REVIEW AND
ANALYSIS OF CATTLE GRAZING EFFECTS IN THE ARID WEST, WITH IMPLICATIONS FOR BLM
GRAZING MANAGEMENT IN SOUTHERN UTAH
A Literature
Review Submitted to the Southern Utah
Landscape
Restoration Project
By Allison
Jones
The Wild Utah
Project
February, 2001
Table of
Contents
ECOLOGICAL
IMPACTS OF GRAZING ON RESOURCES AND BIOLOGICAL VALUES IN THE ARID WEST
Effects on Vegetative Communities
·
Community
composition
·
Proliferation
of exotics
·
Examples
of cattle impact studies (plants) in Utah’s part of Colorado Plateau
·
Cattle
grazing in arid vegetative communities - conclusions
Effects of cattle grazing on faunal communities
·
Large
mammals
·
Small
mammals
·
Avifauna
·
Fish and their habitats
Effects of cattle grazing on endangered species
on Utah’s part of the Colorado Plateau
· Mexican spotted owl
· Gunison sage grouse
· southwestern willow flycatcher
·
Threatened/Endangered
plants in Utah
Discussion
and conclusions - cattle and wildlife on BLM land
Changes
to physical structure of ecosystems
·
Vegetation
structure
· Soil stability/erosion
· Presence of cryptobiotic crusts
Disruption
of critical ecosystem processes
Impacts
to streams: channel morphology and aquatic
·
Bank
stability
·
Channel
morphology
·
Health
of aquatic habitats
Impacts
to wetlands and riparian zones
Associated
effects of cattle grazing: range improvements
·
Fences
·
Stock
Tanks and other water developments
·
Vegetation
treatments
Summary
of grazing effects - an evolutionary perspective
ANALYSIS
OF BLM GRAZING MANAGEMENT IN SOUTHERN UTAH, IN LIGHT OF A REVIEW OF THE
LITERATURE
Compliance
with Standards and Guidelines, and the tools to do so
·
Properly
Functioning Condition assessments of riparian areas
·
Upland
range health - or visual - site assessments
Season of
use / timing of grazing
In
1995 the U.S Department of the Interior issued new Standards and Guidelines for
grazing management that were to be applied to all BLM lands (USDI 1995). Each state BLM office then adopted these
national guidelines in the form of state manuals that were to direct BLM
management at the state level. In Utah,
for example, the new national mandate is outlined in the Utah Standards and Guidelines
for Healthy Rangelands (USGHR),
which outlines four Standards[1] that are to be applied to
all rangelands (Utah BLM 1997):
1. “Upland soils exhibit permeability and infiltration rates that sustain or improve site productivity, considering the soil type, climate, and landform.
2. Riparian and wetland areas are in properly functioning condition. Stream channel morphology and functions are appropriate to soil type, climate and landform.
3. Desired species, including native, threatened, endangered, and special-status species are maintained at a level appropriate for the site and species involved.
4. BLM will apply and comply with water
quality standards established by the State of
Utah (R.317-2) and the Federal Clean
Water and Safe Drinking Water Acts.
the Utah Water Quality Standards
(R.317-2) for surface and ground water.
A key problem with current BLM grazing management in the arid west, and one that has frustrated outside observers and conservationists, is that the BLM will often acknowledge that areas are not meeting these Standards but is unable to definitively state the reasons behind noncompliance. This is problematic because it leads to inaction on the part of the BLM to work to improve conditions on the site. The systems in which the BLM works are highly complex, and it can be difficult to attribute degraded conditions to a specific cause without careful study. In light of these uncertainties, it would behoove the BLM to turn to the scientific literature to better analyze the various reasons behind non-compliance with USGHR in the arid west. The BLM has not yet conducted such a survey of the scientific literature, so one is provided for them here.
The
following extensive review of the ecological literature was conducted to review
the evidence that cattle grazing can impede the BLM from meeting USGHR.
The vast majority of literature considered in this review comes from
studies conducted in the Colorado Plateau and intermountain west, with
particular emphasis on lower elevation sites such as those administered by the
BLM. In addition to a review of the
general literature, we use southern Utah as a “showcase,” not only to highlight
specific studies in the region, but also to analyze certain aspects of BLM
grazing management in light of both USGHR
and the findings of this review.
ECOLOGICAL IMPACTS OF GRAZING ON RESOURCES AND BIOLOGICAL VALUES IN THE ARID WEST
The
scientific evidence that cattle grazing impacts arid western landscapes, and
the sensitive biota found there, is vast.
Yet what is perhaps more disturbing is the pervasiveness of these
deleterious effects. Recent estimates
on federal grazing lands in the western United States concluded that less than
half the vegetation was 50% similar to the presumed climax community (U.S.
General Accounting Office 1988b, 1991).
The negative repercussions associated with cattle grazing on arid lands
such as the Colorado Plateau and southern Utah can be seen in vegetative
communities, faunal communities, abiotic systems, ecosystem processes, and streams
and riparian/wetland habitats. The
following literature review will not only expand on these effects, but will
highlight how each Standard, and Indicators for meeting those Standards, can be
compromised by cattle grazing.
Effects on Vegetative
Communities
Various
vegetative indicators required to meet the Utah
Standards and Guidelines include “sufficient cover and litter [to prevent
erosion and promote soil moisture], an appropriately diverse plant community
that “sustains…properly functioning ecological conditions,” and “diverse age
structure and composition.” As outlined
below, each of these indicators can be severely affected by cattle
grazing. Decreases in native plant
species diversity, cover and density as a result of livestock grazing have been
observed in a wide variety of arid ecosystems in the western U.S, including
those of the Colorado Plateau of southern Utah. Moreover, these kinds of alterations to the vegetative community
can in turn lead to significant repercussions for successional trajectories,
the abiotic environment, and wildlife.
Community
composition: Grazing affects species composition of plant communities in two
ways: 1) active selection by herbivores for or against a specific plant taxon,
and 2) differential vulnerability of plant taxa to grazing. Grazing can also
delay plant phenology, which in turn can have dramatic effects on communities
of pollinators and seed dispersers (Fleischner 1994), thereby further
disrupting the composition of a vegetative community. Studies that have documented significantly greater native
plant species richness in ungrazed areas compared to those that are grazed
include Brady et al. (Arizona - 1989), and Floyd-Hanna et al. (New Mexico -
2000).
While
cattle grazing has been shown to decrease species richness in arid communities,
it similarly affects species evenness,
with considerable secondary effects.
Long-term cattle grazing has been shown to decrease the abundance of
perennial grasses and forbs and increase the amount of annual grasses and weeds
in western deserts (in northern Arizona-Schmutz et al. 1967; the Great Basin-Rice and Westoby 1978; central Utah-Brotherson and Brotherson 1981; California-Hanley and Page 1981; and Nevada-Medin and Clary, 1990). Studies
that have focused chiefly on impacts to perennial grasses have found densities
of these grasses to be significantly decreased by grazing (in central
Utah-Cottam and Evans 1945; New Mexico-Gardner 1950; Arizona-Blydenstein et al. 1957;
Capitol Reef N.P.-Rosenstock 1996). Studies that have shown shrub cover to significantly decrease due
to cattle grazing include Bock et al. (Arizona - 1984) and Jones (Nevada
-1999). Any significant grazing-induced
changes in cover, densities or relative abundances of certain plant species or
guilds can in turn have profound implications at the community level, as these
changes can translate into major conversions of community organization, for
example, transforming grassland to desert
(Schlesinger et al. 1990).
While
it is useful to document various studies that demonstrate the deleterious
effects of cattle grazing on arid plant communities, previously completed
literature reviews comprise the strongest evidence of the impacts cattle have
on vegetation in xeric environments.
For example, in a quantitative literature review involving 50
independent grazed/ungrazed comparisons from 41 different studies performed in
arid environments of the western U.S., Jones (2000) found significant negative
impacts of cattle grazing on shrub cover, grass cover, vegetation biomass, and
seedling survival. Another extensive
literature review by Fleischner (1994) cites numerous cases where grazing was
shown to have deleterious effects on vegetative communities.
Proliferation
of exotics: One particularly insidious
result of cattle grazing in arid
western
ecosystems is the spread of exotic grasses and weeds. Grazing aids the spread and establishment of alien species in
three ways: 1) dispersing seeds in fur and dung (2) opening up habitat for
weedy species and 3) reducing competition from native species by eating them
(Fleischner 1994). Studies that have
found increased densities, cover or biomass of exotic plant species in grazed
versus ungrazed sites include Green and Kaufman (Oregon -1995), Drut (Oregon -
1994) and Harper et al. (Utah - 1996).
It would behoove the BLM to do all in its power to prevent the spread of
exotic weeds, especially in light of President Clinton’s 1999 executive order
that gives federal direction to prevent the introduction of invasive species,
as well as providing for their control and/or elimination (EO 11312).
Once
they are established (often assisted by cattle grazing), weeds negatively
impact western arid ecosystems in numerous ways. Weed infestations reduce biodiversity (Randall 1996), increase
fire frequency (Esque 1999, Brooks et al. 1999), disrupt nutrient cycling
(Vitousek 1990), alter soil microclimate (Evans and Young 1984), reduce
effectiveness of wildlife habitat
(Davidson et al. 1996, Knick and
Rotenberry 1997), and can expedite loss of topsoil in xeric environments (Lacy
et al. 1989).
The
BLM claims that “grazing can help prevent the spread of undesirable plant
species” and can “minimize, or at least have no effect on, the spread of
invasive weeds such as cheatgrass (Bromus
tectorum) (i.e. BLM 2000a). In both
cases the agency cites Sheley (1995), an article that appears in a magazine,
not a peer-referred journal. This paper
is a 2-page set of grazing recommendations, based on no experimental evidence
of its own (or any other studies for that matter), that goes into no detail on
the “proper grazing management practices” that can supposedly control weeds. The BLM has also stated that
livestock can be used to “control” weed invasions that have already occurred
(i.e. USFS and BLM 1997, BLM 2000b). These claims are based on observations made
in systems already so degraded that they might never improve. However, it is irresponsible for the BLM or
anyone else to claim that cattle grazing is in some way neutral or beneficial
in fighting weed infestations without backing these claims up with adequate
studies from the scientific literature.
The reason the BLM is not able to cite the literature on this topic is
because evidence to support the use of cattle to avoid or control weed
infestations in the arid west is scant.
Livestock select bunchgrass over weedy species when given a chance
(Gelbard, in review).
The
evidence for cattle’s implication in spread and establishment of exotic weeds
is certainly greater than any “evidence” to the contrary. A recent extensive review of the literature
on this topic Gelbard (in review), illustrates that this relationship is
insidious and pervasive. Gelbard
outlines how cattle disseminate weed seeds in their fur/hooves, increase the
“invasibility” of sites, and maintain weedy communities by continuing to graze
preferentially on natives. The ability
of cattle to increase a site’s susceptibility to invasion has received the most attention from the
scientific community. Sites become
invasible due to increased bare soils as a result of grazing, which offer
greater opportunity for weed establishment, with less competition (Gelbard, in
review, and references within). Evans
and Young (1972) found that increased soil erosion [shown to be caused by
grazing] also loosens surface soils and helps bury seeds. Exotic seeds adapted to more erosion-prone
environments will benefit from this while natives likely won’t. Deposition of nitrogen-rich livestock dung
also increases invasion of nitrophilous weeds such as cheatgrass by stimulating
germination and enhancing growth over that of native plants (Evans and Young
1975,
Smith and Nowak 1990, Trent et al. 1994, and Young and Allen
1997). Finally, cattle grazing can
further compound the above impacts by creating warmer and drier soil
microclimates, through soil compaction, and loss of plant, microbiotic crust
and litter cover. The resulting warmer,
drier microclimate reduces the competitive vigor of many native grasses
(Piemeissal 1951, Archer and Smeins 1991), thus further increasing viability of
aggressive exotics.
Examples of
cattle impact studies (plants) in Utah’s part of Colorado Plateau: An impressive number of studies
that document the impacts of cattle grazing on southern Utah’s plant
communities can be found in the literature.
These include:
·
A
study by Rawlings et al (1997) that found that the part of Canyonlands National
Park that had been grazed most intensively prior to 1967 has since been
extensively invaded by cheatgrass.
·
A
study of 530 different rangeland sites in southern Utah, where Gelbard (1999)
found that cheatgrass cover was five times greater on sites without
cryptobiotic soils (disturbed by either cattle or motorized use) than on sites
with undisturbed crusts (and 64% of all sites that were disturbed and lacking
crusts were attributed to cattle grazing)
·
A
study by Bich et al. (1995) involving a “grazing gradient” where independent
variables were measured at 4km, 7km, and 9km distant from wells critical for
cattle in the Glen Canyon National Recreation Area. The authors found that both density and basal area of Indian
ricegrass (Orzopsis hymenoides), a
native bunchgrass, increased with decreasing grazing intensity, while density
and foliar cover of snakeweed (Gutierrezia
spp.) increased with increasing grazing intensity.
·
A
study by Kleiner (1983) that found that 10 years of rest from grazing
in Chesler Park within Canyonlands National Park led to increases in litter
cover from 9.8% to 25.7%, and increases in total vegetative cover from 31.6% to
44.5%.
·
A
study in the Kaiparowits Basin by Jeffires and Klopatek (1987) that compared a
heavily grazed site, a light/moderate winter grazed site, a site10 years into
recovery from heavy grazing, and a relict, never-before grazed site. The authors found that the relict site had
significantly more herbaceous cover (comprised mostly of perennial grasses)
than all other sites. There were no significant differences between the heavily
grazed site and the recovering site for any of the measured parameters, leading
the authors to conclude that recovery from grazing can take a very long time
indeed.
·
A
study in Capitol Reef National Park (Cole et al. 1997) which used packrat
middens to describe vegetation changes in the region over time. The authors found that pre-settlement
middens contained abundant macro-fossils of plant species palatable to
livestock, such as winterfat (Ceratoides
lanata) and Indian ricegrass. Their
midden analysis demonstrated that drastic vegetation changes, unprecedented
during the last 5,000 years, occurred in this part of southern Utah between
roughly 1800 and the present. Species
typical of overgrazed range, such as snakeweed, rabbitbrush (Chrysothamnus nauseosus), and Russian
thistle (Salsola iberica) were not
recorded in middens prior to the introduction of grazing animals.
·
Another
study in Capitol Reef National Park (Willey 1994) that documented more (and
taller) native grasses, more (and taller) shrubs, and more forbs on an ungrazed
mesa top compared to a grazed areas within the Park.
·
A
study in Zion National Park (Madany and West 1983) that documented twice as
much forb cover, three times as much grass cover, and more diverse age structure
of trees on a set of ungrazed mesas, compared to a nearby grazed area.
Cattle grazing in arid vegetative communities - conclusions: With the considerable evidence of the negative impacts of cattle
grazing in arid plant communities like those on the Colorado Plateau, it is
surprising that the BLM would claim various beneficial effects of grazing such
as “increased seed dispersal, increased carbohydrate root reserves, increased
plant vigor and increased probability of plants producing seed” (i.e. BLM 2000a). While the BLM doesn’t cite any studies, they
are likely referring to “grazing optimization theory,” a theory that loss of
tissue to herbivores can actually increase total productivity or reproductive
fitness of the grazed plant (see Owen and Wiegert 1976, 1981). This theory was chiefly established through
studies conducted in highly productive and intensely managed systems, such as
the Great Plains or Africa, and has little relevance to the fragile and arid systems
of the Colorado Plateau. More importantly, the grazing optimization (or
overcompensation) theory was debunked by Belsky (1986, 1993, Painter and Belksy
1993), not only through evolutionary arguments, but also by pointing out that
the studies that have shown “evidence of overcompensation” by plants are
extremely few, rely on highly managed conditions (i.e. plants are watered,
fertilized and protected from competition during experiments), and when
replicated, have failed to produce the same results. Furthermore, a local Utah study (Trlica and Cook 1971) found that
Utah desert plants exhibit no beneficial effects from defoliation, and that
most species defoliated in the spring have significantly smaller carbohydrate
reserves than controls by fall quiescence.
Regardless of arguments about the existence of overcompensation in
plants, the optimization theory is notoriously misapplied to rangeland
management. In their landmark book
(Saving Nature’s Legacy, 1994) Noss and Cooperrider conclude that much of
the controversy over the grazing optimization theory is “traceable to confusion
over temporal/spatial scales, and to attempts to generalize over vastly
different rangeland ecosystems and to equate naturally evolved herbivore
grazing [patterns] with current livestock management practices.”
In addition to claiming increased plant vigor,
compensatory growth and seed production due to cattle grazing, you will recall
above that the BLM cited “increased seed dispersal” as a benefit of grazing
(BLM 2000a). Dispersal by fur and
hooves aside (which incidentally disperse non-beneficial weeds just as well as
natives), I came across no studies that cited cattle ingestion of seeds and
their subsequent deposition in cattle dung as a favorable environment for
native seed germination. While cattle
may very well “disperse” native seeds through their feces, this cannot be cited
as a beneficial effect unless the seeds actually germinate. Moreover, any
progeny that germinates as the result of cattle ingestion is likely to grow up
only to be ingested, or destroyed, by cattle as well - often before reaching
reproductive maturity. Native desert
plant seeds are adapted for dispersal by wind, rodent caching activities
(Longland, 1995) or ingestion by certain native
herbivores. There does not appear to be
any literature pertaining to the beneficial role of cattle in native plant seed
dispersal and germination in the arid western U.S.
The BLM also claims that
“grazing…could improve ground cover, vegetative density and diversity, and
plant vigor in some areas better than continuous rest” (i.e. BLM 2000a, BLM
2000c and BLM 2000d). They cite
Holechek et al (1989) and Holechek and Stephenson (1983) as proof that rest
from grazing is not beneficial. In
Holechek’s et al’s (1989) textbook, the authors cite four other studies as
proof that rest form grazing does not improve vegetative conditions. None of these studies were conducted on the
Colorado Plateau. Furthermore, one was
a confounded experiment (fertilizer was added to the grazing-treated sites),
and the other three focused primarily on shrub expansion, an issue that
numerous researchers have had difficulty tying one way or another to grazing,
and that is probably equally influenced by fire suppression and climate change
as it is by grazing (Jones 2000).
The BLM uses Holechek and
Stephenson’s 1983 study as an example where cessation of grazing in a degraded
area did not lead to improvements in forage plants. This is a poor study to use an example of grazing effects (or
lack of them), as there were clear confounding factors at work in this
study. While total shrub density was
significantly greater outside the
exclosure at the lowland site, it was significantly greater inside the exclosure at the upland
site. Furthermore, while cover of some
grasses (i.e. blue grama, Bouteloua
gracilis) was greater outside the exclosures, other grasses (i.e. western
wheatgrass, Elymus smithii) were
actually more common inside the exclosures.
The authors themselves admit that the timing of grazing was one of the
confounding factors in this study - if grazing had occurred in summer rather
than spring they would have expected to see the grama and wheatgrass results
reversed because of the interaction of the species’ phenology with season of
use (blue grama does well with spring grazing and western wheatgrass doesn’t -
Holechek and Stephenson 1983). And
lastly, the authors could not even address the influence of rest on forbs
because there were no forbs in the study area due to past sheep grazing. Holechek et al (1998), when referring to this
study, stated that the authors “considered control of big sagebrush the only
feasible means to improve forage production” at the site - not grazing.
The BLM will need to find
better scientific proof that long-term rest from cattle grazing does little
good for the arid plant communities of the Colorado Plateau. The burden has to be on the BLM since, in
reality, “long-term rest” from cattle grazing is the historic and natural
condition always known by these communities. In most cases studied by scientists removal
of cattle does lead to marked improvements over the long-term. In some cases arid rangelands may take over
100 years to recover (Gardner 1950).
Indeed, most of the studies included in this literature review (and
there are far too many to cite them all here) that compare an “ungrazed” site
to a currently grazed site, are actually comparing a site grazed at sometime in
the past (15, 30 50 years ago…) to one that is currently grazed. As most of these studies report significant
beneficial effects of what is actually cattle removal over the long-term (as
opposed to never being grazed), the positive effects of cattle removal cannot
be dismissed. It is irresponsible for
the BLM or anyone else to claim that grazing is somehow beneficial to the vegetative
components of arid communities without backing these claims up with numerous
references from the scientific literature.
Even the following statement
from another BLM San Juan Resource Area EA (BLM 2000d), while sounding like a
reasonable claim, is suspect: “many studies have shown that grazing can be
accommodated without causing irreparable damage to vegetative resources or
watershed values, and in fact, forage species and site conditions can be
sustained under proper grazing management” (BLM citing CAST 1996). In an extremely thorough review of the 1996
CAST document, renowned ecologist Elizabeth Painter pointed out the document’s
strong bias and unsupported claims. Among
other things, “of the 7 ½ pages of Literature Cited, only 19 citations are from
scientific journals.” In the same San
Jaun EA, the BLM also cites Holechek et al. (1989), claiming that “light grazing can be a useful means of
improving forage production during the early
stages of range deterioration when the desirable forages are still present
but in low vigor.” Most ecologists would be quick to dismiss this careless
statement - the principle reason that arid ranges in the western U.S enter into
“early stages of range deterioration” in the first place is because of grazing. Studies that suggest otherwise are few and
far between.
Effects of cattle grazing on faunal communities
Various faunal indicators required to meet the USGHR include habitat connectivity, and
“frequency, diversity, density, age classes and productivity of…native species
necessary to ensure reproductive capability and survival.”
Grazing in arid environments can exert significant
impacts on animal populations. The
effects range from direct trampling of burrows and nests, to indirect effects
on habitat structure and forage availability, to increased competition with
other native species for significantly reduced water, cover, and space (Donohue
1999). A thorough review of the
literature by Fleischner (1994) documents deleterious effects of grazing on all
vertebrate classes and numerous foraging guilds (i.e. herbivores, granivores,
insectivores, etc.). As illustrated
below, grazing has been shown to have deleterious impacts on desert fauna
ranging from lizards (Jones 1981) to large game (Kraussman 1996).
Large mammals: In the
most thorough review on this topic (Kraussman 1996), livestock are shown to
have insidious effects on large game, chiefly through habitat alteration and
behavioral avoidance. Potential
competition between elk (Cervus elaphus)
and cattle is highest on “ecologically compressed habitats” such as winter
range, where forage quality and quantity are often limited. If such areas are grazed heavily by cattle
in the fall, insufficient forage will remain for elk in the winter (Nogle and
Harris 1966) - a problem that is exacerbated in arid sagebrush ecosystems of
the Colorado Plateau (Hobbs et al. 1996).
It is estimated that perennial grass utilization by cattle of only 25 to
30% in arid western rangelands will trigger forage competition with elk
(Kraussman 1996). Many studies have
demonstrated that elk avoid or decrease their use of areas grazed concurrently
by cattle (Arizona-Wallace and Kraussman 1987;
Frisna 1992; Idaho-Yeo et al.
1993). Mule deer (Odocoileus hemionus) have been similarly shown to avoid areas with
high concentrations of cattle (Wallace and Kraussman 1987, Griffith and Peek
1989, Loft et al. 1991).
Cattle grazing also has deleterious effects on
pronghorn antelope (Antilocapra americana)
and bighorn sheep (Ovis Canadensis). Grazing chiefly impacts pronghorn habitat by
changing vegetative structure and composition. Secondary effects can include reduced fawn production in
modified and degraded habitat (Ellis 1970).
Evidence of negative effects of grazing on pronghorn populations has
been documented in Idaho (Kindschy et al. 1982), New Mexico (Howard et al.
1990), and Nevada (Ellis 1970). Cattle
grazing has also damaged bighorn sheep habitat in many areas of the southwest
(Kraussman 1996). One of the most
serious effects of cattle grazing on bighorns is transmission of
livestock-borne diseases (Menke and Bradford 1992). Whether because of this, or because of habitat degradation,
bighorn sheep demonstrate marked “social intolerance” for livestock, which can
have serious implications; even
seasonal grazing appears to result in effective bighorn habitat fragmentation
(Ralph 1984). There are numerous
examples of bighorn avoidance of livestock in the literature (see Kraussman
1996 and references within).
The BLM feels that vegetation trend studies that
reflect either stable or upward trend conditions (as defined by the BLM) equate
to “no impacts” to native game species like deer and elk (i.e. BLM 2000a). These trend studies primarily take long-term
viability of plant populations into account, as well as the forage needs of
cattle. Yet if an allotment is trending
upwards, this does not guarantee adequate forage for elk and deer. In fact, in
the San Juan Resource Area in southern Utah, for example, all (ungulate only)
wildlife combined receive only 28% of the total allocation of available forage
(BLM 1986), virtually guaranteeing that wildlife needs will scarcely be
accounted for when rangeland health is assessed. Moreover, by claiming that stable or upward
vegetative trends is the only factor that influences health and viability of
native game populations, the BLM is seriously discounting negative effects of
cattle on game that have little or nothing to do with forage; namely behavioral
avoidance, structural alteration of habitat, and competition for water
resources.
Small mammals: Cattle grazing can seriously impact small mammal
communities in arid environments of the western U.S. Hanley and Page (1981) found that grazing decreased rodent
species diversity in California desert grasslands, probably due to a decline in
plant species diversity that resulted from the grazing treatment. Rosenzweig and Winakur (1969) also found a
negative correlation between grazing intensity and rodent species diversity in
arid regions. However, they attributed
this to grazing-induced changes in structural aspects of vegetation, rather
than changes in plant species diversity.
In Utah, Rosenstock (1996) found that both small mammal overall
abundance, and species richness, were greater in ungrazed sites in Capitol Reef
National Park compared to nearby grazed areas.
Researchers have also found changes in relative abundances of certain
key desert rodent species due to cattle grazing effects (Glen Canyon NRA-Bush
et al. 1995; Nevada-Jones 1999). And
Medin and Clary (Nevada-1990) and Bock et al. (Arizona-1984), found a negative
correlation between grazing and overall rodent densities in desert
environments. In a quantitative literature review involving 18 independent
grazed/ungrazed comparisons from 9 different studies performed in xeric
environments of the western U.S., Jones (2000) found significant impacts of
cattle grazing on both rodent species diversity, and richness.
Lagomorphs (Lepus
californicus) are also affected by cattle grazing. Sparks (1968) found evidence of direct
competition for forage between cattle and black-tailed jackrabbits in early
spring when both species prefer green forage like western wheatgrass and
needle-and-thread grass. Jones (in
prep) is currently analyzing potential competition between cattle and
lagomorphs and other small mammals for limited available forage in the Grand
Staircase Escalante National Monument and San Jaun Resource Area in Utah. This analysis is based on fecal pellet
content and stomach contents of these native herbivores, and is assessing
availability of key plants needed by small mammals but usually allotted to
cattle. It is likely that forage
competition is driving what has been found to be reduced populations of
jackrabbits in grazed areas in New Mexico (Norris 1950) and Colorado (Crouch
1982).
Avifauna: Cattle grazing impacts on bird communities are chiefly manifested
through direct effects such as trampling of nest sites for ground-nesting
birds, and indirect effects such as alteration of habitat structure (Taylor
1986) and community composition.
Researchers have found cattle grazing to cause reduced species richness
of all birds (Capitol Reef N.P - Willey 1994), songbirds (northern Utah - Duff
1979), riparian passerines (Oregon - Taylor 1986), and raptors (northern Utah -
Duff 1979). Again, comprehensive
literature reviews are the most telling in terms of grazing effects. One by Saab et al. (1995) concluded that
grazing in the west has led to a decline in abundance of 46% of the 68 neo-tropical
migrants that utilize riparian habitats.
Another review by Bock et al. (1993) similarly reported that on some
western sites up to 40% of riparian birds have been found to be negatively
impacted by grazing.
Fish and their habitats: This topic
has received perhaps the most research attention in regards to grazing. The effects of cattle on native fish species
are almost always the result of indirect effects to physical habitat. Cattle grazing eliminates over-hanging banks
(Behnke and Zarn 1976, Duff 1979, Hubert et al. 1985), which are critical for
protection from predators. In fact,
Clarkson and Wilson (AZ – 1991) found that the amount of ungulate damage to
streambanks consistently explained the greatest amount of variation in native
fish abundance. Grazing also leads to
loss of shrub cover on streambanks (Kovalchic and Elmore 1992, Chaney et al.
1993), which allows water temperatures to increase, which is deleterious to
native fish because of reduced oxygen tension-levels in warmer water (Storch
1979).
In addition to researching grazing impacts to fish
habitats in the arid west, many scientists have investigated grazing effects on
fish populations themselves. Cattle grazing has been shown to reduce total
densities or abundances of native fish in Oregon (Lorz 1974), Utah (Duff, 1977,
1979 and 1983), Colorado (Stuber 1985), and the Great Basin (Claire and Storch
1983). In an extensive review of the
literature, Platts (1982) concluded that livestock grazing was the major cause
of reduced fish populations throughout the western U.S.
In the arid west, effects of
grazing on native trout communities has received particular attention. Cattle grazing has been shown to decrease
native trout size and abundance (Idaho-Keller and Burnham 1982), standing crop
(CO -Stuber 1985), and overall production (Great Basin - Bowers et al.
1979). A review of 21 studies on
grazing impacts on salmonids (Platts 1991) revealed that all but one study
demonstrated that salmonid habitats were degraded from grazing. And in a position paper published by the
American Fisheries Society (Armour et al. 1991) the Society states that,
“overgrazing is considered one of the principle factors contributing to the
decline of native salmonids in the west.”
They feel the most apparent effects of grazing on fish habitats are
changes in water quality and stream morphology, addition of sediment through
bank degradation and off-site soil erosion, and reduction of shade and cover
with resultant increases in stream temperature. These various effects of cattle grazing on native fish habitat
will be discussed further in the “Impacts to Streams and Aquatic Habitats”
section below.
Effects of cattle grazing on endangered species on Utah’s part of the Colorado Plateau
In the Colorado Plateau ecoregion, it is estimated
that the negative ecological impacts caused by grazing have contributed to the
decline of 41% of all threatened and endangered species listed under the
Endangered Species Act (Flather et al. 1994).
Mexican spotted owl: While there have actually not been any studies
that have looked at the effects of cattle grazing on Mexican spotted owls (Strix occidentalis lucida) (MSOs)
themselves, there are studies that have demonstrated impacts of grazing on
other raptors (e.g. Duff 1979, Kochert et al. 1988, Kochert 1989). Furthermore, the indisputable impacts that
cattle have been shown to have on MSO habitat and prey base on the Colorado
Plateau have prompted many ornithologists to voice concern about the potential
effects of cattle on MSOs (Howe 1994, Willey 1999, Stacey - in TWP 2000). When assessing the effects of cattle grazing
on MSOs, two critical life-requirements must be reviewed. One is quality and quantity of suitable
habitat, and the effects that cattle grazing has on these parameters. The other is availability of suitable small
mammal prey, and the effects cattle grazing has on these communities. These same two avenues of analysis were used
in the Mexican Spotted Owl Recovery Plan (USFWS 1995) to determine possible
effects of grazing on owls.
Research has shown that MSOs in Utah require
well-developed riparian vegetation (Rinkevich and Gutierrez 1997) with dense
understories (Rinkevich 1991), and multi-layered, deciduous habitats (Willey
1991 and 1992, and Willey and Van Riper 1993).
Research throughout the Colorado Plateau has revealed that MSOs require high canopy closure, high stand
density, substantial vertical and horizontal diversity, and a multi-layered
canopy resulting from an uneven aged stand (McDonald et al. 1991, Gutierrez et
al. 1995, Zwank 1996, Stacey and
Hodgson 1999). Furthermore, Stacey and
Hodgson (1999) have identified deciduous understory plants as being
particularly important to MSOs, because they offer the greatest vertical riparian
vegetative structure and canopy cover. Almost all researchers who study habitat
requirements for MSOs agree that structural complexity is paramount. Structural complexity allows MSOs to avoid
detection by avian predators such as northern goshawks and great horned
owls. It is also important for creating
cool microsites for the notably “heat intolerant” MSO (Willey 1991 and 1992, Stacey
and Hodgson 1999).
Cattle grazing has been shown to impact all of the
structural habitat attributes, outlined above, that are important to MSOs. Long-term grazing can inhibit or retard an
area’s ability to produce mature trees (USDI 1995). While upper canopy species like large cottonwoods are not
directly impacted by grazing, cattle grazing in riparian zones has been shown
to eliminate or reduce the upper canopy by preventing the establishment of
seedlings, which leads to a loss of the upper canopy over time (Gilinski
1977). In addition, long-term negative
effects of cattle grazing on tree seedling establishment can result in
decreased stand density, and more open, thermally unfavorable micro-sites (Howe
1994). Age structure of riparian trees also becomes even-aged due to cattle
grazing (Kauffman et al. 1983). Reduced
seedling establishment due to ingestion and trampling by livestock has
transformed a variety of southwest riparian systems into even-aged,
non-reproducing communities (Carothers 1977, Davis 1977, Szaro 1989). In general, both vertical and horizontal
structure of riparian areas is simplified due to cattle grazing (Taylor 1986,
Knopf et al. 1988, Medin and Clary 1989), through ingestion and trampling of
seedlings by cattle, and butting/rubbing and browsing shrubs and saplings (Howe
1994). Decreased structural complexity
can in turn impact quality and quantity of perches used by MSOs for hunting,
courtship and territorial defense (Howe 1994).
In addition to impacting quality of MSO habitat
through reduction of structural complexity, there are concerns about cattle’s
impact on quantity of MSO habitat. The
MSO recovery plan voices concern about the decrease in herbaceous ground cover
attributed to cattle grazing, and thus increased chances of catastrophic fire
in MSO habitat (USFWS 1995), with concomitant effects on MSO
foraging/wintering/dispersal/roosting and nesting habitats. The recovery plan also states that grazing
can “generally degrade, and in some cases through erosion and lowering of the
water table, virtually eliminate some riparian areas, and reduce them to a
non-functioning condition, thereby impairing use of riparian areas by
owls.” The Wildlands Project shares
this concern; in a recent publication on a New Mexico/Arizona reserve design
project that uses the MSO as a focal species, TWP states that, “loss of
riparian areas from livestock grazing may be a major factor in continuing
population declines” of the MSO (TWP 2000).
The second critical aspect
of determining potential effects of cattle on MSOs is to explore the impacts of
grazing on the MSO prey base. It has
been suggested that MSOs select habitat based partially on the availability of
prey (USDI 1995). Small mammals by far
make up the most important component of MSO diets (Howe 1994). In southern
Utah, woodrats (Kertell 1977, Rinkevich 1991, Sureda and Morrison 1998) and
white-footed mice, or Peromyscus
species (Rinkevich 1991, USDI 1995, Willey - unpubl data), have been identified
as the most important prey species for MSO. Grazing can influence prey
availability and diversity by altering various habitat conditions for small
mammals (USDI 1995). In terms of Permyscus species, a study in southeast
Utah (Sureda and Morrison) found that these mice were most often found in areas
with heavy brush, a component that is not likely to be present in heavily
grazed areas. In New Mexico, cattle
grazing has been shown to reduce abundance of Mexican voles (Microtus mexicanus) (Ward 1996), another
important prey species for MSOs (personal communication, Peter Stacey). West-wide, cattle grazing has been shown to
reduce both species density (NV - Medin and Clary 1989 and 1990) and diversity
(UT-Duff 1979, NV-Medin and Clary 1989 and 1990) of rodent populations in
riparian areas.
MSOs are not by any means
“restricted” to riparian or forested habitats.
MSOs inhabit relatively open country along the northwest part of the
species’ range in southern Utah. Here,
the MSO is strongly associated with steep sandstone canyonlands that include
relatively open Great Basin desert scrub and woodland communities (Brown 1982,
Willey 1995 and 1998). Pinyon-juniper
habitat has also been identified as an important component of MSO home ranges
during summer and fall (Willey 1992).
These mesa/upland habitats contain substantial rodent populations, which
can be severely impacted by cattle grazing, via impacts to alterations to
vegetative structure and composition (see small mammal section, above). Sureda and Morrison (1998) found that Peromyscus species were significantly
more abundant in mesa habitats than canyon (riparian) habitats in southeastern
Utah.
The impacts of grazing on sage grouse are widely
documented (see Yocom 1956, Autenrieth et al. 1977, Klebenow 1982, Dobkin
1995,). The recent petition for federal
listing of the species has cited grazing of domestic livestock and its
associated operations as likely to be the number one threat to the continued
existence of the species (Webb 2000).
Even light grazing has the potential to reduce food quality for the sage
grouse, as light grazing is known to put stress on the herbaceous plants
favored by livestock, and required by sage grouse (West 1996). The reduction of forbs due to cattle grazing
is particularly harmful to the sage grouse, not only because of the direct food
value to grouse, but also because forbs provide food sources to insects - a
critical dietary component for sage grouse chicks during their early
developmental period (Webb 2000).
Grazing also harms sage grouse by removing sheltering near the nest (Webb
1993), which is known to impact both nesting success and chick survival
(Klebenow 1969, Hein et al. 1980) due to increased predation levels.
Perhaps surprising to some, mesic meadows and
riparian areas are as important to sage grouse as healthy sagebrush habitats -
especially in the summer and fall (Savage 1969, Oakleaf 1971, Autenreith et al.
1982). Unfortunately, these are some of
the habitats impacted most heavily by cattle.
Cattle prefer gentle slopes near water.
These are precisely the same areas needed by sage grouse for nesting and
brooding (Klebenow 1969, Hayden-wing and Ostain 1986). Hens with broods have been shown to avoid
meadows where grazing has caused poor conditions such as eroded streambanks and
low grass/forb availability (Klebenow 1969).
Notably, the riparian areas in the Gunnison basin have many heavily
grazed riparian areas, and have suffered grazing impacts such as weed
encroachment into the riparian zone, and gullying with concomitant secondary
effects such as a lowered water table and loss of soil moisture (GSGCP 1997). These effects are probably having
considerable impacts on Gunnison sage grouse populations (Webb 2000).
In the Bluff Bench EA in the San Juan Resource Area
(BLM 2000c), the BLM stated that “where appropriate, allotments will be managed
under a deferred rotation system to maintain and enhance [sage grouse]
habitats.” While a discussion of the
merits of deferred rotation grazing compared to year-long grazing is beyond the
scope of this review, I came across no studies that demonstrated the benefits
of deferred rotation grazing to sage grouse.
Also, while rest-rotation systems have been found to work reasonably
well in cooler, wetter areas of the western U.S. they can to lead to continued
deterioration in deserts like the Arizona strip that have erratic rainfall and
limited water availability (Hughes 1981).
Southwestern willow flycatcher : There have been a number of studies that have researched the role
of cattle grazing in southwestern willow flycatcher (SWF) (Empidonax tralii extimus) habitat degradation, with associated
effects on SWF populations. Livestock
grazing has been implicated in willow flycatcher habitat loss and habitat
changes (Sogge et al. 1997a), reduced quality
of willow flycatcher habitat (Taylor 1986, Sanders and Flett 1989) reduced nest
productivity (Johnson 1999), and nest failure due to direct impacts by cattle
(Stafford and Valentine 1985, Valentine et al. 1988). In light of these
considerable impacts cattle have on riparian areas in the southwest, the
original petition to list the SWF stated that, “grazing of domestic cattle is
probably the single greatest direct and indirect threat to southwestern willow
flycatcher habitat” (Suckling et al. 1992).
SWFs are riparian obligates (Paradzick et al. 2000) and
require a diverse combination of over - and under-story vegetation (Hubbard
1987). Taylor and Littlefield (1986)
also detected a correlation between Empidonax
tralii abundance and riparian habitat heterogeneity. Knopf et al. (1988) detected a similar
correlation, and primarily attributed reduced abundances of sensitive riparian
passerines to the impact of cattle on the horizontal patterning of the
vegetative community. When willows are
“notched” or “highlined” by cattle, they become top heavy with live branches
above, with few remaining below. Serena
(1982) noted this condition in otherwise suitable habitat in southern
California where willow flycatchers were conspicuously absent.
The evidence that cattle grazing reduces SWF numbers is irrefutable. In southern California, Harris et al. (1987) noticed that SWF numbers increased by 50% during a 5-year period in which The Nature Conservancy acquired the area and greatly reduced the intensity of cattle grazing. And SWF appeared on the Brock Canyon allotment in the Gila National Forest the second year after cattle were removed (Suckling et al. 1992). In southeastern Oregon, AUMs on a test plot were steadily lowered from 1973 to 1982, resulting in SWF presence at the site only at the end of the experiment when AUMs had declined by a factor of four (Taylor and Littlefield 1986). In fact, many of the locations where SWF still occur (i.e. Rio Grande Conservancy land outside Albuquerque, the New Mexico Game and Fish wildlife enclosure, Grand Canyon National Park), are areas from which cattle have been excluded or dramatically reduced.
Livestock grazing can also increase parasitism by
brown-headed cowbirds (Molothrus ater)
(Kimball 1993), an exotic nest parasitizer that has been shown to be a factor
in willow flycatcher nest failure (Whitfield 1990, Sogge et al.
1997b, and Sedgewick and Iko, unpublished manuscript) and population declines
(TWP 2000). Brown-headed cowbirds,
formerly associated with bison (Bison
bison), are now followers of cattle and are attracted to the grass stubble
they leave behind (Suckling et al. 1992).
Not only do cattle bring cowbirds into riparian areas, they can increase
fragmentation of willow habitat, thus creating more edge habitat which makes
SWFs susceptible to cowbird parasitism.
Threatened/Endangered plants in Utah: While most research attention usually focuses on federally listed
animal species, its important not to discount the adverse effects of cattle
grazing on threatened and endangered plants on Utah’s part of the Colorado
Plateau. Southern Utah actually
contains far more T/E plants than one would expect. This is partly the result of intrinsically high rates of endemism
in the Colorado Plateau due to climate, the intersection of different
ecological provinces on the plateau, and distinctive geologic formations and
substrates (Welsh 1978). This results
in very small populations of unique plants that have evolved in relative
isolation and are adapted to specific habitats.
Utah currently has 29 plant species that are either
federally listed under the ESA, or are candidates for listing (UDWR 1998). Because 86% of rare Utah endemics occur in
arid and semi-arid regions of the state (where most BLM holdings exist), a
majority of federally listed plants occur on BLM lands. Groupings of the state’s T/E and sensitive
plants according to vegetation type illustrate that a majority of those species
occur in vegetative communities that typically characterize Utah BLM holdings
(Table 1, next page).
One reason for the preponderance of T/E plants in southern Utah is high intrinsic rates of endemism; the other is that the habitats for these plants have been threatened by many human activities. One of these activities is cattle grazing. With ranges as narrow as those occupied by these rare species, it is conceivable that a whole population of the rarest species could be decimated if it existed within one or two poorly placed grazing allotments. Detrimental impacts of cattle grazing have been documented on Townsend’s aprica (Townsendia aprica), Wright’s fishook cactus (Sclerocactus wrightii), and Winkler’s pincushion (Pediocactus wrinkleri) cactus in Capitol Reef National Park (San Juan College 1994). These impacts primarily consisted of death and damage to plants due to trampling. While trampling may not necessarily kill plants, it often destroys the meristem, and the plant fails to produce flowers, fruit, and seeds. The highest percentage of damaged T/E plants in Capitol Reef were found near water sources.
Discussion and conclusions - cattle and wildlife on BLM land
This section has outlined the pervasive and
insidious effects that cattle grazing can have on native wildlife on arid
rangelands such as those on the Colorado Plateau. The U.S. Department of the Interior agrees with these findings,
as outlined in Rangeland Reform ’94 (USDI 1994): “if grazing were discontinued
on western rangelands 75% of degraded…fish habitat would
be restored, waterfowl populations would increase,…[and both] game and nongame
species would benefit from improved riparian habitat and from increased
vegetation for winter food/cover.”
Changes to
physical structure of ecosystems
Various indicators required to meet the Standards
and Guidelines for physical structure of ecosystems include the appropriate
kinds of vegetative habitats for wildlife, “sufficient cover and litter to
protect the soil surface from…erosion,” and “the absence of indicators of
excessive erosion such as rills, soil pedestals, and actively eroding gullies.”
Vegetation structure: Intact physical structure of arid ecosystems is
very important to native wildlife on large and small scales. Because of grazing, shrub components have
appeared where none were before (Archer 1989, Schlesinger et al. 1990), and
extensive willow stands have been removed from streamcourses (Oregon -
Kovalchik and Elmore 1992), with profound effects on native wildlife. Grazing structurally changes habitat for
ground-dwelling vertebrates, such as snakes and lizards, through the loss of low-height
vegetation (Jones 1981, Szaro et al. 1985).
Grazing similarly affects shrub and woodland riparian forest structure,
with impacts on birds who require diverse habitat structure (Taylor1986, Knopf
et al 1988). Cattle grazing also
removes soil litter from the system (Four Corners region-Orodho et al. 1990,
Capitol Reef National Park-Willey 1994 and Rosenstock 1996, and review by Jones
2000 and references therein), which can impact ground-nesting animals that
require litter for their nests.
Soil stability/erosion: Grazing also contributes to the deterioration of
soil stability in deserts (Warren et al. 1985), thus leading to increased soil
erosion. Soil erosion is further
exacerbated by increased surface runoff triggered by loss of vegetative cover and litter (Ellison 1960),
both of which have been shown to be reduced by cattle grazing (see references
above). As soils take 5,000 to 10,000
years to naturally re-form in arid regions such the Colorado Plateau (Webb
1983), accelerated soil loss caused by grazing is an irreversible loss. The steep slopes with little to no vegetal
cover underlain by highly erodible rock that are common in the rugged landscape
of southern Utah are particularly susceptible to cattle-induced erosion.
Numerous studies have observed severe erosion when
comparing heavily grazed to ungrazed sites in the arid west (Cooperrider and
Hendricks 1937, Croft et al. 1943, Gardner 1950, Kauffman et al.
1983). In a particularly well-designed
study (Lusby 1979), a federal inter-agency committee chose Badger Wash, just
over the border from Utah in western Colorado, as a representative Colorado
Plateau site to assess grazing effects on erosion. The BLM was one of the five agencies cooperating in this 20-year
study, initiated in 1953, which compared four entire ungrazed watersheds to
four others left open to grazing. The
findings indicated that runoff was reduced by 40%, and sediment yield by 63%,
on ungrazed watersheds compared to grazed watersheds. There are a number of good reviews on this topic that describe
the indisputable impact of livestock grazing on soil stability and erosion (see
Gifford and Hawkins 1978, Fleischner 1994, Trimble and Mendel 1995, and Jones
2000).
Presence of cryptobiotic crusts: Cryptobiotic crusts, which were historically widespread in
western arid lands, are being rapidly depleted across rangelands today. These crusts increase the stability of
otherwise easily erodible soils, increase water infiltration in a region that
receives limited precipitation, and increase fertility of xeric soils often
limited in essential nutrients such as Nitrogen and Carbon (Johansen 1993,
Belnap et al. 1994).
Cattle are highly
destructive to these fragile cryptobiotic crusts that exist within many BLM
lands on the Colorado Plateau.
Cryptibiotic crusts are only prominent components of ecosystems where
large-bodied herbivores have been absent from recent evolutionary history; such
as in the Colorado Plateau and many other regions of the arid west. Under these circumstances, cryptobiotic
crusts are easily damaged by livestock (Navajo National Monument, AZ - Johansen
et al. 1981 and Brotherson et al. 1983;
Utah - Anderson et al. 1982; northern
AZ - Beymer and Klopatek 1992; northwest
New Mexico - Floyd-Hanna et al. 2000). While the previously cited studies were
conducted on the Colorado Plateau, the majority of studies that have
investigated the impacts of cattle grazing and other disturbances on
cryptobiotic soils have actually been conducted in southern Utah and have
found:
·
that
heavy grazing reduced crusts by 98.5% and light grazing reduced crusts by 52.3%
at the Desert Experimental Range in southern Utah (Marble 1990)
·
that
cryptobiotic crust cover was seven times greater in an ungrazed part of
Canyonlands National Park compared to a grazed area (Kleiner and Harper 1972)
·
that
Nitrogenase activity levels in cryptobiotic crusts decreased anywhere from 30%
to 100% in disturbed plots relative to undisturbed plots, and that threshold
friction velocities (the force required to detach soil particles from the
surface) were significantly higher in undisturbed cryptobiotic crusts than in
disturbed plots (Moab area - Belnap 1996, Belnap and Gillete 1997)
·
that
a relict, never-grazed site in the Kaiporwits Basin had significantly more
cryptobiotic crust cover than both a light-moderately winter grazed site and a
site that had not been grazed for 10 years (Jeffries and Klopatek 1987)
·
that
cryptobiotic crust cover more than doubled over a ten year period of rest from
grazing in Canyonlands National Park (Kleiner 1983)
·
and
Jayne Belnap, the respected authority on cryptobiotic soils, reports that
cattle grazing has greatly impacted cryptobiotic crust integrity within the new
Grand Staircase Escalante National Monument (Belnap 1997).
The deleterious effects of cattle on cryptobiotic
crusts are not easily repaired or regenerated.
The recovery time for the lichen component of crusts has been estimated
at about 45 years (Belnap 1993). At
this time the crusts may appear to have regenerated to the untrained eye. However, careful observation will reveal
that the 45 year-old crusts will not have recovered their moss component, which
will take an additional 200 years to fully come back (Belnap and Gillette 1997).
There are numerous secondary effects once crusts are
trampled by cattle. Destruction of
crusts increase wind and water erosion of surface soils that were previously
protected by the crusts (personal communication with Howard Wilshire). This can in turn trigger rapid loss of the
underlying topsoil (Webb 1983). The destruction of cryptobiotic soils by cattle
can reduce nitrogen fixation by cyanobacteria, and set the nitrogen economy of
these nitrogen-limited arid ecosystems back decades. A severe loss of nitrates
to plants is a significant threat in typically Nitrogen poor arid environments,
and may even eventually lead to desertification (Belnap 1995). Once crusts are
destroyed, ecosystem structure can be furthered altered when bare ground is
available for colonization by exotic weeds (see Gelbard, in review, and
references within). In addition, the breaking up of physical and microbiotic
soil crusts increases surface roughness, which favors cheatgrass germination
(Tisdale and Hironaka 1981, Stohlgren in press). The relationship of crust destruction and weeds is further
supported by evidence that intact cryptobiotic crusts reduce or prohibit weed
establishment by preventing weed seed germination (Eckert et al. 1986, Mack
1989). Even small reductions in crusts
can lead to diminished productivity and health of the associated plant
community, with cascading effects on plant consumers (Davidson et al. 1996).
The BLM has stated numerous times that grazing and cryptobiotic crusts are compatible. In many of their EAs accompanying term permit renewals in the San Juan Resource Area (i.e. BLM 2000a), the BLM states that “properly managed grazing [does] not damage crusts to the point that ecological processes associated with the crusts…would be negatively affected.” Yet, instead of citing studies that examine the effects of grazing on ecological processes associated with crusts, the BLM simply cites (first) a study that documented slightly more microphytic cover in a single grazed versus ungrazed comparison (Anderson 1994), and (next) a conclusion by one author (Schofield 1985) that reduction of