Download No Population Can Grow Indefinitely: J-Curves and

Biodiversity, Species Interactions,
and Population Control
Chapter 5
5-3 What Limits the Growth of Populations?
 Concept 5-3 No population can continue to grow
indefinitely because of limitations on resources
and because of competition among species for
those resources.
Populations Have Certain Characteristics
 Populations differ in their:
•
•
•
•
Distribution
Numbers
Age structure (proportion of individuals in diff. age groups)
Density (number of individuals in a given area)
 Population dynamics – the study of changes in
population characteristics due to:
•
•
•
•
Temperature
Presence of disease organisms or harmful chemicals
Resource availability
Arrival or disappearance of competing species
Most Populations Live Together in
Clumps or Patches
 Most populations live in clumps although other
patterns occur based on resource distribution.
• Clumps can help provide protection, aid in capture of
prey, or be for mating or caring for young.
Populations Can Grow, Shrink,
or Remain Stable
 Populations increase through:
• Births
• Immigration = arrival of individuals from outside the population
 Populations decrease through:
• Deaths
• Emigration = departure of individuals from the population
Populations Can Grow, Shrink,
or Remain Stable
 The proportion of individuals at various ages is the
population’s age structure.
• Pre-reproductive age: not mature enough to reproduce.
• Reproductive age: those capable of reproduction.
• Post-reproductive age: those too old to reproduce.
 How fast a population grows or declines depends on
its age structure:
• Young populations can grow fast
• Old populations decrease over time
• Even age distribution will result in constant population size
No Population Can Grow Indefinitely
 Biotic Potential
• The intrinsic rate of increase (r) is the rate a
population would grow if it had unlimited resources.
• Maximum rate of population increase under ideal
conditions.
• In nature, biotic potentials are rarely reached
 Individuals in populations with high r :
•
•
•
•
Reproduce early in life
Have short generation times
Can reproduce many times
Have many offspring each time they reproduce
No Population Can Grow Indefinitely
In general,
 Populations with large individuals have a low biotic potential
• Humans, elephants, blue whales
 Populations with small individuals have a high biotic potential
• Bacteria, insects, microorganisms
 Examples
• Rabbits are sexually mature at 5 months of age. Their gestation period
averages 31 days and the average litter size is 6 offspring. It is possible
to have 8 litters per year. If we start with 1 fertile female and assume
that 0 die and 50 percent of the litter is female, at the end of twelve
months we have approximately 1,850 rabbits.
• Bacteria populations can double every 20 minutes
• For humans, the biotic potential (for females age 14 years to 50 years)
can average as high as 12 per female.
• Elephants have a 22 month gestation period and can usually only give
birth once every 4 years.
No Population Can Grow Indefinitely
 In nature, all rapidly growing populations will
eventually reach a size limit imposed by various
limiting factors such as:
•
•
•
•
•
•
•
Light
Water
Space
Nutrients
Competition
Predators
Infectious diseases
No Population Can Grow Indefinitely
 No population can increase its size indefinitely.
Environmental Resistance – the sum of all the factors
that act to limit the growth of a population
• Limiting factors can be food, water, light, space,
nutrients, competitors, predators, disease, etc.
• These factors act as a negative feedback on the
population size.
Carrying capacity (K): the maximum population of a
given species that a particular habitat can sustain
indefinitely without degrading the habitat.
No Population Can Grow Indefinitely:
J-Curves and S-Curves
Linear Growth
Exponential Growth
 We will be using
these terms in our
population growth
discussions
Logistic Growth
 Know them!!
No Population Can Grow Indefinitely:
J-Curves and S-Curves
 Populations grow
rapidly with ample
resources – starts off
as exponential growth
(J-Curve).
 As resources become
limited, its growth rate
slows and levels off.
No Population Can Grow Indefinitely:
J-Curves and S-Curves
 As a population levels off
(forming the S-Curve) it
often fluctuates slightly
above and below the
carrying capacity.
 This fluctuation can be small,
large, regular, or irregular.
When a Population Exceeds Its Habitat’s
Carrying Capacity…
 Populations which overshoot
their carrying capacity suffer a
crash (or dieback)
 This situation is made possible
by a reproductive time lag
• The time needed for birth/death
rates to change in response to
environmental changes.
 Overshooting the carrying
capacity can adversely affect
the ecosystem as well
• Decrease the carrying capacity
Under Some Circumstances Population
Density Affects Population Size
 Population density: the number of individuals in a
population found in a particular area.
• Some population controls are density-dependent.
This means that they have a greater effect as the
population density increases.
• e.g. biotic factors, predation, competition, disease,
scarcity of food
• Other population controls are density-independent
and are not affected by population density.
• e.g. abiotic factors like weather, pollution, fire
Species Have Different Reproductive Patterns
 r-selected species – Large
number of smaller offspring
with little parental care.
 They overcome the massive
loss of offspring by having so
many that even if only very
few survive – it is enough.
 Opportunists – reproduce
and disperse at a rapid rate
(r) when conditions are
favorable.
Species Have Different Reproductive Patterns
 K-selected species – Fewer,
larger offspring with higher
invested parental care.
 Produce offspring later in life
and put a lot of energy into
one offspring.
 Competitors – can compete
well for resources when the
population is close to the
carrying capacity (K).
Species Have Different Reproductive Patterns
Species Have Different Reproductive Patterns
Answer:
R-selected species – they are more adaptive to
changing environmental conditions.
Since they have many offspring and much faster,
they can also evolve much more quickly.
Population Change Curves in Nature
 Population sizes often vary in regular cycles when the
predator and prey populations are controlled by the
scarcity of resources.
 These populations are regulated through:
• Top-down population regulation – predation, parasitism
• Bottom-up population regulation – the scarcity of resources
Survivorship Curves:
 The way to represent the age structure of a
population is with a survivorship curve.
• Late loss population (Type I)
• Live to an old age
• Humans, elephants
• Constant loss population (Type II)
• Die randomly at all ages
• Most birds
• early loss population (Type III)
• Most members die at young ages
• Fish, amphibians, many insects