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BIO120H1
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Melody Neumann
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Chapter 8

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Biology

BIO120H1

Melody Neumann

Fall

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8.1 Population Growth and Regulation
Chapter Concepts
Populations grow by multiplication rather than addition
Age structure influences population growth rate
A life table summarizes age-specific schedules of survival and
fecundity
The intrinsic rate of increase can be estimated from the life table
Population size is regulated by density-dependent factors
o Demography: is the study of populations and to intensive study of natural
and laboratory populations to learn about mechanisms of population
regulation
8.2 Populations grow by multiplication rather than addition
o Population growth depends on the reproduction and deaths of individuals,
biologists conveniently describe the rate of growth on a per-individual or per
capita basis. A population growing at a constant per capita rate expands ever
faster as the number of individuals increases
o Biologists can use a continuous-time approach: an approach to population
modeling assuming that the time flows continuously and that change can
occur at every instant
to model the way in which populations change instantaneously.
More convenient to work with time intervals that match the natural cycles of
activities in populations and the ways in which ecologists sample
populations. This method is referred to as a discrete-time approach: an
approach to population modeling that uses discrete time intervals, generally
corresponding to intervals between reproductive periods
o One must count individuals at the same time each year, so that all counts are
separated by the same cycle of birth and death processes. Such an increase
(or decrease) over discrete intervals is referred to as geometric growth:
increase or decrease in a population as measured over discrete intervals in
which the increment is proportional to the number of individuals at the
beginning of the interval
8.4 A life table summarizes age-specific schedules of survival and fecundity
o Life tables: a summary of the probabilities of survival and the fecundities of
the individuals in a population by age, can be used to model the addition and
removal of individuals in a population (in the absence of immigration and
emigration)
o When reproduction is continuous, as it is in the human population, each age
class can be designated arbitrarily as comprising individuals between ages x-
1/2 and x + ½
o The fecundity: the number of offspring produced per reproductive episode
of females is often expressed in terms of female offspring produced per
reproductive season or age interval and is designated by b x
o Life tables portray mortality in several ways. The fundamental measure of
mortality is the probability of survival (x ) between ages x and x+1 o Probabilities of survival over many age intervals are summarized by
survivorship to age x, designated by l wxich is the probability that a
newborn individual will be alive at age x. Survivorship is the most important
variable in the life table for calculating the growth rate of a population with a
stable age structure.
o For an individual of age x, the number of additional years that it can expect to
live is the exception of further life (X ), which depends on the probability of
survival through each subsequent age interval. The expectation of further life
measures the probability that a particular individual will be a member of a
population in the future. If the survival rate (sx) were the same for all adults
regardless of age, then each individual would have the same expectation of
further life
o Life tables can be constructed in two ways:
1. cohort (dynamic) life table: a life table that follows the fates of a group of
individuals born into a population at the same time from their birth to
death of the last individual
-method is readily applied to populations of plants and sessile
animals, in which marked individuals can be continually tracked over
the course of their life spans
-disadvantage: individuals can not life to long or are not to mobile to
track easily. Time and age are confounded, so it i

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