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Grazing management influences the subsidy of terrestrial prey to trout in central Rocky Mountain streams (USA)


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1. Research in forest and grassland ecosystems indicates that terrestrial invertebrates that fall into streams can be an important prey resource for fish, providing about 50% of their annual energy and having strong effects on growth and abundance. However, the indirect effects of land uses like cattle grazing on this important prey subsidy for stream salmonids are unclear. 2. During summer 2007, we compared the effects of three commonly used grazing systems on terrestrial invertebrate inputs to streams in northern Colorado and their use by trout. Cattle graze individual pastures for about 120 days under traditional season‐long grazing (SLG), about 35–45 days under simple rotational grazing and 10–20 days under intensive rotational grazing in this region. We also compared these effects to a fourth group of sites grazed only by wildlife (i.e. no livestock use). 3. Overall, rotational grazing management (either simple or intensive), resulted in more riparian vegetation, greater inputs of terrestrial invertebrates, greater biomass of terrestrial invertebrate prey in trout diets, a higher input compared to trout metabolic demand and more trout biomass than SLG. However, these differences were frequently not statistically significant owing to high variability, especially for trout diets and biomass. 4. Despite the inherent variability, riparian vegetation and terrestrial invertebrates entering streams and in trout diets at sites managed for rotational grazing were similar to sites managed for wildlife grazing only. 5. These results indicate that rotational grazing systems can be effective for maintaining levels of terrestrial invertebrate subsidies to streams necessary to support robust trout populations. However, factors influencing the effect of riparian grazing on stream subsidies are both spatially variable and complex, owing to differences in microclimate, invertebrate and plant populations and the efforts of ranchers to tailor grazing systems to specific riparian pastures.
Content may be subject to copyright.
Grazing management influences the subsidy of terrestrial
prey to trout in central Rocky Mountain streams (USA)
Department of Fish, Wildlife, and Conservation Biology, and Graduate Degree Program in Ecology, Colorado State University, Fort Collins,
CO, U.S.A.
1. Research in forest and grassland ecosystems indicates that terrestrial invertebrates that fall into
streams can be an important prey resource for fish, providing about 50%of their annual energy
and having strong effects on growth and abundance. However, the indirect effects of land uses like
cattle grazing on this important prey subsidy for stream salmonids are unclear.
2. During summer 2007, we compared the effects of three commonly used grazing systems on
terrestrial invertebrate inputs to streams in northern Colorado and their use by trout. Cattle graze
individual pastures for about 120 days under traditional season-long grazing (SLG), about 35–
45 days under simple rotational grazing and 10–20 days under intensive rotational grazing in this
region. We also compared these effects to a fourth group of sites grazed only by wildlife (i.e. no
livestock use).
3. Overall, rotational grazing management (either simple or intensive), resulted in more riparian
vegetation, greater inputs of terrestrial invertebrates, greater biomass of terrestrial invertebrate
prey in trout diets, a higher input compared to trout metabolic demand and more trout biomass
than SLG. However, these differences were frequently not statistically significant owing to high
variability, especially for trout diets and biomass.
4. Despite the inherent variability, riparian vegetation and terrestrial invertebrates entering
streams and in trout diets at sites managed for rotational grazing were similar to sites managed for
wildlife grazing only.
5. These results indicate that rotational grazing systems can be effective for maintaining levels of
terrestrial invertebrate subsidies to streams necessary to support robust trout populations.
However, factors influencing the effect of riparian grazing on stream subsidies are both spatially
variable and complex, owing to differences in microclimate, invertebrate and plant populations
and the efforts of ranchers to tailor grazing systems to specific riparian pastures.
Keywords: food web subsidies, livestock grazing, riparian ecology, stream salmonids, terrestrial insects
Habitat degradation is the leading cause of biodiversity
loss worldwide (Vitousek et al., 1997; Dirzo & Raven,
2003), and humans have now modified >75%of the ice-
free land surface (Ellis & Ramankutty, 2008). Negative
effects on plant and animal populations may result
directly from habitat loss, but also indirectly from decou-
pling important linkages among habitats and communi-
ties (Foley et al., 2005). These linkages are especially
important in streams, which have small area but long
boundaries with adjacent riparian areas, so they are
strongly influenced by fluxes from terrestrial habitats
they drain (Wallace et al., 1997; see Baxter, Fausch &
Saunders, 2005 for review). However, the extent to which
different riparian land uses influence stream food webs by
Correspondence: W. Carl Saunders, Department of Watershed Sciences, Utah State University, Logan, UT 84322, U.S.A. E-mail:
Freshwater Biology (2012) 57, 1512–1529 doi:10.1111/j.1365-2427.2012.02804.x
1512 2012 Blackwell Publishing Ltd
altering fluxes of terrestrial materials to streams is still
poorly understood.
Livestock grazing is a dominant land use globally and
can have strong negative direct effects on aquatic habitats
if poorly managed. Nearly a quarter of the land surface is
used for grazing (22%; Ramankutty et al., 2008), including
>344 million hectares in the United States (GAO, 1988;
NRCS, 2002), primarily in the west. High densities or poor
distribution of cattle across the landscape remove vege-
tation and trample and compact soils, which in riparian
areas leads to bank erosion, increased turbidity, siltation
of streambed gravel, infilling of pools and reduced habitat
complexity (Platts, 1981; Kauffman & Krueger, 1984;
Belsky, Matzke & Uselman, 1999). This can reduce aquatic
invertebrate production, growth and reproduction of
trout in coldwater streams and ultimately trout abun-
dance and production.
Current riparian grazing management is designed to
protect stream-bank stability by maintaining a minimum
stubble height of riparian grasses and forbs (e.g. 10 cm,
Clary & Leininger, 2000). The goals are to sustain roots that
bind banks and prevent cattle from over-browsing shrubs
(Clary & Webster, 1989; Clary & Kruse, 2004), thus
preventing erosion that destroys aquatic habitat (Wyman
et al., 2006). This logic has been supported by demonstra-
tion projects that showed large increases in streamside
vegetation (Rickard & Cushing, 1982; Dobkin, Rich & Pyle,
1998; Hansen & Budy, 2011) and trout populations (Keller
& Burnham, 1982; Knapp & Matthews, 1996; see Platts,
1991 for review) within 5 years after cattle grazing was
eliminated from overgrazed riparian zones. However, full
recovery of stream habitat from overgrazing, including
stabilising banks that provide overhead cover and scour-
ing silt from deep pools and the gravel in riffles, often
requires more than 5 years to achieve (Kondolf, 1993; Sarr,
2002). Therefore, other mechanisms in addition to restor-
ing instream habitat destroyed by erosion and siltation are
probably important in causing increases in trout popula-
tions in rangeland streams protected from overgrazing.
Two indirect pathways by which improved grazing
practices that increase riparian vegetation may influence
trout are increased inputs of terrestrial insects and of
detritus that fuels secondary production of aquatic insects.
Terrestrial invertebrates that fall or blow into streams often
account for 50–85%of trout diets during summer months
(Dineen, Harrison & Giller, 2007; Utz & Hartman, 2007)
and provide about 50%of their annual energy budget
(Kawaguchi & Nakano, 2001; Nakano & Murakami, 2001;
Sweka & Hartman, 2008). Moreover, experimental reduc-
tion of terrestrial prey using mesh greenhouses in a
Japanese stream reduced growth of salmonids by 25%
(Baxter et al., 2007) or caused half the biomass of salmonids
to emigrate (Kawaguchi, Nakano & Taniguchi, 2003;
Fausch, Baxter & Murakami, 2010). Bioenergetic simula-
tions yielded similar conclusions about the importance of
terrestrial prey (Sweka & Hartman, 2008).
Although this research highlights the importance of
riparian vegetation in supplying terrestrial invertebrates
that help sustain stream salmonids, less is known about
how land uses like cattle grazing alter these prey subsi-
dies. Edwards & Huryn (1996) found that streams
traversing ungrazed native tussock grasslands in New
Zealand received more terrestrial invertebrate biomass
than grazed pastures. Saunders & Fausch (2007) showed
that terrestrial invertebrate inputs to Wyoming streams
with riparian zones under a high-density short-duration
grazing system were more than double that for paired
streams under season-long grazing (SLG), and supported
more than twice the trout biomass. What is needed now
are comparisons of the most common prescribed grazing
management systems in other biomes and their effects on
these indirect pathways by which livestock influence trout
The goals of this study were to (i) evaluate the potential
for the three most common grazing management systems
used in the western United States, and livestock exclusion,
to support terrestrial prey inputs to streams, and (ii) to
determine the extent to which these terrestrial prey are
used by, and support, trout populations. We predicted
that the two rotational grazing systems (or livestock
exclusion), which limit the potential for cattle to congre-
gate in riparian areas, would result in greater terrestrial
invertebrate inputs and support greater trout biomass
than a season-long system which allows continuous
riparian grazing throughout the summer. We show that
sites under rotational grazing systems and those from
which cattle were excluded provided more terrestrial prey
that sustained trout than sites managed for SLG. How-
ever, high variability among sites owing to differences in
microclimate, riparian vegetation, invertebrate popula-
tions and local management limited the consistency of
some differences among grazing systems.
Study reaches
We selected 16 study reaches on 14 streams in the North
Platte, Colorado and Yampa river drainages of north-
central Colorado (Fig. 1), three to five under each of four
grazing management systems on a mixture of private, state
and federal lands (Table 1). Livestock grazing is believed
Grazing management influences stream subsidies 1513
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
to have the strongest effects on fish populations in lower-
gradient channels with extensive riparian zones (Wyman
et al., 2006), so all were second- to fourth-order streams
with relatively low gradient [1.7%±0.18%(mean ± 1 SE)],
moderate width (6.4 ± 0.7 m), in mid-altitude shrublands
(2 552 ± 26 m a.s.l.), where mean water summer temper-
atures were suitable for trout (15.1 ± 0.7 C during July
and August). All streams had gravel substratum, mixed
with either cobble or fine materials, and consisted primar-
ily of long runs and short pools, with few riffles. All had
naturally reproducing populations of brook trout (Salveli-
nus fontinalis Mitchell) or brown trout (Salmo trutta L.),
and four streams also had low densities of wild rainbow
trout (Oncorhynchus mykiss Walbaum) or cutthroat trout
(Oncorhynchus clarkii Richardson).
Grazing management
We compared the effects of traditional season-long cattle
grazing to two rotational systems and cattle exclusion
[wildlife-only grazing (WO)]. Under SLG, cattle are put in
large pastures to achieve a certain stocking rate and allowed
to roam freely for the entire grazing season until mid- to late
autumn (grazing duration of about 120 days in north-
central Colorado; Table 1). In contrast, under rotational
grazing, managers tailor the timing, intensity and duration
of grazing to range conditions. Under simple rotational
grazing (SRG), cattle graze individual pastures only once
each year for 35–45 days and are rotated among three to five
moderately sized pastures, in different orders each year.
Intensive rotational grazing (IRG) is more variable, but
cattle are generally placed in smaller pastures at higher
densities for 10- to 20-day intervals. They graze individual
pastures at most twice during a single year (total grazing
pressure was 17–21 days). The IRG mimics aspects of
historical grazing of large herds of native ungulates like elk
(Cervus canadensis Erxleben) and bighorn sheep (Ovis
canadensis Shaw), which typically move from lower to
higher altitude and graze new vegetation growth as it
appears (Burkhardt, 1996). Many private ranchers did not
record data on cattle density, although they did record
timing of use (Table 1), and no data were available on
wildlife densities. However, our estimates of utilisation of
herbaceous vegetation provide the best estimate of riparian
grazing pressure because they integrate the effects of
density and distribution of cattle and wild ungulates, and
vegetation regrowth, at sites under all four grazing systems.
All pastures had been under the same grazing system
for at least 7 years. However, managers regularly adjust
the timing, duration or intensity of grazing to local range
conditions each year, so each grazing system is similar
across sites but they are not identical or static treatments.
Sites managed for IRG were most common in the eastern
North Platte River drainage (Fig. 1), but one site was
selected 80 km west in the Colorado River drainage to
increase the spatial distribution. Suitable sites managed
for WO were rare because the cattle exclosures were
usually too small (Bayley & Li, 2008). Only three sites of
sufficient size were found, and these were either in
headwaters or surrounded by heavily grazed pastures.
Wild ungulates also had access to riparian vegetation at
the other study sites. However, they were rarely observed
and likely had minor effects on riparian vegetation, except
for elk frequently seen grazing riparian vegetation at the
Illinois River IRG study site. In each pasture, a 200-m
study reach was selected that maximised distance to
changes in geomorphology and grazing management. In
each reach, we sampled riparian vegetation, terrestrial
prey input, fish diets and fish abundance during summer
2007, using methods in Saunders & Fausch (2007) except
as described below.
Fig. 1 Map of the 16 study sites in three drainage basins in northern
Colorado (bold lines show drainage divides). Data were collected at
four sites managed under season-long grazing (triangles), four sites
under simple rotational grazing (squares), five sites under intensive
rotational grazing (circles) and three sites under wildlife-only grazing
(stars, see text).
1514 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
Riparian vegetation sampling
Riparian vegetation measurements included clipped bio-
mass, utilisation of herbaceous species, overhead cover,
community composition and ground cover. Most were
measured during July at peak standing crop biomass,
before plants matured. Standing crop of aboveground
biomass was estimated separately for graminoids
Table 1 Characteristics of physical habitat in the 16 study sites in northern Colorado
Site name Location
width (m)
temperature (C)
(m a.s.l.)
species Cattle use
Intensive rotational grazing
Canadian R.
Lat: 4037¢58¢¢N
Lon: 10601¢45¢¢W
Private 5.4
2564 1.2 BNT (97)
RBT (3)
Early June (10 days)
Sep. (10 days)
Canadian R.
Lat: 4038¢32¢¢N
Lon: 10602¢21¢¢W
Private 5.2 16.3 2555 2.6 BNT (100) Mid-July (17 days)
Michigan R. Lat: 4037¢42¢¢N
Lon: 10606¢27¢¢W
Private 13.0 16.4 2580 1.0 BNT (94)
RBT (6)
Sep. (21 days)
Illinois R. Lat: 4032¢01¢¢N
Lon: 10613¢22¢¢W
Private 8.3 17.5 2564 1.4 BNT (100) Late July (19 days)
Floyd Cr. Lat: 4047¢53¢¢N
Lon: 10659¢16¢¢W
Private 2.0 15.7 2458 1.2 RBT (82)
BKT (11)
CUT (7)
Mid-June, July, Aug.
(6 days each)
Simple rotational grazing
Arapaho Cr. Lat: 4024¢32¢¢N
Lon: 10623¢28¢¢W
USFS 3.1 –
2683 1.4 BNT (100) July–mid-Aug.
(42 days)
Rock Cr.
Lat: 4023¢58¢¢N
Lon: 10612¢47¢¢W
USFS 3.0 11.2 2751 2.2 BKT (100) Mid-July–Aug
(40 days)
Rock Cr.
Lat: 4002¢10¢¢N
Lon: 10639¢27¢¢W
USFS 6.3 16.6 2600 1.4 BNT (100) Aug. (35 days)
Trout Cr.
Lat: 4016¢00¢¢N
Lon: 10703¢38¢¢W
Private 6.3 15.3 2343 2.8 BNT (51)
BKT (36)
RBT (4)
CUT (9)
(43 days)
Season-long grazing
North Fork North
Platte R.
Lat: 4052¢56¢¢N
Lon: 10632¢59¢¢W
Private 5.0 15.9 2591 1.2 BNT (99)
BKT (1)
(150 days)
Shafer Cr. Lat: 4051¢49¢¢N
Lon: 10633¢00¢¢W
Private 4.6 10.5 2576 2.4 BNT (88)
BKT (12)
(150 days)
East Fork
Troublesome R.
Lat: 4011¢26¢¢N
Lon: 10613¢30¢¢W
USFS 7.3 –
2475 0.8 BNT (100) 1 July–15 Sep.
(77 days)
Newcomb Cr. Lat: 4035¢43¢¢N
Lon: 10636¢05¢¢W
USFS 11.2 14.9 2656 1.2 BKT (59)
BNT (41)
(123 days)
Wildlife-only grazing
Hinman Cr. Lat: 4046¢07¢¢N
Lon: 10648¢57¢¢W
USFS 6.6 13.5 2375 1.4 BKT (92)
RBT (8)
Trout Cr.
Lat: 4013¢45¢¢N
Lon: 10606¢00¢¢W
USFS 6.4 14.1 2508 3.2 BKT (85)
CUT (13)
BNT (1)
RBT (1)
Grizzly Cr. Lat: 40’26¢¢0N
Lon: 10629¢00¢¢W
Private 8.5 18.4 2554 1.2 BNT (100)
*Sites were on lands owned privately or by the U.S. Forest Service (USFS) or Bureau of Land Management (BLM).
No temperature data were available for 2007.
Data were collected at five sites managed under intensive rotational grazing (IRG), four sites under simple rotational grazing (SRG), four sites
under season-long grazing (SLG) and three sites under wildlife-only grazing (WO, see text). Mean summer water temperature was calculated
from hourly water temperatures recorded during July and August 2007. Trout species present (BNT, brown trout; BKT, brook trout; RBT,
rainbow trout, CUT, cutthroat trout) are listed in order of prevalence with the percentage each species contributed to the salmonid community
in parentheses. Cattle use indicates the timing and duration cattle were present in pastures containing study sites.
Grazing management influences stream subsidies 1515
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
(hereafter grasses) and forbs by clipping all plants to
ground level within four randomly located 0.25-m
circular plots 1.5 m from the bankfull channel. Utilisation
of herbaceous vegetation by ungulate grazers (both cattle
and wildlife) was estimated with two 1-m
(13 ·20 cm wire mesh), each placed at a random distance
along one bank and 1.5 m from the channel. Paired 0.25-
circular plots inside and outside of the exclosure were
clipped in late August or early September. Estimated
utilisation was the difference in biomass between the pair,
divided by the biomass inside the exclosure, expressed as
a percentage. Clipped vegetation was dried (55 C for
48 h) and weighed (nearest 0.1 g). Exclosures were con-
structed before livestock grazing in all but three cases,
owing to logistical constraints. At the North Fork North
Platte River and Shafer Creek (both SLG), cattle grazing
began 3 weeks before exclosures were built. The site at
Lower Trout Creek (SRG) was selected after most grazing
was completed, so no exclosures were built.
Overhead cover was measured using hemispherical
photographs taken at 10-m intervals along each reach,
with random starting points (see Saunders, 2010 for
details on equipment and image analysis). Images were
taken towards the zenith from 50 cm above the ground or
stream surface each 2 m along a transect perpendicular to
the channel that extended 4 m into the riparian zone on
each bank. Per cent overhead cover was estimated using a
100-point sampling grid, and points were classified as
either open (no vegetation) or covered by grasses, forbs,
shrubs or trees. From 16 July until 10 August 2007, plant
community composition was characterised by identifying
all species present and estimating their ground cover in 10
equally spaced quadrats (20 ·50 cm) along two 30-m
Daubenmire-type transects placed 1.5 m from each stream
bank (see Stohlgren, Bull & Otsuki, 1998). Transects
started at randomly chosen distance and followed the
stream bank.
Invertebrate sampling
Inputs of terrestrial and adult aquatic invertebrates were
measured using plastic pan traps placed just above the
stream surface. Traps were filled with 5 cm of filtered
stream water, to which 5 mL of unscented, biodegradable
surfactant was added to reduce surface tension (Wipfli,
1997; Nakano, Miyasaka & Kuhara, 1999b). Five clear pan
traps (100 cm ·41 cm ·15 cm deep) were randomly
allocated to locations within the wetted perimeter of the
stream in proportion to two strata of overhead cover
(>35%or <35%cover; see Saunders, 2010). Three were
randomly allocated next to the bank and two were located
mid-channel, to sample both invertebrates that tumble in
from banks or fall in from vegetation or when flying. Pan
traps were supported so that the top of the trap was no
more than 20 cm above the water surface. Invertebrates
were sampled during middle and late summer (2–16 July
and 9–16 August 2007) when terrestrial inputs to north
temperate streams are greatest (Cloe & Garman, 1996;
Kawaguchi & Nakano, 2001; Allan et al., 2003). Inverte-
brates were collected for a 6-day period divided into two
3-day samples, to optimise sampling effort based on a
variance analysis of previous samples (see Saunders &
Fausch, 2007). Invertebrates were sieved with a 250-lm
net and preserved in 70%ethanol.
Sampling fish diets and populations
Stomach contents were collected from salmonids in each
reach during July and August 2007 to estimate the
biomass and proportion of terrestrial invertebrates. Diets
were collected from 10 to 20 fish captured by electrofish-
ing just after the period of peak terrestrial invertebrate
input (c. 1500–1900 h; Nakano et al., 1999a; Hieber, Rob-
inson & Uehlinger, 2003). These afternoon diet samples
reflected the biomass of terrestrial invertebrates in trout
diets throughout the diel period, based on samples
collected during August at 00:00, 06:00, 12:00 and
18:00 hours at half the study sites (all SLG and IRG sites;
unpubl. data). Gastric lavage was used to collect diets
from fish of 110–350 mm fork length (FL). Diets could not
be sampled effectively from smaller fish, and larger fish
become piscivorous. Stomach contents were sieved
through 333-lm mesh and preserved in 70%ethanol.
On average, gastric lavage removed 98%(SE = 0.9) of
invertebrate biomass in diets (N= 19 preserved trout;
mean 221 ± 17.1 mm FL).
Abundance and biomass of trout were estimated in
each 200-m reach at baseflow in August 2007, using three-
pass depletion electrofishing conducted at night to
improve accuracy (Saunders, Fausch & White, 2011).
Two backpack electrofishing units (LR-24, SmithRoot Inc.,
Vancouver, WA) were used in wide streams (7m) to
increase efficiency. Mass (nearest 0.1 g) and length (near-
est 1 mm FL) were recorded for all trout. Age-0 fish are
not sampled effectively and were not measured. Removal
estimates of capture probability and abundance for age-1
fish were calculated using the Huggins closed-population
estimator in Program MARK (White & Burnham, 1999;
Saunders et al., 2011), incorporating fish length as an
individual covariate and pooling data across the 16 sites to
improve estimates of capture probabilities. Trout biomass
was estimated by multiplying population abundance by
1516 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
mean fish mass, and variances were calculated using the
finite population variance (Riley & Fausch, 1995).
Invertebrate identification and biomass estimation
Invertebrates were identified to the taxonomic level
necessary to assign their origin as terrestrial versus
aquatic (generally Family, see Saunders & Fausch, 2007
for details). Biomass (nearest 0.3 mg) of invertebrate taxa
in pan trap samples was measured after drying at 60 C
for 48 h. Biomass of invertebrate prey items in trout diets
was estimated using published length–mass regressions
based on total invertebrate length or head capsule width
(Rogers, Buschbom & Watson, 1977; Smock, 1980; Meyer,
1989; Burgher & Meyer, 1997; Benke et al., 1999; Johnston
& Cunjak, 1999; Sabo, Bastow & Power, 2002). This
reconstruction of stomach contents provides a consistent
comparison of the amount of prey eaten by fish at sites
where water temperatures, and thus evacuation rates,
differ. Lengths were measured for up to 15 of each taxon
in each fish diet, and the mean mass of these 15 was used
to estimate biomass when more were counted.
Terrestrial invertebrate input versus fish metabolic demand
We evaluated the importance of terrestrial invertebrate
prey by comparing their estimated energy content to the
metabolic requirements of trout for each site during July
and August. We estimated the mean daily energy input
(J day
) of terrestrial invertebrates for each month
by converting estimates from pan traps using published
energy densities for invertebrate orders (Cummins &
Wuycheck, 1971; Gray, 2005; McCarthy et al., 2009). We
expanded these estimates to the entire reach and summed
them over the 2-month period. Adult aquatic insects were
For comparison, we estimated the energy required for
metabolism to sustain trout at each site during July and
August 2007. Total metabolic demand was estimated for
all fish collected during the August population survey.
Metabolic demand was estimated using respiration equa-
tions from the Wisconsin Bioenergetics Model (Hanson
et al., 1997) parameterised for brook, brown or rainbow
trout (Dieterman, Thorn & Anderson, 2004; Hartman &
Cox, 2008; Van Poorten & Walters, 2010), using average
daily water temperature and a daily time step. Fish
weight was assumed constant, because no growth data
were available, and was based on the August sampling.
At two sites, temperatures were estimated from a nearby
site with similar trout species and valley configuration,
owing to temperature logger malfunction. Temperatures
at the Lower Canadian River site (IRG) were estimated
from a temperature logger 2.4 km upstream, and those for
East Fork Troublesome Creek (SLG) came from Southern
Rock Creek, a stream with similar aspect and altitude in
the Colorado River drainage. None were available close to
Arapaho Creek (SRG), so no estimates of respiration were
Estimates from the Wisconsin Bioenergetics Model
account for fish activity by multiplying the standard
respiration rate by a species-specific activity multiplier
(see Hanson et al., 1997). Estimates of respiration (g O
per day) were converted to Joules using an
oxycalorific conversion factor of 13.56 kJ per g oxygen
consumed (Elliott & Davison, 1975). Estimates of total
metabolic demand are conservative because they do not
account for fish estimated to have been in the study reach
but not captured. However, nearly all (96 ± 1%) age-1 and
older fish were estimated to have been captured in three
Stream habitat characteristics
Stream habitat characteristics were measured in each
study reach for use as covariates in models, including
stream width, pool and run area, water temperature and
map gradient. Wetted and bankfull widths (nearest 0.1 m)
were measured at 10-m intervals, and the length and
width of each pool and run were measured to estimate
area. Temperature loggers (HOBO WaterTemp Pro, Onset
Computer Corporation, Pocasset, MA) deployed in
shaded locations in the middle of each reach recorded
water temperature hourly from June through September.
Stream gradient for a 500-m segment containing each
study reach was estimated from U.S. Geological Survey
7.5-min topographic maps.
Data analysis
Linear mixed models were used to investigate the factors
influencing grazing utilisation, vegetation biomass, over-
head cover, invertebrate input, fish diets and fish
biomass, and were evaluated using an Information
Theoretic approach (Burnham & Anderson, 2002). We
hypothesised that site-level habitat and vegetation mea-
surements (e.g. stream width, biomass of herbaceous
vegetation, overhead cover and species richness of
riparian vegetation) would be better predictors of
terrestrial invertebrate input to streams, and in fish
diets, than the four categorical grazing systems, so we
compared the models based on these two different
themes. The goal was to assess (i) how much variance
Grazing management influences stream subsidies 1517
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
could be explained by the two sets of covariates, and (ii)
whether mechanistic variables (e.g. vegetation character-
istics) would be supported by model selection. For each
theme, we fit models with all combinations of covariates
to allow model averaging across a balanced set (Doherty,
White & Burnham, 2012). Afterwards, a limited explor-
atory analysis was conducted to assess whether the top
model using grazing system covariates could be
improved by adding site-level covariates. Models were
ranked using the Akaike Information Criterion corrected
for small sample sizes (AIC
). Multimodel inference was
conducted by model-averaging either parameter esti-
mates or model predictions using Akaike weights (w
account for model selection uncertainty (Burnham &
Anderson, 2004).
In all cases, models constructed with site-level covari-
ates performed poorly, relative to grazing system models,
and the exploratory analysis indicated that site-level
covariates failed to improve the fit of models constructed
with grazing system covariates (Saunders, 2010). For
example, comparison between the best approximating
model constructed of system versus site covariates using
an AIC
evidence ratio (Burnham & Anderson, 2002)
indicated that there was 52 times more support for only
grazing system covariates for predicting terrestrial inver-
tebrate input and >2500 times more support for only
grazing system covariates for predicting terrestrial inver-
tebrate biomass in trout diets. Furthermore, 90%confi-
dence intervals for model-averaged parameter estimates
for site-level covariates all overlapped zero substantially,
indicating that these covariates provided little predictive
power. As a result, here we present results only for the
analysis of grazing system models.
Modelling response variables. Terrestrial invertebrate input
and invertebrate biomass in trout diets were sampled
multiple times per site. Therefore, we first used model
selection, using the restricted maximum likelihood
(RMEL), to evaluate whether a random effect was needed
to account for autocorrelation among samples and to
select the appropriate variance structure (see Saunders,
2010 for analysis description). This analysis showed that
the best approximating model of variance structure
incorporated homogeneous variance within two sets of
grazing systems (SLG and WO versus SRG and IRG) but
heterogeneous variance between the sets, so this structure
was used in further analyses of input and fish diets. In
contrast, vegetation characteristics and fish biomass were
measured during a single sampling event at each site, and
so values were considered independent and random
effects were not evaluated. We evaluated fixed effects
(e.g. grazing systems) for all response variables, using the
full maximum likelihood.
Models with categorical covariates were developed with
a standard intercept, representing the mean response
value across all sites, so the sum of each column of the
design matrix associated with categorical variables equal-
led zero (see Saunders, 2010). This allows a consistent
interpretation of parameter values for categorical covari-
ates across all models while facilitating model averaging to
account for model uncertainty (D. R. Anderson, personal
communication). Parameter estimates were determined by
model-averaging all models that included the covariate
(i.e. non-shrinkage model averaging; Burnham & Ander-
son, 2002), and unconditional standard errors were calcu-
lated that included model selection uncertainty. Contrasts
between parameters (e.g. those representing different
grazing systems) were estimated as the difference between
the two model-averaged parameters, and the uncondi-
tional standard error of this difference was estimated by
summing appropriate variances and covariances (see
Burnham & Anderson, 2004). Because this is the first
study to evaluate effects of a broad variety of grazing
systems, we used 90%confidence intervals to avoid type II
statistical error (failing to detect real effects). For param-
eter contrasts, confidence intervals that do not include zero
indicated that differences between grazing systems were
consistent across the study sites an estimated 90%of the
time. This effect size is considered biologically important
and is described throughout as a ‘consistent’ difference.
Global models for vegetation measurements, inverte-
brate input, fish diets and biomass, and the bioenergetic
comparison contained a parameter for each grazing
system. For the analysis of invertebrate input and biomass
of invertebrates in fish diets, all models also included
month, but we made separate predictions by month only
when the 90%confidence interval on this parameter did
not overlap zero. Models of terrestrial invertebrate bio-
mass in trout diets also included covariates for trout
species and relative length. Salmonids in streams form
distinct size-based interspecific dominance hierarchies
(Nakano, 1995), which determine foraging positions and
access to large, profitable prey. Therefore, larger fish
within populations become dominants and are expected
to ingest greater prey biomass than subordinate fish.
Relative length was defined as individual fish length
divided by the 90th percentile fish length for the site, after
pooling trout of all species. Individuals 90th percentile
were assigned a relative length of 1. To reduce heteroge-
neous variance, invertebrate inputs were transformed
using natural logarithms, and biomass in trout diets was
transformed using the square root. Standard errors for
1518 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
back-transformed data were estimated using the Delta
Method (Degroot & Schervish, 2002).
Riparian vegetation
Utilisation of herbaceous vegetation was greatest at SLG
sites, so sites managed for rotational (SRG and IRG) and
WO grazing tended to have more riparian vegetation than
sites managed for SLG. Sites managed for SLG had 3.3
times the total per cent utilisation measured at sites
managed for IRG, and 6.5 times that at WO sites (Fig. 2).
Non-overlapping 90%CI indicated these two differences
were consistent throughout the study region. Sites man-
aged for either rotational or WO grazing also had, on
average, 3.1 times the percentage of overhead cover
measured at sites managed for SLG (Fig. 3a), and these
differences were consistent throughout the region. Woody
vegetation was the predominant functional group con-
tributing to overhead cover, although tall grasses and
forbs contributed about 5–10%of the cover at IRG and
WO sites (unpubl. data). Sites managed for IRG also had
2.9 times the aboveground herbaceous biomass of sites
managed for SLG, on average, and 1.8 times the biomass
of sites managed for SRG (Fig. 3b). In contrast, we could
not detect a difference in vegetation biomass between sites
under WO and other grazing systems.
Overall, there were small differences among vegetation
communities at sites under each of the four grazing
systems, but these were generally inconsistent owing to
high variability among sites. On average, green vegetative
cover was highest at sites managed for SLG (78%), lowest
at WO sites (63%) and intermediate at sites managed for
SRG (66%) and IRG (71%; Appendix S1). In contrast, the
area covered by plant litter was higher at SRG, IRG and
WO sites (mean = 22%) than at SLG sites (14%). In
general, forbs accounted for as much or more vegetative
ground cover (range: 23–32%) as other functional groups,
except at IRG sites where grasses (33%) covered more
than forbs. Woody vegetation (predominantly willows,
see Appendix S1 for scientific names) provided the least
ground cover (range: 9–15%) and also tended to be the
most variable, although these estimates do not account for
overhead cover from woody vegetation rooted outside
sample plots. In general, species evenness was higher at
WO sites where grass and forb species that contributed
2%to total ground cover accounted for only 34%of the
ground cover contributed by grasses and 25%of the
ground cover contributed by forbs. In contrast, at sites
grazed by cattle, a relatively few forbs (e.g. white clover
Fig. 2 Predicted average per cent utilisation (± unconditional SE, see
text) of herbaceous riparian vegetation at 15 sites in northern
Colorado under season-long (SLG), simple rotational (SRG),
intensive rotational (IRG) or wildlife-only (WO) grazing (no
estimates could be made for one simple rotational grazing site, see
text). Different letters indicate consistent differences between sites,
based on 90%confidence intervals (see text).
Fig. 3 Predicted average per cent overhead vegetative cover (a) and
aboveground dry biomass (b) (± unconditional SE) of herbaceous
vegetation at 16 riparian sites under season-long (SLG), simple
rotational (SRG), intensive rotational (IRG) or wildlife-only (WO)
grazing. Different letters indicate consistent differences between
sites, based on 90%confidence intervals (see text).
Grazing management influences stream subsidies 1519
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
and dandelion) and grasses (e.g. tufted hairgrass, Ken-
tucky bluegrass, timothy and beaked sedge) that individ-
ually contributed 2%to total ground cover accounted for
>50%(range = 50–74%) of the ground cover contributed
by each vegetation functional group.
Falling invertebrate input
Terrestrial invertebrate biomass falling into streams dur-
ing July and August 2007 was greatest at SRG and WO
sites, and lowest at SLG sites. The biomass was similar
during July and August 2007, so the data were pooled. On
average, sites received 25.0 mg m
per day of terrestrial
invertebrate biomass. Sites managed for SRG received, on
average, 5.4 times the invertebrate biomass measured at
SLG sites, and 3.0 times that at sites managed for IRG
(Fig. 4). Sites managed for WO received 3.1 times the
invertebrate biomass as SLG sites, but no consistent
differences could be detected compared with sites man-
aged for rotational grazing. Twelve orders of terrestrial
invertebrates were collected in common among all sites,
whereas Orthoptera were collected only at rotational
grazing sites. Averaged across all the sites, Diptera (34%),
Heteroptera (17%), Lepidoptera (14%, adults and larvae
combined), Coleoptera (12%), Hymenoptera (9%) and
Homoptera (8%, Auchenorrhyncha and Sternorrhyncha
combined) accounted for 94%of terrestrial invertebrate
biomass entering streams (Saunders, 2010).
In contrast to terrestrial invertebrates, the input of adult
aquatic insects returning to streams was variable among
sites, but was greater during July than August 2007. In
general, sites received 4.0 times the biomass of adult aquatic
insects during July [averageinput = 26.1 ± 6.9 mg m
day (mean ± SE)] as August (6.5 ± 0.89 mg m
per day),
and this difference was consistent among all grazing
systems. In contrast, the biomass of adult aquatic insects
falling into streams during summer 2007 did not differ
consistently among grazing systems, owing to high vari-
ability among sites during July and low levels of input
across all sites during August. However, during July, when
input of aquatic adults was greatest, sites managed for SLG
(12.9 ± 7.14 mg m
per day) and SRG (17.5 ± 9.61 mg m
per day) tended to receive less adult aquatic insect biomass
than sites managed for either IRG (30.7 ± 15.08 mg m
day) or WO (43.4 ± 27.54 mg m
per day). Overall,
the most common adult aquatic insects found entering
streams were Trichoptera, Chironomidae, Ephemeroptera,
Ceratapogonidae, Tipulidae and Coleoptera (Saunders,
Invertebrate biomass in trout diets
During both July and August 2007, trout at sites managed
for SRG had more terrestrial invertebrate biomass in their
diets than trout at sites under other grazing systems, and
this difference was consistent during August (Fig. 5a). In
general, the biomass of terrestrial invertebrates in trout
diets increased between July and August at SRG and WO
sites, whereas it remained constant at SLG sites and
decreased at IRG sites. These differences were consistent,
based on the importance of the model that included a
grazing system by month interaction (AIC
weight = 99.4%; Saunders, 2010). During July, trout at
SRG sites had 7.4 times the biomass of terrestrial prey as
those at WO sites. However, during July, even relatively
large differences (e.g. 2–3 times) between SRG and SLG or
IRG were not consistent, owing to high variability.
Differences among grazing systems were greater in
August, when fish at SRG sites had, on average, 4.7 times
the terrestrial invertebrate prey in their stomachs as trout
at SLG sites, 15.2 times that at IRG sites and 5.2 times that
at WO sites. Overall, terrestrial invertebrate prey made up
31%of the biomass in trout diets during July, and 43%in
August, based on model-averaged estimates across
grazing systems [range within grazing systems 17%
(WO July) – 76%(SRG August)]. Only 23 of 507 diets
(4.5%) contained remains of vertebrate prey, primarily
small fish.
In contrast to terrestrial invertebrates, trout at sites
under all grazing systems had similar amounts of aquatic
invertebrate biomass in their diets, but had more during
Fig. 4 Predicted average terrestrial invertebrate input (± uncondi-
tional SE) to 16 streams in northern Colorado under season-long
(SLG), simple rotational (SRG), intensive rotational (IRG) or wildlife-
only (WO) grazing. Values reflect estimates back-transformed to the
original scale, and standard errors were estimated using the Delta
Method (see text). Different letters indicate consistent differences
between sites, based on 90%confidence intervals (see text).
1520 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
July than August (Fig. 5b). Of this biomass, 42%was
adult aquatic insects, 41%was immature and pupating
macroinvertebrates and 17%was other aquatic inverte-
brates (e.g. snails, bivalves, and annelids). On average,
trout consumed 1.8 times more aquatic invertebrate
biomass during July than August 2007, based on the
relative importance of the model that included a grazing
system by month interaction versus all models without
the interaction (evidence ratio for next best model without
interaction = 7.2, AIC
weight for interaction mod-
el = 40.2%; Saunders, 2010). However, no differences
among grazing systems in aquatic invertebrate biomass
in trout diets could be detected.
There was considerable variation in the biomass of
terrestrial and aquatic invertebrates in diets among
individual fish, and part of this could be explained by
fish length and trout species. Overall, invertebrate bio-
mass in trout diets increased with relative fish length for
both terrestrial and aquatic prey. However, there was a
strong interaction between fish length and species of fish
for the biomass of terrestrial invertebrates in trout diets,
such that the effect of relative length was greater for
rainbow and cutthroat trout and brook trout than for
brown trout (Fig. 6a). The top model for predicting
terrestrial biomass in trout diets, which incorporated an
interaction between relative length and fish species, held
95%of the model weight. Furthermore, this top model
had 520 times more support (based on an evidence ratio)
than the simpler model containing no effect of relative
length (Delta AIC
= 12.5), 196 times more support than
the model with no species effects (Delta AIC
= 10.6) and
143 times more support than the model with an additive
Fig. 5 Predicted average terrestrial (a) and aquatic invertebrate bio-
mass (b) (± unconditional SE) in trout diets collected during mid-
afternoon for 16 streams in northern Colorado under season-long
(SLG), simple rotational (SRG), intensive rotational (IRG), or wildlife-
only (WO) grazing. Bars show invertebrate biomass in the average
trout diet (relative length = 0.73; N= 507 trout) and account for
species differences. Values reflect estimates back-transformed to the
original scale, and standard errors were estimated using the Delta
Method (see text). Different letters above estimates indicate consis-
tent differences between grazing systems based on 90%confidence
intervals, but comparisons are valid only within months.
Fig. 6 Relationship between relative fish length (see text) and dry
biomass of (a) terrestrial and (b) aquatic invertebrate prey in trout
diets sampled in 16 streams in northern Colorado. The relationship
represents the average across sites under four different types of
grazing management. Values reflect model predictions estimated
using square-root transformed data, which were back-transformed to
the original scale. Samples sizes were 318 brown trout, 151 brook
trout, and 38 rainbow and cutthroat trout.
Grazing management influences stream subsidies 1521
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
effect of relative length and fish species (Delta AIC
= 9.9).
In general, rainbow and cutthroat trout, and brook trout,
had greater terrestrial invertebrate biomass in their diets
than brown trout of similar size. For example, rainbow
and cutthroat trout had, on average, 57.8 mg of terrestrial
invertebrates per fish (making up 52%to their diet),
whereas brook trout had 31.2 mg (44%) and brown trout
had 19.1 mg (30%). In contrast, the biomass of aquatic
invertebrate prey in trout diets was largely influenced by
fish size (Delta AIC
> 28 for models not including
relative fish length), whereas there was little support for
a difference among fish species. The top five models,
which differed only in the structure of the effect of fish
species, differed by only 2.5 AIC
Terrestrial invertebrate input versus trout metabolic demand
In general, energy from terrestrial invertebrate inputs to
streams met the metabolic demands of trout populations
during July and August. The total energy entering streams
as terrestrial invertebrates followed the same general
pattern as the biomass inputs (Figs 4 & 7). However, sites
managed for WO grazing tended to receive more energy
than expected based on the biomass of terrestrial inver-
tebrates entering streams, owing to one site that received a
large amount of Hemiptera inputs during August. In
contrast, the metabolic demands of trout populations
were similar across all grazing management systems. On
average, the energy entering streams as terrestrial inver-
tebrates exceeded the metabolic demand of trout during
July and August by 2.4, 1.1 and 3.6 times at sites managed
for SRG [difference (90%CI) = 39 495 kJ ()1420, 80 410)],
IRG [1325 kJ ()31 655, 34 304)] and WO grazing [58 128 kJ
(22 094, 94 162)], respectively. In contrast, at SLG sites
metabolic demand of the trout population was slightly
greater than the energy input from terrestrial inverte-
brates [)1913 kJ ()34 031, 30 206)]. However, the differ-
ence was consistent only at WO sites, owing to high
variability among sites within grazing systems. The trout
in Floyd Creek (an IRG site) had migrated from Steamboat
Lake to spawn and were not resident fish, so this site was
Trout density and biomass
Density and biomass of adult trout <350 mm was greatest
at SRG and IRG sites, but no differences were consistent
because of high variability. Biomass of trout <350 mm
varied considerably both among sites under the same
grazing system [average coefficients of variation (CV) =
0.37] and among different grazing systems (Fig. 8). Trout
biomass at sites managed for SRG and IRG were, on
average, 1.7 and 1.5 times that at sites under SLG,
although these differences were not consistent. Trout
biomass at sites receiving no livestock grazing (WO)
tended to be lower than at sites grazed by cattle, but these
results were strongly influenced by low trout abundance
at Grizzly Creek, a site adjacent to heavily grazed reaches
upstream and downstream. Trout density under the
Fig. 8 Predicted average trout biomass for age-1 and older fish
<350 mm (± unconditional SE) estimated in late summer 2007 at 15
streams in northern Colorado under season-long (SLG), simple
rotational (SRG), intensive rotational (IRG), or wildlife-only (WO)
grazing. The estimate from one IRG site (Floyd Creek) was consid-
ered an outlier and not included in the analysis (see text).
Fig. 7 Comparison of the energy entering sites as terrestrial inver-
tebrates and the metabolic demand (± unconditional SE) of trout
populations in 16 streams in northern Colorado under four different
grazing systems (see Fig. 2 for abbreviations). The difference between
input and demand is consistent only for WO sites, based on 90%
confidence intervals (see text). No temperature data were available at
one simple rotational grazing site to estimate of metabolic demand.
Data from one intensive rotational grazing site were not used to
estimate metabolic demand owing to the adfluvial life history of most
1522 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
different grazing systems showed a similar pattern as
trout biomass, but tended to be even more variable
(average CV = 0.41). As in the analysis of bioenergetic
demand, data from Floyd Creek were not included owing
to the adfluvial trout there. Large trout (>350 mm), mostly
brown trout, were also excluded because they tend to be
mobile among different reaches (Clapp, Clark & Diana,
1990; Young et al., 1997) and are likely less dependent on
prey resources at the scale of 200-m study reaches. Large
trout were present at 10 of 15 sites but contributed, on
average, only 3%(SE = 1.1%) of the biomass.
Both styles of rotational grazing management (simple or
intensive) resulted in consistently more overhead cover
and intensive rotation resulted in more aboveground
vegetation biomass than traditional SLG. These greater
levels of riparian vegetation at sites managed for rota-
tional grazing were often associated with greater inputs of
terrestrial invertebrates, greater biomass of terrestrial prey
in trout diets and more trout biomass than traditional
SLG, but these differences were frequently not statistically
consistent owing to high variability. In contrast, the
responses at sites with wildlife grazing only were usually
similar to those under rotational grazing. Therefore, the
results presented here and by Saunders & Fausch (2007)
indicate that rotational grazing systems can be effective
for maintaining riparian vegetation that supplies terres-
trial invertebrate inputs necessary to support trout, but
that the effects of riparian grazing on these subsidies are
complex and highly variable.
Riparian vegetation
In general, grazing pressure, measured as per cent
utilisation, decreased with increasing management inten-
sity (i.e. rotation frequency) and was lowest at sites
managed for wildlife grazing only. Sites where cattle
grazed throughout a 120-day season (SLG) had both the
highest per cent utilisation values and lowest above-
ground vegetation biomass and vertical vegetation struc-
ture, measured as per cent overhead cover, which is
similar to our results for west central Wyoming (Saunders
& Fausch, 2007). These results indicate that rotational
grazing, when applied to riparian areas which support
plant growth for longer periods than uplands, can
produce both greater herbaceous vegetation biomass
and more complex vegetation communities than SLG.
Variability in the response of vegetation to grazing
systems at individual sites was likely caused by differ-
ences in historical grazing management; in the timing,
intensity and duration of grazing applied; and in weather
throughout the large region studied (Saunders, 2010).
Nevertheless, the greater vegetation biomass and vertical
structure at more intensively managed sites likely pro-
vides more food and cover for terrestrial invertebrates,
thereby supporting greater densities. A greater variety of
common plant species at these sites also likely supports a
greater diversity of invertebrate species (Morris, 2000;
Soderstrom et al., 2001; Zurbrugg & Frank, 2006). In-
creased structural complexity also increases the probabil-
ity that terrestrial invertebrates, and recently emerged
aquatic insects, fall into streams (Wipfli, 1997; Baxter et al.,
2005; Saunders & Fausch, 2007). Furthermore, greater
streamside vegetation at more intensively managed sites
will likely increase aquatic prey through greater litter
inputs (Wallace et al., 1997), suggesting that the effects of
grazing management on invertebrate prey for trout are
complex and involve multiple food web pathways.
Invertebrate input
Input of terrestrial invertebrates was relatively constant
throughout the summer, but was generally greater under
rotational grazing systems and at WO sites compared
with SLG sites. In contrast, input of adult aquatic insects
was greatest during early summer and varied less among
grazing systems. Sites managed for rotational grazing
(both SRG and IRG) received two to five times more
terrestrial invertebrate biomass than sites managed for
SLG, although the difference between IRG and SLG sites
was not statistically consistent owing to high variability
within grazing systems. Nevertheless, this pattern was the
same as for rangeland streams in Wyoming, where sites
under IRG received more than twice the biomass of
terrestrial invertebrates as those under SLG (Saunders &
Fausch, 2007). Furthermore, in Colorado the biomass of
terrestrial invertebrates entering streams grazed only by
wildlife was three times that at SLG sites, but within the
range of that at sites under rotational grazing. These
results suggest prescribed rotational grazing may provide
an alternative to livestock exclusion, which is not always
feasible at large spatial scales, for conserving stream-
riparian linkages that support aquatic consumers.
One paradox was that sites managed under IRG had, on
average, the highest aboveground vegetation biomass and
overhead cover of all grazing systems, yet received lower
terrestrial invertebrate input than either SRG or WO sites.
One reason may be that simple metrics of riparian
vegetation are insufficient to predict terrestrial inverte-
brate inputs to streams, as indicated by the lack of support
Grazing management influences stream subsidies 1523
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
for models predicting input from site-level variables.
Instead, invertebrate subsidies may depend more on
particular attributes of riparian vegetation than the abso-
lute amount of biomass or cover. A second reason may be
that insect phenology is rapid during summer, so our
relatively short sample duration (6 days month
) may
have missed peak inputs by chance. For example,
sampling at Grizzly Creek (WO) captured a large input
of terrestrial hemipterans during one 3-day sample that
made up 76%of the total input for this stream in August,
which might have been missed during a different 6-day
period. Finally, the close proximity of four of five IRG
sites by necessity may have increased the influence of
insect phenology and sample timing on our measures of
terrestrial invertebrate input.
Our data show that streams in northern Colorado
received similar terrestrial invertebrate subsidies as
streams in other regions. For example, summer inputs to
northern Colorado rangeland streams (25 mg m
day) were less than those measured in similar-sized
streams in eastern United States and northern Japanese
deciduous forests (c. 112–450 mg m
per day; Cloe &
Garman, 1996; Nakano et al., 1999b), but were similar to
those in Scotland (c. 25–30 mg m
per day; Bridcut, 2000)
and Japan during a cool wet year (c. 40 mg m
per day;
Baxter et al., 2005). Furthermore, the terrestrial inverte-
brate input to grassland streams in northern Colorado
was similar to a grassland reach in Japan (30 mg m
day; Kawaguchi & Nakano, 2001) and rangeland streams
in Wyoming (c. 20–46 mg m
per day; estimated from
data in Saunders & Fausch, 2007; using wet:dry weight
regression for invertebrate orders (Kawaguchi & Nakano,
2001; C.V. Baxter unpubl. data)], but greater than grazed
and ungrazed grassland streams in New Zealand
(c. 4mgm
per day; Edwards & Huryn, 1995, 1996).
Therefore, terrestrial invertebrate prey are likely to be as
important to salmonids in western U.S. grassland streams
as in many other regions, but levels may vary consider-
ably in grassland streams across semi-arid regions world-
wide. Moreover, the overall importance of this resource to
trout foraging, growth and density has been amply
demonstrated in regions with similar inputs through both
comparative studies (Wipfli, 1997; Dineen et al., 2007;
Sweka & Hartman, 2008) and field experiments (Kawag-
uchi & Nakano, 2001; Kawaguchi et al., 2003; Baxter et al.,
2004, 2007; see Fausch et al., 2010 for review).
Fish diets
Terrestrial invertebrates contributed about 30–45%of the
biomass of prey in trout diets when averaged across
grazing systems in north-central Colorado, with higher
values in August when fewer adult aquatic insects were
emerging. Trout at SRG sites had more terrestrial inver-
tebrate biomass in their diets than those at sites with other
types of grazing, although this difference was consistent
primarily for August. These differences generally
reflected the terrestrial invertebrate biomass entering
streams, although trout at IRG and WO sites had less
than expected based on inputs. In contrast, there were no
consistent differences in aquatic invertebrate prey in trout
diets under different grazing management. Nevertheless,
an average of 42%of the aquatic invertebrate biomass in
trout diets (24%of the total invertebrate biomass) was
adult aquatic insects, many of which rely on streamside
vegetation for shelter and resting sites (Wallace et al.,
1997; Huryn, Wallace & Anderson, 2008). Although the
inputs of terrestrial and adult aquatic invertebrates were
similar in Wyoming and Colorado, trout in Wyoming
rangeland streams (2004–05) had more terrestrial inverte-
brate biomass in their diets (20–46 mg fish
; estimated
from Saunders & Fausch, 2007) than trout in Colorado
(2007; except for those in SRG sites), and less aquatic
invertebrate biomass (8–23 mg fish
The biomass of invertebrates in trout diets under
different grazing systems may have been influenced by
the spatial arrangement of sample sites and timing of
sampling, especially at IRG sites, and the composition of
salmonid communities. Different trout species composi-
tion among sites combined with differences in their diets
may have confounded detecting differences among graz-
ing systems. For example, at the four IRG sites described
above, diets were collected from only brown trout
because they dominated abundance (94–100%), and this
species ate less terrestrial invertebrate prey than brook,
rainbow or cutthroat trout (Fig. 6). In contrast, diets were
sampled from more brook trout at sites under the other
three grazing systems, and some rainbow and cutthroat
trout at WO and SRG sites. Although our models
accounted statistically for diet differences among species,
substantial differences in species composition limited the
ability to hold trout species constant when evaluating the
effects grazing management on trout diets. Ideally, sites
selected would have similar species composition, but this
was impossible because few riparian areas under IRG
and WO management were suitable or accessible for
High variability in the biomass and composition of prey
in trout diets limited our ability to detect all but the largest
differences among grazing systems (e.g. >4.5 times
different). Such high variability is common, especially
for terrestrial prey (Hunt, 1975; Saunders & Fausch, 2007;
1524 W. C. Saunders and K. D. Fausch
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
Utz & Hartman, 2007), and likely results from differences
in insect phenology, spatial variability of streamside
vegetation and local weather. Additionally, fish domi-
nance hierarchies can allow a few large fish to usurp most
terrestrial prey, which tend to be larger than aquatic prey
(Furukawa-Tanaka, 1985; Nakano et al., 1999a), while
smaller fish are excluded from optimal positions and
subsist on smaller drifting or benthic aquatic invertebrates
(Nakano, 1995; Fausch, Nakano & Kitano, 1997). Our data
confirmed that larger fish consumed more terrestrial
invertebrates than smaller fish, and this relationship was
strongest for brook, rainbow and cutthroat trout.
Terrestrial invertebrate input versus trout metabolic demand
Comparison of the energy entering streams as terrestrial
invertebrates versus that required for trout active metab-
olism during July and August provided further evidence
that SLG management provided the fewest terrestrial
resources to support trout populations. On average, the
input of terrestrial invertebrates under SLG management
was slightly less than the metabolic requirements of trout,
leaving these fish dependent on aquatic prey for energy to
sustain growth and reproduction. Although we did not
estimate the biomass of benthic invertebrates, it also tends
to be lower in streams under SLG management owing to
siltation from bank erosion (Meehan, 1991) and is lower in
all streams during late summer when primarily small
instars are present (Nakano & Murakami, 2001). More-
over, we measured less biomass of adult aquatic insects
returning to streams under SLG management, probably
owing to less riparian vegetation that provides refuge
(Saunders & Fausch, 2007).
In contrast, under SRG and WO grazing, the energy
input from terrestrial invertebrates was 2.4–3.6 times that
required to support trout active metabolism, on average.
This energy, along with that supplied by aquatic prey,
would be available to support growth and reproduction.
Input at IRG sites was only slightly more than trout
metabolic demand, perhaps influenced by insect phenol-
ogy and spatial arrangement of study sites, as described
above. Sweka & Hartman (2008) demonstrated with
bioenergetics simulations that a 50%reduction in con-
sumption of terrestrial invertebrates by brook trout would
result in a 50%reduction in growth. Our data show a 67%
reduction in available energy at SLG sites compared with
the average for other grazing systems studied here. These
results indicate that SLG management has strong poten-
tial to reduce the energy available from terrestrial inver-
tebrate prey and hence limit growth and reproduction in
rangeland trout populations.
Trout density and biomass
Fish density and biomass were higher at sites managed
under rotational grazing systems than SLG, although high
variability among sites rendered these differences not
significant. This difference was similar to the same
contrast reported by Saunders & Fausch (2007), although
mean fish biomass was slightly greater in Colorado. The
high variability among sites within grazing systems
suggests that larger-scale processes also influenced fish
populations and reduced local effects. Research evaluat-
ing effects of livestock exclosures on fish populations has
produced inconsistent results (Platts, 1991; Sarr, 2002),
probably because many were too small to provide the
diverse habitats necessary for salmonids to complete their
life history (Bayley & Li, 2008). WO exclosures were also
small and difficult to find for our study and were often
either located on small headwater streams or bounded on
both ends by SLG, which may explain why results for
invertebrates and fish under this grazing system were
different than we predicted.
These spatial factors are likely most important for large
trout, which often need large home ranges to find
adequate prey resources (typically other fish) and critical
habitats (Young, 1999). These fish depend less on inver-
tebrates at the reach scale than on specific food and
habitat resources dispersed across the riverscape (Fausch
et al., 2002). As a result, these large trout may be found
only temporarily in a given habitat while they use specific
resources, which is why we did not include them in
biomass estimates.
Management implications. The results presented here, and
those of Saunders & Fausch (2007), show that rotational
grazing systems support more riparian vegetation com-
pared with SLG and can increase terrestrial invertebrate
inputs to streams and the biomass of these prey in trout
diets. However, high variability among sites within
grazing systems caused by differences in the application
by ranchers attempting to optimise rangeland condition
and cattle performance, natural variability in the pro-
cesses that produce invertebrates and deliver them to
streams and factors governing trout foraging behaviour
and movement limited the ability to detect differences
among grazing systems. This variability also increases
along the trophic pathway, making it more difficult to
detect increasingly indirect effects of grazing on trout.
Thus, the CV were smaller for the effects of grazing on
riparian vegetation (CV ¼0:22 for all vegetation measure-
ments) and the biomass of terrestrial invertebrates enter-
ing streams (CV ¼0:21) than for the effects on biomass of
Grazing management influences stream subsidies 1525
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
terrestrial invertebrates in fish diets (CV ¼0:37) and trout
biomass (CV ¼0:37).
Our results show the value of rotational grazing
systems for supplying terrestrial prey that sustain trout
growth and reproduction, but it is unlikely that any single
grazing system will be universally suited for all streams.
In more arid rangelands such as those we studied
previously (Saunders & Fausch, 2007) near Lander,
Wyoming (mean annual precipitation 355 mm, 17 C
mean summer air temperature; NCDC, 2010) IRG may
be necessary because growing seasons are shorter and
riparian vegetation more sensitive to recovery periods
after defoliation. In contrast, in the more mesic rangelands
studied here near Steamboat Springs and Walden, Colo-
rado (mean annual precipitation 475 mm, 11 C mean
summer air temperature), longer, wetter growing seasons
may support less management-intensive systems like
SRG, as well as a diversity of other rotational systems
available to ranchers. The information here can be used to
design specific grazing systems to favour terrestrial
invertebrate subsidies that sustain trout populations,
which is an increasingly important goal for large ranches
in the western United States seeking to optimise income
from both cattle production and angling recreation.
We thank P. Bell, S. Bourdon, C. Craft, A. Gendernalik, B.
Dritz, T. Elm, J. Fike, A. Hansen, B. Heinold, W. Hibbs, C.
Jandreau, I. Lopez, L. Luzania, S. Mahlum, J. Mazzone, S.
Meyers, B. Pate, D. Pruitt, A. Romanyshyn, R. Santiago-
Govier, E. Saunders, V. Sednek, K. Weglars and L. Young
for assistance in the laboratory and field. The Buffalo
Creek, Silver Spur, Betchlor, Fetcher, and Trout Creek
ranches, and the North Park Fishing Club and Rick Whal
provided access to study sites. D. Anderson and
P. Chapman gave statistical advice. Financial support
was provided by the Natural Resources Conservation
Service, U.S. Forest Service, Bureau of Land Management,
and Wyoming Game and Fish Department, and graduate
scholarships from Colorado State University, Trout
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Supporting information
Additional Supporting Information may be found in the
online version of this article:
Appendix S1. Mean percentage ground cover of
prevalent plant species at study sites along 16 rangeland
streams in northern Colorado managed for season-long
(SLG), simple-rotational (SRG), intensive-rotational (IRG),
or wildlife-only (WO) grazing.
As a service to our authors and readers, this journal
provides supporting information supplied by the authors.
Such materials are peer-reviewed and may be re-organ-
ised for online delivery, but are not copyedited or typeset.
Technical support issues arising from supporting infor-
mation (other than missing files) should be addressed to
the authors.
(Manuscript accepted 8 April 2012)
Grazing management influences stream subsidies 1529
2012 Blackwell Publishing Ltd, Freshwater Biology,57, 1512–1529
... For example, larger, more dominant fish hold positions near the heads of pools and are likely to have greater access to drifting terrestrial insects. Relative length for individual fish was defined as their SL divided by the 90 th percentile SL among all fish at a given site (Saunders & Fausch, 2012. Individuals at the 90 th percentile or larger were assigned a relative length of 1.0. ...
... Although Schluter (2000) reported that it is never possible to rule out all environmental factors that might explain a pattern of divergence, we found no evidence that differences in habitat or prey availability could explain the morphological divergence we found. For example, we expected that the greater riparian vegetation and higher air temperatures at the lower-elevation sympatric site in Poroshiri Stream would result in input of more terrestrial invertebrates (Baxter et al., 2005;Saunders & Fausch, 2012), yet Dolly Varden head and mouth morphology shifted to adapt them to foraging on benthic prey at this site, which is opposite that expected if such differences in prey taxa drove morphological divergence. ...
Similar species that overlap in sympatry may diverge in characters related to resource use as a result of evolution or phenotypic plasticity. Dolly Varden charr (Salvelinus malma) and whitespotted charr (S. leucomaenis) overlap along streams in Hokkaido, Japan, and compete by interference for invertebrate drift-foraging positions. Previous research has shown that as drift declines during summer, Dolly Varden shift foraging modes to capture benthic prey, a behaviour facilitated by their subterminal jaw morphology. We compare body and jaw morphology of Dolly Varden in sympatry vs. allopatry in two locations to test for character displacement. Statistical analysis showed significant divergence in characters related to foraging, which was correlated with variation in individual charr diets. Dolly Varden in sympatry had shorter heads and lower jaws than in allopatry, and even within sites charr with these characteristics fed less on drifting terrestrial invertebrates but more on benthic aquatic invertebrates. Those in allopatry had longer heads and lower jaws, and fed more on terrestrial invertebrates. The close proximity of sites in one stream suggests that Dolly Varden may display phenotypic plasticity similar to other charr, allowing rapid responses in morphology to the presence of competitors. These morphological shifts probably help them maintain positive fitness when competing with whitespotted charr in Hokkaido streams.
... For example, O'Gorman et al. (2021 demonstrated that the warming of stream waters and the surrounding soil, as expected by climate change, intensified the input of terrestrial insects into streams. Furthermore, the degradation of riparian zones, for example from livestock grazing (Saunders and Fausch 2012) or deforestation (Kawaguchi and Nakano 2001), significantly decreased the reliance of fish on terrestrial insects. However, another aspect of land use with a strong impact on our river ecosystems and the potential to alter food web dynamics is the one from mining activities. ...
Full-text available
Fish communities of streams and rivers might substantially be subsidized by terrestrial insects that fall into the water. Although such animal-mediated fluxes are increasingly recognized, little is known on how anthropogenic perturbations may influence the strength of such exchanges. Intense land-use, such as lignite mining may impact a river ecosystem due to the flocculation of iron (III) oxides, and thus altering food web dynamics. We compared sections of the River Spree in North-East Germany that were greatly influenced by iron oxides with sections located below a dam where passive remediation technologies are applied. Compared to locations below the dam, the abundance of benthic macroinvertebrates at locations of high iron concentrations above the dam was significantly reduced. Similarly, catch per unit effort of all fishes was significantly higher in locations below the dam compared to locations above the dam and juveniles of piscivorous pike Esox lucius were significantly smaller in size in sections of high iron concentrations. Using an estimate of short-term (i.e., metabarcoding of the gut content) as well as longer-term (i.e., hydrogen stable isotopes) resource use, we could demonstrate that two of the three most abundant fish species, perch Perca fluviatilis, and bleak Alburnus alburnus, received higher contributions of terrestrial insects to their diet at locations of high iron concentration. In summary, lotic food webs above and below the dam greatly differed in the overall structure with respect to the energy available for the highest tropic levels and the contribution of terrestrial insects to the diet of omnivorous fish. Therefore, human-induced environmental perturbation such as river damming and mining activities represent strong pressures that can alter the flow of energy between aquatic and terrestrial systems, indicating a broad impact on the landscape level.
... Emerging adult aquatic insects and streamside invertebrate inputs were sampled concurrently with benthic macroinvertebrates. Sampling was conducted in summer because streamside inputs of invertebrates and emergence of adult aquatic insects are generally greatest during this period (Saunders & Fausch 2012). Emerging aquatic adults were sampled using 0.3 m 2 floating emergence traps (Cadmus et al. 2016) deployed in scour pools, with three replicate nets at each station sampled for two consecutive 24-hour periods. ...
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Stream habitat restoration has the potential to rehabilitate degraded stream communities, but the bioenergetic impact of restoration on prey resources and utilization by salmonids has been understudied. We measured the responses of Brown trout (Salmo trutta) populations, aquatic and terrestrial prey resources, and prey resource utilization by trout, before (2008–2014) and after (2015–2020) large-scale habitat restoration in the Upper Arkansas River, a previously listed US EPA superfund site located in the western United States. We observed significant increases in Brown trout populations following restoration, as well as complex alterations to invertebrate prey resources that generally resulted in increased benthic biomass but reductions in adult aquatic insects and terrestrial invertebrates. Population-level metabolic demand by trout increased 25% after restoration, and prey utilization shifted with reduced consumption of adult aquatic insects and greater consumption of terrestrial invertebrates, relative to their availability. We observed reduced biomass of prey resources consumed by trout with increasing trout population size, and while the total number of invertebrates consumed increased after restoration, actual invertebrate biomass consumed decreased. Our results suggest that restoration was effective in increasing targeted Brown trout populations, but prey resources were altered by physical changes in habitat and increased resource utilization by trout. Habitat restoration projects that are primarily designed to increase trout populations may benefit by focusing on improving prey resources and utilization to support the bioenergetic needs of the restored fishery.
... Whereas, in Scotland, livestock has an indirect effect on fox activity amplitude mediated by decreasing their prey (Evans et al., 2015). The negative effects of grazers are not only restricted to terrestrial predators, they can also spread to other habitats, such as aquatic predators (Saunders & Fausch, 2012). Our results also differed from other studies in which positive effects of livestock on predators were observed. ...
Livestock impact is one of the main causes of habitat loss globally. However, the effects of livestock on flora and fauna diversity have been contradictory, observing cases with positive, neutral, and negative effects. We performed a meta-analysis of the scientific information published in the last 15 years, using Google Scholar and WoS for the search. The inclusion criteria were if the studies presented a) changes in abundance, richness, biomass, plant cover, and consumers; b) included replicas; c) the size of the sample; d) study on domestic cattle, and e) reported the mean and standard deviation of effects of each treatment. We found 2450 scientific publications of which we selected 67 publications that reported the effects a) of grazing on the richness, abundance, cover, and biomass of plants (producers), and b) on richness and abundance of primary and secondary consumers, comparing grazed and non-grazed (or weakly grazed) environments. Grazing did not significantly affect the abundance of the plants or animals studied, regardless of whether they were primary or secondary consumers. The magnitude and direction of the observed effects on plants and consumers could be influenced by livestock type, the natural environments evaluated (forests, grasslands, or scrublands), the spatial and temporal scales involved, and the plant species origin (i.e., native versus non-native). The significant effect of livestock on plants and consumers, also can be differentiated in the characteristics of the species (e.g., life-history traits, etc.) that go beyond their position in the food chains. Evaluating the livestock grazing effect in more than one trophic level helps understand how grazing affects the species according to their way of life, in contrast to evaluations of a single trophic level.
... Pan traps (n = 3) were deployed for two consecutive 24-h periods in mid-August, coinciding with the deployment of emergence nets. We chose this time period because inputs of terrestrial insects into rivers are generally the greatest during summer (Saunders & Fausch, 2012). Each pan trap was filled with 4 L of stream water and approximately 5 ml of unscented, biodegradable surfactant that reduced the surface tension of the water and trapped flying insects. ...
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Quantifying the success of stream remediation projects that are designed to improve water quality or restoration projects designed to improve habitat is often challenging because of insufficient post‐treatment monitoring, poorly defined restoration goals, and failure to consider fundamental aspects of ecological theory. We measured effects of habitat restoration on aquatic and terrestrial prey resources in a system recovering from the long‐term effects of mining pollution. The study was conducted in the Upper Arkansas River, a Rocky Mountain stream located in central Colorado (USA). Remediation of California Gulch, a U.S. EPA Superfund Site that discharged metals from past mining operations into the stream, was completed in 2000, resulting in significant improvements in water quality, benthic macroinvertebrate communities, and brown trout (Salmo trutta) populations. A large‐scale restoration project designed to improve habitat and increase density and biomass of brown trout was completed in 2014. To assess the effectiveness of these habitat improvements on invertebrate communities in this system, for 9 years we sampled sites before (2010‐2014) and after (2015‐2018) restoration was completed. In contrast to our expectations, we observed few changes in abundance of aquatic or terrestrial invertebrates after restoration. The most common response was an overall reduction in abundance resulting from significant instream disturbances during and immediately after restoration, followed by a gradual return to pre‐treatment conditions. Despite reductions in prey abundance, the number of prey items in the diet of brown trout increased significantly after restoration. We discuss several explanations for these responses, including effects of residual metals, increased predation by brown trout, and the recalcitrance of novel communities dominated by metal‐tolerant species. Our results suggest that the effectiveness of remediation and restoration differed between macroinvertebrates and fish. Benthic macroinvertebrates were more dependent on water quality improvements at the watershed‐scale, whereas brown trout populations responded to both improvements in water quality and reach‐scale improvements in habitat. This article is protected by copyright. All rights reserved.
... From a management perspective, our results indicate riparian buffers around headwater streams that provide foraging habitat with an abundant and diverse invertebrate community may be necessary to sustain salamander populations in streams with high SC from MTR-VF and other land uses. Revegetation practices have been shown to increase the amount of terrestrial prey that enter the stream (Wipfli, 1997;Kawaguchi and Nakano, 2001;Saunders and Fausch, 2012;Wipfli and Baxter, 2010). An increase in terrestrial prey subsidies could potentially provide sufficient resources for the larvae to reach metamorphosis and to survive to reproductive age, thus aiding population persistence (Clements et al., 2010;Kraus et al., 2016). ...
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Changes in land use, such as mountaintop removal mining with valley fills (MTR-VF), often results in headwater streams with elevated specific conductivity (SC). Stream salamanders appear to be particularly sensitive to elevated SC, as previous studies have shown occupancy and abundance decline consistently among all species and life stages as SC increases. Yet, the proximate mechanism responsible for the population declines in streams with elevated SC have eluded researchers. We sampled salamander assemblages across a continuous SC gradient (30–1966 μS/cm) in southeastern Kentucky and examined the diet of larval and adult salamanders to determine if the ratio of aquatic to terrestrial prey (autochthony), total prey volume, aquatic prey importance (Ix), and body condition are influenced by SC. Further, we asked if threshold points for each diet component were present along a gradient of SC. Larval salamanders experienced a 12–fold decline in autochthony at 153 μS/cm, a 4.2–fold decline in total prey volume at 100 μS/cm, a 2.2-fold decline in aquatic Ix at 135 μS/cm, and a rapid decline in body condition as SC increased. Adult salamanders experienced a 3–fold decline in autochthony at 382 μS/cm, no change in prey volumes, a 2-fold decline in aquatic Ix at 163 μS/cm, and a decline in body condition as SC increased. Our results indicate that SC indirectly affects stream salamander populations by changing the composition of diet, which suggests that food availability is a proximate mechanism that leads to reduced population occupancy, abundance, and persistence in streams with elevated SC.
To inform riparian restoration, research, and monitoring and to provide management recommendations, we reviewed published studies evaluating the physical and biological effectiveness of livestock exclusion and grazing reduction on various metrics in riparian and aquatic areas. We identified 95 North American studies that reported the effects of livestock grazing reduction on physical habitat (channel morphology, mesohabitats, substrate, and bank stability), biological assemblages (riparian vegetation, macroinvertebrates, fish, and birds), and water quality metrics (temperature, nitrates, phosphorus, and turbidity). Most studies reported that methods to reduce or exclude livestock decreased channel width, width‐to‐depth ratio, bank erosion, soil bulk density, bare ground, water temperature, nitrogen, and phosphorus and increased riparian vegetation (cover, height, productivity, biomass, and abundance), riparian bird abundance, and young‐of‐the‐year fishes. Results for channel depth, instream substrate, mesohabitats, water depth, juvenile and adult fishes, and macroinvertebrates showed no consistent response to exclusion. Project success was influenced by the time since exclusion; whether there was complete exclusion or continued grazing; and local climate, geology, and soils. Apart from bank erosion and stability, most of the physical and biological metrics took more than a decade to respond to livestock exclusion. However, coupling exclusion with planting and other restoration measures decreased the recovery time. Complete exclusion of livestock produced more consistent improvements in riparian condition and other metrics than rest–rotation or other grazing management strategies. Understanding how physical and biological metrics respond to livestock exclusion will require (1) focused, long‐term studies using before–after or before–after, control–impact designs; and (2) monitoring of metrics that most consistently respond to exclusion. Ultimately, the design of exclusions should be driven by local climate, geology, biophysical conditions, and management history. Our results highlight the need for watershed‐scale approaches to excluding livestock from broad areas and the need for implementation monitoring to ensure that fencing and other exclusion measures continue to exclude livestock and produce the desired responses.
• Understanding risks to aquatic systems posed by changing drought regimes is particularly important for the conservation of already threatened taxa. However, little is known about how local environmental conditions, especially those in heavily human‐influenced situations, interact with regional shifts such as droughts to alter realised impacts on aquatic communities, including threatened top predators. • Here, we investigated the combined effects of stream drying intensity and riparian canopy cover on the trophic interactions of critically endangered kōwaro, or Canterbury mudfish (Neochanna burrowsius) in an agricultural area of New Zealand. Fish populations and their potential prey, both terrestrial and aquatic, as well as environmental variables, including riparian canopy cover and drying measured with stage loggers, were sampled over eight visits to 24 sites spanning orthogonal drying and canopy gradients. Stable isotope ratios, ¹³C/¹²C and ¹⁵N/¹⁴N, were used to investigate trophic links between mudfish and their terrestrial and aquatic prey across these gradients. • When non‐native willows (predominantly Salix fragilis) dominated the riparian canopy, increased tree cover led to elevated drying intensity, probably driven by their relatively high water demands compared to other trees. However, in the absence of willows, canopy cover had no effect on drying intensity. Although this was the only direct link between these two environmental factors, they had opposing effects on kōwaro populations, which will be important for management under drought. • Increased drying intensity contributed to elevated abundance of microcrustacea and aquatic Diptera larvae, and an increase in the relative abundance of kōwaro juveniles. However, drying‐affected kōwaro populations also had fewer large reproductive adults and elevated δ¹⁵N values, probably driven by physiological limitations and an increase in kōwaro cannibalism, respectively. • By comparison, increased canopy cover enhanced input of terrestrial invertebrates, a food resource for larger kōwaro, leading to elevated kōwaro δ¹³C values, no effects on δ¹⁵N values, and higher relative abundance of large kōwaro in shaded streams compared to unshaded streams. Thus, the riparian canopy cover was able to offset some of the effects of drying. • Overall, we found no interactions between drying intensity and canopy cover affecting kōwaro. However, their opposing effect highlights the important role local conditions such as riparian canopies play on aquatic communities and their potential role as a restoration tool to mitigate the effects of large‐scale shifts such as drought.
Technical Report
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The U.S. Fish and Wildlife Service (Service) is required by section 4(c)(2) of the Endangered Species Act (ESA) to conduct a status review of each listed species at least once every 5 years. The purpose of a 5-year review is to evaluate whether or not the species’ status has changed since it was listed (or since the most recent 5-year review). Based on the 5-year review, we recommend whether the species should be removed from the list of endangered and threatened species, be changed in status from endangered to threatened, or be changed in status from threatened to endangered. Our original listing of a species as endangered or threatened is based on the existence of threats attributable to one or more of the five threat factors described in section 4(a)(1) of the ESA, and we must consider these same five factors in any subsequent consideration of reclassification or delisting of a species. In the 5-year review, we consider the best available scientific and commercial data on the species, and focus on new information available since the species was listed or last reviewed. If we recommend a change in listing status based on the results of the 5-year review, we must propose to do so through a separate rule-making process defined in the ESA that includes public review and comment.
This book describes stream ecosystems and how they relate to salmonid habitats, life histories and distributions of salmonids throughout North America, responses of fish populations to the changes brought about by land-management activities (e.g., timber harvesting, silviculture, use of forest chemicals), planning strategies used to integrate fish habitats into natural resource management, and general approaches to managing salmonid habitats. Although the book emphasizes anadromous fish and their freshwater habitats in western North America, information on resident salmonids has been included, and attempts have been made to expand the applicability of the discussions to other regions of North America including the Atlantic and Great Lakes states and provinces.
Recent radiotelemetry studies demonstrated that stream-dwelling trout are mobile, but few have compared sympatric species. We used radiotelemetry to simultaneously monitor positions of 20 brown trout and 21 rainbow trout from May or June 1994 to February 1995 in Silver Creek, a small spring-fed stream in south central Idaho. Our biweekly observations from May to September indicated that rainbow trout had larger home ranges (medians, 606 m v. 131 m) and moved greater distances (medians, 1109 m v. 208 m) than brown trout. Furthermore, rainbow trout used more positions than brown trout (means, 7 v. 3) over this interval. Hourly diel monitoring revealed no significant difference in 24-h home ranges of rainbow trout and brown trout (means, 77 m v. 105 m). However, activity patterns of the 2 species differed; rainbow trout activity was usually highest during the day, whereas brown trout activity tended to peak at night. Differences in foraging strategies and response to disturbance may be responsible for differences in mobility.
Most chapters of this book concern interactions of land and water masses, or land-water energy exchanges involving primary and secondary trophic levels. In this chapter I review one example of a land-water interaction at the tertiary trophic level: use of terrestrial invertebrates as food by fish, primarily members of the Salmonidae family. Salmonids (salmon, trout, whitefish, grayling) constitute one of the most important families of sport and commercial fishes, especially in North America, England, and northern Eurasia.