Transcript Control of Populations Powerpoint

How do humans control
agroecosystems?
1. Manipulation of the food species
• This is done to increase its saleability, yield and
suitability for cultivation.
eg by selective breeding, GM, use of hormones
2. Control of the environment
This is done to maximise yield and marketability.
eg control of water and nutrient supplies, temperature,
light levels, pest populations
Principles
Of the thousands of plant species we could eat, 80% of world grain
production relies on just 8 species
Why?
• They were easy to grow
• High yields
• Easy to cook
• Nutritious
In terms of the animals we grow to kill, other factors include:
• Social: Europeans don’t eat dog meat, but people in N. Korea do
• Religious: Cows are sacred species for Hindus
• Ethical: Most of the world’s population don’t regard eating whale meat
as acceptable
• Technological: We know how to culture fungi to produce single-cell
protein
Principles
Wild varieties of plants and animals have been manipulated to increase
their yield, saleability and ease of cultivation
e.g. domestic pigs look and behave very differently from the wild boar
from which they were developed
Wild boar
Domestic pig
The environment in which plants and animals are grown is
controlled to maximise productivity
Principles: Factory Farming
 Movement is very restricted so that they do not waste energy on muscle
action, and so gain more biomass
 Fed with optimum amounts of food to give maximum growth
 Animals are often injected with steroids and antibiotics to speed up growth
and reduce disease
 The temperature of their environment is strictly controlled so that the
animals do not use much energy in keeping warm and so gain more
biomass
This is how chickens are kept in
traditional farming
This is how chickens are kept in factory
farming
Manipulating Food Species
Title Page
Manipulating food species1.
Population Control
•
Increasing the population density of the
food species can increase productivity
but can increase competition and make
the spread of disease easier.
•
The optimum livestock stocking densities
depends upon the agroecosystem
involved. eg different stocking densities
for lowland and upland sheep.
Q: If you are a cattle rancher in Texas, is it
possible to have too many cattle in any
given area (or is more always better)
A: Population densities> carrying capacity
may result in:
• Overgrazing
• Soil compaction & erosion
• Disease
Manipulating Food Species
We also need to control the density at which we grow crops
There is intraspecific and interspecific competition for:
• Light
• Water
• Soil minerals
• Rooting space
• Carbon dioxide
e.g. The effect of stem density on potato yield
Q: Summarise the data
As stem density increases up to 15-20 stems/m2
total yield increases
At higher densities, total yield stays about the
same but the average tuber size decreases
(relatively more small potatoes!)
Manipulating Food Species
e.g Effect of tree density on cocoa yield in Sri lanka
600
Yield of dry
beans per acre
(pounds wt)
500
400
300
400 trees per acre
200
300 trees per acre
200 trees per acre
100
5
100 trees per acre
10
15
20
Age of trees (Years)
25
Q: Summarise the data
Irrespectiveof density, optimum harvest time stays about the same
Maximum yield at 400 trees per acre
Manipulating Food Species
e.g. Effects of frequency of weeding on the growth and yield of
long-fruited jute in Nigeria
Technique
Plant height (cm)
Stem girth (cm)
Edible yield g/m2
No weeding
39.50
7.53
17.33
Weeding once per
week
77.02
6.63
126.93
Weeded once two
weeks after sowing
67.97
4.93
109.60
Weeded once 4
weeks after sowing
52.10
4.86
60.80
Source: A.R. Adenawoola, R.D. Aladesanwa and T.D. Adenowuro (2005) Effects of frequency of weeding on the growth and yield of long-fruited
jute (Corchorus olitorius) in a rainforest area of southwestern Nigeria Crop Protection, Volume 24, Issue 5, May 2005, Pages 407-411
Q: Summarise the data
Any weeding increases plant height, stem girth and edible yield of crop
Weeding once per week gives best yield
Early weeding better than later
Monocultures
Most habitats contain a variety of plant species, monocultures are
relatively rare
However, in agriculture and forestry, we often plant monocultures
Monocultures
Advantages
Easy planting,
weeding,fertilising
and harvesting
Can lead to pest
epidemics
Disadvantages
Favours mechanisation
so low labour input
May look
visually
unattractive
May lead to
exhaustion of
particular soil
nutrients
Offers economies of scale –
cheaper to buy 10,000 tree
seedlings of one species than
1000 of each of ten species
Mechanisation leads to
job losses and is
dependent upon fosiil
fuels
Plenary
Write down 4 key learning points
from this lesson
• 1.
• 2.
• 3.
• 4.