Ecology

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9
Population Growth
and Regulation
9 Population Growth and Regulation
• Case Study: Human Population Growth
• Life Tables
• Age Structure
• Exponential Growth
• Effects Of Density
• Logistic Growth
• Case Study Revisited
• Connections in Nature: Your Ecological
Footprint
Figure 9.2 Explosive Growth of the Human Population
Introduction
One of the ecological maxims is “No
population can increase in size forever.”
Ecologists try to understand the factors
that limit or promote population growth.
Figure 9.3 Dash to the Sea
Life Tables
Concept 9.1: Life tables show how survival
and reproductive rates vary with age, size, or
life cycle stage.
Information about births and deaths is
essential to predict trends or future
population size.
Life Tables
A survivorship curve is a plot of the
number of individuals from a
hypothetical cohort that will survive to
reach different ages.
Survivorship curves can be classified into
three general types.
Figure 9.5 Three Types of Survivorship Curves
Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 1)
Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 2)
Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 3)
Age Structure
Concept 9.2: Life table data can be used to
project the future age structure, size, and
growth rate of a population.
A population can be characterized by its
age structure—the proportion of the
population in each age class.
Age structure influences whether a
population will increase or decrease in
size.
Figure 9.7 Age Structure Influences Growth Rate in Human Populations
Age Structure
Life table data can be used to predict age
structure and population size.
Figure 9.8 A Growth of a Hypothetical Population
Age Structure
The growth rate (λ) can be calculated as
the ratio of the population size in year t
+ 1 (Nt+1) to the population size in year t
(Nt).
N t 1

Nt
Figure 9.8 B Growth of a Hypothetical Population
Age Structure
When age-specific survival and fecundity
rates are constant over time, the
population ultimately grows at a fixed rate.
The age structure does not change from
one year to the next—it has a stable age
distribution.
Loggerheads – protect eggs, but even w/
100% protection, species would still
decline. Have to protect juveniles and
adults.
Exponential Growth
Concept 9.3: Populations can grow
exponentially when conditions are favorable,
but exponential growth cannot continue
indefinitely.
Exponential Growth
Geometric growth:
N t 1  N t
λ = geometric growth rate; also known
as the (per capita) finite rate of
increase.
Exponential Growth
Geometric growth can also be
represented by
N t  N 0
t
This predicts the size of the population
after any number of discrete time
periods.
Exponential Growth
In many species, individuals do not
reproduce at discrete time intervals,
they reproduce continuously
The growth in these populations is
exponential growth.
Exponential Growth
Exponential growth is described by
dN
 rN
dt
dN
dt
= the rate of change in population size
at each instant in time.
r is the exponential population growth
rate or the (per capita) intrinsic rate of
increase.
Figure 9.9 A Geometric and Exponential Growth
Figure 9.9 B Geometric and Exponential Growth
Figure 9.10 How Population Growth Rates Affect Population Size
Effects Of Density
Concept 9.4: Population size can be
determined by density-dependent and
density-independent factors.
Under ideal conditions, λ > 1 for all
populations.
But conditions rarely remain ideal. What
factors cause λ to fluctuate over time?
Figure 9.12 Weather Can Influence Population Size
Figure 9.13 Comparing Density Dependence and Density Independence
Effects Of Density
In an experiment where eggs of the flour
beetle Tribolium confusum were placed
in glass tubes, death rates increased as
the density of eggs increased.
Logistic Growth
Concept 9.5: The logistic equation
incorporates limits to growth and shows how
a population may stabilize at a maximum size,
the carrying capacity.
Logistic growth: Population increases
rapidly at first, then stabilizes at the
carrying capacity (maximum
population size that can be supported
indefinitely by the environment).
Figure 9.17 An S-shaped Growth Curve in a Natural Population
Figure 9.18 Logistic and Exponential Growth Compared
Figure 9.20 Faster than Exponential (Part 2)
Figure 9.21 United Nations Projections of Human Population Size
Figure 9.22 The Human Carrying Capacity
Connection in Nature: Your Ecological Footprint
The environmental impact of a population
is called its ecological footprint.
Ultimately, every aspect of our economy
depends on the ecosystems of Earth.
Connection in Nature: Your Ecological Footprint
The ecological footprint approach
highlights the fact that all of our actions
depend on and affect the natural world.
Find yours at
http://www.conservation.org/act/live_gree
n/Pages/ecofootprint.aspx
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