Transcript Ch37_Food_Limiting factor

Ch37: Food as a limiting Factor
Higher Human Biology
How does land
like this…
…turn into
this?
Primary Succession
Colonisation of a previously uninhabited area
Image source: www.racerocks.com
e.g. lichens on
bare rock.
The lichens
make acids that
breakdown the
rock
Image source: www.countrysideinfo.co.uk
This allows
mosses to
become
established.
Image source: spectrum.troy.edu
Over many
years the rock
disintegrates
& dead plants
accumulate
making a layer
of soil.
Infertile
soil
Pioneer
species
Succession
Soil
fertility
increases
Climax community
e.g. Oak woodland
Cause of succession
•Succession occurs because each community acts
on and modifies its habitat.
•After a short time of stability the community
makes the habitat less favourable for itself & a
more favourable community succeeds it.
Each stage makes the soil more fertile. As growing
conditions improve larger grasses and shrubs
choke and shade out smaller plants. These are
then replaced by small trees and eventually the
climax community.
Try the activity: Scholar Unit 3, Table 10.1: Increasing complexity in
succession
http://courses.scholar.hw.ac.uk/vle/scholar/session.controller?action=viewContent&contentGUID=
3aa25eb0-80da-ba3f-3012-a9fa8a329c7f
As succession proceeds
•
•
•
•
Climax
Community
Greatest diversity
Soil becomes more fertile
of animal species
Soil becomes deeper
Height of vegetation increases & most complex
food webs.
Biomass of vegetation increases
Varies from
climate to climate
Image source: www.wyco-rpc.net
Effect of Land use on
Natural Succession
400
Britain was completely covered in
years ago
Oak Woodland
Now
Image source: www.wiltshirewildlife.org
Vast areas cleared for
agriculture & human settlements
Halt on the process of natural succession
Developing Countries
More agricultural land
needed to support
population
Human population
increasing
Land used for
cash crops &
cattle ranches
Land overused &
soil fertility
drops
More land needed
so trees deforested
Cash Crops
Instead of using deforested land to
grow basic food for locals, most of
the land is used to grow cash crops
By selling
e.g.
these to
rich countries,
they try to
earn the
money needed
to support
their fragile
economy.
Cocoa
Coffee
Growing Cash Crops
– A Risky Business
If the price
of the cash
crop drops,
the poor
country is
left in debt
and short of
food for local
people.
More
forests
cleared to
grow food
and cash
crops
Country
borrows from
international
banks to buy
food, seed &
fertiliser, so
gets into
more debt
Fuel Sources
Developed Countries
Developing Countries
Wood 
encourages
more
deforestation
Fossil fuels
Animal dung
Crop stubble
Nuclear power
 leads to a lack of natural
fertiliser & reduces soil fertility
Increased food production
Monoculture
A monoculture is a large cultivated population of
one crop species.
• needed to support the increasing human population
• forests cleared to make space for crops
• usually all plants are genetically identical
• very susceptible to pests/disease
e.g.
Wheat
Maize
Rice
Potatoes
Increased food production:
Fertilisers
Advances in agriculture due to the use of chemicals,
has supported the increasing human population.
Harvesting crops causes the Carbon and Nitrogen
cycle to become unbalanced because most of the dead
plant material is removed. So nutrients don’t get
released back into the soil and the soil becomes less
fertile.
Adding fertilisers (Nitrates,
Phosphates and Potassium)
increases the nutrient level of soil.
Promotes growth of a particular crop
Increased food production:
Chemicals
Pesticides
Herbicides
Increase
in
Crop Yields
Increase
in
food supply
Fertilisers
In developed countries…
Use of fertilisers have eliminated the need for crop
rotation as the same piece of land can be continuously
used for the same crop.
Disadvantages of using
fertilisers
• Soil structure & humus content not maintained
• Excess nitrates can be washed (leached) by rain,
into rivers, which can affect drinking water quality
• Leached fertilisers or sewage effluent running
into water can become over-rich in nutrients
(e.g. nitrate) leading to
eutrophication (rapid
growth of algae, drop in
oxygen)
Herbicides
Crop plants compete with weeds for:
• water
• mineral nutrients
• light
• space
Herbicides (weedkillers) - control unwanted weeds.
2 types:
• Selective
• Non-selective - …..
Selective Herbicides
Plants with broadleaves (e.g.
dandelion) absorb a lot, but
plants with narrow leaves (e.g.
grasses) don’t, so this often
used on lawns.
Herbicides cause the plant to grow rapidly, using
up its food reserves, then it dies of starvation.
View the animation: Scholar Unit 3, Figure 4.15: Use of IAA as a
selective weedkiller
http://courses.interactiveuniversity.net/vle/scholar/session.controller?action=viewContent&conte
ntGUID=c55d4bc9-c029-987d-d6ab-a0701c9431b6
Non-Selective Herbicides
These herbicides have a
destructive but short-lived
effect on all green plants, so
are used to clear an area of
plants completely
e.g. before sowing crops.
Pesticides
Pests reduce crop yield by:
• Feeding directly on part used as food
• Attack roots
• Attack leaves
Growth fails
Pests also effect the animals that depend on that
plant whose populations will decrease.
Fungicides
Insecticides
Fungicides
Used to kill fungi
Sprayed
• before fungal attack
OR
• to kill fungal spores – that absorb chemical when
germinate then die
• Rain washes fungicide off
• Needs to be applied often
• New leaves are vulnerable because they aren’t
protected
Importance of Pesticides
• Reduced loss of crops due to pests & disease
• Increased food production
•Allows production of high quality crops
Good pesticides should be:
• Specific
• Act quickly / Short-lived
• Safe
• Should breakdown in environment becoming
harmless
Scientists are now trying to genetically engineer
pesticides.
Selective breeding
Selective breeding is repeated selection of
individuals possessing desirable characteristics
to be used as parents for the next generation.
Such selection also prevents plants lacking the
desirable characteristics from breeding.
Most plants that are eaten by humans have been
selectively bred from wild species.
There are 2 methods…..
Selective breeding
Inbreeding
Cross closely related members of the same species
• In plants: self-pollination
• maintains uniformity in future generations
• can result in reduced vigour (inbreeding
depression)
Selective breeding
Outbreeding
Crossing unrelated members of the same species
• In plants: cross-pollination
• can result in hybrid vigour (better than it’s
parents)
• can’t be depended on to produce good offspring
• process needs to be repeated for each
generation
Selective breeding in plants
Selective breeding in plants (e.g. wheat, barley,
potatoes, tomato) has produced:
• higher yields
• resistance to disease
• fruit with better flavour
Several varieties have been developed through
selective breeding of the cabbage plant.
Images source: http://en.wikipedia.org
Green Revolution
New varieties have formed due to selective
breeding (e.g. drought resistant wheat) so food
production has increased.
But genetically uniform crops are more susceptible
to disease – have no natural resistance, So to save
them pesticides must be applied rapidly.
These are often high yielding, rapid-response
strains which can be harvested 2-3 times per
year, but only with massive use of fertilisers.
Cost of fertiliser leads to debt for many
farmers in developing countries!
Genetic Engineering
Genetic engineering is the transfer of one or more
genes from one organism to another.
Genetic engineering allows the genes of
completely unrelated organisms to be combined
into one organism. This allows a new variety to be
produced in just one generation.
This scientific process is still very controversial
and has prompted much debate due to the moral
and ethical issues that surround it.
Genetic Engineering Terminology
Genome - the single set of haploid chromosomes
typical of a species.
Recombinant DNA technology = genetic
engineering
Transgenic – an organism whose genes have been
altered by genetic engineering
Somatic Fusion
• The somatic (body) cells of 2 different plant
species have their walls removed using the
enzyme cellulase.
• These cells, now called protoplasts, are then
subjected to an electric current and fuse.
• The new hybrid formed is then induced to
form a cell wall and divide into a mass of
undifferentiated cells.
• A hormone then induces the callus to develop
into a hybrid of the 2 different plants.
Somatic Fusion
Sexual incompatibility prevents many species from
interbreeding successfully, but scientists can
overcome this using somatic fusion.
Somatic cells = body cells
Protoplasts = cells with their cell wall removed
Cellulase = enzyme that breaks down cell walls
Callus = a group of undifferentiated (unspecialised) cells.
View the animation: Scholar Unit 2, Producing plant hybrids by somatic fusion, Fig 7.13:
http://courses.scholar.hw.ac.uk/vle/scholar/session.controller?action=viewContent&contentGUID=2f0602b8-0992f0f4-f203-75a4d069e8df
Somatic Fusion Example
Potato: A new variety of potato plant, produced by
somatic fusion, has:
Tuber-bearing fruit
Image source: www.bbc.co.uk
Resistance to the
potato leaf roll virus
Image source:draf.bretagne.agriculture.gouv.fr
Effects of Food Shortage
Famine
• many deaths due to lack
of food
• applies to animals
as well as humans
Effects of Food Shortage
Starvation
Undernutrition
(lack of food)
deficiency
diseases
Malnutrition
(unbalanced diet)
more
susceptible
to disease
e.g. kwashiokor
(lack of protein)
Unequal Distribution of Food
Developed countries
produce surplus food
which is stored for
future use.
Keeps the price
high, too expensive
for developing
countries
Unequal Distribution of Food
…populations
become helpless &
need to depend on
emergency aid for
survival.
The global
unequal
distribution
of food leads
to….
Starvation
Sometimes
adequate
food
supplies
exist but there is unequal
distribution locally.
Overeating & Long Food Chains
Diet developed
countries
• 1.5x kilojoules
• 2x protein
• 5 x more protein
from animals
Vs Diet developing
countries
Grain can support many
people directly but if the
same quantity is fed to
livestock it will support far
fewer people
because energy is lost at
each stage of the food chain.
If less meat is eaten in developed countries,
more food would be available for people in
developing countries.