AP Biology
Chapter 52 Population Ecology
Population Ecology:
The study of populations in relation to the environment,
including environmental influences on population density and
distribution, age structure, and variations in population size
Population:
A group of individual of a single species living in the same
general area
Density:
The number of individuals per unit area or volume
Density is a result of dynamic interplay between processes that
add individuals to a population and those that remove
individuals from it
Dispersion:
A pattern of spacing among individuals within the boundaries
of a population
Immigration:
The influx of new individuals from another area
Emigration:
The movement of individuals out of a population
Patterns of dispersal:
Clumped: most common pattern of dispersal with individuals
aggregated in “patches” Ex. Wolves, mushrooms Some habitat
patches are more suitable for a species than others
Uniform: evenly spaced, not as common as clumped dispersion
patterns. A pattern may be a result of direct interactions
between individuals of a population. Ex. Territoriality
Random: unpredictable spacing occurs in the absence of strong
attractions or repulsions among individuals of a population or
where key physical or chemical factors are homogenous across
a study area. Ex. Dandelions wind dispersed
Demography:
Study of vital statistics of populations and they change over
time. Particularly, birth and death rates (especially of females)
Demographers use life tables, age-specific summaries of the
survival patterns of a population.
A cohort is a group of individuals of the same age, from birth to
death.
Survivorship curve: a plot of the proportion or numbers in a
cohort still alive at each age. There are 3 types:
Type I, is flat at the start, reflecting low death rates for early
through midlife, then drops steeply as death rates increase with
age.
Type II, are intermediate, with a constant death rate over the
organisms life span.
Type III, drops severely at the start, reflecting very high death
rates for the young, but then flattens out as the death rate
declines with the individuals that survived to a certain critical
age.
Reproductive Table: or fertility schedule, is an age specific
summary of the reproductive rates in a population. The best
way to construct this type of schedule is to measure the
reproductive output of a cohort from birth to death.
Life History: the traits that affect an organisms schedule of
reproduction and survival (from birth through reproduction to
death)
Big bang reproduction or Semelparity- one shot reproduction
Ex. Salmon after maturing at sea, return to their birth stream to
spawn, produce 1000s of eggs and then die
Iteroparity or repeated reproduction: Ex. Some lizards produce
a few eggs in their second year of life and repeat the
reproductive act annually for several years
Factors that contribute to the evolution of semelparity or
iteroparity:
Critical factor is survival rate of offspring
Where survival rates are low (unpredictable environments)
semelparity is favored. Large number of offspring so some will
survive.
In more stable environments, iteroparity is favored.
Organisms have finite resources so there are “tradeoffs”
Ex. trade-off between reproduction and survival- Red deer who
calved in the spring had a higher mortality rate the following
winter.
Exponential model:
It is useful to study population growth in an ideal, unlimited
environment because these studies reveal the capacity of
species and the conditions in which the capacity may be
expressed.
Per capita Rate of Increase:
Change in population size during a time interval = births during time – deaths during time
Zero Population Growth: (ZPG) = when per capita birth and
death rates are equal
Exponential Population Growth:
Members of a population have access to abundant food and are
free to reproduce at their physiological capacity (ideal
conditions)
Carrying Capacity: the maximum population size that a
particular environment can support. It is not fixed but varies
over time with the abundance of limiting resources. Energy,
shelter, refuges from predation, soil nutrients, water, and
suitable nesting sites can all be limiting factors.
Logistic population growth: changes in growth rate as the
population nears its carrying capacity. The per capita rate of
increase declines as the carrying capacity is reached
Logistics Model and Real Populations:
Some basic assumptions of the logistics model don’t apply to all
populations. The logistics model assumes that populations are
grown in a constant environment lacking predators, and other
species that may compete for resources. It also assumes that
populations adjust instantaneously to growth and approaches
carry capacity (K) smoothly.
In natural populations, there is a lag time before the negative
effects are realized. Ex. If food becomes a limiting factor, the
females might use their reserves to continue to reproduce for a
short time. This may cause the population to overshoot its
carrying capacity before settling to a relatively stable density.
The logistics model is a good starting point for thinking how
populations grow. It’s also good for conservation biology to
estimate how fast a particular population might increase in
numbers after it has been reduced to a small size. It’s also
good for estimating sustainable harvest rates for fish and
wildlife populations.
Logistics model and life histories:
Logistics model predicts different per capita growth rates for
populations of low or high density relative to carrying capacity
of the environment. Selection for life history traits that are
sensitive to population density is known as K-selection, or
density dependent selection. K-selection tends to maximize
population size and operates in populations living at a density
near the limit imposed by their resources (carrying capacity K)
The opposite, selection for life history traits that maximize
reproductive success in uncrowded environments (low
densities) is called r-selection or density independent selection.
r-selection tends to maximize (r), the rate of increase, and
occurs in environments in which the population density
fluctuate well below the carrying capacity or individuals are
likely to face little competition.
The concepts of r and K selection have been criticized as over
simplification. The characteristics of most species places them
somewhere in between the extremes of r and k selections.
Populations are regulated by biotic and abiotic factors:
2 main questions about regulation of population growth: 1st
what environmental factors stop a population from growing?
Second, why do some populations show radical fluctuations in
size over time while others remain more stable?
Density independent: a birth rate or death rate that does not
change with population density
Density dependent: a death rate that rises with the population
density.
Density-dependent birth and death rates are an example of
negative feedback. Without some type of negative feedback
between population density and vital rates of birth and death, a
population would not stop growing.
Density-dependent population regulation:
Competition of resources: In crowded populations, increasing
populations intensifies intraspecific competition for declining
nutrients and other resources.
Territoriality: may limit density in many vertebrates and some
invertebrates. Ex. Suitable nesting sites
Health: If the transmission rate of a disease depends on a
certain level of crowding in a population, the disease impact
may be density dependent. Tuberculosis (TB) spread through
the air spreads faster in the city than a rural environment.
Predation: as a prey population builds up, predators may feed
preferentially on that species. Ex. Trout feeding on a particular
hatch of insects emerging from its aquatic larval stage
Toxic wastes: metabolic by-products of micro-organisms
accumulate as the population grows, poisoning the organisms
Intrinsic Factors: (physiological) factors An example is white
footed mice will multiply from a few to 30-40 individuals, but
reproduction will decline until the population stops growing.
This is due to increased aggressive behavior brought about by
the population density even with abundant food and shelter.
Question #2: why do populations fluctuate over time?
Study of population dynamics focuses on the complex
interactions between biotic and abiotic factors that cause
variation in population size.
Immigration and emigration can influence populations when
considering grouped populations a metapopulation. This
concept is important in understanding populations in “patchy”
habitats.
Population cycles:
Many populations fluctuate at unpredictable intervals while
others go in “boom or bust” cycles fluctuating in density with
remarkable regularity. Long term experimental studies will
help unravel the complex causes of population cycles.
Human population growth has slowed after centuries of
exponential growth:
Demographic transition: the movement from exponential
growth to logistical growth. This is associated with an increase
in quality health care, and sanitation as well as improved access
to education, especially for women.
Factors:
Family planning and voluntary contraception
Age structure: the relative number of individuals of each age
Infant mortality and life expectancy
Ecological capacity: the actual resource base per country.
Based on this, the USA is over its carrying capacity. The
calculation of 8.4 ha/person and we only have 6.2ha/person.