Transcript Population Growth

Population
A population is a group of organisms of
a single species that lives in a given
area.
Density and Distribution
Population density = number of
individuals per unit area.
* Populations of different species often
have very different densities, even in the
same environment.
Ex. A population of ducks in a pond may
have a low density, while fish and other
animals in the same pond community may
have higher densities.
Density and Distribution
– Distribution = how individuals in a
population are spaced out across the
range of the population
– randomly
– uniformly
– mostly concentrated in clumps
Growth Rate
– A population’s growth rate determines
whether the population size increases,
decreases, or stays the same.
Age Structure
– Age structure = the number of males and
females of each age a population contains.
Population Growth
The factors that can affect population size are
– Birthrate/ death rate
– Immigration = move into an area
– Emigration = move out of an area
Calculating population change:
(Births – Deaths) + (Immigration – Emigration)
Exponential Growth
Under ideal conditions with unlimited
resources, a population will grow
exponentially.
In exponential growth, the larger a population
gets, the faster it grows. The size of each
generation of offspring will be larger than the
generation before it.
The Logistic Growth Curve
This curve has an S-shape that represents
what is called logistic growth.
Logistic growth occurs when a population’s
growth slows and then stops, following a period
of exponential growth.
.
Population growth may slow for several
reasons:
1. Growth may slow if the population’s birthrate
decreases or the death rate increases—or if
births fall and deaths rise together.
2. In addition, population growth may slow if the
rate of immigration decreases, the rate of
emigration increases, or both.
Carrying capacity =maximum number of
individuals of a particular species that a
particular environment can support.
Once a population reaches the carrying
capacity of its environment, a variety of factors
act to stabilize it at that size.
• Limiting Factor = any factor that limits the
existence of an organism in a particular area.
May be:
1. Density-Dependent (affected by population
size)
2. Density-Independent (not affected by
population size)
* Limiting factors are selective pressures that
drive natural selection!!
Include:
Include:
*Competition
*Natural disasters
*Predation
*Unusual weather
*Disease and Parasites
*Overcrowding
Predator-prey population dynamics are connected
Predators kill prey  affects prey death rate
• prey population size depends on number of predators
• with few predators, prey population grows
• with many predators, prey population shrinks
Predator-prey population dynamics are connected
Predators kill and eat prey affects prey death rate
 affects predator birth rate
• with few predators, prey population grows
• with many prey, predator population grows
• with many predators, prey population shrinks
• with few prey, predator population shrinks
N
time
Population Cycling
Real world predator and prey populations can cycle
in size.