Download Changes in Population Size

UNIT 5: Population Dynamics
12.3: Changes in Population Size over Time:
Modeling Population Growth
pg. 600 – 609
Animal populations change over time due to births, deaths, and the
immigration and emigration of individuals between different populations.
Population dynamics are affected by environmental conditions and other
regulating factors. Changes can occur, hourly, daily, seasonally, and
annually. These factors lead to unpredictability of population changes, but
being able to predict, understand how and why populations change overtime
will allow for species management.
Population Dynamics – is the change in a population over time.
Exponential Model of Population Growth
Ecologists are interested in the population growth rate: the fractional rate at
which the number of individuals in a population increases or decreases. The
change of a population over a period of time and is expressed in as
percentage of number of individuals in the population at the beginning of
this period is known as the population growth rate.
Population = (births + immigration) - (deaths + emigration)
Growth rate
initial population
X 100
Positive growth rate indicates that the population is increasing in size.
Negative growth rate indicates that the population is decreasing in size.
If there is no change in the population size over time, then the growth rate is
zero.
Populations that can reproduce continuously, that do not have set times of
reproduction (seasonal) have the ability to increase their populations
exponentially, a constant ratio per unit time, e.g.: bacteria. If no bacteria die
between successive generations, then the population has the potential to
double for each generation. If there is an ideal temperature, unlimited food
supply, and ample space, bacteria will continue to increase in size.
Exponential Model of Population Growth – is a pattern of population
growth in which organisms reproduce continuously at a constant rate.
Human populations also have the same potential as bacteria to reproduce
exponentially. Ignoring migration at this time will allow for the following to
hold true;
Change in population size = number of births – number of deaths
Per Capita Growth Rate (r) – is the difference between the per capita birth
rate and the per capita death rate of a population.
Zero Population Growth (ZPG) – is a situation in which the death rate and
birth rate are in equilibrium.
Carrying Capacity (K) – is the number of individuals in a population that
the environment resources can support.
Logistic Model of Population Growth – is a model that describes limited
population growth, often due to limited resources or predation.
Sigmoid Curve (Logistic) – is an S-shaped curve, typical of population
growth that starts slow, accelerates or grows rapidly, and then levels out
over time.
Figure 2: Bacterial population growth.
Figure 3: Exponential population growth produces a J-shaped curve when population size
is plotted against time. Although the per-capita growth rate (r) remains constant, the
population growth increases every month because more individuals are responding.
Figure 5: A geometric growth curve.
Figure 6: a) laboratory population of the grain borer beetle showed logistic growth when
its food was replenished weekly. b) European mouflon sheep introduced into Tasmania
exhibited logistic population growth. The data represented five-year averages, smoothing
out the annual fluctuations in population size.