This document discusses threats to biodiversity, including current extinction rates and causes of past mass extinction events. It notes that the current extinction rate among mammals is 45 times the natural background rate, indicating we may be in the midst of a sixth mass extinction event caused primarily by human activities. Key threats include habitat loss, pollution, overharvesting, and introduction of invasive species. Species are more vulnerable if they have small populations, specialize in limited resources, reproduce slowly, face human or natural predators, or occupy high positions in food chains. Conservation status is determined by population size trends, habitat quality and fragmentation. Examples are given of extinct, critically endangered, and recovering species.

1. Topic 3
Biodiversity and Conservation
3.3 Threats to Biodiversity

2. Current Biodiversity
• Nobody knows the number of species on the planet
– (its somewhere between 5 and 100 million)
• New species are still being dicovered today
• Less than 2 million species have been named
• Most species are animals and are terrestrial (since
there are more niches in land biomes)
• Two thirds of all species live in tropical rainforests
• Most species are small (e.g. bacteria, insects and
nematode worms), not exactly cuddly and therefore
don’t elicit much attention and emotion (other than
disgust)

3. Current Extinction Rates
• On average a mass extinction event occurs
about once every 100 million years
– (the last was 65 mya)
• In between mass extinctions there is a gradual
trickle of natural extinctions as species reach
‘evolutionary dead-ends’ (they are no longer
suited to a changing ecosystem
– This is known as the Background Extinction Rate

4. Mass
Extinction
Events
?

5. Mass Extinctions (5 events)
• Ordovician-Silurian (439 mya)
– A drop in sea levels due to glaciers forming, followed by sea-level rise as they melted
– 57% of all genera became extinct
• Late Devonian (364 mya)
– Cause currently unknown
– 50% of genera became extinct
• Permian-Triassic (251 mya)
– Possibly due to asteroid impact
– 83% of genera became extinct
• End Triassic (214 – 199 mya)
– Due to increased volcanic activity on a global scale
– 48% of genera became extinct
• Cretaceous-Tertiary (65 mya)
– Asteroid impact in the Gulf of Mexico that ended the dinosaurs
– 47% of marine genera and 18% of terrestrial genera became extinct
– The mammals filled the niches vacated by the dinosaurs

6. The Sixth Mass Extinction Event?
• Mammal species on average last 1 million years
• There are about 5000 known extinct and extant mammal
species
• Therefore background extinction rate is exivalent to the loss of
one mammal species every 200 years on avarage
• In the past 400 years there have been 89 mammal species
extinctions and 169 species are currently critically endangered
• This extinction rate is 45 times the background rate (and bear
in mind this is for mammals only)
• Are we therefore currently living through the sixth mass
extinction event? What is causing it? Is it just affecting
mammalian species?
The biologist Edward Wilson has claimed that at current rates of
biodiversity loss, 30-50% of all species alive today will be extinct in 100
years time

7. Succession
1. Pioneer Species colonise bare rock
2. Growth of these species cause changes in the
environment, soil is created
3. New species colonise soil and replace pioneers
4. Growth of root systems stabilise soil
5. Animals begin to colonise
6. A climax community is created
7. New disturbances result in new waves of succession
Primary
Succession
Secondary
Succession
Note that it is communities that succeed each other, not simply individual species
Each new community is called a sere. The final stable sere is called the climax community

8. Disturbance
• 3 factors determine how quickly an ecosystem
can recover from a disturbance (via succession):
– Inertia – resistance to alteration
– Resiliance – ability to recover
– Diversity – number and proportions of species

9. Disturbance
• Tropical rainforest
– High diversity
– High inertia
– Low resiliance (takes a long time to recover)
• Grassland
– Low diversity
– Low inertia
– High resiliance (takes a shorter time to recover)
Complex ecosystem therefore has
many ways to respond to
disturbances. However, soils thin and
low in nutrients, and eaily washed
away after disturbance
Fairly simple ecosystem. Thick soils
with lots of nutrients stored in them.
Recover fairly quickly after
disturbance
Rainforests recover well from limited, short-term disturbance. However anthropogenic
disturbance is not generally short-term and is not limited to small pieces of habitat.

10. Non-anthropogenic disturbances
• Wildfires
• Movement of plates (leading to change in
local climate)
• Volcanic eruptions
• Meteorite strikes
• Changes in orbit of the Earth and its tilt (lead
to regular changes in climate)

11. Anthropogenic Disturbances
• Introduction of alien species
• Overharvesting
• Islandisation (Fragmentation)
• Loss of habitat
• Pollution
Some of these may occur naturally, some are purely anthropogenic
Can you think of some that are not anthropogenic?
Humans are having a big effect on the least resilient ecosystems

12. Why are rainforests vulnerable?
• They are not resilient to disturbance
• They occur close to the equator where conditions
are stable (they have evolved in stability)
• They have high biodiversity often with rare species
• Their resources (or land) are in high demand (e.g.
for wood, soya, beef, biofuels)
• Succession takes a very long time
• They have thin, nutrient poor soils which are often
washed away once root systems are destroyed
Coral reefs are the only other ecosystem with comparable biodiversity

13. Biodiversity Hotspots
• Rainforests are thought to contain
up to 50% of all the species on Earth
• Many contain biodiversity hospots
(where there are large numbers of
endemic species)
• Another famous hotspot is Feynbos
near Cape Town
• This is the area of a few football
fields and is home to 2000 species
of flowering plants, many of them
endemic

14. What makes some species prone to extinction?
1. Small population size and small habitat size
Case study:
The slender-billed grackle (Cassidix palustris)
• It only inhabited Valle de Mexico and Toluca
Valley in small numbers
• It became extinct due to habitat loss and draining
of marshland
• The last one was recorded in 1910

15. What makes some species prone to extinction?
2. Specialisation
Case study:
Giant Panda (Ailuropoda melanoleuca)
• Dependent on bamboo although it is in
order carnivora (it will switch to other
food sources but very rarely)
• It is endangered due to habitat loss (loss of
bamboo forest)

16. What makes some species prone to extinction?
3. Low reproductive capacity
Case study:
Black Rhinoceros (Diceros
bicornis)
• Gestation period of 15-16
months and only give birth to
one calf
• It is critically endangered and
one sub-species, the western
black rhino was declared extinct
in 2011

17. What makes some species prone to extinction?
4. Poor competitors
Case study:
Dodo (Raphus cucullatus)
• Flightless and slow-moving
• Unable to protect itself against the
pressures of human hunting and
introduced species (eg pigs)
• The last one was recorded on
Mauritius in 1662

18. What makes some species prone to extinction?
5. Prone to hunting by humans (value to humans)
Case study:
Blue Whale (Balaenoptera musculus)
• Fairly easily hunted and provide huge
quantities of meat and blubber
• Now critically endangered in the Arctic and
have threatened status in the Pacific
• Recovering since they received protection
in 1955 from the International Convention
for the Regulation of Whaling

19. What makes some species prone to extinction?
6. Altruism
Case study:
Carolina Parakeet (Conuropsis carolinensis)
Monogamous pairs that live together (therefore
often hunted together) and come to each others
aid if threatened
• Last one killed in the wild in 1901
• Last captive pair (Incas and Lady Jane) died in
1917 and 1918, respectively

20. What makes some species prone to extinction?
7. Clumping
Case study:
Passenger pigeon (Ectopistes migratorius)
Native to North America. Lived in large groups for
protection and to find food. When split into smaller
groups, the population was no longer viable and
declined further
• Flocks were estimated to exceed 3 billion
individuals in the early 1800s
• Their habitat was fragmented and they provided a
cheap and plentiful food source (especially for
slaves!)
• Last one (Martha) died in Cincinati Zoo in 1914

21. What makes some species prone to extinction?
8. Position in the food chain
Case study:
Bald Eagle (Haliaeetus leucocephalus)
Top predators are always rare due to the loss of
energy through the food chain. They are also
vulnerable to the bioaccumulation and
biomagmification of non-polar toxins in the food
chain
• Bald eagles were subject to hunting, habitat loss
and the effects of a number of toxins
• By the 1960s there were less than 500 nesting
pairs in the US and Canada
• They were especially vulnerable to the effects of
DDT (banned in the US in 1972 and Canada in
1973)

22. Determining Conservation Status
• This is carried out by the International Union for the
Conservation of Nature (IUCN)
• Their red list is a constantly updated list of threatened
species
• To compile this they take into consideration:
– Population size
– Reduction in population size
– Number of breeding individuals
– Size of habitat and degree of fragmentation
– Quality of habitat
– Probability of extinction

23. Questions
1. Make a list of a number of species currently
on the IUCN red list
2. Link each of the species directly to one of the
eight reasons that make a species prone to
extinction

24. Case Studies: Extinct
• Tasmanian Tiger (Thylacinus
cynocephalus)
• Originally extant in Australia,
New Guinea and Tasmania
• Habitat: grassland and
Eucalyptus forest
• Nocturnal, top predator (but
was in competition with many
other species such as the
Tasmanian Devil)
• Hunted to extinction (largely by
farmers to protect livestock)

25. Case Studies: Critically Endangered
• Black Rhinoceros (Diceros bicornis) – There are
4 subspecies (Western Black Rhino now extinct)
• Largely Kenya and Namibia, East Africa
• Habitat: Grassland and desert
• Hunted close to extinction (European sport
hunters, farmers protecting crops, poachers)
• At the start of the 21st century there were less
than 50
• There are now around 4 500
• Protection:
– Existing protected areas have been established
and old ones expanded
– Improved security to deter poachers
– International law enforcement has been
improved to restrict the sale of rhino horn
– Ecotourism is an expanding and lucrative market
in Africa

26. Case Studies: Back from the Brink
• Bald Eagle (Haliaeetus leucocephalus)
• All of mainland USA and Canada
• Habitat: Lakes, marshes and coast
• Came close to extinction due to
hunting, habitat destruction and
pesticide poisoning (especially DDT)
• By the start of the 1960s there were
less than 500 nesting pairs. There are
now around 10 000 pairs
• Protection:
– DDT outlawed in the USA in 1972 and
restricted in Canada in 1973
– Hunting was prohibited in 1940, but
ilegal hunting continued to some extent

27. Case Studies: A Threatened Area
• The Great Barrier Reef
• Stetches for 2300 km along the Queensland Coast
• Ecological, socio-political and economic pressures
• Ecotourism is helping conservation but in some ways
hindering it
• There is a serious problem of terrestrial runoff as the
coast becomes more industrially and agriculturally
developed
• Rising sea temperatures are a problem since corals
survive within a very narrow band of temperature.
Climate change may also cause fish migration
• The Australian government allows limited fishing and
trawling within the national park which can be
destructive
Protecting the Great Barrier Reef

28. Questions
1. List 5 factors that lead to loss of biodiversity
2. Why is rainforest particularly vulnerable?
3. How many extinction events have there been?
What were the causes? How do the first 5 differ
from the possible sixth?
4. Why do biologists have difficulty estimating
current rates of extinction?