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GCSE level biology exam revision notes on ECOLOGY
Ecosystems 6.
Some
more examples explained of how a population might change in size when a
biotic or an abiotic factor changes
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(6)
Some examples
of how a population might change in size when a
biotic or abiotic factor changes
A change in the environment can be due to either a
new biotic or abiotic factor.
Such a change can therefore affect the size of the
population of one or more organisms in a community.
This can have 'knock-on' effects because of the
nature of interdependence of organisms in a given habitat.
What the graph shows from the 'timing points'
a, b, c and d
From points
a to b the
population of the organism X
grows steadily -
perhaps a good supply of food and good weather. The
population growth can be exponential (see Example 5)
At point
b some
biotic/abiotic factor changes with a
negative
effect. on the organism X, hence a
decline in the population.
At
c, the population of X plateaus and then begins to
recover and from
c to d
increases at a rate similar to before the negative impact of some
biotic/abiotic factor.
However, the outcome is often uncertain and the population may
not recover - see possible outcomes in Example 5.
Example 1. The emergence of a new predator
From
a to b,
no new predator, population of X growing steadily.
At
b
new predator enters the habit of the organism X.
From
b to c,
population of organism declines due to predator attack on organism
X.
In some cases the population of X may not
recover in this particular habitat and the graph
stops at c.
However, food for the predator is becoming
scarcer, so its population either declines or this particular
predator moves on to another 'hunting ground'.
This allows the population of the organism X
to start to grow steadily again from
c to d.
Example 2. The emergence of a new pathogen
From
a to b,
no new pathogen around in the habitat of X, so population of X grows
steadily.
At
b a
new pathogen enters the habit of the organism X.
From
b to c,
population of organism declines because organism X is susceptible to
attack from the new pathogen.
However, some of the X organisms may have
some genetic resistance to the pathogen, either already present
in the gene pool or gained through mutation - this effect
begins to show up at point
c.
The X organisms which are resistant to the new
pathogen begin to multiply and more frequently as they grow in
number, so the population of the organism X to starts to grow
steadily again from c to
d.
Example 3. A change in the weather
From
a to b,
the weather is warm and sunny with plenty of food around in habitat
of insect X, so population of insect X grows steadily.
At
b
the weather deteriorates, temperature falls, less sun and less food
available.
From
b to c,
population of insect X declines because its colder and less food
available.
At point
c, the weather
improves, temperature increases, more sun and more food.
Therefore the population of the insect X to
starts to grow steadily again from
c to d.
Example 4. The emergence of a new
competitor Y
From
a to b,
no new competitor for the same food source, population of
animal X grows steadily.
At
b a
new competitor for the same food, animal Y enters the habit
of animal X.
From
b to c,
population of animal X declines due to be outcompeted for the food
source by animal Y.
In some cases the population of X may not
recover in this particular habitat and the graph
stops at c.
This has happened with the introduction of
the grey squirrel into the UK in the 1870s.
The grey squirrel outcompetes the native
red squirrel for the same food source - mainly nuts.
Whole areas of the UK have no red
squirrels.
However, by trapping and shooting grey
squirrels, such management of selected woodland habitats has
allowed the red squirrel population to rise - sometimes drastic
measures are called for in conservation projects.
(Not sure whether you call the trapping
and shoot a biotic or abiotic factor!)
Example 5. The sigmoid
population growth curve
A sigmoid growth curve shows the change in population as
circumstances change in terms of:
(i) biotic factors: predators, pathogens (disease),
food resources (food supply)
(ii) abiotic factors: climate often in terms of
temperature, rainfall
The population of any organism is always subject to one or
more limiting factors, often in complex situations, since
any ecosystem is complex.
Sigmoid patterns help us understand how populations can
change and therefore important to conservation scientists.
A sigmoid pattern is often observed in conditions that are
generally consistent, but where the population of a species
successively increases exponentially, then linearly, and at last
asymptotically giving an S-shaped graph curve (the first 'half'
of the graph above).
There are three primary phases to the S-shaped sigmoid curve
graph from the original population after the lag phase:
(i) an accelerated
stage/period
(ii) a transitional stage/period
(iii)
plateau stage/period.
The sigmoid growth curve can be applied to biomass as
well as population, since the two will be closely related. It
can even be applied to the growth of an individual e.g. the
height of a person.
BUT please note the following
Most organisms in a natural environment (complex
ecosystems) are unlikely to show a sigmoid population growth
graph because the population is affected by so many other
factors.
e.g. changing temperature, population of predators,
occurrence of pathogens causing disease, immigration of
individuals/species moving into the area or emigration of
individuals/species leaving the area.
The 'immigrants' or
emigrants' may be a non-competitive food supply or
competitive predators!
Lag phase
This represents an initial stable population number where
there is no significant change in the ambient conditions
(biotic/abiotic factors) e.g. stable climate conditions, steady
supply of food/energy and the birth rate (natality) is roughly
the same as the death rate (mortality).
The graph line is ~horizontal, then something changes!
Exponential growth phase (logarithmic or log phase)
Here the growth of the population is rapid with increasing
'acceleration' (increasing positive slope of graph line), the
growth in population gets faster and faster.
This happens when birth rate (natality) exceeds death rate
(mortality).
This may be caused by more favourable weather e.g. warmer
climate, a sudden greater abundance in the food/energy supply so
more individual organisms survive to reproduce.
e.g. an increase in rich nutrient concentrations can cause an
algal bloom at sea or in lakes (the latter often due to
nitrogen/phosphorus compounds from fertilisers or sewage.
Transition phase
Here the population continues to increase (graph line still
upwards in direction), but at an increasingly slower rate.
e.g. the food/energy supplies may be scarcer, ambient
temperature changes unfavourably.
Birth rate is declining and death rate increasing, but birth
rate still more than death rate.
Stationary phase or plateau phase
Eventually the growth rate drops to more or less zero and the
population number stabilises (graph line is ~horizontal).
(The stationary phase can show minor fluctuations in
population.)
Birth rate is similar to death rate as conditions stabilise
e.g. steady food supply, no great change in weather/climate
patterns, no significant increase/decrease in predators.
The plateau/stationary phase is due to one or more
limiting factors.
Death phase
Here the population number goes into decline and this may be
exponential too, with initially the death rate getting faster
and faster.
Here, conditions have become very unfavourable for the
population number of the species.
This can be due to a relatively sudden fall in the principal
food/energy resource and insufficient to maintain a stable
population number.
It could also be due to a build-up of toxic metabolic waste
material from the organism itself (this applies to fermentation
- see last section).
Final outcome? (extended dotted graph lines G, S, L and E)
There are various possible outcomes in population depending
on the effects of biotic and/or abiotic factors.
G: The population number may stabilise at a greater
value than the original number.
S: The population number may stabilise at a similar
value to the original number.
L: The population number may stabilise at a smaller
value than the original number.
E: The population may completely collapse and the
species becomes extinct.
Examples of sigmoid S-shaped growth curves
Yeast growth
When a culture of yeast cells is
inoculated in a fresh nutrient growth medium they are
biochemically active, but no cell division takes place.
They are
metabolising in preparation for cell division.
When cell
division starts, the population grows rapidly with a good supply
of food/energy.
Eventually the metabolism and rate of cell
division slow down and the population number stabilises.
In the case of fermentation, the increase in alcohol
concentration (ethanol is toxic), the yeast population
declines and you can design process conditions to produce a
particular % of alcohol in the fermented sugary solution.
Bacteria growing on a rotting resource (plant or
animal)
After an initial lag at the start the bacteria population can
grow rapidly with good food/energy supply.
However, as the
bacteria use up the resources available (less organic material
left), their rate of population growth slows and eventually will
fall when the nutrient resource becomes more and more scarce.
Key points
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Key
ideas and examples of how populations are affected by changes in biotic factors
of an ecosystem
Population
Changes in Response to Biotic and Abiotic Factors
Understanding how
populations change in response to environmental factors is crucial in
ecology.
These changes can be influenced by
biotic factors
(living organisms) and abiotic factors (non-living
environmental elements).
Here are detailed notes on examples of how
populations might fluctuate due to these factors.
1.
Biotic Factors
Affecting Population Size
Biotic factors relate to
interactions among living organisms. Examples include:
A.
Competition
-
Example:
In a woodland ecosystem, rabbits and deer may compete for available
vegetation. If food resources become scarce, one species might
experience population decline while the other thrives.
-
Impact:
Increased competition reduces population size due to limited
resources.
B.
Predation
-
Example:
If a wolf population increases, the number of deer may decline due
to higher predation rates.
-
Impact:
A rise in predator numbers decreases prey populations, leading to
fluctuations in ecosystem stability.
C.
Disease
-
Example:
If a fungal infection spreads among a population of frogs, many
individuals might die, reducing the population size.
-
Impact:
Disease can drastically decrease populations, affecting food chains
and ecosystem dynamics.
D.
Food
Availability
-
Example:
If an insect population experiences a boom, birds that feed on them
may increase in number due to higher food availability.
-
Impact:
A population grows when food sources are abundant but declines when
they become scarce.
2.
Abiotic
Factors Affecting Population Size
Abiotic factors refer to
non-living environmental conditions. Examples include:
A.
Temperature
-
Example:
If global temperatures rise, polar bear populations may decline due
to melting ice habitats.
-
Impact:
Changes in temperature can lead to shifts in species distribution
and survival rates.
B.
Water
Availability
-
Example:
In drought conditions, plant populations may shrink, affecting
herbivore species relying on vegetation.
-
Impact:
Reduced water availability leads to ecosystem-wide population
reductions.
C.
Pollution
-
Example:
Increased water pollution in a river may decrease fish populations
due to toxic contamination.
-
Impact:
Pollution causes habitat degradation, leading to population
declines.
D.
Natural
Disasters
-
Example:
After a volcanic eruption, ash deposits may kill local plant
species, reducing food sources for herbivores.
-
Impact:
Natural disasters can create sudden population declines and force
migration.
Importance of
Understanding Ecosystem Population Changes
Studying population
changes helps ecologists and conservationists by:
-
Predicting
Ecological Imbalances
– Recognizing population shifts aids in forecasting potential
ecological disruptions.
-
Managing
Conservation Efforts –
Population trends allow conservationists to protect endangered
species.
-
Understanding
Human Impact – Human
activities, such as deforestation and pollution, alter ecosystems,
affecting population sizes.
-
Improving
Agricultural Practices
– Farmers monitor pest populations to optimize food production while
maintaining biodiversity.
Understanding population
dynamics is essential for maintaining healthy ecosystems.
Examining the influence of both
biotic and abiotic factors
provides insight into nature’s delicate balance and how species interact
with their environment.
Summary of learning objectives and key words or phrases
Be able to describe, or given information, some examples of how a population might change in size
when a biotic or abiotic factor changes.
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