Topic 2 – The Ecosystem
2.3 – Changes
2.3.1 - 2.3.4
n 2.3.1 – Explain the concepts of limiting factors
and carrying capacity in the context of
n 2.3.2 – Describe and explain ‘S’ and ‘J’
population growth curves.
Population curves should be sketched,
described, interpreted and constructed from
n Nearly 1.6 million people
join the human population
n 84 million people join every
n In three years the human
population grows by an
amount nearly equivalent to
the entire U.S population.
n By 2025 the world
population could exceed 8
n The study of any population is an important aspect of
n Studies on both human populations and smaller
ecosystem populations are carried out in depth.
n We are going to concentrate on population control of
ecosystems but these theories can also be applied to
n By taking samples and counting the
numbers of organisms in a
particular habitat, ecologists can
study the affects of any factor on
the size of a population.
n The factors affecting a population
size may be biotic or abiotic.
n Together they affect the rate at
which population grows, and also
it’s final size.
Biotic Factors Affecting Population
n How many biotic factors can you think of that
might affect population size?
n How many abiotic factors can you think of
that might affect population size?
Biotic and Abiotic Factors
1. Food – both quantity and 1. Temperature – higher
quality of food are temperatures speed up
important. enzyme-catalyzed reactions
2. Predators – refer back to and increase growth.
predator prey relationships. 2. Oxygen Availability –
3. Competitors – other affect the rate of energy
organisms may require the production by respiration.
same resources from an 3. Light Availability – for
environment. photosynthesis and
4. Parasites – may cause breeding cycles in animals
disease and slow down the and plants.
growth of an organism. 4. Toxins and pollutants –
tissue growth may be
Biotic and Abiotic Factors
All of these things come under the category of
n When a small population grows in a particular
environment, the environmental resistance is almost
n This is usually because there is plenty of food and no
accumulation of poisonous wastes.
n Look at the graph of population growth.
n This shows how population growth is eventually
inhibited by environmental resistance and the
environment reaches it’s carrying capacity.
n The carrying capacity (K) is the maximum
number of a species that the habitat can hold.
n Once the carrying capacity is reached, unless the
environmental resistance is changed, e.g. by a new
disease, the size of the population will only fluctuate
n Think of your brine shrimps!?
n The graph we have just been looking at is an
example of an ‘S’ curve.
n This is the type of graph that is almost always
seen in nature.
n As the energy resources become more scarce
the population size levels off at the carrying
n Just as in the ‘S’ curve example, a population
establishing themselves in a new area will undergo
rapid exponential growth.
n This type of growth produces a J shaped growth
n If the resources of the new habitat were endless then
the population would continue to increase at this rate.
n This type of population growth is rarely seen in
n Initially exponential growth will occur but eventually
the increase in numbers will not be supported by the
n Can you think of any examples where ‘J’ curve
population growth would be extremely desirable.
Is there a Carrying Capacity for Homo
n ‘As we have seen, the human population growth curve is
currently following an exponential curve or a "J-shape”.
Common sense tells us that such growth cannot continue -
otherwise within a few hundred years every square foot of the
Earth's surface would be taken up by a human.
n Furthermore, experience with other species tells us that,
ultimately, resource limitations and/or habitat degradation will
force the human population curves to approach an upper limit
- the carrying capacity, often symbolized as " K" by ecologists.
n It is very natural to ask the linked questions - does humanity
have a carrying capacity and, if so, what is it - and when will
we reach or overshoot this
n Complete the activity – The new zoos
2.3.3 – Describe the role of density-dependent
and density-independent factors, and internal
and external factors, in the regulation of
Density Independent Factors
n The following factors are classed as density-
n Forest Fires
n These factors exert their effect irrespective of the size
of the population when the catastrophe struck.
Density Independent Factors
This graph shows the decline in the
population of one of Darwin's finches
(Geospiza fortis) on Daphne Major, a
tiny (100-acre) member of the
The decline (from 1400 to 200
individuals) occurred because of a
severe drought that reduced the
quantity of seeds on which this
The drought ended in 1978, but even
with ample food once again available
the finch population recovered only
Density Dependant Factors
n Intraspecific Competition -
competition between members of
the same species.
n Read the information about the
n Many rodent populations (e.g.,
lemmings in the Arctic) also go
through such boom-and-bust
Density Dependant Factors
n Interspecific Competition – this is competition
between different species for different resources.
n This can include food, nesting sites, sunlight.
n This occurs when two species share overlapping
ecological niches, they may be forced into
competition for the resource(s) of that niche.
n 2.2.4 – Describe the principles associated with
survivorship curves including K- and r -
n “I once ploughed up an old
field and allowed it to lie
fallow. In the first season it
grew a large crop of
n Ragweed is well adapted to
exploiting it’s environment
in a hurry – before
competitors can become
n Ragweed’s approach to continued survival is
through rapid reproduction.
n We say that they have a high value of ‘r’
n They are called r-strategists
n Can you think of any other animals that may
n In general, r-strategists share a number of
1. Usually found in disturbed and/or transitory
2. Have short life spans
3. Begin breeding early in life
4. Have short gestation times
5. Produce large numbers of offspring
6. Take little care of their offspring (infant mortality
7. Have efficient means of dispersal to new habitats
n When a habitat become filled with a diverse
collection of creatures competing with one another
for resources, the advantage shifts to K-Strategists
n K-strategists have a stable population that is close to
n There is nothing to be gained from a high r.
n The species will benefit the most by a close
adaptation to the conditions of the environment.
n K-strategists share these qualities:
1. Found in a stable habitat
2. Long life spans
3. Begin breeding later in life
4. Long gestation times
5. Produce small numbers of offspring
6. Take good care of their young – infant mortality
7. Have evolved to become increasingly efficient at
exploiting an ever-narrower slice of their
n The graph shows 4 representative survivorship curves.
n Curve A – characteristic of organisms that have low
mortality until late in life when aging takes its toll.
n Curve B – typical of populations in which factors such
as starvation and disease inhibit the effects of aging
and infant mortality is high.
n Curve C – a theoretical curve for an organism
whereby the chance of death is equal at all stages
n Curve D – typical of organisms that produce huge
numbers of offspring accompanied by high rates of
n K-strategists usually have survivorship curves
somewhere between A and C.
n R-strategists usually have D survivorship
n The Californian side-blotted lizard
n 2.3.5 – Describe the concept and processes of
succession in a named habitat.
n 2.3.6 – Explain the changes in energy flows, gross
and net productivity, diversity and mineral cycling in
different stages of succession.
n 2.3.7 – Describe factors affecting the nature of climax
Succession – An intro
n The gradual process by which the species population
of a community changes is called ecological
n A forest following a disturbance such as a fire.
n Succession takes places as a result of complex
interactions of biotic and abiotic factors.
n Early communities modify the physical environment
causing it to change.
n This in turn alters the biotic community which further
alters the physical environment and so on.
Succession – What happens?
n Each successive community makes the
environment more favourable for the
establishment of new species.
n A succession (or sere) proceeds in seral
stages, until the formation of a climax
community is reached.
n Refers to colonization of regions where there
is no pre-existing community.
n Can you think of examples where this would
n You will be studying glacial moraines in detail
as well as the succession occurring on bare
n Community changes on a glacial moraines
n Study the information on glacial moraines and
answer the following questions:
Questions – Glacial Moraines
n During succession there is a change in species composition
of a community. There are also changes in species diversity,
stability of the ecosystem, and in gross and net production
until a climax community is reached.
1. Explain what is meant by a climax community.
2. Explain each of the following changes which occur during
a) Species diversity increases
b) Gross production increases
c) Stability of the ecosystem increases
3. Give two reasons why farmland in the UK does not reach a
Primary and Secondary Succession
n Primary Succession – occurs on newly
formed habitats that have not previously
supported a community.
n Secondary Succession – occurs on sites that
have previously supported a community of
Primary Succession – Bare Rock
Bare Rock Lichens, Grasses and
and annual shrubs
After 100-200 years
Slower growing growing
broadleaf species trees e.g.
e.g. oak Ash
Example for a Northern Hemisphere lithosere: a succession on bare rock
In Summary - the 1 Invaders!
n These are usually fast growing plants that photosynthesize
well in full sunlight.
n We call these pioneer species making up the pioneer
n Examples = lichens, grasses, herbs
n As these species begin to grow well, they produce shade.
Their own seedlings grow more poorly than shade-adapted
n Plants that grow well under full sun are replaced by plants that
germinate and grow better in deeper shade.
n This type of succession takes place after a land
clearance (e.g. from fire or landslide).
n These events do not involve loss of the soil.
n Secondary succession therefore occurs more rapidly
than primary succession.
n Humans may deflect the natural course of succession
in these circumstances (e.g. by mowing or farming).
n This leads to the development of a different climax
community than would otherwise develop naturally.
Secondary Succession – Cleared
Primary Bare community Grasses and low
Earth (annual grasses) growing
Time to develop: Years
Young broad Scrub: shrubs
leaved woodland and small trees
mainly oak 150+ = climax community
n As the plant community changes, the soil will also undergo
changes (abiotic factors will change).
n Decomposers will join the community as well as animal
n Animal species have a profound affect on the plant species
occurring within a habitat.
n Changing conditions in the present community allows for new
species to become established (the future community).
n Succession continues until the climax community is reached.
n Wetland areas present a special case of
n Wetlands are constantly changing:
Open water Plant invasion Siltation and
• Wetland ecosystem may develop in a variety of ways:
• In well drained areas, pasture or heath may
develop as a result of succession from fresh
water to dry land.
n In non-acidic, poorly drained areas, a swamp
will eventually develop into a fen.
n In special circumstances, a an acid peat bog
may develop. (may take 5000+ years).
n Think back to the work on food webs/chains
n It is often useful to know how much energy is
passing through a trophic level over a period of time.
n This is called productivity
n Productivity is a measure of the amount of energy
incorporated into the organisms in a trophic level, in
an area, over a certain period of time.
n 2.2.4 – Define the terms gross productivity, net
productivity, primary productivity, secondary
productivity, gross primary productivity and
net primary productivity.
n 2.2.5 – Calculate the values of gross and net
productivity from given data
n The area is normally one square metre and the
time is usually one year.
n It is therefore measured in units of kilojoules
per square metre per year (kJm-2year-1)
n The rate at which producers convert light
energy into chemical energy is called primary
n Gross Productivity (GP) – is the total gain in energy or
biomass per unit time.
n This is sometimes shown as GPP – Gross Primary Productivity
n It is related to the total amount of chemical energy
incorporated into the producers.
n The producers use some of this energy during respiration and
energy needs which is eventually lost to the environment as
n The remaining energy is available to the herbivores and is
known as net primary productivity (NPP)
Recap of Definitions!
n Productivity = production per unit time
n Primary Productivity = The rate at which energy/biomass is formed through
n Secondary Productivity = The rate at which primary material is synthesised
into animal tissue per unit area in a given time.
n Gross Productivity (GP) = the total gain in energy/biomass per unit time.
n Gross Primary Productivity (GPP) = the total gain in energy of the producers.
n Net Productivity (NP) = the gain in energy/biomass per unit time remaining
after allowing for respiration (R) loses.
n Net Primary Productivity (NPP) = the gain in energy/biomass per unit time
remaining after allowing for respiration loses which is passed onto the
n Primary productivity varies greatly in different
n The rate at which plants can convert light energy into
chemical energy is affected by many factors:
n Amount of nutrients
n In natural ecosystems primary productivity
tends to be highest in tropical regions.
n This is due to good light levels and high
temperatures in the tropics.
n In the oceans however, the most productive
areas are in cold regions due to the up-welling
of water bringing plant nutrients with it.
n We can calculate GPP as follows:
GPP = NPP + R
n We can calculate NPP for both producers and
NPP = GPP – energy used in respiration
n In addition, the equation for consumers only is:
GP = food eaten – faecal losses
Calculating Productivity Values
n Some easy ones to start you off!
n What is the % energy from sunlight that is fixed as GPP if the
total energy from the sun in 3 x 106 and the gross primary
productivity = 2.8 x 104?
n What is the GPP of an ecosystem if the NPP is 1660 kJm-2yr-1
and the energy lost during respiration is 573 kJm-2yr-1 ?
n What is the NPP if the GPP is 2700 kJm-2yr-1 and the energy
used in respiration is 1850 kJm-2yr-1?
Calculating Productivity Values
Now for some slightly harder ones!
Energy Flow Diagrams
Energy flow diagrams illustrate energy flow
through communities and include values for
respiratory losses and energy flow through
Information from energy flow diagrams can be used to
calculate ecological efficiencies
Ecological Efficiency is the net production of new
biomass at each trophic level as a percentage of the
total energy flowing through that trophic level
Therefore, for photoautotrophs, photosynthetic
efficiency is determined as:
Photosynthetic Efficiency =
Net production ÷ Light Energy Absorbed
Use information from the energy flow diagram to:
• Explain the meaning of the term Gross Primary Production
• Explain the meaning of the term Net Primary Production
• Calculate the Photosynthetic Efficiency of the phytoplankton
Gross Primary Production is the total energy fixed Photosynthetic
by photoautotrophs during photosynthesis Efficiency =
Net Primary Production is the energy stored as 3.7 x 104
biomass (gross production – energy lost as heat ------------ x 100
172 x 104
in respiration) = 2.15%
800 kJ m-2year-1
efficiency = 1.3%
24 x106 kJ m-2year-1 NPP = NPP =
69.7 x 103 kJ m-2year-1 200 kJ m-2year-1
114 x 103 kJ m-2year-1
Finally Back To Succession!
n The NPP and GPP of any ecosystem is going to
fluctuate. This is especially the case during each
n As ecosystems become more diverse, the overall GPP
is also going to increase.
n This is because climax communities are better
adapted to an efficient rate of utilisation of their
n They become stable.
The Early Stages
n Gross Productivity = Low
n This is due to the initial conditions and the relatively low
density of producers.
n Net Productivity = High
n This is due to low respiration rates of the initial producers and
therefore a lot of energy available to be passed on.
n This allows the system to grow and biomass to accumulate.
The Later Stages
n Gross Productivity = High
n This is due to an increase in the consumer community
who can synthesise a lot of energy from the food they
n Net Productivity = Low
n Increased rates of respiration and other energy
sapping activities by consumers means that NP will
begin approaching zero.
n Succession comes to an end with the establishment of
a mature, relatively stable community – the climax
n Climax communities are more stable that the seral
stages that preceded them.
n Ultimately, the climate will be responsible for
affecting the nature of the climax community unless
human or other factors maintain an equilibrium at a