1. Topic 2.6
Changes
http://www.youtube.com/watch?v=
xMQ0Ryy01yE

2. Limiting Factors and Carrying Capacity
• Limiting Factors
–
–
–
–
Variables which constrain the growth of populations
These include temperature, light, water and nutrients
Wide range of environmental factors
= stenoecious species
Narrow range of environmental factors = euryoecious species
• Carrying Capacity
– The maximum number of organisms of a particular species that
a given area or ecosystem can sustainably support
population
size
carrying capacity
time

3. Limiting Factors and Carrying Capacity
oligotypic species
mesotypic species
polytypic species

4. Temperature
• Plants have different niches with regard to
temperature
• Different temperatures may be limiting at
different stages in a plant’s lifecycle
• e.g. The South African Protea flower only
germinates after bush fires activate its seeds, kill
off competitors and provide ash to enrich the soil
with nutrients

5. Water
• Too little water may cause water
stress in plants:
– Germination may fail
– Seedlings may die
– Seed yield may be reduced
• Plants have developed adaptations to
survive in different levels of water
stress:
– Hydrophytes – water tolerant plants
– Mesophytes –plants that survive in
moist environments
– Xerophytes – plants that survive in dry
environments

6. Population Growth

7. Population Growth
• When a species is introduced into a new
environment its population grows in a characteristic
way.
• The curve is generally called a sigmoid growth curve
or S-curve

8. Population Growth
S-Curve
• The S-curve has three phases
(1,2,3)
– 1. Exponential growth phase
Resources are plentiful. Birth rate
(natality) is greater than death rate
(mortality)
– 2. Transitional phase
Natility falling, mortality rising. But natality
is still greater than mortality
– 3.Plateau phase
natality = mortality. Something is now
limiting the birth rate. Eg predation,
competition for resources, food
shortage, disease. In reality the
population wobbles around k, with
overshoot and die-back
k
k = carrying capacity
The s-curve is typical of longlived species that are able to
reach their carrying capacity –
e.g. top predators

9. Population Growth
J-Curve
• The J-curve only shows
exponential growth
• The species does not reach
the transitional or plateau
phases, therefore growth
does not slow down
The J-curve is typical of shortlived species that exploit a ‘boom
and bust’ strategy – e.g. rodents,
pest insects and microbes

10. Density-dependent Limiting
Factors
• These factors increase mortality rate as population
density increases
– e.g. competition for resources, space, disease, predation,
parasitism
– Predators are attracted to more dense populations,
disease spreads more easily
• They act as negative feedback mechanisms and lead to
stability around the carrying capacity
• They also act as natural selection factors which help to
increase the fitness of the population
• They include internal factors such as densitydependent fertility or size of available breeding
territory and external factors such as predation or
disease

11. Density-independent Limiting Factors
• These factors increase mortality rate but are not
related to population density – they are generally
abiotic factors. They may also reduce birth rate
– e.g. extremes of weather (drought, hurricanes), longterm climate change (natural and anthropogenic),
geophysical events such as tsunamis and volcanic
eruptions
• They may act in conjunction with densitydependent factors, but do not act to stabilise
populations

12. Human Influences on Limiting Factors
• Human activities may cause population increases by:
–
–
–
–
Increasing availability of resources (e.g. fertiliser use)
Reducing competition (e.g. pesticide use)
Reducing predation (e.g. by over-hunting)
Introducing animals to new areas
• Human activities may cause population decreases (or
even extinction) by:
– Habitat destruction
– Introduction of alien species
– Over-hunting

13. r- and K-Strategists
• Slow growing, long-lived species which exploit S-curve
growth are called K-strategists
• Fast growing, short-lived species which exploit J-curve
(boom and bust) growth are called r-strategists
• Species which employ a strategy somewhere between
these extremes are called C-strategists
• K-strategists tend to be large, have a small number of
offspring and high level of parental care (e.g.
mammals)
• r-strategists tend to be small, have large number of
offspring and very little parental care (e.g. insects)

14. r and K Survivorship Curves
K-strategists
Number of individuals surviving
1000
100
C-strategists
10
r-strategists
1
0
50
Percentage of lifespan
100

15. r and K Survivorship Curves
• K-strategists tend to survive for their entire
potential lifespan and then die over a short time
period (e.g. humans, salmon)
• r-strategists tend to die at a young age, and those
that don’t are likely to live a long time (e.g.
turtles, oysters)
• Limiting factors that affect the shape of the curve
include:
– Competition for resources
– Adverse environmental conditions
– Predator-prey relationships

16. Ecological Succession
• The long-term change in a community after it
starts to colonise bare substrate
• Pioneer community
climax community
• Bare rock
lithosere
• Freshwater
hydrosere
• Dry habitat
xerosere
http://www.bbc.co.uk/programmes/p0038s03

17. Succession Model (Lithosere)
Bare uncolonised rock exposed
Colonisation by lichens, weathering of rock and build up of dead organic material
Growth of moss, further weathering; soil begins to form
Growth of small plant such as grasses, leading to further improvement of soil
Larger plants can grow in the deeper and more nutrient rich soil
Climax Community of shrubs and trees

18. Succession
• Primary Succession
– Occurs on a previously uncolonised substrate (e.g. a
newly formed bit of bare igneous rock, newly exposed
surfaces such as landslips, newly quarried rock face,
newly exposed sanbanks or sanddunes)
• Secondary Succesion
– Occurs in places where a previously existing
community has been destroyed (e.g. by a forest fire,
newly exposed sediment at the edge of a lake)
– This occurs faster since soil and seed already exist

19. Succession
• Succession occurs in stages
• The first colonisers are pioneer species
– Fast growing species such as lichens and mosses which don’t require thick
soil
• Secondary species (especially grasses) then take over
– These are slower growing and are able to outcompete the pioneer species
for light and nutrients
• Tertiary species (especially herbaceous plants) then take over
– These take advantage of thicker soil appearing as litter breaks down
• Quaternary species then take over
– Largely shrubs appear which are able to outcompete the herbaceous
plants
• Finally a climax community is produced (e.g. oak forest)
– Thick soil has been produced, trapping more moisture and nutrients
– A community of soil organisms develops
– An animal community develops to exploit the available niches
http://edroness.blogspot.mx/2014/02/ecologicalsuccession.html
Private Life of Plants – episode 4

20. Succession
Feature
Start of Succession
End of Succession
Amount of organic matter
Small
Large
Amount of Nutrients
Small
Large
Nutrient conservation
Poor
Good
Amount of detritus
Small
Large
Niches
Few but wide
Many but small
Size of colonisers
Small
Large
Life cycles
Simple
Complex
Growth patterns
r-strategists dominate
K-strategists dominate
Diversity
Low
High
Stability
Low
High
Productivity
Low and unstable
High and stable
pH
High
Low
Biodiversity
Low
High

21. Productivity
• In the early stages of succession, GPP is low due to the low
density of producers. However, NPP is fairly high due to the
low respiration rates in the whole community
• As succession proceeds, GPP increases as soils become
more structured and the density of organisms increases.
NPP stabalises as niches become occupied
• The ratio of GPP/NPP is called the P/R ratio. In the climax
community, overall productivity is balanced by overall
respiration – therefore as succession proceeds, the P/R
ratio approaches a value of 1

22. Productivity
1
P/R Ratio
Climax Forest
Agricultural Crop
0
time

23. Plagioclimax
• An interruption to succession is termed plagioclimax
• Humans are often the source of this
• Sometimes it is accidental and sometimes on purpose (e.g.
farmers do not want forests to become established on their
land)
• The greatest impact is through direct deforestation
• Humans also affect succession by:
–
–
–
–
–
Erosion of footpaths
Trampling of plants
Hunting
Land clearance
Introduction of grazing animals

24. Questions
1. Briefly describe the stages of primary
succession
2. What is P/R ration and how does it change as
succession proceeds
3. List the characteristics of a typical climax
community
4. What is plagioclimax. Give examples of how
humans may cause it