Grazing

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Grazing
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Grazing

• A form of exploitation where the prey (primary
  producer) is not killed.
• Typically involves low vegetation (grasses,
  herbs, algal mats), not taller, woody plants
  (browsing).

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Global distribution of grazed land
Grazed grasslands are the largest single
component (25) of the Earth's 117 million km2 of
vegetated lands. In the last 300 years, managed
grazing systems have increased 6fold by land
cover, and they are rising still.
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Grazing by biome
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1.53 billion (2001) cattle occupy nearly 24
percent of earths landmass. Add sheep goat
3.3 billion (2001) They weight more than the
entire human population.

• Forest converted to pasture or production of
  cattle feed.
• Desertification of marginal rangelands in
  semi-arid and arid regions.
• Production of greenhouse gases (CO2, methane,
  nitrous oxide) 18 of emissions.
• Decreased water quality through runoff from
  fertilized fields and feed lots.
• Introduction of invasive species.

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In the 17 Western States 1870 4.1 million
beef cattle 4.8 million sheep 1900 19.6
million beef cattle 25.1 million sheep 1930
great drought
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Overgrazing a global issue
Since 1945, 3.8 of Earths vegetated land have
been seriously degraded by overgrazing. The rate
of abandonment of dryland due to degradation is
1 million km2 per decade and this rate is
probably accelerating.
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Overgrazing fundamentally changed the face of
western states
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San Pedro River, AZ in 1984
San Pedro River in 1998 Cattle-free for 12 years
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Grazing in the Texas Hill Country
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Woody Encroachment linked to Grazing

• Grazing reduces grass biomass, so that seedlings
  of woody plants achieve higher growth rates.
• Fire is suppressed on rangelands, so that
  seedlings and saplings suffer lower mortality.
• Grazers compact soils and increase erosion by
  wind and water, all of which lower grass
  productivity.
• Grazers often facilitate seed dispersal.

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Mechanics of grazing

• Cow has a specific bite width given by the size
  of the mouth
• Cow takes off a fixed percentage of the height of
  the pasture
• Cow takes a certain amount of time tearing off a
  bitefull of grass
• Cow takes time chewing proportional to bite size

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Co-evolution of grass and grazer Many large
grazers evolved after a global climate change
which replaced forests with grasslands at
mid-latitudes Grazers evolved several
specialized organs to deal with low quality,
abrasive food
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Co-evolution of grass and grazer Grasses evolved
strategies to escape from grazers
Below-ground storage of carbohydrate to rapidly
regrow after grazing Altered morphology under
heavy grazing highly branched and low canopy
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Red Queen Hypothesis (the evolutionary arms race
between consumer and consumed)
"It takes all the running you can do, to keep in
the same place." The Red Queen said in Lewis
Carroll's Through the Looking-Glass.
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Intake as a function of pasture height
Cattle take about 50 of the standing biomass
within the bite area in one bite.
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Handling time as a function of bite size
Chewing time is proportional to biomass per bite.
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Functional response curve for grazing
Reflects limits of grazing time (ca. 13 hours per
day) and digestive capacity (ca. 8 kg dry matter)
Daily rate of consumption per animal
Grass height
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Isoclines for free-breeding grazers
this is the effect of the ungrazeable horizon
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Free-breeding grazers
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Free-breeding grazers the grass refuge
stabilizes
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Taurine cattle (without hump) were domesticated
twice In the near-east, giving rise to all
European breeds (humpless cattle) In India
(zebu, humped cattle) Zebu cattle probably
entered Africa from India.
Old-world grasses co-evolved with domesticated
cattle for about 7000- 8000 years. New-world
grasses did not. This may explain the
vulnerability of American prairies to domestic
cattle.
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Alien plant origins
Bromus spec Europe (near-east) Botriochloa
Europe (near-east) Salsola spec Eurasia Timothy
Europe Wild oat Europe Lehman lovegrass South
Africa Buffelgrass (India and Africa) Bermudagras
s (Africa)
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How about captive animals with fixed stock
density?
Total daily rate of grass consumption
Increasing stocking rates
Grass height
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grass growth function
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two cows per acre
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four cows per acre
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five cows per acre
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• One and the same stocking rate can lead to two
  stable states
• the desired state, where the sward is
  fast-growing and the cows eat all they can
  every day,
• the overgrazed state, where the sward is
  growing slowly and
• the cows remain hungry.

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Less productive years can easily set the system
into the overgrazed state
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Recovery from overgrazing is achieved only by
drastically reducing stock density
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A safe stocking density is well below the maximal
sustainable yield for average conditions
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• Summary so far
• In natural grazing systems, grasses and grazers
  have co-evolved to sustain one another. Example
  the ungrazeable horizon prevents grasses from
  being eaten to extinction.
• Grazers are controlled by the state of the
  grassland and vice versa. For example, grazer
  numbers decline when there is not enough forage,
  either by death or migration. Grazers disappear
  before they irreversibly damage the grassland.
• In managed grazing systems, grazer numbers
  (stocking rates) are controlled by the rancher,
  making it possible for grass and soil system to
  be damaged beyond repair.
• Things that are harmful
• Supplementary feeding during drought. This
  maintains an unrealistically large herd.
• Restocking too soon after drought. This subtracts
  time for grasses to repair and recover.

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• Summary so far
• Theory suggests that harvesting a resource at the
  maximal rate maximizes the risk of
  over-exploitation.
• The pre-cautionary principle therefore suggest to
  set stocking rate far below the value that would
  maximize production in an average year.
• However, economic forces tend to work in the
  opposite direction.

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The Tragedy of the Commons
Multiple individuals, acting independently and
rationally in their own self-interest, will
ultimately deplete a shared limited resource,
even when it is not in anyone's long-term
interest for this to happen. Hardin 1968.
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Rancher 1 Rancher 2 Rancher 3
Each rancher, 10 cows Sell 10 calves of 100 kg for 1000 Sell 10 calves of 100 kg for 1000 Sell 10 calves of 100 kg for 1000
Rancher 1 decides to add one cow Sell 11 calves of 99 kg for 1089 Sell 10 calves of 99 kg for 990 Sell 10 calves of 99 kg for 990
Ranchers 2 and 3 follow the lead. Sell 11 calves of 97 kg for 1067 Sell 11 calves of 97 kg for 1067 Sell 11 calves of 97 kg for 1067
Some time later All ranchers have 20 cows. Sell 20 calves of 40 kg for 800 Sell 20 calves of 40 kg for 800 Sell 20 calves of 40 kg for 800
The tragedy is the result of privatizing profits,
while sharing the costs.
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• Summary
• Co-evolution of grass and grazer produces stable
  grazing systems.
• Lack of co-evolution often causes species
  extinctions and reorganization of biodiversity.
• Overstocking causes irreversible loss of species
  diversity, soil quality and quantity.
  Unfortunately, there are economic incentives for
  overstocking.
• This is what happened when cattle, sheep, goat
  were introduced to the Americas, Australia and
  the Pacific Islands.
• Cattle production is still on the rise globally,
  at the cost of forest conversion to species-poor
  grasslands, the spread of exotic, invasive
  species (which often did co-evolve with cattle),
  methane production (a greenhouse gas),
  irreversible land degradation.