Ch37: Food as a limiting FactorCh37: Food as a limiting Factor

Higher Human BiologyHigher Human Biology

How does land How does land like this… like this…

……turn into turn into this? this?

Primary SuccessionPrimary Succession

e.g. lichens on bare rock.The lichens make acids

that breakdown

the rock

Image source: www.racerocks.com

This allows mosses to become

established.

Image source: www.countrysideinfo.co.uk

Over many years the

rock disintegrates & dead plants accumulate making a

layer of soil.

Image source: spectrum.troy.edu

Colonisation of a previously uninhabited area

SuccessionSuccessionPioneer species

Climax community

e.g. Oak woodland

Infertile soil

Soil fertility increases

Cause of successionCause 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

Image source: www.wyco-rpc.net

• Soil becomes more fertile• Soil becomes deeper• Height of vegetation increases• Biomass of vegetation increases

Climax Climax CommuniCommuni

tytyGreatest diversity of

animal species & most complex

food webs. Varies from climate to climate

As succession proceedsAs succession proceeds

Effect of Land use on Effect of Land use on Natural SuccessionNatural Succession

Britain was completely covered in Oak Woodland

Vast areas cleared for agriculture & human

settlements

Halt on the process of natural succession

Now

400 years ago

Image source: www.wiltshirewildlife.org

Developing Developing CountriesCountries

Human population increasing

More agricultural land needed to

support population

Land overused & soil fertility

drops

More land needed so trees deforested

Land used for cash crops &

cattle ranches

Cash CropsCash CropsInstead of using deforested land to grow basic food for locals, most of the land is used to grow cash crops

Coffee

Cocoae.g. By selling these to rich countries, they try to earn the money needed to support their fragile economy.

Growing Cash Growing Cash Crops – A Risky Crops – A Risky

BusinessBusiness

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 SourcesFuel SourcesDeveloped Countries

Developing Countries

Animal dung

Crop stubble

Wood encourages more deforestation

leads to a lack of natural fertiliser & reduces soil fertility

Fossil fuels

Nuclear power

Increased food productionIncreased food productionMonocultureA 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/diseasee.g.

Wheat Maize Rice Potatoes

FertilisersFertilisersAdvances 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.

Increased food Increased food production:production:

Adding fertilisers (Nitrates, Phosphates and Potassium) increases the nutrient level of soil.

Promotes growth of a particular crop

Increase in Crop Yields

Increased food Increased food production: Chemicalsproduction: Chemicals

Pesticides

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.

Increase in

food supply

Disadvantages of Disadvantages of using fertilisersusing 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)

HerbicidesHerbicidesCrop plants compete with weeds for: • water• light

Herbicides (weedkillers) - control unwanted weeds.

2 types:

• Selective

• Non-selective - …..

• mineral nutrients• space

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&contentGUID=c55d4bc9-c029-987d-d6ab-a0701c9431b6

Herbicides cause the plant to grow rapidly, using up its food reserves, then it dies of starvation.

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.

Selective HerbicidesSelective 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.

Non-Selective HerbicidesNon-Selective Herbicides

PesticidesPesticidesPests reduce crop yield by:

• Feeding directly on part used as food

• Attack roots

• Attack leaves Growth fails

Fungicides Insecticides

Pests also effect the animals that depend on that plant whose populations will decrease.

FungicideFungicidessUsed 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 PesticidesImportance of Pesticides• Reduced loss of crops due to pests & disease

• Increased food production

•Allows production of high quality crops

Scientists are now trying to genetically engineer pesticides.

Good pesticides should be: • Specific• Act quickly / Short-lived• Safe• Should breakdown in environment becoming harmless

Selective breedingSelective breedingSelective 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 breedingSelective breedingInbreeding

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 breedingSelective breedingOutbreeding

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 plantsSelective breeding in plantsSelective 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 RevolutionGreen RevolutionNew 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 EngineeringGenetic EngineeringGenetic 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 Genetic Engineering TerminologyTerminology

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 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 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=2f0602b8-0992-f0f4-f203-75a4d069e8df

Sexual incompatibility prevents many species from interbreeding successfully, but scientists

can overcome this using somatic fusion.

Somatic Fusion Example 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

Image source: www.bbc.co.uk

Effects of Food ShortageEffects of Food ShortageFamine • many deaths due to lack of food

• applies to animals as well as humans

Effects of Food ShortageEffects of Food ShortageUndernutrition(lack of food)

Starvation

Malnutrition (unbalanced

diet)

deficiency

diseases

e.g. kwashiokor (lack of protein)

more susceptible to disease

Unequal Distribution of FoodUnequal Distribution of Food

Keeps the price high, too

expensive for developing countries

Developed countries produce surplus food

which is stored for future use.

Unequal Distribution of FoodUnequal Distribution of Food

The global unequal distribution of food leads to….

Starvation

…populations become helpless & need to depend on emergency aid for survival.

exist but there is unequal distribution locally.

Sometimes adequate food supplies

Overeating & Long Food ChainsOvereating & Long Food ChainsDiet developed

countries

• 1.5x kilojoules• 2x protein• 5 x more protein from animals

Diet developing countries

Vs

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.