Transcript Characteristics of population growth ppx

Characteristics of
Population
Growth
A) Growth rate is affected by:
#of births
#of deaths
immigration and emigration
Growth rate = birth rate – death rate
Growth rate may be
positive, negative, or 0
B) Exponential Growth
Populations will grow
exponentially if:
• resources are unlimited
• individuals reproduce at
a constant rate
ex: E. Coli bacteria reproduce
asexually every 20 minutes
in 6 hours – over 200,000
in 2 days – enough to cover the
planet!
WHY DON’T THEY?
Exponential growth rate:
J curve
C) Logistic Growth
Limited resources will cause the growth
to slow down or stop
Carrying Capacity – the largest number
of individuals of a population that the
environment can support
Logistic Growth rate:
S curve
K = carrying capacity
Fictional "Tribbles" from Star Trek:
One of the more peculiar species encountered, the
defining characteristic of the Tribbles is their extreme
reproductive rate. Over half of a Tribbles metabolism is
devoted to reproduction, allowing them to bear a litter of
young every twelve hours.
With an average litter of ten, a single Tribble can
therefore create a population of 1,771,561 within three
days,
and an amazing
304,481,639,541,400,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000 in thirty days!
Did the “Tribbles” have logistic
or exponential growth?
D) Limiting Factors on Human
Populations:
Famine, Disease, War
Will we reach carrying capacity and
level off?
What are the consequences if we
don’t?
E) Density Independent Factors
Affect all populations,
regardless of size or density
Ex: weather
natural disasters
seasonal cycles
certain human activities
clearcutting forests, damning rivers,
filling wetlands, building roads, etc.
F) Density-Dependent Factors
Effect depends on the
population size or density
Ex: competition for food or shelter
predation
disease/parasites
spread easily in dense populations,
may wipe out small populations
Predator/Prey Populations
Lynx and Snowshoe hare
Small populations are very
vulnerable to extinction
• If only a few remain, inbreeding will
occur. This reduces genetic
variations.
• Populations with low genetic variability
are less able to adapt to changing
environmental conditions.
The genetic diversity of cheetahs is so low that
biologists think their population was reduced to
a very small size in the past.
Survivorship Curve:
If birth rates are constant, then age structure can be used
to calculate survivorship.
(patterns in age-specific death rates)
• Some species produce many offspring, with most dying
young (Type III).
• While other species produce few offspring with most
surviving to maturity (Type I).
• This indicates that there is an evolutionary trade-off
between energy invested to reproduction versus energy
invested to survival.
Page 123 – Compare the Fruit Fly population
growth and the Rabbit population growth.
1. Which one shows logistic growth and has
reached carrying capacity?
2. How would the growth curve change for the
rabbit population if more predators moved into
their habitat?