1. Ch37: Food as a limiting Factor
Higher Human Biology

2. How does land like this…
…turn into this?

3. Primary Succession Colonisation of a previously uninhabited area
Image source:
Image source:
Image source: spectrum.troy.edu
e.g. lichens on bare rock.
The lichens make acids that breakdown the rock
This allows mosses to become established.
Over many years the rock disintegrates & dead plants accumulate making a layer of soil.

4. Climax community e.g. Oak woodland
Soil fertility increases
Infertile soil
Succession
Pioneer species
Climax community e.g. Oak woodland

5. 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

6. As succession proceeds
Climax Community
As succession proceeds
Greatest diversity of animal species & most complex food webs.
Varies from climate to climate
Soil becomes more fertile
Soil becomes deeper
Height of vegetation increases
Biomass of vegetation increases
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7. Effect of Land use on Natural Succession
400 years ago
Britain was completely covered in Oak Woodland
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Vast areas cleared for
agriculture & human settlements
Now
Halt on the process of natural succession

8. Developing Countries Human population increasing
More agricultural land needed to support population
Land overused & soil fertility drops
Land used for cash crops & cattle ranches
More land needed so trees deforested

9. 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 these to rich countries, they try to earn the money needed to support their fragile economy.
Cocoa
e.g.
Coffee

10. 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.
Country borrows from international banks to buy food, seed & fertiliser, so gets into more debt
More forests cleared to grow food and cash crops

11. Fuel Sources Developed Countries Developing Countries
Fossil fuels
Wood  encourages more deforestation
Animal dung
Nuclear power
Crop stubble
 leads to a lack of natural fertiliser & reduces soil fertility

12. 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.
Potatoes
Maize
Rice
Wheat

13. 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

14. Increased food production: Chemicals
Pesticides
Increase in Crop Yields
Increase in food supply
Herbicides
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.

15. 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)

16. Herbicides Crop plants compete with weeds for: water light
mineral nutrients
space
Herbicides (weedkillers) - control unwanted weeds.
2 types:
Selective
Non-selective - …..

17. 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

18. 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.

19. 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.
Insecticides
Fungicides

20. 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

21. Scientists are now trying to genetically engineer pesticides.
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.

22. 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…..

23. 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)

24. 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

25. 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.
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26. 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!

27. 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.

28. 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

29. 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.

30. 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.
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View the animation: Scholar Unit 2, Producing plant hybrids by somatic fusion, Fig 7.13:

31. Somatic Fusion Example
Potato: A new variety of potato plant, produced by somatic fusion, has:
Image source:draf.bretagne.agriculture.gouv.fr
Resistance to the potato leaf roll virus
Tuber-bearing fruit
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32. Effects of Food Shortage
Famine
many deaths due to lack of food
applies to animals as well as humans

33. Effects of Food Shortage
Starvation
Undernutrition
(lack of food)
Malnutrition (unbalanced diet)
deficiency diseases
e.g. kwashiokor (lack of protein)
more susceptible to disease

34. Keeps the price high, too expensive for developing countries
Unequal Distribution of Food
Developed countries produce surplus food which is stored for future use.
Keeps the price high, too expensive for developing countries

35. Unequal Distribution of Food
…populations become helpless & need to depend on emergency aid for survival.
Starvation
The global unequal distribution of food leads to….
exist but there is unequal distribution locally.
Sometimes adequate food supplies

36. Overeating & Long Food Chains
Diet developed countries Vs
Diet developing countries
1.5x kilojoules
2x protein
5 x more protein from animals
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.