POPULATION
ECOLOGY
UNIT 3
FUN FACT
The current human population is
approximately 8 billion people. It is
expected to grow to 9.7 billion by 2050.
POPULATION ECOLOGY
Population ecology explores how biotic and abiotic
factors influence the density, distribution, size, and
age structure of populations. Populations evolve as
natural selection acts on heritable variations among
individuals, changing the frequencies of alleles and
traits over time. Evolution remains a central theme
as we now view populations in the context of
ecology.
Population = group of individuals of a
single species living in same general
area
ex. human population, ant colony
Members of a population rely on the same resources, are
influenced by similar environmental factors, and are likely to
interact and breed with one another. Populations are often
described by their boundaries and size (the number of individuals
living within those boundaries). Ecologists usually begin
investigating a population by defining boundaries appropriate to
the organism under study and to the questions being asked. A
population’s boundaries may be natural ones, as in the case of an
island or a lake, or they may be arbitrarily defined by an
investigator.
Density: number of individuals / area
Dispersion: pattern of spacing between
individuals
Determining population size and density
Count every individual
Random sampling
Mark-recapture method
Density is not a static property but changes as individuals are added to or
removed from a population. Additions occur through birth (which we define
here to include all forms of reproduction) and immigration, the influx of new
individuals from other areas. The factors that remove individuals from a
population are death (mortality) and emigration, the movement of individuals
out of a population and into other locations. While birth and death rates
influence the density of all populations, immigration and emigration also alter
the density of many populations.
Patterns of Dispersal
Clumped – most common; near required resource
Uniform – usually antagonistic interactions
Random – unpredictable spacing, not common in
nature
Demography: is the study of vital
statistics of populations and how they
change over time
Additions occur through birth, and
subtractions occur through death.
Life table: age-specific summary
of the survival pattern of a
population
Survivorship Curve
A graphic method of representing some of the data in a life table is a
survivorship curve, a plot of the proportion or numbers in a cohort still alive
at each age.
Type I: low death rate early
in life (humans)
Type II: constant death rate
over lifespan (squirrels)
Type III: high death rate
early in life (oysters)
Change in Population Size
Imagine a population consisting of a few individuals living in an ideal,
unlimited environment. Under these conditions, there are no external limits
on the abilities of individuals to harvest energy, grow, and reproduce. The
population will increase in size with every birth and with the immigration of
individuals from other populations, and it will decrease in size with every
death and with the emigration of individuals out of the population.
N = population size B= birth
t = time D= death
Zero population growth (ZPG) occurs when the per capita birth and death
rates are equal (r = 0). Births and deaths still occur in such a population, of
course, but they balance each other exactly.
Population Growth Models
A population whose members all have access to abundant food and are free to
reproduce at their physiological capacity. Population increase under these
conditions, called exponential population growth.
Exponential Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
Sample Problem:
A certain population of mice is growing exponentially. The
growth rate of the population (r) is 1.3 and the current
population size (N) is 2,500 individuals. How many mice are
added to the population each year for 3 generations?
Exponential Growth Equation
Exponential Growth Equation
Exponential Growth Equation
The exponential growth model assumes that resources remain abundant,
which is rarely the case in the real world. As population density increases,
each individual has access to fewer resources. Ultimately, there is a limit to
the number of individuals that can occupy a habitat. Ecologists define the
carrying capacity, symbolized by K, as the maximum population size that a
particular environment can sustain. Carrying capacity varies over space and
time with the abundance of limiting resources. Energy, shelter, refuge from
predators, nutrient availability, water, and suitable nesting sites can all be
limiting factors.
Logistical Growth
In the logistic population growth model, the per capita rate of
increase approaches zero as the population size nears the carrying
capacity.
Logistical Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
K = carrying capacity
Sample Problem:
If a population has a carrying capacity (K) of 900, and the
growth rate (r) is 1.1, what is the population growth when the
population (N) is 425? And until what generation until it hits the
carrying capacity?
Logistical Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
K = carrying capacity
Logistical Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
K = carrying capacity
Logistical Growth Equation
dN/dt = change in population
r = growth rate of pop.
N = population size
K = carrying capacity
Logistical Growth Equation
FACTORS THAT LIMIT POPULATION GROWTH
Density-Dependent factors: population matters
i.e. Predation, disease, competition,
territoriality, toxic wastes, physiological
factors
Density-Independent factors: population not a
factor
i.e. Natural disasters: fire, flood, weather
DENSITY DEPENDENT FACTORS
DENSITY DEPENDENT FACTORS
DENSITY INDEPENDENT FACTORS
CHALLENGE Qs
If a species has a long life span, how might
this impact their population growth?
AGE STRUCTURE
Another important demographic variable in
present and future growth trends is a country’s
age structure, the relative number of individuals of
each age in the population. Age structure is
commonly graphed as “pyramid
Age-structure diagrams not only predict a
population’s growth trends but also can illuminate
social conditions.
AGE STRUCTURE
AGE STRUCTURE
Based on the diagrams in Figure 53.24, we can
predict that employment and education
opportunities will continue to be a problem for
Afghanistan in the foreseeable future.
In the United States and Italy, a decreasing
proportion of younger working-age people will
soon be supporting an increasing population of
retired “boomers.”
AGE STRUCTURE
AGE STRUCTURE
High Birth Rate:
Implication: A broad base signifies that the country has a high
number of births relative to the number of people in older age
groups. This suggests a young population with a high fertility rate.
Rapid Population Growth:
Implication: Because there are more people in the younger age
groups, the population is likely to grow rapidly if current birth rates
and death rates remain unchanged. This is due to the large number
of individuals entering the reproductive age in the future.
Youthful Population:
Implication: A perfect pyramid indicates a large proportion of the
population is young. This can mean higher demands on resources
for education and child services and potentially higher dependency
ratios as the population ages.
AGE STRUCTURE
Future Aging Population:
Implication: As the population ages, the pyramid will start to narrow.
Over time, the large cohort of young people will age and eventually
make up a larger proportion of the older age groups, leading to
increased demands on healthcare and pension systems.
Potential Economic Challenges:
Implication: A youthful population can present both opportunities
and challenges. While a large working-age population can drive
economic growth, there may also be challenges related to creating
sufficient job opportunities and providing adequate social services.
