ArticlePDF Available

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

July 2012

DOI:10.1111/j.1365-2427.2012.02804.x

Authors:

W. Carl Saunders

Utah State University

Kurt Fausch

Colorado State University

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.

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).

…

Characteristics of physical habitat in the 16 study sites in northern Colorado

…

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).

…

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).

…

Comparison of the energy entering sites as terrestrial invertebrates and the metabolic demand (± unconditional SE) of trout populations in 16 streams in northern Colorado under four different grazing systems (see 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 fish.

…

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APPLIED ISSUE

Grazing management inﬂuences the subsidy of terrestrial

prey to trout in central Rocky Mountain streams (USA)

W. CARL SAUNDERS AND KURT D. FAUSCH

Department of Fish, Wildlife, and Conservation Biology, and Graduate Degree Program in Ecology, Colorado State University, Fort Collins,

CO, U.S.A.

SUMMARY

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

Introduction

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:

carl.saunders@usu.edu

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

populations.

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.

Methods

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

Land

ownership*

Bankfull

width (m)

Mean

summer

temperature (C)

Altitude

(m a.s.l.)

Map

gradient

(%)

Trout

species Cattle use

Intensive rotational grazing

Lower

Canadian R.

Lat: 4037¢58¢¢N

Lon: 10601¢45¢¢W

Private 5.4 –

†

2564 1.2 BNT (97)

RBT (3)

Early June (10 days)

Sep. (10 days)

Upper

Canadian R.

Lat: 4038¢32¢¢N

Lon: 10602¢21¢¢W

Private 5.2 16.3 2555 2.6 BNT (100) Mid-July (17 days)

Michigan R. Lat: 4037¢42¢¢N

Lon: 10606¢27¢¢W

Private 13.0 16.4 2580 1.0 BNT (94)

RBT (6)

Sep. (21 days)

Illinois R. Lat: 4032¢01¢¢N

Lon: 10613¢22¢¢W

Private 8.3 17.5 2564 1.4 BNT (100) Late July (19 days)

Floyd Cr. Lat: 4047¢53¢¢N

Lon: 10659¢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: 4024¢32¢¢N

Lon: 10623¢28¢¢W

USFS 3.1 –

†

2683 1.4 BNT (100) July–mid-Aug.

(42 days)

Northern

Rock Cr.

Lat: 4023¢58¢¢N

Lon: 10612¢47¢¢W

USFS 3.0 11.2 2751 2.2 BKT (100) Mid-July–Aug

(40 days)

Southern

Rock Cr.

Lat: 4002¢10¢¢N

Lon: 10639¢27¢¢W

USFS 6.3 16.6 2600 1.4 BNT (100) Aug. (35 days)

Lower

Trout Cr.

Lat: 4016¢00¢¢N

Lon: 10703¢38¢¢W

Private 6.3 15.3 2343 2.8 BNT (51)

BKT (36)

RBT (4)

CUT (9)

June–July

(43 days)

Season-long grazing

North Fork North

Platte R.

Lat: 4052¢56¢¢N

Lon: 10632¢59¢¢W

Private 5.0 15.9 2591 1.2 BNT (99)

BKT (1)

June–Oct.

(150 days)

Shafer Cr. Lat: 4051¢49¢¢N

Lon: 10633¢00¢¢W

Private 4.6 10.5 2576 2.4 BNT (88)

BKT (12)

June–Oct.

(150 days)

East Fork

Troublesome R.

Lat: 4011¢26¢¢N

Lon: 10613¢30¢¢W

USFS 7.3 –

†

2475 0.8 BNT (100) 1 July–15 Sep.

(77 days)

Newcomb Cr. Lat: 4035¢43¢¢N

Lon: 10636¢05¢¢W

USFS 11.2 14.9 2656 1.2 BKT (59)

BNT (41)

July–Oct.

(123 days)

Wildlife-only grazing

Hinman Cr. Lat: 4046¢07¢¢N

Lon: 10648¢57¢¢W

USFS 6.6 13.5 2375 1.4 BKT (92)

RBT (8)

–

Upper

Trout Cr.

Lat: 4013¢45¢¢N

Lon: 10606¢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: 10629¢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

exclosures

(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

excluded.

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

made.

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

ﬁsh

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

passes.

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

)to

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).

Results

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).

(a)

(b)

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

per

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

per

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,

2010).

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

model

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

(a)

(b)

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.

(a)

(b)

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

units.

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

excluded.

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

ﬁsh.

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.

Discussion

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

per

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

per

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

study.

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.

Acknowledgments

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

Unlimited and the American Fisheries Society.

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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

Evaluating a pattern of ecological character displacement: Charr jaw morphology and diet diverge in sympatry versus allopatry across catchments in Hokkaido, Japan

Article

Jan 2020
BIOL J LINN SOC

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.

Effects of mining activities on fish communities and food web dynamics in a lowland river

Preprint

Full-text available

May 2023

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.

Bioenergetic responses of a stream food web to habitat restoration: interactions between Brown Trout and invertebrate prey resources

Article

Full-text available

Mar 2023

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.

Effects of livestock grazing on biodiversity: A meta-analysis on three trophic levels

Article

Jan 2022
J NAT CONSERV

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.

Habitat restoration for brown trout ( Salmo trutta ) has limited effects on macroinvertebrate communities in a historically metal‐contaminated stream

Article

Full-text available

Aug 2021

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.

Diet composition: A proximate mechanism explaining stream salamander declines in surface waters with elevated specific conductivity

Article

Full-text available

Sep 2021

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.

Effects of Livestock Exclusion on Stream Habitat and Aquatic Biota: A Review and Recommendations for Implementation and Monitoring

Article

Mar 2023

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.

Disentangling the multiple effects of stream drying and riparian canopy cover on the trophic ecology of a highly threatened fish

Article

Sep 2020

• 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.

Paiute Cutthroat Trout (Oncorhynchus clarkii seleniris) 5-year Status Review

Technical Report

Full-text available

Oct 2020

Chad Mellison

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.

Distribution and Status of Trout and Char in North America

Chapter

Aug 2019

Trout and char of North America

TERRESTRIAL–AQUATIC LINKAGES: RIPARIAN ARTHROPOD INPUTS ALTER TROPHIC CASCADES IN A STREAM FOOD WEB

Article

Oct 1999

Livestock Grazing in Riparian Areas: Environmental Impacts, Management Practices and Management Implications

Chapter

Jul 2003

Caloric Equivalents for Investigations in Ecological Energetics: With 2 figures and 3 tables in the text

Article

Jan 1971

Habitat, life history, secondary production, and behavioral adaptations of aquatic insects

Article

Jan 2008

The relationship between body length parameters and dry mass in running water invertebrates

Article

Dec 1989
Arch Hydrobiol

Elisabeth I. Meyer

Influences of forest and rangeland management on salmonid fishes and their habitats

Article

Jan 1999

W.R. Meehan

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.

Contrasting movement and activity of large brown trout and rainbow trout in Silver Creek, Idaho

Article

Jul 1997

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.

Food Relations and Behavior of Salmonid Fishes

Chapter

Jan 1975

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.

Herbivory in the Intermountain West: An overview of evolutionary history, historic cultural impacts and lesson from the past. Moscow, ID, USA: Forest, Wildlife and Range Experiment Station, University of Idaho

Article