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Grazing
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”).
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.
Grazing by biome
• 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.
1.53 billion (2001) cattle occupy nearly 24 percent of earth’s landmass.
Add sheep & goat: 3.3 billion (2001)
They weight more than the entire human population.
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
Overgrazing – a global issue
Since 1945, 3.8% of Earth’s 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.
Overgrazing fundamentally changed the face of western states
18 years without cattle
30 years without cattle
San Pedro River, AZ in 1984 San Pedro River in 1998Cattle-free for 12 years
Grazing in the Texas Hill Country
1932193220002000
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.
Overgrazing
Shrubs replace grasses
Soil compaction,loss of soil stability
Wind & Water erosion
Nutrient lossIncreased spatial
heterogeneity
Irreversible land degradation.
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
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:
high-crowned teeth to increase the ability to tear apart abrasive grass leaves
Ruminant digestive system
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
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.
Grass height (“prey density”)
Inta
ke p
er b
ite
Cattle take about 50% of the standing biomass within the bite area in one bite.
But there is an ungrazeable
horizon.
Intake as a function of pasture height:
Bite size
Han
dlin
g tim
eChewing time is proportional to biomass per bite.
Taking a bite takes the same amount of time, independent of
bite size.
Handling time as a function of bite size:
Reflects limits of grazing time (ca. 13 hours per day) anddigestive capacity (ca. 8 kg dry matter)
Dai
ly r
ate
of c
onsu
mpt
ion
per
anim
al
Grass height
Functional response curve for grazing:
Grazing limited by time
Grazing limited by digestivecapacity
Non-grazeable horizon
Victim density
Pre
dato
r de
nsity
Pre
dato
r is
oclin
e:
Isoclines for free-breeding grazers
this is the effect ofthe ungrazeable horizon
Victim density
Pre
dato
r de
nsity
Pre
dato
r is
oclin
e:
Free-breeding grazers
Victim density
Pre
dato
r de
nsity
Pre
dato
r is
oclin
e:
Free-breeding grazers: the grass “refuge” stabilizes
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.
Alien plant origins:
Bromus spec Europe (near-east)Botriochloa Europe (near-east)Salsola spec EurasiaTimothy EuropeWild oat EuropeLehman lovegrass South AfricaBuffelgrass (India and Africa)Bermudagrass (Africa)
To
tal d
aily
ra
te o
f g
rass
co
nsu
mp
tion
Grass height
Increa
sing sto
cking ra
tes
How about captive animals with fixed stock density?
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
one cowper acre
Stable grass height for grazing at 1 animal per acre.
grass growth function
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
two cowsper acre
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
four cowsper acre
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
five cowsper acre
unstable equilibrium statestable
equilibrium state
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
five cowsper acre
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.
1
2
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
Theoretically maximal yield:
not stable!
drier year
Less productive years can easily set the system into the overgrazed state:
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
Recovery from overgrazing is achieved only by drastically reducing stock density:
Gra
ss g
row
th o
r co
nsu
mp
tion
ra
te
Grass height
A safe stocking density is well below the maximal sustainable yield for average conditions:
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.
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.
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.
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.
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.
