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Chapter 10

BIOL150 Chapter Notes - Chapter 10: Herbivore, Survivorship Curve, Maximum Life Span

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Rebecca Rooney

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Biology 150 Chapter 10 Population Ecology
Organismal and Evolutionary Ecology Reading notes
10.1 Demography
- The number of individuals present in a population depends on 4 processes: birth,
death, immigration and migration
- Demography: the study of factors that determine the size and structure of
populations through time
- Generation: the average time between a mother’s first offspring and her daughter’s
first offspring
- Life Table: summarizes the probability that an individual will survive and reproduce
in any given time interval over the course of its lifetime.
(1) Zootoca Vivipara: A Case Study
(2) Survivorship: a key component of a life table, is defined as the proportion of
offspring produced that survive, on average, to a particular age.
Cohort: a group of the same age that can be followed through time (how
many individuals would survive to age 1, 2, 3...?
In most cases, only females will be kept tracked of when calculating life
table data because the number of males present rarely affects population
dynamics. There are almost always enough males present to fertilize all of
the females in breeding condition, so population growth depend entirely
on females.
To analyse general patterns in survivorship, biologist plot the logarithm of
the number of survivor vs ageit is called survivorship curve
3 general types of survivorship curves:
I. Survivorship throughout life is highmost individuals approach
the species’ maximum life span, for example, humans
II. Individuals have about the same probability of dying in each year
of life, e.g. blackbirds and other songbirds
III. A pattern defined by extremely high death rates for seeds and
seedlings but high survival rates later in life, e.g. many plants
(3) Fecundity: the number of female offspring produced by each female in a
Age-specific fecundity: the average number of female offspring produced
by a female in age class x
Age class: a group of individuals of a specific age, e.g., all female lizards
between 4 and 5 years old
- The Role of Life History
(1) Fitness Trade-offs: occur because every individual has a restricted amount of
time and energy at its disposalmeaning that its resources are limited. If a
female lizard devotes energy to producing a large number of offspring, it is not
possible for her to devote that same energy to herself for survival. In the graph
of “probability of survival to following year” vs “clutch size”, (a) no points in the
upper right corner of the graph because it is impossible to have both high
fecundity and high survivorship; (b) no points in the lower left corner because
species with low survivorship and fecundity have low population growth and go
extinct. A female can maximize fecundity, maximize survival, or strike a balance
between the two.

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Biology 150 Chapter 10 Population Ecology
Organismal and Evolutionary Ecology Reading notes
(2) Life history is based on resource allocation
Life history: describes how an individual allocates resources to growth,
reproduction, and activities or structures that are related to survival.
Life history is shaped by natural selection in a way that maximizes an
individual’s fitness in its environment. (fitness trade-off is key)
(3) Patterns across species
Individuals from species with high fecundity tend to grow quickly, reach
sexual maturity at a young age, and produce many small eggs or seeds
Individuals from species with high survivorship tend to grow slowly and
invest resources in traits that reduce damage from enemies and increase
their own ability to compete for resources
10.2 Population Growth
- Quantifying the Growth Rate
(1) A population’s growth rate is the change in the number of individuals in the
population(N) per unit time(t)
(2) If no immigration and emigration, a population’s growth rate is equal to the
number of individuals in the population times the difference between the birth
rate per individual(b) and death rate per individual(d).
(3) Difference between the birth rate and death rate per individual is per capita rate
of increase(r), per capita means for each individual
(4) Population and r value
Positive r
Per capita birth rate is greater
Population grows
Negative r
Per capita death rate is greater
Population declines
(5) Intrinsic rate of increase, rmax: maximum growth rate, birth rates are as high as
possible and death rates are as low as possible, r reaches a maximum value. A
population’s growth rate is expressed as ∆N/∆t= rmaxN
(6) Each species has a characteristic rmax that does not change
- Exponential Growth
(1) Exponential population growth: occurs when r does not change with population
size or density
(2) Exponential growth rate does not depend on the number of individuals in the
population, and is called density independent
(3) It adds an increasing number of individuals as the total number of individuals, N,
gets larger. The rate of increase is the same, but the number of individuals added
is not.
(4) It is observed in 2 circumstances: (a) a few individuals found a new population in
a new habitat or (b) a population has been devastated by natural disasters and
then begins to recover, starting with a few surviving individuals
(5) Many populations in unstable or unfavourable environments have periods of
exponential growth, but it is not possible for exponential growth to continue
(6) Population density: the number of individuals per unit area
(7) When the population density gets very high, the population’s per capita birth
rate will decrease and the per capita death rate will increase, causing r to
decline, this growth is density dependent
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