6. Population Density and
Spatial Distribution
 Population density is the number of individuals
per unit area.
• High population density
– injures all individuals within the population
– because they compete for resources
7. 7.2 A Population Growth Curve
 Biotic potential is the inherent reproductive
capacity of a species
• (biological ability to produce offspring).
 biotic potential is much above replacement level.
• natural tendency for increase
11. Population growth curve
 Lag Phase:
 first portion of the curve
 slow population growth
 Few births
12. Population growth curve
 Exponential Growth Phase (Log Phase):
 More organisms reproducing
 causing accelerated growth
 continues if birth rate exceeds death rate
13. 7.2 Population Growth Curve
• Deceleration Phase:
– The population growth rate slows
– death rate and birthrate equal one another
14. 7.2 Population Growth Curve
 Stable Equilibrium Phase:
 The death rate and birth rate become equal
 the population stops growing
15. 7.3 Factors That Limit Population Size
 Limiting Factors--prevent unlimited population
growth
• Extrinsic limiting factors
• Intrinsic limiting factors
• Density-dependent factors
• Density-independent factors
16. 7.3 Factors That Limit Population Size
 Extrinsic limiting factors
 Come from outside the population
•
•
•
•
Predators
Loss of food source
Lack of sunlight
Accidents of nature
17. 7.3 Factors That Limit Population Size
 Intrinsic limiting factors
• factors that originate within the population
• exercise control over it
• Behavioral changes amongst the population cause
lower birthrates and higher death rates.
18. Other limiting factors
 Density-dependent limiting factors
• become more effective as the density of the
population increases.
• Denser population
– Predators more effective
19. Other limiting factors
 Density-independent limiting factors
• population-controlling influences not related to the
density of the population.
• Accidental
• Extrinsic factors
20. Density dependent or independent?
 Mutualism between two species.
 A wolf eating rabbits.
 A large fire burns down many of the trees in a
forest.
 A disease kills all the mice in a local radius and
the foxes have nothing to eat.
21. 7.4 Categories of Limiting Factors
 For most populations, limiting factors recognized
as components of environmental resistance
•
•
•
•
Raw material availability
Energy availability
Accumulation of waste products
Interactions among organisms
24. 7.5 Carrying Capacity
 Carrying capacity is the maximum sustainable
population for an area.
 It is not an inflexible number; it can be
influenced by environmental differences
27. 7.6 Reproductive Strategies
and Population Fluctuations
 Species divided into two broad categories based
on their reproductive strategies:
• K-strategists
• r-strategists
28. 7.6 Reproductive Strategies
 K-strategists: Organisms that typically reach a stable
population as the population reaches the carrying
capacity.
 K-strategist characteristics:
 Usually occupy relatively stable environments
• Large organisms
• Long-lived
• Produce few offspring
29. 7.6 Reproductive Strategies
and Population Fluctuations
 K-strategist characteristics:
• Provide substantial parental care
• Reproductive strategy
– invest a great deal of energy in producing a few offspring
that have a good chance of living to reproduce.
 K-strategists
• controlled by density-dependent limiting factors.
31. 7.6 Reproductive Strategies
 r-strategist characteristics include:
•
•
•
•
•
Small, short-lived organisms
Produce many offspring
Little if any parental care
Exploit unstable environments
Usually do not reach carrying capacity (boom-bust
cycles)
35. Summary
 The birthrate (natality) is the number of individuals
entering the population by reproduction during a
certain period.
 The death rate is the number of deaths in a
population in a certain period.
 A typical population growth curve shows a lag
phase followed by an exponential growth phase, a
deceleration phase, and a stable equilibrium
phase at the carrying capacity.
37. 11.3 Invasive Species
 Impact populations (affect population size)
 Some introductions of exotic species are
purposeful, while others are accidental.
 Globalization is responsible for spreading
thousands of invasive alien species around the
world.
38. 11.3 Invasive species
 The IUCN estimates about
• 30% of birds and 15% of plants are threatened
• because they are unable to successfully compete against
invasive exotic species.
 Various insects have had an effect on ecosystem
structure.
• Asian long horned beetle
 Freshwater ecosystems have been greatly affected.
• Zebra mussel