Ch37: Food as a limiting FactorCh37: Food as a limiting Factor
Higher Human BiologyHigher Human Biology
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.