UNIT 5: Population Dynamics
Chapter 12: Natural Population Dynamics
pg. 582 -
The term dynamic is used to describe populations of organisms, because
they are ever changing, and fluctuating as factors influence these
adjustments though time. Habitat, feeding relationships, and the ability to
reproduce are some such factors that affect populations.
Why do populations change? What factors play a role in determining the
size, density, and distribution of populations?
12.1: Population Characteristics
pg. 584 – 592
Population Ecology – a sub - field of ecology that focuses on the dynamics
of populations and how populations interact with their environment.
Population Ecology is the study of populations, their size, density,
distribution and changes over time. A population ecologist will monitor,
evaluate, model changes in populations and determine the health of the
species, by gathering data that will help them predict growth trends of a
population.
Population Ecologist’s research is usually liked to other areas of science,
such as; genetics, physiology, anatomy, behaviour, paleontology, and
evolution, geology, geography and environmental science.
The Ministry of Natural Resources of Ontario has a mandate to monitor
wildlife and protect the rich biodiversity of the province, by determining if
an invasive specie is present or if a native species is expanding its range
because of human intervention, must determine is a species has reached its
ideal population size for its ecosystem, and maintain healthy, sustainable
population levels using wildlife management strategies.
Distribution of Populations
There are three factors that describe distribution of populations through out
an ecosystem; Geographic Range, Population Size and density, and its
Dispersion.
Geographic Range
Geographic range – is the overall spatial boundaries within which a
population lives.
Habitat – is the place where an organism normally lives.
The geographic range of an organism is the total area that is occupied by a
population. These ranges can vary greatly. The habitat of an organism is a
specific environment in which the organism lives, characterize by the biotic
and abiotic features.
Population Size and Density
Population Size (Nt) – is the number of individuals of a specific species that
occupies a given area or volume at a given time.
Population density (D) – is the number of individuals of the same species
that occurs per unit area or volume.
Crude Density – is the population density measured in terms of the number
of organisms of the same species within the total area of the entire habitat.
Ecological Density – is the population density measured in terms of the
number of individuals of the same species per unit area or volume that is
actually used by the individuals.
The population size; the number of individuals that actually make up the
population at a specific time, and the population density; which is the
number of individuals per given space, are what are studied by biologists.
Smaller animals versus large animals require different sized areas to survive;
large animals require larger spaces to acquire food for survival.
Simple analysis of population density and size can be misleading because it
includes usable and unusable space within a habitat. Therefore the area that
includes the usable and unusable area within its analysis as Crude Density,
while the study of the number of organisms within the usable space as
Ecological Density, per unit area.
Figure 2: As the body size of animals increase, the number of animals per unit area tends
to decrease.
Population size and density are related, the population density is more
important because it can provide more information to the population’s
relationship to other resources it uses.
Population Density (D) = Number of Individuals (N)
Space Occupied (S)
D = N
S
e.g.: there are 860 sugar maples growing in a 4.0 hectare (ha) area. What is
the density?
D = N
S
D = 860
4.0
D = 215 trees/ha
It was also estimated that there were 75 000 maple seedling in a 0.5 ha plot.
What is the density?
D = N
S
D = 75 000
0.5
D = 150 000 seedlings/ha
Comparing the data by hectare, it appears the density of trees is less dense
then the density of the saplings. This can even be analyzed at smaller areas,
such as; per square metre. Data would show that a hectare is 10 000 m2, that
are 15 seedlings per meter squared.
This data can also be used to compare other organisms to each other.
Dispersion of Populations
Dispersion – is the pattern of distribution in which a population exists; may
be clumped, uniformed, or random.
There are three population dispersion patterns; clumped, random and
uniformed.
Clumped dispersion occurs when the individuals in a population are more
concentrated in certain parts of the habitat and can occur under three
situations:
1. Suitable living conditions are often distributed in patches because of
the living environmental conditions
2. Populations clump because mates are easier to locate within the
group. Groups of individuals can support each in rearing the
offspring, supplying food, and protection.
3. Organisms which have limited seed dispersal or reproduce asexual
while form clumped populations.
Figure 3: a) a clumped pattern of dispersion is evident in fish that live in social groups. b)
a random pattern dispersion, seen here in Australian rainforest, is rare in nature. c) a
nearly uniform pattern is demonstrated by creosote bushes near Death Valley, California.
Random dispersion occurs in environments where there are very little
variations within the habitat and little competition between individuals.
Individuals are distributed unpredictably.
Uniform dispersion occurs when individuals are distributed equally
throughout the habitat, and can occur because of competition between
individuals for food, breeding, and nesting grounds.
The dispersion of animals will vary over time, as a response to changing
natural environments, changes in food supply, seasonal cycle and its
availability of resources.
Studying Populations
Population size and density are studied to understand more about population
dynamics and to monitor and manage populations of endangered species,
economically important species, and agricultural pests.
Population characteristics, at times make it difficult to accurately count their
numbers to determine populations sizes or densities. Scientists use sample
data and their own observations to estimate population size and density.
When accurate estimates are required or populations are mobile within their
habitat or not readily observable, ecological sampling methods are used.
Sampling Populations
Quadrat – is a sampling frame that is used for estimating population size,
the frame can be real or virtual.
Population size and density are important characteristics, but are difficult to
measure accurately. A common sampling technique for estimating for small
or immobile organism is the quadrat sampling.
A quadrat is a square apparatus that is used to isolate a sample. The size of
the quadrat depends on the size of the population being sampled. E.g.:
dandelions. If the organisms are larger then the quadrat would need to be
larger.
Quadrats are to be placed randomly to avoid bias and must represent the
entire area being studied.
Estimated Pop’n. Density = total number of ind.
Sampling area
Tutorial #1, page 588
Mark – recapture Method – is a sampling technique for estimating
population size and density by comparing the proportion of marked and
unmarked animals that are captured in a given area; sometimes called the
captured-recaptured method.
In other situations animals are in continuous motion, seeking food,
protection, and shelter in hard to see areas. This would make it very difficult
to count the number of organisms. The common sampling technique used in
this situation is the mark-capture method.
The first sampling is used to capture, count, mark (or tag) the organisms and
then release them back into their habitat. After a period of time, the marked
organisms have intermingled with the unmarked organisms. At this time the
organisms are
recaptured (marked and unmarked) and counted. The
proportion of marked and unmarked organism is determined and this
proportion is used to estimate the population size.
The accuracy of this method is based on 5 assumptions:
1. There is an equal and constant opportunity to capture both marked and unmarked
individuals in the population. (marked individuals must not be easier to capture)
2. The proportion of marked and unmarked individuals remains the same between
captures. There are no new births or immigration and deaths or emigration.
3. There is adequate time for marked and unmarked individuals to intermingle randomly
throughout the population.
4. The individuals are not adversely affected by their marks.
5. The marked individuals cannot lose their marks.
Total number marked (M) = number of recaptured (m)
Total pop’n. (N)
size of second sample (n)
Or
N = Mn
m
N = estimate of the total population size
M = total number of animals captured, marked, and released on the first visit.
n = total number of animals captured on the second visit.
m = number of marked animals that were recaptured on the second visit.
Tutorial #2, page 589