G eo Factsheet
January 2002
Number 125
Grassland Biomes
This Factsheet will describe the structure and function of both tropical and temperate grassland ecosystems and provide
a case study of a temperate grassland - the North American Prairies, which illustrates the impact of human activity.
Introduction
The prime factors in influencing the distribution of grasslands are climatic;
grasslands can survive areas of low rainfall, or where heat and high rates
of evaporation reduce the effectiveness of the rainfall, which would be
unsuitable for trees. Grasses are well adapted with a rapid life cycle, and
a dense root network to areas with unpredictable rainfall patterns. Locally
impoverished soils may lead to grasslands, in areas which are wet enough
for forests. In many other areas grasslands are found as a result of human
factors, for example the trees have been destroyed by fire and are therefore
a plagioclimax vegetation (human-induced) not a climatic climax.
Grasslands are one of the Earth’s major biomes (bioclimatic zone), i.e.
regional groupings of plant and animal communities that inhabit a large
geographical area whose climate largely determines its vegetation type.
Grasslands are found on every continent except Antarctica and have local
names. As Fig. 1 shows, grasslands can be sub-divided into temperate and
tropical grasslands based on their distinctive climatic characteristics.
Grasslands are largely devoid of trees in both temperate and tropical areas,
although in many areas conditions are wet enough for some trees to survive.
Fig. 1 Distribution of grasslands biomes.
Grasslands provide an enormous range of goods and services, in
particular food. Cereal crops are all varieties of grass, and grasslands are
the main feeding/grazing grounds for meat-producing animals which
humans hunt or reap.
Steppes
9.4
Prairies
Tropic
of Cancer
72.0
Equator
Sahel
Savanna
Tropic of Capricorn
Key:
Temperate grassland
Tropical grassland
9.4 % remaining as grassland
The two biomes compared
Cerrado/
Campos
2.1
Pampas
Veld
73.0
56.7
As can be seen from Fig. 2 there are both similarities and differences
between the two grassland biomes.
Rangelands
Exam Hint: When analysing data tables, always highlight key
points and then spend one or two minutes selecting data to focus
on the question. For example, compare and contrast the
environments and human activities in the two biomes.
Bush
Fig. 2 The summary of tropical and temperate grasslands.
Tropical grasslands
Approximate latitude 5° - 20° N and S
Climate
22° - 33°C
• temperature
Variable growing season: 3 - 8 months.
Limited by lack of rainfall in winter - dry season.
Adaptation rapid life cycle.
300 - 1500mm - concentrated in summer
• precipitation
• climate type
Soils
Tropical: wet - dry
Largely ferruginous tropical red earths - includes latosols.
Incomplete leaching.
Upward capillary action - concentrates iron.
Native species
Land use
Numerous game, e.g. antelope, wildebeest
Traditional herding (Masai)
Wild game parks
Cash crops: cotton ranching, peanuts
Drought resistant xerophytes in driest savanna (Sahel),
e.g. thorn scrub, acacia
Fire resistant plants and trees (bark), e.g. baobab.
Plant adaptations
Main issues
Threats to nomadism: overpopulation pressurises grazing,
fuel wood and water supplies in African areas
Challenges of cash cropping, e.g. cotton, coffee
Game reserve management - illegal hunting/poaching
Increasing unpredictability of rainfall - desertification.
1
Temperate grasslands
40° - 55° N and S
-15° - 20°C
Average 5 months growing season.
Limiting factor: low winter temperatures.
Adaptation rapid life cycle.
250 - 600mm: Spring (late) summer maximum.
Winter precipitation as snow.
Temperate continental
Long grass prairie: chernozems (black earth)
Short grass prairie: chestnut soils
Leaching after spring - snow melt
Capillary action - accumulation of calcium
Bison, buffalo
Traditional herding and hunting, e.g. in Mongolian Steppes.
No longer in N. America.
Intensive arable farming for cereals/soya beans and ranching.
Nearly all grass - wind chill from strong winds and
physiological drought limits trees
Loss of biodiversity - introduced species
Under threat from agriculture, especially in Americas.
Issues of hi-tech farming, eutrophication etc.
Extreme fragmentation - few natural areas left, especially in USA.
Excessive farming of marginal lands - massive soil erosion
issues in Steppes and Prairies (dust bowls).
Grassland biomes
Geo Factsheet
Figs 3a and b. Food chains for tropical and temperate grasslands.
The two ecosystems compared – structure and functioning
•
Fig. 3a Tropical grasslands
Energy loss by respiration etc.
Top carnivores:
humans
Fig. 3b Temperate grasslands
Consumers:
carnivores,
e.g. lions
Consumers:
herbivores,
e.g. wildebeest
Primary producer: grass
Top carnivores:
humans
•
Consumers:
carnivores,
e.g. wolves
Consumers:
herbivores,
e.g. buffalo
As can be seen from Fig. 5 net primary productivity is one and a half
times as much in savannas as in prairies largely because of the much
larger biomass produced each year because of the higher temperatures
and greater amounts of moisture.
• Net primary productivity is the new growth available (generated by
the producers) to feed other levels of the food chain. NPP = GPP – R
(where GPP = gross primary productivity and R = respiration).
• Biomass is the term used to describe the energy stored as a dry
weight. This energy is transferred through the ecosystem giving rise
to a food chain. Each organism in the chain feeds on and gains energy
from an organism preceding it e.g. lions from wildebeests, and itself
provides energy for a predator.
• Each link is termed a trophic level. There are normally no more than
four trophic levels because energy is lost via respiration, decay and
excretion – hence the pyramid shape of Figs 3a and 3b, as a result of
the decreasing width of each trophic level.
Primary producer: grass
NB: width represents number of organisms
Figs 4a and 4b Generalised Gersmehl diagrams showing relative
nutrient cycling in grasslands.
Fig. 4a Tropical grasslands
Precipitation
B
Key:
B = Biomass
S = Soil
Fallout
L = Litter
L
n
Fig. 5 Net primary productivity of grasslands.
itio
Plant
take up
os
mp
Biome
co
De
S
Runoff
Leaching
Weathering
The soil contains fewer nutrients, with the majority remaining in the litter
throughout the year. This is because decomposition is slow during the dry
season, when there is very little moisture and high temperatures inhibit
bacterial decomposition. During the wet season leaching removes nutrients
from the soil that can result in a hard crust forming that hinders plant
growth. Biomass varies seasonally, with elephant grasses growing up to 2
metres high in the wet summer season.
Precipitation
Key:
B = Biomass
S = Soil
Fallout
600 (0.6 kk/m2/yr)
Temperate
grassland (prairie)
1.6
2.7
5.4
Tropical grassland 900 (0.9 kk/m2/yr)
(savanna)
4.0
4.4
13.5
Tropical rainforest 2800 (2.8 kk/m2/yr)
(for ref)
45.0
19.4
39.4
L = Litter
Plant
take up
Thus a high number of consumers (heterotrophs) can be supported.
These are animals which obtain their food by eating plants or other
animals. With higher primary productivity tropical grasslands support
more large mammals than temperate ones:
• herbivores eat plants
• carnivores eat other animals
• omnivores eat plants and animals
• detrivores feed on dead plant and animal matter
L
Decomposition
S
Weathering
NPP (net primary Biomass Rates of World
(kg/m2) biomass: NPP %
productivity)
NPP
g/m2/yr
The key to the ecosystem are the primary producers – green plants
capable of producing their own food by photosynthesis, i.e. autotrophs.
Grass (gramineae) is a highly successful plant with 9000 species which
can grow in a wide variety of environments. Also, unlike most plants
grass thrives on being eaten (grazed), burned and trampled because it has
multiple growing points or meristems. It is for this reason that grasslands
have productivity rates as high as some trees such as coniferous forest.
Fig. 4b Temperate grasslands
B
Ecosystem structure describes the ways in which its biotic (living)
and abiotic (non-living) environment are arranged and interact. A
sensible way to show structure is by showing a trophic level diagram
(see Figs 3a and 3b).
Ecosystem functioning refers to how energy is transferred through it
(energy flow) and how nutrients are cycled within it. Models and
diagrams help in understanding these functions, e.g. energy flows can
be shown in trophic pyramids or food chains and nutrient cycling by
Gersmehl diagrams (see Figs 4a and 4b).
Runoff
Leaching
Nutrient recycling is the process by which bacteria and fungi feed off
dead organic matter (DOM) and release the nutrients essential to plant
growth such as carbon, hydrogen and potassium. These nutrients are
available for plant uptake. Figs 4a and 4b compare nutrient cycling in the
two ecosystems.
The cold winters and subsequent short growing season means that the amount
of nutrients stored in biomass is small. Large amounts of bacteria return
nutrients from the litter to the soil, which is the largest store of nutrients in
temperate grasslands. The relatively dry climate ensures that nutrients are
not leached away. The nutrient soil is very fertile which is the reason why
temperate grasslands have been ploughed up around the world.
2
Grassland biomes
Geo Factsheet
Case Study: Temperate Grassland - the North American Prairies
Fig. 8 Sequence of development in the prairies.
PHASES
INCIDENTS
1.
• Little tribal
settlement
Long grass prairie,
e.g. between 10 - 20 cm high
3000m
2.
• People of
European
origin
E
encroach
Productive farmland
Ranching
Great Plains (prairies)
Central Lowlands
1000m
1000km
West
600 - 1000mm
300 - 600mm
600 - 800mm
Average Annual Precipitation
800 - 1000mm
3.
• Settlers move
on to
E
moister
grasslands
• Mechanical
aids in use
The rainfall varies from the drier west to the wetter east allowing a
wide range of grass species to grow. During summer the effect of
continentality results in temperatures of over 30°C in July, leading to a
soil moisture deficit, which can result in drought. Average monthly
winter temperatures are 10°C, but fall to -10°C further north. The
wetter eastern Prairies support tall grass up to 2m while in the drier
west only shorter species less than 0.5m grow.
Fig. 7 Soil profile of a chernozem
5.
• Demands D
fluctuate
• Monoculture
• Migration from
margin
E
grasslands
• Results of
earlier
malpractices
0
Dense root system forms thick sod
layer (up to 80% of biomass)
Black horizon
Black/dark brown crumb structure
with many earthworms.
Some leaching occurs after heavy
rain or spring snow melt.
Brownish horizon
Concentration of Ca, Mg, Na and
K in nodules as a result of upward
capillary action.
6.
• Increasing D
world demands
for grain, soya
beans
• High
E
energy costs of
intensive farming
Weathered parent material.
These soils form the basis of an ecosystem whose vast populations
supported beetles, ants, grasshoppers and worms that decomposed the
organic debris into humus. The primary consumers of the vast amounts
of grass were large herbivores like buffalo and antelope and smaller
ones like jackrabbits and marmots. At the next stage of the food chain
were carnivores like bears, wolves and coyotes.
• Review of more
sustainable
methods
INFLUENCES
Climatic variability
Grazing
Fire
Greater tribal majority Climatic variability
Pioneer
settlement
Grain
replaces
natural
vegetation
Exploration
of mountain
west
Grazing
Fire
Native fauna
under pressure Firearms
Conflict with
tribal groups
Ranching (low
quality stock)
Climatic variability
Fire
Firearms
Native herbivores Settlement rapidly
decimated, such developing, with
as buffalo/ bison routeways inland
Tribal reserve areas Climatic variability
Grain growing
established
Ranching
Dry lands
ploughed
Organised
land-holdings & Stock rearing
settlement using
checkerboard system
Grain growing Stock rearing
in favourable
locations
Arable dry farming
Ranching
Dust bowl
Serious erosion
New technologies
Efficient transport
& communications
TIME
4.
• Worldwide D
demands for
produce
• Settlement E
extends
• Mechanisation
• Energy
E
input
Since grasses are short-lived plants, grassland soils accumulate large
amounts of organic debris that turns into humus resulting in fertile
soils known as chernozems (see Fig. 7).
Depth
1m
Nomadic tribal
hunters/herdsmen
Ranching (low
quality stock)
East
Factors Influencing the Ecosystem
2m
DRY
Limited overall impact on
balance between plant
associations/ herbivores/
predators. System in
comparative harmony
Fig. 6 The North American Prairies
Short grass prairie,
e.g. up to 5cm high
MOIST GRASSLANDS
TIME
The Prairies extend over a vast area of the continental interior of North
America. from Canadian Alberta in the north to Texas in the south – a
distance of 4000km, limited by the northern coniferous forests and the
southern deserts, and extending from the Rockies in the west towards
the eastern seaboard 800km (see Fig. 6).
Climatic variability
Effects of
over-cultivation
& over-grazing
Soil deterioration
Grain-growing extended
Climatic variability
Climate change
Subsidies review
Mixed
farming
Use of
legumes
Irrigated pastures
& fodder crops
Feedlots (intensive
cattle farms)
Soil improvement
Scientific erosion
control
Energy/supply
costs affect
development
Investment in
structures/projects
The Human Impact
For thousands of years the only humans on the Prairies were nomadic
hunter-gatherer Native Indians. Their impact is uncertain because some
researchers suggest that the Indians burned ancient forests to
encourage grazing land for buffalo, which they hunted. If this is the case
then the grassland of the Prairies would not be the climatic climax
vegetation, but a human induced pyro-climax, i.e. arrested by fire.
The arrival of European settlers during the 19th century had a
devastating ecological impact on the existing Prairie ecosystem. For
example, the vast buffalo herds, which numbered several million, were
reduced to just fifty wild buffalo by 1883. Encouraged by the 1862
Homestead Act, six million settlers began to exploit the fertile soils of
the Prairie grasslands by ploughing them and planting wheat.
3
Grassland biomes
Geo Factsheet
Case Study: Temperate Grassland - the North American Prairies
(continued)
USA Management
However, the low and variable rainfall during summer led to severe droughts
in the 1890s and 1910s. In addition, the lack of vegetation cover during the
winter after the crops were harvested meant that soil was vulnerable to rain
and wind which gradually eroded away the humus rich topsoil. The situation
became an environmental crisis during the 1930s when drought and overcultivation resulted in vast areas of the Prairies becoming the so-called ‘Dust
Bowl’(see Fig. 9). Dust clouds rose 20,000 feet above the Prairies and the dry,
windblown soil buried abandoned farmhouses in 10 foot drifts blowing dust
as far away as Chicago. Fig. 10 is a timeline of events that details the
environmental degradation of the Prairies during the 1930s.
In response the US Forest Service runs the National Grassland strategy
that oversees four million acres of grassland in twenty sites e.g. Little
Missouri National Grassland in Dakota is the largest with over one
million acres. They promote sustainable land management policies
adapted to the grassland ecosystem, but since the land is a mix of
private and public ownership they do not have the protection of federal
law so are also threatened by activities like road building and mining the world’s largest coalmine is located within Thunder Basin National
Grassland in Wyoming. A more recent management approach is the
Great Plains Program which is currently trying to ensure that the
grassland ecosystem survives its human population.
Fig. 9 Extent of the 1930s ‘Dust Bowl’
Canadian Management
The North American Prairies extend into the southern Canadian
provinces of Alberta, Saskatchewan and Manitoba. Over-cultivation
during the early twentieth century led to the Canadian equivalent of the
‘Dust Bowl’ – the ‘Dirty Thirties’ characterised by large-scale loss of
topsoil.
N
Nebraska
Colorado
New Mexico
In 1983 the PFRA (Prairie Farm Rehabilitation Administration)
identified three major soil degradation issues still posing a problem:
1. Soil erosion
2. Decreasing humus content (organic matter)
3. Increasing salinity
Kansas
Oklahoma
Texas
Together, these were costing over $700 million per year in lost farm
revenues. To combat these problems the government funded 49 local
soil conservation groups to work with farmers on a program of soil
conservation measures (see Fig. 11). This program was replaced in
1993 with the National Green Plan which included soil conservation
under the wider remit of ‘sustainable agriculture’ along with related
Prairie management issues e.g. water quality, wildlife conservation,
pollution, etc.
Fig. 10 Timeline to illustrate the ‘Dust bowl’ decade.
1931 Severe drought occurs in the mid-western states, e.g. Colorado,
New Mexico, Oklahoma, Kansas and Texas. As crops die, dust
from the over-cultivated land is blown away in dust storms.
1932 38 dust storms blow increasing amounts of top-soil away.
1934 The drought becomes the most severe in US History, affecting 75% of
the land area. 35 million acres of cultivated land are now essentially
useless for crop production. Afurther 225 million acres is losing top-soil.
1935 The US forms the Soil Conservation Service that developed
conservation programs that retained topsoil and prevented
irreparable damage to the land. Farming techniques such as strip
cropping, terracing, crop rotation, contour ploughing and cover
crops were advocated. Farmers were paid to practice soilconserving farming techniques.
1937 The government begins the Shelterbelt Project, which was large-scale
planting of trees across the Prairies, stretching in a 100-mile wide zone
from Canada to Northern Texas, to protect the land from erosion.
1938 The soil conservation methods result in a 65% reduction in the
amount of soil blowing. However, the drought continues.
1939 The autumn rains end the drought and the land is planted with wheat.
The 1991 Census of Agriculture recorded that 30% of farmers (farming
25% of the Prairies) were implementing soil conservation measures.
While this has reduced soil erosion and degradation in some areas, not
all the grassland is so managed and soil erosion remains an issue.
Fig. 11 Soil conservation techniques
ZERO TILLAGE SEEDING
Problem Tilling soils with ploughs reduces the organic matter (the
perennial nature of native grassland meant that the soil was
not exposed, therefore not eroded).
Solution Specially designed equipment reduces soil disturbance by
combining ploughing, seeding and fertilising in a single operation.
Benefits
SHELTER BELTS
The Prairies Today
In total over 90% of the original North American Prairies have been
lost to cultivation and erosion. Despite the better land management
practices introduced in the wake of the ‘Dust Bowl,’ the cultivated
Prairies continue to face environmental problems:
1. Soil erosion - continues due to overgrazing by cattle ranching
especially in Western areas
2. Loss of biodiversity – 214 species are threatened. The afroecosystems replacing them lack biodiversity
3. Aquifer levels decreasing – the world’s largest aquifer, the
Ogalalla beneath the Prairies, has lost 3 to 30m of water due to
irrigation pumping.
4. Climate change could result in increased drought as temperatures
rise leading to decreasing crop yields.
Decreased erosion, improved water retention, decreased loss
of humus and less disturbed wildlife habitats.
Problem Lack of trees led to exposed soil blowing away during drought.
Solution Planting rows of trees, e.g. ash, Scots Pine.
Benefits
Trees reduce wind speed which decreases erosion, better crop
yields due to improved micro-climate temperatures, provide
additional wildlife habitat.
CONSERVATION FALLOW
(leaving land to ‘rest’ for a season, i.e. no crops)
Problem Conventional farming leaves land fallow by ploughing it,
which leaves soil exposed to erosion.
Solution Use herbicides to control weed growth instead.
Benefits
4
Incorporates organic matter that improves crop yields and
decreases wind speed which reduces erosion.
Grassland biomes
Geo Factsheet
The Future for Grasslands
Question – A2 style
The long-term survival of the world’s temperate grasslands is vital if they
are to remain agriculturally viable and supply grain to a growing
population. The use of soil conservation methods is likely to assume
increased importance if climate change occurs. It is predicted that the
Prairies, Pampas and Steppes would become drier as temperature rose,
perhaps leading to severe drought and a fall in grain crop yields.
1. (a) Analyse the distibution of the Worlds Grassland Biomes
12 marks
Further Research
1. (a) If possible provide a sketch world map or very detailed locations
(see Fig 1). Give details of the physical factors such as rainfall,
but also consider the role of soils (edaphic climax) and human
factors such as burning and cultivation. Include precise details of
locations such as the Prairies. Fig 2 provides a basic summary.
(b) Compare and contrast the structure and functioning of tropical
and temperate grasslands.
13 marks
Answer - guidelines
www.fs.fed.us/grasslands - National Grasslands website
www.epa.gov/ecoplaces/part1/site9.html - Great Plains Program
www.aceis.arg.ca/pfra/pfintroe.htm - Canadian Grassland Management
www.wri.org/wr2000grasslands is a superb summary of grasslands.
(b) Learn diagrams such as the Gersmehl models (see Fig 4) and the
trophic pyramids (Fig 3) to help you make comparisons more
easily. Fig 3 can be used for structure and also for energy flow
(part of functioning). Fig 4 can be used to show nutrient cycling.
Further Activities
Research a contrasting tropical grassland case study for example looking at
issues in the African Sahel (see Geo Factsheet) or alternatively a game park
environment in East Africa such as Ngorongoro (Tanzania) or Tsara (Kenya).
As a starting point use www.unesco.org/whc/sites/39htm which looks at
all world heritage sites.
For a perspective on Steppes grassland read Grassland ecosystems –
sustaining the Steppe (pages 212 – 224), People and Ecosystems World
Resources 2000 – 2001 published by Elsevier Science.
Question – AS style
1. Compare the climates of temperate and tropical grasslands. 4 marks
2. For either temperate or tropical grasslands describe and explain the
nutrient cycle. (You should include a diagram of the nutrient cycle)
4 marks
3. Describe two ways in which grass species are adapted to their
environment.
4 marks
4. How has the impact of commercial farming affected the relationship
between the climate, soils and vegetation of the North American
Prairies?
4 marks
5. How can agriculture on the Prairies be made more sustainable?
4 marks
Note: look at weightings (marks) very carefully to gauge the depth of
answer required.
Answers
1. Give temperature and precipitation for each type – possibly name clmate
type.
2. Provide details of how the climate affects processes such as leaching
and decomposition and include Gersmehl diagram.
3. The grass species has meristems which enables it to survive being
eaten, burned, etc; tolerant of a wide range of climatic conditions –
e.g. grows in temperatures ranging from –10 to 35°C; also tolerant of
high salinity e.g. Spartina on saltmarshes.). Use of rapid growth
cycle, extensive root systems.
4. Agriculture has disturbed the biome by ploughing up the dense sod of
grass roots which protected the soil being eroded by winter winds;
ploughing also decreases the fertility of the soil because the organic
matter from the die back of annual grasses cannot be incorporated
into the soil; habitats for wildlife are destroyed so beneficial insects
are lost resulting in the need for pesticides.
Acknowledgements;
This Factsheet was researched by Debra Jowitt, an A-level examiner who works at
Holy Cross College, Bury.
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5. Use farming techniques that least disturb the soil e.g. zero tillage
seeding, shelter belts, conservation fallow.
5