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Higher Biology
Unit 3
3.7- Mass Extinctions and
Biodiversity
Extinction
• When an organism is classed as extinct
there are no living members of that
species left
• Extinction is a natural process where
organisms that are less suited to an
environment die out
• Throughout the history of earth there
have been several mass extinction
events
Mass Extinctions
Mass Extinction
• Mass extinctions are closely related to
changes in global climate
• These changes in climate result in
different selection pressures being places
on organisms
• Only the organisms best suited to the new
conditions will survive and reproduce
• Evidence of mass extinctions comes in the
form of fossil records
Impact on Biodiversity
• Immediately following a mass extinction
biodiversity may be reduced
• Over time speciation will occur and
biodiversity will increase as new species
emerge
• For examples, the diverse number of
mammalian species that now exist
evolved from a small number of common
ancestors
Extinction Rate
• Extinction rates can be difficult to
measure accurately so are very
approximate
• Human impacts on the environment are
increasing the rate of extinction across
many species
• Fossil records show that the arrival of
Homo sapiens in a new area was quickly
followed by extinction of the largest
animals (megafauna)
Ecosystem Degradation
• Humans are changing the environment to
make space and resources available for
the increasing human population
• Over hunting and habitat destruction
have resulted in the loss of hundreds of
species of birds and mammals
Measuring Biodiversity
• When looking at biodiversity there is no
single measure used to compare the
differences between living things
• Biodiversity measures are useful in
helping us to monitor the changes in
populations and ecosysystems
• There are 3 measurable components of
biodiversity
Genetic Diversity
• Genetic diversity results from the
variation shown by the number and
frequency of alleles possessed within a
population
• By measuring the number and frequency
and alleles we can observe loss in genetic
variation within a population
• This may be a problem for a species as the
loss of certain alleles may limit their
ability to adapt to changing conditions in
the future
Species Diversity
• When measuring the species diversity of
an ecosystem 2 measures are taken into
account
• Species Richness- the number of different
species present
• Relative abundance- the proportion of each
species in the ecosystem
• Ecosystems can share the same species
richness but differ in their relative
abundance of each species
Ecosystem Diversity
• Ecosystem diversity refers to the
number of distinct ecosystems present
in a defined area
• Within a geographical area there can
exist a number of different ecosystems
Island Biogeography
• Island biogeography is the study of the
factors that affect the distribution and
diversity of species on islands
• Islands can refer to:
• A true island- a land mass surrounded by
water permanently
• A habitat island- an area surrounded by a
different ecosystem that cannot be
colonised by the island species
• The more isolated an island the lower the
diversity found among its species
• The smaller the surface area of the island
the lower the biodiversity