Science 14
Chapter 13: Populations 3
Notes
Introduction
-population has increased dramatically over the last 100 years – it is estimated that the world’s current
population of approximately 6 billion (1999) could double in the next 100 years
-this raises several serious questions (i.e. where will 12 billion people live, how will we produce twice as
much food as we do today? How will we generate twice as much electricity and find fuel for twice as
many vehicles? Where will we get rid of twice as much garbage?
13.1 Changing Populations
-Population – group of individuals of the same species living together in the same place, at the same
time – an individual is one member of the population
-to identify a population we must know three things: species, where they live(d), when they live(d)
-populations can change in size over time – can determine how healthy a population is by calculating
rate of increase or decrease
-Find Out Activity p. 256
13.2 Population Growth and Decline
-four factors to determine the size of a population
-death rate – the number of individuals that die during specified time (i.e. a year) – can be
affected by predation, disease, and competition for food
-immigration – coming into an area (i.e. where there is a greater food supply) – increases
population
-birth rate – number of young produced by a population over a period a time (i.e. a year) –
affected by environmental conditions, nutrition, population density, number of young produced
by each mature female
-emigration – leaving an area – decreases population
13.3 Limiting Factors
-if members of a population never died the population would continue to grow – however, this is not
the case – organisms die for many reasons
-if resources such as light, food, water, space become scarce a population dies or does not reproduce
-these are called limiting factors – they limit the growth of a population
-most populations cannot increase beyond a certain size – eventually a population size will level off
because the resources in the environment stop growing – when the populations levels off it is called the
carrying capacity – the number of individuals a given environment can support
Abiotic Limiting Factors
-non-living limiting factors
-water – all organisms need water – many even live in water
-matter – need carbon/nitrogen – without these plant growth is slowed
-weather – extreme cold or heat, too much or too little rain, etc… affect birth and death
rates of most living things
13.4 Biotic Limiting Factors
-ways that interactions among living organisms might affect the size of a population (i.e. competition,
predation, parasitism)
-these factors help control the populations of organisms in an ecosystem
-competition – organisms compete for resources such as food (animals), nutrients (plants), shelter, light
and water
-animals compete within their own species – stronger animals get the best food – can also compete with
other species that eat the same food
-plants compete for resources – light, water, space, nutrients
-predation – an animals that catches and kills and eats another animals is called a predator – the animal
it hunts is call prey – benefits mostly predators but can also help prey – many predators pick on sick or
weak animals leaving more food for healthy animals in the prey population
-parasitism – organisms that live on or in another living organism are called parasites – the organism
they live in is called the host
-external parasites live on the skin of the host – eat hair, feathers, scales or skin of the host – some suck
the blood
-internal parasites live inside the host
-most only weaken the host; not kill it
Reproduction Rate
-reproductive rates are not as high as they might be due to factors that cause death of newborns
-example – houseflies – can be eaten by birds, killed by parasites and diseases, killed by people, larvae
die of starvation (competition for food), adult houseflies only live to be two weeks old
13.5 Human Impact
-population of humans in the world puts a strain on the natural world
-when seeds/plants or animals are brought from other countries, they are called exotic species
-exotic species are foreign to an ecosystem – they compete with native species for resources – native
species do not generally eat or predate on exotic species so there is a low death rate – this allows them
to reproduce faster than native species and possible even replace them
-environmentalists are concerned that exotic species interbreed with native species – i.e. salmon farms
in BC – farmers prefer to use Atlantic salmon because they grow faster – they might escape and breed
with wild salmon – this could damage the ability of wild salmon to breed
-other exotic species have potential to do damage
-chesnut blight – fungus that has killed native chestnut trees
-english sparrows – use same nesting sites as native bludebirds – more aggressive and replaced
bluebirds in some areas
-sea lamprey – moved through the St Lawrence into the great lakes – attach themselves to fish
and suck on their blood and weaken them (later causing death)