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?