Effects of Grazing Management on Terrestrial Invertebrate Subsidies to Trout in Wyoming Rangeland Streams

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Effects of Grazing Management on Terrestrial
Invertebrate Subsidies to Trout in Wyoming
Rangeland Streams

• W. Carl Saunders
• Department of Fishery and Wildlife Biology
• Colorado State University

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Acknowledgements
Advisor Dr. Kurt Fausch Graduate
Committee Dr. Will Clements Dr. Wayne
Leininger Bob Budd Tony Malmberg Joe Perkins
Erica Barr Field and Lab Assistance Colden
Baxter, Mark Coleman Joe Connery, Gregor Dekleva,
Becky Dritz, Jessica Evans, Matt Cook, Tim Gump,
Jeb Hammer, McLain Johnson, Angela Klein, Amy
Nesler, and Virginia Sednek
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Presentation Outline

• Introduction
• Grazing effects and management
• Subsidies as a prey resource
• Grazing systems and prey subsidies
• Study design
• Results
• streamside vegetation
• Terrestrial invertebrate input
• Use of terrestrial prey by fish
• Fish biomass
• Conclusions

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Cattle Grazing in the United States

• Extent of grazing in US
• Cattle use of riparian areas
• Cattle vs. wild ungulates
• Difficulty of fisheries conservation

NRCS 2001
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Effects of Cattle Grazing on Streams and Fish

• Loss of shading vegetation
• Loss of bank stability
• Siltation of stream habitat
• Alteration of water chemistry

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Rangeland Research and Management for Fisheries

• Focus on the instream habitat
• Importance of rangeland rest
• General management guidelines
• Aboveground biomass
• Stubble height
• Rapid response

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Food Web Subsidies
Baxter, Fausch, and Saunders 2005
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Terrestrial Invertebrate Inputs to Streams
Terrestrial invertebrate input (mg x m2 x day)
Reference
Winter
Fall
Summer
Spring
Riparian Vegetation
Location
Cloe Garman, 1996
10.0
50.0
450.0
50.0
Deciduous forest
Second order streams, Virginia
Wipfli, 1997


37.0

Coniferous forest
Headwater streams, Alaska
Kawaguchi Nakano, 2001
0.07
17.4
29.9
8.7
Mown grassland
Horonai Stream, Hokkaido, Japan
Kawaguchi Nakano, 2001
0.2
10.7
71.8
12.3
Deciduous forest
Horonai Stream, Hokkaido, Japan
Edwards Huryn, 1996


1.3

Pasture
Headwater Streams, New Zealand
Edwards Huryn, 1996


5.7

Tussock grassland
Headwater Streams, New Zealand
after Baxter, Fausch, and Saunders 2005
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Fish Consumption of Terrestrial Invertebrates
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Experiment Excluding Terrestrial Prey
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Streamside Vegetation and Terrestrial Prey

• Terrestrial prey important to fish
• Vegetation supports terrestrial invertebrates
• Potential for grazing effects

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Research Objectives

• Streamside Vegetation
• Terrestrial invertebrate input
• Fish diet
• Fish biomass

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Study Design

• Season Long Grazing (SLG)
• Widely used
• Ambient, unmanaged
• Large pastures
• Use of riparian areas
• Cattle feeding behavior

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Study Design

• High-Intensity / Short-Duration (HISD)
• Management intensive rotational system
• Electric fencing or herd riders
• Mimics historical ungulate grazing
• Single defoliation

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Study Design
Wyoming

• West central Wyoming
• Paired study design
• 5 pairs (n10 streams)
• Matched for physical characteristics
• 100-m reaches in continuous management
• Sampled monthly June -August

20 miles
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2004 and 2005 Sampling

• 2004
• Selected best pairs for Observational Study
• HISD spatially limited
• Elevation difference
• 2005
• Searched for additional replicates
• 3 high elevation HISD sites
• No low elevation Season Long grazing with
  Salmonids

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Site Pairing
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Measurement of Riparian Vegetation

• Above ground biomass (g)

Overhead cover ()
Species richness
Ground cover ()
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Above Ground Biomass
- HISD
- Season Long
Biomass
Functional Group Biomass
P 0.006
120
80
Dry Biomass (g)
40
0
Shrub
Grass
Forb
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Overhead Cover
- HISD
- Season Long
Vegetative Cover
Dominant Vegetation Types
Percent
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Species Richness and Ground Cover
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Riparian Vegetation Summary
Season Long
HISD
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Measurement of Invertebrate Input and Use
Falling invertebrate input
Fish diet
Biomass estimates
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Pan Trap Methods

• Sampled monthly
• Collected two 3-day samples
• Stratified by overhead vegetation
• Separated terrestrial and aquatic

Kurt Fausch 2004
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Pan Trap Terrestrial
- HISD
- Season Long
P 0.0003
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Pan Trap Aquatic and Total Input
P 0.0001
P 0.02
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Fish Diet Methods

• Monthly
• Gastric lavage
• Goal of 10 fish per stream
• Adults 120mm-300mm

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Fish Diet Terrestrial Biomass
P 0.05
HISD
Season Long
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Fish Diet Aquatic Biomass
P 0.009
HISD
Season Long
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Elevation and Fish Diets
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Fish Diet

• Effect of grazing management
• Terrestrial consumption highly variable
• 2005 Diet Samples
• Importance of terrestrial prey
• 53 of diet (12-71)

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Fish Biomass Methods

• 3 pass depletion estimates
• 2004 daytime estimates
• July
• 100 m reaches
• One shocker
• 2005 night time estimates
• June / July
• 200 m reaches
• 2 shockers on large streams

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Total Fish Biomass
P 0.02
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Summary
3 times greater in HISD

• 2 times greater in HISD

• 70 more terrestrials in HISD

• 85 more Terr. in HISD

• 2 times greater in HISD

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Concluding Remarks

• Grazing management can improve streamside
  vegetation
• HISD increased terrestrial input and terrestrial
  biomass consumed by trout
• Terrestrial invertebrate inputs are an important
  resource for salmonids
• First study terrestrial invertebrate prey
  resources
• field experiment necessary

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Concluding Remarks Grazing and Subsidies
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Concluding Remarks Grazing and Subsidies
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Concluding Remarks Unanswered Questions

• How important is grazing on riparian vegetation
  through other pathways?
• Relative importance of terrestrial and aquatic
  prey
• Short vs. long-term recovery with change in
  grazing management

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Questions ?