ungrazed, continuously grazed, wet-season grazed and dry-season grazed) on vernal-pool plant and aquatic faunal diversity in the Central Valley of California. After 3 years of treatment, ungrazed pools had 88% higher cover of exotic annual grasses and 47% lower relative cover of native species than pools grazed at historical levels (continuously grazed). Species richness of native plants declined by 25% and aquatic invertebrate richness was 28% lower in the ungrazed compared with the continuously grazed treatments. Release from grazing reduced pool inundation period by 50 to 80%, making it difficult for some vernal-pool endemic species to complete their life cycle. My results show that one should not assume livestock and ranching operations are necessarily damaging to native communities. In my central California study site, grazing helped maintain native plant and aquatic diversity in vernal pools.

Full text

Effects of Cattle Grazing on Diversity in Ephemeral
Wetlands
JAYMEE T. MARTY
The Nature Conservancy, Cosumnes River Preserve, 13501 Franklin Boulevard, Galt, CA 95632, U.S.A., email jmarty@tnc.org
Abstract: Cattle are usually thought of as a threat to biodiversity. In regions threatened by exotic species inva-
sion and lacking native wild grazers, however, cattle may produce the type of disturbance that helps maintain
diverse communities. Across 72 vernal pools, I examined the effect of different grazing treatments (ungrazed,
continuously grazed, wet-season grazed and dry-season grazed) on vernal-pool plant and aquatic faunal
diversity in the Central Valley of California. After 3 years of treatment, ungrazed pools had 88% higher cover
of exotic annual grasses and 47% lower relative cover of native species than pools grazed at historical levels
(continuously grazed). Species richness of native plants declined by 25% and aquatic invertebrate richness was
28% lower in the ungrazed compared with the continuously grazed treatments. Release from grazing reduced
pool inundation period by 50 to 80%, making it difficult for some vernal-pool endemic species to complete
their life cycle. My results show that one should not assume livestock and ranching operations are necessarily
damaging to native communities. In my central California study site, grazing helped maintain native plant
and aquatic diversity in vernal pools.
Key Words: biodiversity, grazing, land management, species richness, vernal pools
Efectos del Apacentamiento de Ganado sobre la Diversidad en Humedales Ef´ımeros
Resumen: Generalmente se piensa que el ganado es una amenaza para la biodiversidad. Sin embargo, en
regiones amenazadas por la invasi´
on de especies ex´
oticas y carentes de apacentadores silvestres nativos, el
ganado puede producir el tipo de perturbaci´
on que ayuda a mantener a diversas comunidades. Examin´
eel
efecto de diferentes tratamientos de apacentamiento (sin apacentamiento, apacentamiento continuo, apacen-
tamiento en ´
epoca de lluvias y apacentamiento en ´
epoca de sequ´
ıa) sobre la diversidad de plantas y fauna
acu´
atica en 72 charcos primaverales en el Valle Central de California. Despu´
es de tres a˜
nos de tratamiento,
las charcas sin apacentamiento ten´
ıan 88% de m´
as cobertura de pastos anuales ex´
oticos y 47% de menos
cobertura relativa de especies nativas que charcas con apacentamiento en niveles hist´
oricos (apacentados
continuamente). La riqueza de especies de plantas nativas declin´
oen25% y la riqueza de invertebrados
acu´
aticos fue 28% menor en los tratamientos sin apacentamiento que en los apacentados continuamente. El
cese de apacentamiento redujo el per´
ıodo de inundaci´
on entre 50 y 80%, haciendo que a algunas especies
end´
emicas de charcos primaverales se les dificultara completar su ciclo de vida. Mis resultados muestran que
no se debe asumir que la operaci´
on de ganado y de ranchos necesariamente es da˜
nina para las comunidades
nativas. En mi sitio de estudio en el centro de California, el apacentamiento ayud´
oamantener la diversidad
acu´
atica y de plantas nativas en charcos primaverales.
Palabras Clave: apacentamiento, biodiversidad, charcos primaverales, gesti´on de tierras, riqueza de especies
Paper received June 16, 2004; revised manuscript accepted October 11, 2004.
1626
Conservation Biology 1626–1632
C
2005 Society for Conservation Biology
DOI: 10.1111/j.1523-1739.2005.00198.x

Marty Cattle Grazing in Wetlands 1627
Introduction
Disturbance plays a critical role in maintaining the di-
versity, structure, and function in many ecological sys-
tems (White 1979; Sousa 1984; Hobbs & Huenneke 1992).
Grazing is an important widespread disturbance in natural
grasslands (McNaughton et al. 1989; Milchunas & Lauen-
roth 1993; Perevolotsky & Seligman 1998); native herbi-
voreshave been extirpated from many grassland ecosys-
tems, however, and replaced with non-native ungulate
species. Poorly managed grazing has negatively affected
biodiversity in some ecosystems (Painter & Belsky 1993;
Fleischner 1994; Freilich et al. 2003), but livestock can
also serve as a functional equivalent to large herbivores
that historically grazed grasslands and play an essential
role in maintaining biodiversity (Collins et al. 1998; Har-
rison 1999; Maestas et al. 2003).
One might expect grazing to be most beneficial in areas
where keystone herbivores have been removed and can-
not be reintroduced for practical or economic reasons.
In fact in some regions domestic livestock such as cat-
tle may be the only grazer available for the promotion of
biodiversity (Knapp et al. 1999). Although a number of
studies have recorded higher biodiversity in grazed rela-
tive to ungrazed systems (Noy-Meir et al. 1989; Harrison
1999; Hayes & Holl 2003) and have quantified that graz-
ing can effectively control invasive species (Stohlgren et
al. 1999; DiTomaso 2000), none has sampled both plant
and aquatic diversity in the same system, quantified inva-
sive species, and taken measurements that allow one to
consider altered hydrology. In other words, few grazing
studies consider other measures of ecosystem response to
grazing treatments beyond biodiversity of a single trophic
level.
Ver nal pools occur throughout California’s Central Val-
ley grasslands, where the soils restrict water percolation.
These seasonal wetlands fill with water in the winter
rainy season, dry down in the spring as rainfall decreases
and temperatures rise, and remain desiccated throughout
the hot, dry summer. These extreme conditions create
a unique ecosystem that harbors high species diversity
and endemism: more than 100 vascular plant species and
more than 34 crustacean species (King et al. 1996; Keeley
&Zedler 1998). Consequently, vernal pools have become
atarget for conservation organizations and are the sort of
target that often prompts conservationists to think they
need to remove cattle from the land (Barry 1998; Griggs
2000). Vernal-pool habitats are disappearing rapidly in
California. In the last 20 years more than one-third of the
vernal-pool habitat in Sacramento County alone has been
lost to development (Holland 1998). It is a matter of ur-
gency that the last remaining vernal pools be protected,
and the science informing these conservation efforts has
lacked an experimental foundation. A major controversy
surrounding the conservation of vernal pools concerns
grazing. Some argue we need to remove cattle from the
vicinity of these threatened habitats, and others argue cat-
tle should be used to manage vernal-pool grasslands. In
fact, cattle grazing was implicated as a major contributing
factor to the decline of four vernal-pool crustaceans listed
under the U.S. Endangered Species Act (USFWS 1994)
with little to no supporting scientific data.
I controlled grazing timing in vernal-pool grasslands
in California’s Central Valley to determine the effects of
grazing on native plant and aquatic faunal diversity. I hy-
pothesized that removing grazing would negatively im-
pact plant species diversity but would have either neutral
or positive effects on aquatic invertebrate and vertebrate
diversity. My experiments involved natural vernal pools
across two different soil types and a wide range of ver-
nal pool sizes and depths. I removed grazing either com-
pletely or seasonally from groups of pools and compared
the response of plant species cover and diversity, pool
hydroperiod, soil compaction, and aquatic invertebrate
diversity to pools grazed at historical levels.
Methods
Study Site
I conducted my experiments on a 5000-ha parcel of land
in eastern Sacramento County, California (USA) (38◦38N,
121◦02W; elevation, 75 m). The climate of this region
is Mediterranean, with average annual rainfall of 56 cm
occurring between October and May. Less than 2 cm of
rain falls from June through September.
The area is flat with low-lying hills that rise in elevation
from 50 to 160 m. Vernal pools occur in the lower, flatter
areas and are abundant on approximately one-third of the
area. The site has been grazed by cattle for the past 100
or more years. Cattle graze the entire site each year from
approximately October through June at a stocking rate of
1 animal unit (cow-calf pair) per 2.4 ha.
Experimental Design
In 2000 I selected 24 groups of pools (4 treatments ×6blo-
cks) for use in this study based on soil maps, aerial pho-
tographs, and available geographic information system
mapping of vernal pool occurrence. Each treatment plot
contained three nested pools (24 treatment plots ×3
pools =72 pools) of varying sizes (range: 70–1130 m2,
mean: 252 m2)and shapes. I stratified the treatments
across two major geologic formations occurring on the
ranch (the Laguna and Valley Springs formations) and ran-
domly assigned treatments to the four plots within each
block. Three of the blocks (12 plots) were located on
the Valley Springs formation and the other three on the
Laguna formation.
I applied four grazing treatments to the plots dur-
ing the 2000–2001, 2001–2002, and 2002–2003 grazing
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1628 Cattle Grazing in Wetlands Marty
seasons: (1) ungrazed, released from grazing; (2) dry-
season grazed, grazed October through November and
mid-April through June; (3) wet-season grazed, grazed De-
cember through mid-April; and (4) continuously grazed,
control, grazed during the historical grazing season (Oc-
tober through June). Electric fencing was used to ex-
clude cattle from the ungrazed and seasonally grazed treat-
ments. Cattle exclosures ranged in size from 0.33 to 0.80
ha, which included an average 14-m buffer around each
pool and a 9:1 ratio of upland to pool area. Pool area and
depth did not differ significantly among treatments at the
beginning of the experiment.
I collected data on plant species composition each year
in permanently marked 35 ×70 cm quadrats after the
pools had dried and the majority of the plant species
were flowering (April—May). Quadrats were randomly
located and sampled along three transects for each pool
in three different pool zones (3 quadrats ×3 zones =9
quadrats per pool). The three zones were the (1) deep-
est part of the pool, (2) the edge of the pool (selected in
the first year based on the high-water mark of the pool),
and (3) upland area (5 m from the adjacent edge quadrat).
Each plant species occurring in the quadrat was recorded
and given a modified Daubenmire cover class value (Bar-
bour et al. 1987). I pooled quadrat data for each zone in
each pool. Cover class values were converted to midpoint
cover values to calculate percent cover. I calculated rela-
tive cover of native species to assess the relative effect of
grazing treatments on native species in relation to exotic
species. Soil compaction was measured with a penetrom-
eter (Geotest Instrument Corporation, Evanston, Illinois)
at each of the vegetation sampling points (n=9 per pool)
in October 2003, before the first rainfall of the season.
Once the pools filled with water, I took weekly depth
measurements at a permanent marker located in the deep-
est part of each pool. The weekly presence or absence of
water was used to calculate total and maximum inunda-
tion period for each pool and the number of dry-down
periods during the grazing season. In 2001 I did not start
collecting data on pool depth until February, so period
of inundation data are not available for the first season of
the study.
Isampled for aquatic invertebrates once in January and
once in March of each year in all pools that held sufficient
water. In 2003 I also sampled a random subset of eight of
the pools every 2 weeks until they were dry. I collected
aquatic invertebrates and vertebrates in a D-shaped dip-
net (500-µm mesh and 0.1-m2aperture). Each pool was
proportionally sampled based on the surface area of water
in the pool estimated in the first year of the study with a
global positioning system (TSC1 Asset Surveyor, Trimble
Navigation Limited, Sunnyvale, California) with submeter
accuracy. I used the same sampling scheme throughout
the study and adjusted it during pool dry down to reflect
the reduced pool surface area. Ten percent of the area of
each pool was sampled by sweeping the dipnet as close to
the pool bottom as possible along a randomly located tran-
sect. The number of transects and length of each transect
per pool were established in the first year of the study. For
example, a 210-m2pool would receive seven, 3-m-long,
randomly located dipnet sweeps. All contents collected
in the net from a pool were consolidated and surveyed
for threatened and endangered species before preserving
the sample in 70% isopropyl alcohol. All threatened and
endangered species were recorded, removed from the
sample, and returned to the pool. Samples were stored at
room temperature until they could be sorted and identi-
fied to the lowest taxonomic level in the laboratory. With
only five exceptions (n=42), all invertebrate taxa were
identified at least to genus.
Statistical Analyses
I used a two-way, nested analysis of variance (ANOVA) mo-
del with experimental block and grazing treatments as
main effects and pool nested within grazing treatment
to test for significant nested effects for each variable
measured in each year for all pool zones combined. If
the nested effect was not significant, I pooled the data
for each plot and then performed the statistical analy-
sis with block and grazing treatment as main effects. I
used repeated-measures multivariate analysis of variance
(MANOVA) to test for treatment effects across years. For
the vegetation data, I also tested for grazing effects within
each pool zone separately. Pairwise comparisons were
made with Tukey’s honestly significant difference (HSD)
test (Sokal & Rohlf 1994). Stepwise multiple regression
analysis was used to determine the relationship between
faunal richness and pool area and depth. I report diversity
as species or taxa richness. All analyses were conducted
using JMP statistical software (version 4.0, SAS Institute
2001).
Results
The continuously grazed (CG) pools had the highest rel-
ative cover of native species across all 3 years of the ex-
periment (n=69, F=10.54, p<0.0001). In 2001 the
relative cover of natives was 20% higher in the CG treat-
ment than in the ungrazed (UG) and dry-season grazed
(DG) treatments (n=72, F=5.50, p<0.01). In 2002
and 2003 relative native cover was 26 to 47% higher in
the CG treatment than in the wet-season grazed (WG) and
UG treatments (n=72, 2002: F=7.01, p<0.001; 2003,
F=10.12, p<0.0001). The effects of grazing on relative
native cover in the pool zone were not significantly dif-
ferent in any of the 3 years. By 2003 relative native cover
in CG treatments in the edge zone was 80% higher than
in the UG treatment (n=24, F=11.93, p<0.001) and
160% higher than in the UG treatment in the upland zone
(n=24, F=8.18, p<0.01, Fig. 1a).
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Marty Cattle Grazing in Wetlands 1629
Figure 1. Effect of grazing treatments in three
vernal-pool zones in 2003 on (a) relative cover of
native species and (b) absolute cover of exotic annual
grasses (CG, continuously grazed; DG, dry-season
grazed; WG, wet-season grazed; UG, ungrazed; n.s., not
significant; ∗∗p<0.01, ∗∗∗ p<0.001).
Exotic annual grass cover increased dramatically in the
pools with grazing removal during the experiment (n=
69, F=10.74, p<0.0001). In 2001 exotic grass cover
was 67% higher in the UG versus CG treatments (n=
72, F=3.86, p=0.01). In 2002 exotic grass cover was
60 to 130% higher in the UG treatment than in all other
treatments (n=72, F=11.83, p<0.0001). In 2003 the
DG and UG treatments had 60 and 88% more exotic grass
cover, respectively, than the CG treatment (n=72, F=
7.76, p<0.001). This effect was significant in the pool
zone only in 2002 (n=24, F=3.30, p=0.05) and in the
upland zone in 2002 (n=24, F=14.78, p<0.001) and
2003 (N=24, F=7.15, p<0.01) but was not significant
in the edge zone in any year (Fig. 1b).
I measured a strong increase in cover of grasses relative
to forb cover in the ungrazed treatments (n=69, F=
7.41, p<0.001). The ratio of grass cover to forb cover
was not significantly different among treatments in 2001
(n=72, F=2.10, p=0.10) but was nearly two times
higher in UG than in all other treatments in 2002 (n=72,
F=4.29, p<0.01) and three times higher in UG than in
the CG treatment in 2003 (n=72, F=7.02, p<0.001,
Fig. 2). By 2003 the ratio was four times higher in UG
Figure 2. Effect of grazing treatments on the grass to
forb cover ratio over the 3 years of the experiment
(CG, continuously grazed; DG, dry-season grazed; WG,
wet-season grazed; UG, ungrazed; n.s., not significant;
∗∗p<0.01, ∗∗∗ p<0.001).
pools than in CG pools (n=24, F=2.95, p=0.06) and
two to three times higher in UG and DG uplands than in
CG uplands (n=24, F=4.85, p=0.001).
Native plant species richness either increased or re-
mained the same over the 3 years of the experiment in
the CG, WG, and DG treatments, whereas it declined in
the UG treatments (n=72, F=7.61, p<0.001, Fig. 3).
The change in native species richness in the pool zone
was not significantly different among grazing treatments.
In the edge zone, the UG pools had on average three fewer
native species per quadrat than the CG pools (n=24, F
=9.49, p<0.001). The UG upland zone had one to two
fewer native species per quadrat than the WG and CG
uplands s. This change in diversity represented a loss of
25% of the average native species richness in the edge
and upland zones over the 3 years of the experiment.
Figure3. Change in native species richness (s) per
quadrat between 2001 and 2003 in the four grazing
treatments (CG, continuously grazed; DG, dry-season
grazed; WG, wet-season grazed; UG, ungrazed; n.s., not
significant; ∗p<0.05, ∗∗p<0.01).
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1630 Cattle Grazing in Wetlands Marty
In 2003 the CG pools had a total inundation period that
wasonaverage 49 days longer than the UG pools and 24
days longer than the WG pools (n=72, F=7.30, p<
0.001). More important, maximum inundation period in
the CG pools was on average 115 (±9) days, whereas
UG pools were inundated for an average of 65 (±8) days,
DG pools for 78 (±7) days and WG pools were inun-
dated for 65 (±8) days (n=72, F=10.53, p<0.0001).
The sporadic rainfall pattern in 2003 led to several of
the pools drying and refilling during the rainy season.
The CG pools dried on average less than one time during
the season (0.61 ±0.14 dry-down periods), whereas the
UG, DG, and WG pools dried on average twice during
the season (2.11 ±0.25, 1.61±0.18, 1.67 ±0.21 dry-
down periods, respectively). Hydroperiod did not differ
significantly among grazing treatments in 2002. Soil com-
paction was significantly lower in the UG treatment than
in all other grazing treatments (n=72, F=20.19, p<
0.001). Compaction ranged from 3.88 ±0.10 kg/cm2in
the UG treatment to 4.48 ±0.01 kg/cm2in the CG pools.
In 2003 the UG pools had the lowest invertebrate taxa
richness with an average of 10 (±0.80) invertebrate taxa,
whereas the CG, DG, and WG pools had an average of 14
(±0.50), 12 (±0.72), and 11 (±0.62) taxa, respectively
(n=109, F=3.41, p=0.02). Pool depth and period
of inundation were the strongest predictors of total in-
vertebrate taxa richness in 2003 (n=109, r2=0.25,
p<0.0001; p=0.03, respectively). Taxa richness was
not significantly different among treatments in 2001 or
2002.
Discussion
My field experiments indicate that when cattle are re-
moved from grazed vernal pool grasslands, diversity de-
clines and non-native species abundance increases. The
decline in native plant cover and diversity in the ungrazed
treatments was most likely caused by the significant in-
crease in grass cover. Exotic grasses maintain dominance
primarily by competing for soil moisture and light re-
sources (Barbour et al. 1993) and accumulation of thatch
(Huenneke et al. 1990). Many of the native vernal pool
plants (e.g., Lasthenia spp., Downingia spp., Ver onica
spp.) are small and require an open environment to suc-
cessfully germinate and reproduce (Linhart 1988). Cattle
grazing may be particularly effective at reducing exotic
grass cover because cattle selectively forage on grasses
(Kie & Boroski 1996; Knapp et al. 1999) and help main-
tain a more open canopy (Weiss 1999).
Most of the exotic grasses in this system cannot tolerate
extended periods of inundation, so hydrology plays a ma-
jor role in controlling grass encroachment into the pools
(Gerhardt & Collinge 2003). My results show, however,
that prolonged inundation in the absence of grazing is not
enough to keep exotics out of the pools. Moreover, the
decreased inundation period in the pools may make the
habitat more suitable for exotic grass growth and invasion.
The edge and upland zones were the most negatively af-
fected by grazing removal with marked declines in native
species richness and relative cover of natives. The loss of
native plant species diversity on the edge of the pool in
particular may adversely affect other trophic levels such
as specialist pollinators that depend on the pollen of the
plants which grow only in that pool zone (Thorp & Leong
1998).
The primary cause of the dramatic decrease in pool
hydroperiod in the ungrazed and seasonally grazed treat-
ments may be increased evapotranspiration rates that
resulted from the abundance of vegetation, principally
grasses, in and around the pools. This conclusion is sup-
ported by studies in the Midwest and northern Great
Plains that documented higher evapotranspiration rates
in ungrazed grasslands relative to grazed grasslands (Bre-
mer et al. 2001; Frank 2003). Results from studies in other
habitats show significant negative effects of abundant veg-
etation on hydrology in wetlands and streams (Bliss &
Comerford 2002; Moorhead 2003).
Although decreased soil compaction in UG pools does
not appear to be a major factor in reducing inundation pe-
riod, I cannot rule out changes in the water-holding capac-
ity of the soil as a contributing factor. The pattern of wa-
ter depths for 2003 in the pools showed little difference
between grazed and ungrazed pools during the initial in-
undation period, but a sharp decline in depth quickly
followed by complete drying in the ungrazed pools oc-
curred once the primary vegetation growth period began
in early March ( J.T.M., unpublished data). If decreased soil
compaction was a major contributing factor, then the hy-
drology in the UG pools should have differed significantly
throughout the season rather than just at the end of the
season.
It is important to remember that this is a relatively
short-term study in a system driven by variable rainfall
and temperature regimes. I expect the effects of grazing
removal on vernal pool hydrology to be less prominent in
years with consistent, above-average rainfall and in areas
with much higher and much lower annual rainfall than
my study site (Pyke & Marty 2005).
The decline in invertebrate taxa richness in the un-
grazed pools was most likely due to the altered pool hy-
drology, particularly the increased number of dry-down
periods. Pools with shorter hydroperiods tend to be in-
habited by species with rapid development cycles, mainly
because the pools dry up before longer-lived species are
able to complete their life cycles (King et al. 1996). Al-
though the invertebrate communities in all the study
pools developed quickly, reaching maximum taxa rich-
ness approximately 6 weeks after first inundation for all
treatments ( J.T.M., unpublished data), each dry and re-
fill cycle in the UG pools essentially arrested community
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Marty Cattle Grazing in Wetlands 1631
development, therefore contributing to a lower number
of taxa in the next sampling cycle. The invertebrate com-
munities in the continuously grazed pools had sufficient
time to develop and presumably provide for the success-
ful growth and reproduction of the longer-lived taxa.
In addition to negatively affecting community diversity,
this shift in hydrology has important implications for the
individual species that inhabit these pools. In particular,
the decreased hydroperiod in the ungrazed and season-
ally grazed pools may not support the long reproductive
cycles of some of the rare invertebrate and vertebrate
species that depend on these pools to complete their life
cycles. For example, California tiger salamanders (Am-
bystoma californiense)require pools with continuous
inundation periods of 70–90 days (Shaffer & Trenham
2004). The Western spadefoot toad (Spea [Scaphiopus]
hammondii ) that inhabits these vernal pools could be
negatively affected by hydroperiod reduction. They re-
quire pools with on average 80 days of inundation (Morey
1998). The ungrazed pools in my study could not provide
suitable toad and salamander habitat, and the seasonally
grazed pools could provide only marginal habitat.
Why is cattle grazing so clearly beneficial to biodiver-
sity in these vernal pools, when conservationists often
advocate severe grazing restrictions? The answer to this
question comes in part from the fact that California grass-
lands have a long history of extensive grazing dating back
to the Pleistocene but were most recently grazed by herds
of tule elk (Cervus elaphus nannodes) and pronghorns
(Antilocarpa americana) before livestock introduction
in the late 1800s (Edwards 1996). Hence, the pool species
are adapted to some level of grazing. In addition, the plant
species composition of California Central Valley grass-
lands has changed significantly since European settlement
and is now dominated by exotic annual grasses (Barbour
et al. 1993). Thus, a long history of grazing coupled with
the altered plant community yields a system that is now
adapted to the changes brought about by cattle and one
that becomes quickly degraded when cattle are removed.
Conclusion
The conservation of rare habitats and the species that rely
on them depends not only on staving off development but
also on implementing appropriate management regimes.
It is not enough to simply monitor change in these sys-
tems as we guess at what the appropriate management
formula might be. Experimental studies testing a variety
of management options are the best hope for success-
ful conservation. In the vernal pool grasslands I studied,
Ifound that livestock grazing played an important role
in maintaining species diversity. There were significant
negative effects on the native plant community, pool hy-
drology, and the aquatic invertebrate community with the
removal of grazing. Grazing should be considered one of a
variety of important tools for land managers interested in
the maintenance of biodiversity. The debate over grazing
needs to move beyond the simple dichotomy of whether
it is good or bad and be properly evaluated through ex-
perimental studies of practical alternatives.
Acknowledgments
Iamindebted to all of The Nature Conservancy interns
who worked on this project and L. Serpa for his aquatic
invertebrate expertise and field support. I thank J. Chance
for allowing continued access to the property and J. Spar-
rowkfor the use of his cattle. I thank P. Kareiva, C. Pyke,
and two anonymous reviewers for valuable comments on
the manuscript. Funding support for this research was
provided by the Anderson Grasslands Research Fund, Eliz-
abeth and Stephen Bechtel Jr. Foundation, Bernard Osher
Foundation, and a private donor.
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