Changes 1-Populations

Topic 2.6

Factors that increase population size:•Natality (Birth rate) is recruitment to a population through reproduction•Immigration from external populations e.g. Bird migration

Factor reducing population size:•Mortality (death rate) which is the death rate from any source e.g. predation•Emigration, where individuals leave the population for another habitat

PopulationImmigration Emigration

Death

Birth

For a stable population:

Immigration Emigration+ Natality + Mortality =

• Lag phase: Population growth begins slowly from a few individuals.• Exponential Phase: Exponential growth occurs, the conditions are ideal and

maximum growth rate is reached.• Transitional Phase: Growth rate begins to slow down as factors such as food,

water and space become limiting.• Plateau phase: Carrying capacity for the population has been reached and the

population number becomes stable. The carrying capacity is the population size that can be supported by a particular environment.

• Decline phase: If there is a sudden change in the environment meaning that the environment can no longer support the population, such as a drought causing food shortage, the population will crash and the whole process begins again.

A Sigmoid (s - shaped) Population CurvePo

pula

tion

size

Time

Pla

teau

pha

se

Dec

line

phas

e

Tran

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hase

Exp

onen

tial P

hase

Lag

phas

e

Carrying Capacity.• An ecosystem can only support a certain

size of a population – its carrying capacity.• Carrying capacity is affected by the

amount of energy available to the organism – more energy available means a higher population!

Population size can be affected by the following:

• Density dependent factors: These are any factors, dependent on the density of the population in question. Generally their effects increase as population increases. Some examples of these are predation, disease and competition. They tend to form a negative feedback mechanism that controls population.

• Density independent factors: These are any factors, not dependent upon the density of the population in question. Some examples of these are climate or weather and catastrophe.

Internal and external factors affecting populations

• Internal – factors that act within a species. Eg. limited food supply, availability of territory, density dependent fertility. (Take a look at this interesting article about overcrowding by Aidan Sammons – link )

• External – factors that act between different species. Eg. Predation and disease.

Case studies

•Rabbits•The Lynx and the Snowshoe Hare.

Rabbits in Australia

• In 1859, Thomas Austin brought out 24 rabbits, 5 hares and 72 partridges and released them on his property, in Victoria.

• In 1887, Rabbits were first sighted in Northern Territory.

The gestation period for a rabbit averages 31 days.

A rabbit can live as long as 10 years

Rabbits can start breeding as early as 3-4 months of age

European Rabbits have an average litter size of 7 – 9 but may be as high as 15!

8 Rabbits eat as much grass as 1 sheep!

• By 1907 Rabbit numbers had reached plague proportions (up to an estimated 600million) causing untold environmental damage:

So from 1901 -1908 they tried to build a….

It didn’t work….. So in 1950, after research carried out by Frank Fenner, Myxomatosis was deliberately released into the rabbit population, causing it to drop from an estimated 600 million to around 100 million.

Genetic resistance in the remaining rabbits allowed the population to recover to 200-300 million by 1991.

• To combat this trend, Commonwealth Scientific and Industrial Research Organisation (CSIRO) scientists developed and accidentally released calicivirus (also known as Rabbit Haemorrhagic Disease or RHD) in 1996. The Australian Government

refuses to legalise a vaccine to protect pet rabbits against Myxomatosis, for fear the rabbit may escape and transmit resistance to the disease. A legal vaccine exists in Australia for RHD. There is no cure for either Myxomatosis or RHD, and many affected pets are euthanised.

Activity:Using the information in this power-point:• Draw a sketch of the population curve for

Rabbits in Australia from 150 to present day.

• Label relevant parts of the curve and include approximate dates and population sizes.

• Discuss with a friend whether the methods of control were ethically or environmentally justifiable.

Predator Prey Relationships

•The lynx and the Snow Hare

A lynx

-Eats Snowshoe hares

A Snowshoe Hare.- Eaten by Lynxs.

Activity:

• How will increasing numbers of Snowshoe hare affect the numbers of lynx?

• How will increasing numbers of Lynx affect the numbers of Snowshoe hare?

• How will decreasing numbers of Snowshoe hare affect the numbers of lynx?

• Does this explain the population graph?

J – population curvesShow a ‘Boom and Bust’.

• Exponential growth followed by ‘Dieback’.

• Growth is not limited by density dependent factors.

• Often ‘overshoots’ the carrying capacity of the ecosystem.

• Examples: Bacteria, algae, invertebrates, many fish species and some small mammals.

Pop

ulat

ion

Time

Pop

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ion

Time

There are 2 groups of species with differing reproductive strategies. Those that go for rapid growth (r-strategists) and those that go for Karrying Kapacity (K-strategists). These are extremes – most species fall somewhere in the middle.

r-strategists k-strategistsLife span Short Long

Growth rate Rapid Slow

Age of maturity Early Late

How many offspring? Loads Few

How large offspring? Small Large

Parental care Little or none Lots

Stability of environment it lives in

Unstable Stable

Internal or external population regulation?

External Internal

Trophic levels Lower Higher

Example Weeds, plankton Oak trees, elephants

Survivorship curves.• This is the fate of groups of individuals

through their life span.

Age

% P

opul

atio

n of

su

rviv

ors

K- strategists – Few young, lots of parental care, so mortality at young ages is low. Most organisms live to average lifespan.

r- strategists – many young, limited parental care, so high infant mortality. Few organisms live to full lifespan.

Most organisms come somewhere in between.

• What is the lifespan of these birds?

• What is the potential population size?

• How many survivors are there after the 1st year?

• What is the percentage mortality after the1st year?

• What is the percentage mortality after the 2nd year?

• What is the percentage mortality after the 3rd year?

• What would the survivorship curve for this population look like?

• Is this a K or an r strategist?

Activity:

200 eggs laid

100 50 25 12

Deaths:100

young and eggs

Deaths:100 birds

Age in years

Population pool- birds of breeding age.

Question form Page 164 – Environmental Systems and Societies Course companion – Jill Rutherford – Oxford University press 2009 -

1 2 3 40

A survival model for a species of bird

Picture sources:• Slide 4 http://www.s-cool.co.uk/alevel/biology/ecological-concepts/populations.html• Slide 7 http://www.creationsbydawn.net/pi/tutorials/rabbit.jpg• Slide 8

http://wwp.greenwichmeantime.com/time-zone/australia/_derived/map.htm_txt_australia-map.gifhttp://library.thinkquest.org/03oct/00128/en/rabbits/history.htm

• Slide 9 http://highlandviewrabbitry.tripod.com/sitebuildercontent/sitebuilderpictures/breeding.jpghttp://www.threelittleladiesrabbitry.com/rabbitfacts.php

• Slide 10 http://upload.wikimedia.org/wikipedia/en/3/3e/Rabbit-erosion.jpg• Slide 11 http://www.onesteelwaratah.com.au/media/81204/rabbit%20fence%20sign.jpg• Slide 12 http://en.wikipedia.org/wiki/Rabbits_in_Australia#Biological_measures

http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/t978/T978475A.jpg• Slide 13 http://members.iinet.net.au/~rabbit/rcd2.jpg• Slide 16

http://4.bp.blogspot.com/_W9mOaqcNMB4/SVVbtbqziJI/AAAAAAAABgc/WXsJp9Ysgpo/s400/lynx+young+walking+on+snow.jpg

• Slide 17 http://www.ngsprints.co.uk/images/M/100148.jpg• Slide 18

https://fp.auburn.edu/sfws/ditchkoff/images/Lecture%20Images/Carnivores/lynx-hare_cycle.gif