Download Population ecology summary

Population ecology summary (part 1):

Populations have a measurable density (the number of organisms per area or volume);
population density may vary with time.
1. To calculate density use D = N/A, where N is the number of individuals and A is
the area
2. To determine how this density is changing use R = change in Density / change in
Time

Population density is influenced by external environmental factors.
1.
2.
3.
4.

Natality (birth rate)
Mortality (death rate)
Immigration (inward migration)
Emigration (outward migration).
Several other factors influence population density. Such factors modify natality, mortality
and migration.
1. Biotic factors: availability of food; crowding; parasites; predator pressure;
competition; host density; allelopathy; etc.
2. Abiotic factors: space; sunlight; temperature; soil quality; nutrients; pH; toxic
chemicals; precipitation; etc.
A combination of all the above factors or terms controls changes in the number of individuals
within a population. This can be expressed mathematically as:
Population Growth = (births + immigration) – (deaths + emigration) / Initial # of organisms *
100
This will give the population growth as a percent. It is often more useful to discuss this increase or
decrease with respect to a unit of time. This would be the growth rate and would be found as
follows:
Growth rate = change in Population Growth / change in time.
If the growth rate is positive - the population is increasing; negative – decreasing.
Population ecology summary (part 2):


All species have a relatively high biotic potential. This is the maximum population growth
possible for that species.
1. Factors regulating biotic potential are: maximum number of offspring per birth,
chances of reaching reproductive age, age at which reproduction begins, number
of times per year of reproduction.
2. Biotic potential is essentially never reached due to environmental resistance.
(Environmental resistance is responsible for the difference between biotic
potential and actual population size.)
What factors curtail population growth? Two categories of factors are often recognized.
1. Limiting factors: some necessary growth factor depletes with increased
population density, limiting more growth.

For example, the nutrients nitrogen and phosphorous are often limiting
factors for plant populations in western North America.
2. Toxic factors: organisms have a tolerance limit for certain factors in the
environment. When this is exceeded, population growth will stop (or, in some
cases, crash).
More on Abiotic Factors and Tolerance Limits
Most species appear to be limited in at least part of their geographic range by abiotic factors,
such as temperature, moisture availability, and soil nutrients. No species is adapted to survive
under all conditions found on the Earth. All species have specific limits of tolerance to physical
factors that directly effect their survival or reproductive success. The portion of the abiotic factor's
range of variation which a species can survive and function in is commonly defined as the
tolerance range. The level within the tolerance range at which a species or population can
function most efficiently is termed the optimum.
In 1840, J. Liebig suggested that organisms are generally limited by only one single physical
factor that is in shortest supply relative to demand. At one time ecologists accepted this idea so
completely that they called it Liebig's Law of the Minimum and tried to determine the single
limiting factor that controls the growth of numerous species. However, many studies have shown
that Liebig's concept is inadequate to account for the distributional limits of a large number of
species. In these cases, scientists believe that complex interactions between several physical
factors are responsible for distribution patterns.
Population ecology summary (part 3):

Populations show characteristic growth patterns depending upon resources available and
environmental resistance.
1. A J-shaped population growth curve (characteristic of many populations) is
characterized by a slow growth stage, followed by exponential growth, then die
off.
2. An S-shaped (sigmoid) growth curve shows slow growth, followed by exponential
growth, then essentially no growth (equilibrium).
 The equilibrium state reached in a sigmoid population growth curve
represents carrying capacity. Carrying capacity is the maximum number
of organisms an ecosystem can carry in the long-term without ecosystem
damage.
Here are some questions to think about:
1. Do we have evidence to indicate whether the human population growth curve is
sigmoid or J-shaped?
2. Does the Earth have a carrying capacity for humans?
3. Does human carrying capacity differ in different parts of the world?
4. What happens as the carrying capacity is approached?

Changes in population density are often cyclic due to changes in environmental carrying
capacity. Changes in carrying capacity are generally due to changes in biotic or abiotic
factors.
1. Populations are often interdependent and an increase in one may cause an
increase or a decrease in the other. Examples:
 Predator-prey cycles (lions and deer in western North America, coyotes
and rabbits, etc.)
 Think about humans and cereal grains.

Think about humans and the rotoviruses (or any other host-parasite
interaction).
Population Growth and Density


a population gains individuals by natality and immigration, and loses them by mortality
and emigration
whether a population grows, declines, or remains the same size depends upon the
balance of these factors
Change in Population Size (D N) = (births + immigration) - (deaths + emigration)
("natality" means births, while "mortality" means deaths)
e.g. from data from table 25.2 on page 582 of Biology, calculate the change in the Sandhill crane
population at the Banks Island breeding site in 1991
births = 40
immigrations = 0
deaths = 55
emigration = 0
initial no. = 200
N = (40 + 0) - (55 + 0) = - 15 individuals

population growth in percent is calculated by the formula:
Percent growth = [b + i] - [ d + e] x 100%
N
(do calculation for above example)
PG% = [40 + 0] - [55 + 0] x 100% =
200

population density refers to the number of organisms per unit area (or per unit volume):
this measurement is often more useful to ecologists than actual size
Dp = N/A or Dp = N/V
Density = number of organisms, A= area, V = volume
e.g. 5 elk/ km2, 135 cod/km3
rate of change can be calculated by the formula R = D/t
t = time, D = density