Transcript File - Mr. Shanks` Class

C. FACTORS AFFECTING POPULATION CHANGE
1. Density Dependent Factors
– factors that have a greater
effect when the
pop density is greater
a. Intraspecific competition
- individuals compete for nest space, food, light, etc,
with others of the same species
- eg. high density trees – lots of shade, little water,
few nutrients per tree
b. Predation
from a predator viewpoint:
- more prey / predator -- predator has more food
less prey / predator -- predator has more
intraspecific competition
Lots of prey, so you get to eat
what you kill.
Prey is scarce so lots of fights and
less time to hunt.
from a prey view point:
- more prey / predator – prey has less chance of being eaten
- less prey / predator – prey has more chance of being eaten
With lots to choose from, the
predator is unlikely to pick ‘you’!
With no choice, you are ‘lunch’!
c. Allee effect
reproduction fails to equal mortality due to low density
eg. can not find a mate
eg. species that rely on ‘overwhelming predator’
can’t do so when numbers low
Boat engine
noises
prevent
whales from
finding
mates by call
Passenger pigeons were so
numerous that millions of deaths were
not a problem, now extinct!
d. Minimum viable populations size
- enough individuals so that the pop can cope with
variations in births & deaths
eg with human help – 23
whooping cranes
was enough
eg in wild – 200 elephant seals
was enough
for many species – 200 + are needed
for this
2. Density Independent Factors
- the number or density of individuals
does not affect the outcome
a. Limiting factor – one part of the life cycle that is
limited and can not be compensated
for by surpluses elsewhere
eg. if seeds need 3 days of frost to scarify them and we
have a warm winter with no frost, none grow even if
it is a good Spring!
The rows missing were not scarified.
eg. DDT makes hawk egg shells thinner, regardless of
how many hawks are in an area or how many
good nest sites & how much food!
DDT tainted eggs
crushed when parent
attempted to incubate
do Questions on handout Part
A
pg 8