BIO205H5 Lecture Notes - Exponential Growth, Logistic Function, Introduced Species
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Chapter 54 – Population Ecology
54.1 How Do Ecologists Study Populations?
•A population consists of the individuals of species within a given area.
•The members of a population are distributed over space, and they differ in age
•The age distribution of individuals are spread over the environment describe its
•The number of individuals of a population per unit of area is its population
•The structure of a population changes continually because demographic
events are common occurrences.
Population densities can be estimated from samples
•Estimating population densities is easiest for sedentary organisms.
•Counting mobile organisms is much more difficult because individuals move
into and out of census area. (write equation here).
Birth and death rates can be estimated from population density data
•(Write equation here).
•Individuals born at the same time cohort.
•The number of a population still alive at a given period of time in the future
54.2 How Do Ecological Conditions Affect Life Histories?
•An organism’s life history describes how it allocates its time and energy
among the various activities that occupy its life.
•Ecological interactions influence the evolution of life histories.
54.3 What factors Influence Population Densities?
All populations have the potential for exponential growth
•All populations have the potential for explosive growth. As the number of
individuals in a population increase, the number of new individuals added per
unit of time accelerates (even if the rate of increase is expressed on a per
individual basis remains constant).
•If birth and deaths occur continuously and at constant rates, a graph of the
population size over time forms a continuous upward curve exponential
•(Write equation here).
•The difference between the average per capita birth rate and the average per
capita death rate of a population produces the net productive rate of a
•The highest possible value for the net productive rate is called rmax, or the
intrinsic rate of increase, expressed: (write equation here).
•For very short periods, some populations may grow at rates close to the
intrinsic rate of increase.
Population growth is limited by resources and biotic interactions
•No real population can maintain exponential growth for very long. As a
population increases in density, environmental limits cause birth rates to drop
and death rates to rise.
•An environment can support no more than a certain number of individuals of
any particular species per unit of area environmental carrying capacity,
K. It is determined by the availability of resources as well as by diseases,
predators and sometimes social interactions.
•Growth of a population typically slows down as its density approaches the
environmental carry capacity b/c resource limitations and the activities of
predators and pathogens lower birth rates and increase death rates logistic
Population densities influence birth and death rates
•Because each additional individual typically makes things worse for other
members of the population in an environment with limited resources, per
capita birth and death rates usually change together with changes in
population density density dependent.
•Birth rates and death rates may be density dependent for several reasons:
oAs a species increases in abundance, it may deplete its food supply.
oPredators may be attracted to areas with high densities of their prey.
oDiseases can spread more easily.
•Factors that change per capita birth and death rates in a population
independently of its density are said to be density independent (e.g.
•All populations fluctuate less than the theoretical maximum, but the sizes of
some populations fluctuate remarkably little.
•Stable populations are seen in species with long lived individuals that have low
•Small, short lived individuals are generally more vulnerable to environmental
changes than long-lived individuals.
•EPISODIC REPRODUCTION GENERATES POPULATION FLUCTUATIONS
oSome years are better than others for reproductive success.
oPopulation densities increase following years of good reproductive
success and decrease following years of poor reproduction.
•RESOURCE FLUCTUATIONS GENERATE CONSUMER FLUCTUATIONS
oDensities of populations of species that depend on a single or just a few
resources are likely to fluctuate more than those of species that depend
on wide variety of resources.
Several factors explain why some species are more common than others
•Many factors determine why typical population densities vary so greatly among
species, but four of them—resource abundance, the size of individuals, the
length of time a species has lived in an area, and social organization—exert
especially strong influences.
oSpecies that use abundant resources generally reach higher population
densities than species that use scarce resources.
oSpecies with small body sizes generally reach higher population
densities than species with large body sizes.
oSome newly introduced species reach high population densities (i.e.
species that have recently escaped from the control of certain factors, or
introduced into a region where their regular predators and pathogens
oComplex social organization may facilitate high densities.
•A new species can originate in several ways.
•A species that arises by polyploidy inevitably begins with a very small, local
•Species that arise thought founder events typically begin their history with only
a few individuals.
•Most species, on the other hand, that arise from a vicariant event begins with
large populations and ranges.
•As a species declines toward extinction, its range shirks until it vanishes.
•Interactions with other species may also limit both the densities and ranges of
54.4 How Do Spatially Variable Environments Influence Population Dynamics?
Many populations live in separated habitat patches
•Most populations are divided into separated, discrete subpopulations that live
in distinct habitat patches—areas of a particular kind of environment that are
surrounded by other kinds.
•The larger population is the metapopulation.
•Local disturbances and random fluctuations in numbers of individuals are more
likely to cause the extinction of a subpopulations that the extinction of an
•If individuals move frequently between subpopulations, immigrants may
prevent declining subpopulations from becoming extinct rescue effect.
Distant events may influence local population densities.
54.5 How Can We Manage Populations?
Demographic traits determine sustainable harvest levels
•Populations that have high reproductive capacities can persist even if harvest
rates are high. In such population, each female may produce thousands or
millions of eggs. The growth rates of individuals are often density-dependent.
Thus, if preproductive individuals are harvested at a high rate, the remaining
individuals may grow faster.
Demographic information is used to control populations
•At densities well below carrying capacity populations typically have high birth
rates and therefore can withstand higher death rates than they can when they
are closer to carrying capacity.
•When population dynamics are influenced primarily by factors that operate in a
density-dependent manner, killing part of a population reduces it to a density
at which it reproduces at a higher rate.
•A more effective approach is to remove its resources thereby lowering the
carrying capacity of its environment.
•Sometimes and introduced predator or parasite simply fails to control its host;
more serious consequences occur when a species introduced to control and
exotic pest not only attacks the pest species but also destroys other species
that are considered valuable.
Population densities can be estimated from samples: estimating population densities is easiest for sedentary organisms, counting mobile organisms is much more difficult because individuals move into and out of census area. (write equation here). Birth and death rates can be estimated from population density data (write equation here). Individuals born at the same time cohort: the number of a population still alive at a given period of time in the future survivorship. 54. 2 how do ecological conditions affect life histories: an organism"s life history describes how it allocates its time and energy among the various activities that occupy its life, ecological interactions influence the evolution of life histories. All populations have the potential for exponential growth: all populations have the potential for explosive growth. As the number of individuals in a population increase, the number of new individuals added per unit of time accelerates (even if the rate of increase is expressed on a per individual basis remains constant).