Threats to biodiversity
(i) Overexploitation
Suggested learning outcomes for content
delivered in this sub-section

Describe the exploitation and recovery of whale or fish populations.

Investigate the reductions of populations and if species can recover to
previous levels.

Explain the effect of small populations on evolutionary responses to
environmental changes (the bottleneck effect).

Analyse the rates of reproduction within small populations and
examine the effects of inbreeding.

Investigate the relationship between genetic diversity and viable
populations.
Threats to biodiversity
(ii) Habitat loss
Suggested learning outcomes for content
delivered in this sub-section

Measure/describe species richness data from habitat
fragments within the Caledonian Forest or for tiger populations.

Describe the processes that cause habitat fragmentation.

Investigate the impact of degradation of habitat fragments on
interior species.

Explain the use of habitat corridors for recolonising habitats.
Key terms
Overexploitation and habitat loss
Genetic variation
o
Variation in alleles of genes.
o
Occurs both within and among populations.
o
Genetic variation is important because it
provides the ‘raw material’ for natural
selection.
Genetic diversity
•
Measure of genetic differences within
and between individuals, populations
and species.
•
The variety of genetic material within a
single species of organism that permits
the organism to adapt to changes in the
environment.
The bottleneck effect
o
A population bottleneck (or genetic
bottleneck) is an evolutionary event in
which a significant percentage of a
population or species is killed or
otherwise prevented from reproducing.
Habitat fragmentation
o
The separation of an organism’s
preferred habitat.
o
Habitat fragmentation can be caused
by geological processes that slowly
alter the layout of the physical
environment or by human activity.
Habitat fragmentation
o
Pathways of natural habitat occur
within larger areas that have been
fragmented.
o
They attract species and act as safe
passages for species between
neighbouring habitats.
Reduction of population numbers: can
species come back from the brink?

American bald eagle
removed from the
endangered species list
in 2007.
Population number
The bottleneck effect
Recovery of species
Extinction of species
Time
Loss of genetic variation means that the population may not be able to adapt to
environmental changes or pressures, such as climate change or a loss of available
resources. The genetic variation needed for natural selection will have drifted out of
the population, which could result in extinction. If there is enough genetic variation
then the species can still recover but will lack genetic diversity, making it susceptible
to disease.
An example of a population
bottleneck

Northern elephant seals
An example of a population
bottleneck



Northern elephant seals have reduced genetic
variation, probably due to a population bottleneck
humans inflicted on them in the 1890s.
Hunting reduced their population size to as few as
20 individuals at the end of the 19th century.
Their population has since risen to over 30,000, but
their genes still carry the marks of this bottleneck:
they have much less genetic variation than a
population of southern elephant seals that was
not so intensely hunted.
Investigating habitat fragmentation
http://www.treesforlife.org.uk/forest/humanimpacts/fragmentation.html