Limits to Growth
5-2
Limiting Factors
• Remember, primary productivity of an
ecosystem can be reduced by limiting
nutrients
• In the context of populations, limiting
factors can cause population size to
decrease
• A resource base that is limited can also
affect the long-term survival of a species
• Ex: pandas and bamboo
Density-Dependant Factors
• Limiting factors that depend on population
size
• Density-dependent factors become limiting
only when population density reaches a
certain level
• Do not affect small, scattered populations
• Examples of density-dependent limiting
factors:
– Competition
– Predation
– Parasitism
– Disease
Competition
• Occurs when populations become
crowded
• All organisms in a population need
sunlight, food, water, space, etc.
• Competition among members of the same
species is density dependent
• The more members in the population, the
faster the resources are used up
• Competition also happens between
members of different species
– Major force behind evolutionary change
• When 2 species compete for same
resource, they are under pressure to
change
– Over time, evolve to occupy diff. niches
Predation
• Populations in nature are controlled by
predation
– Known as predator-prey relationship
• Ex:
– sea otters and urchins
– Sea otters and whales
Isle Royale
• Well known example of predator-prey
relationship is the moose/wolf populations on
Isle Royale in Lake Superior
– When moose numbers are high, wolf #’s increase
– As wolves feed on moose, moose #’s decrease,
causing a decrease in wolves
– With fewer wolves, moose #’s can increase again
Parasitism & Disease
• Parasites can also limit population growth
• Parasites are similar to predators – weaken and
kill hosts
• Large populations can be under stress making
them more susceptible to diseases
• More members = easier to pass “things” around
Density-Independent Factors
• Factors that affect populations regardless
of their size
• Examples:
– Unusual weather
– Season cycles
– Human activities (damming rivers, clear
cutting)
• Populations usually respond to such
factors with a crash in population size
• After crash, numbers may increase
quickly, or stay low for awhile
• Storms and hurricanes can wipe out
populations of insects
• Extreme cold or hot weather can also take
a toll on populations
• Droughts can affect entire populations of
vegetation, which can affect populations of
consumers
• Environments are always changing
• Most populations can adapt to a certain
amount of change (grow or shrink)
• Major upsets in ecosystems can lead to
long-term decline in certain populations
• Human activities have caused some of
these upsets
5-3 Human Population Growth
• Like the populations of many other
living organisms, the size of the human
population tends to increase with time.
• For most of human existence, the
population grew slowly
• Until fairly recently, only half the children in
the world survived to adulthood.
• About 500 years ago, the human population
began growing more rapidly.
• Agriculture and industry made life easier and
safer.
• The world's food supply became more reliable,
and essential goods could be shipped around
the globe.
• Improved sanitation, medicine, and health care
dramatically reduced the death rate and
increased longevity.
• With these advances, the human population
experienced exponential growth, as shown in
the figure at right.
Patterns of Population Growth
• English economist Thomas Malthus
observed that human populations were
growing rapidly.
• Malthus predicted that such growth would
not continue indefinitely.
• Instead, according to Malthus, war,
famine, and disease would limit human
population growth.
• Scientists have identified a variety of other
social and economic factors that can affect
human populations.
• The scientific study of human populations
is called demography
• Birthrates, death rates, and the age
structure of a population help predict
why some countries have high growth
rates while other countries grow more
slowly.
Demographic Transition
• Over the past century, population growth
in the United States, Japan, and much of
Europe has slowed dramatically.
• Demographers have developed a
hypothesis to explain this shift.
• According to this hypothesis, these
countries have completed the
demographic transition, a dramatic
change in birth and death rates.
Age Structure Diagram
• Demographers can predict future growth using
models called age-structure diagrams, or
population profiles.
• Age-structure diagrams show the population of a
country broken down by gender and age group.
• Each bar in the age-structure diagram
represents individuals within a 5-year group.
Percentages of males are to the left of the center
line and females to the right in each group.
Future Population Growth
• To predict how the world's human
population will grow, demographers must
consider many factors including:
– the age structure of each country
– prevalence of life-threatening diseases