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 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.

Most Populations Live Together in Clumps or Patches

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

Populations Can Grow, Shrink, or Remain Stable

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

Linear Growth

Logistic Growth

Exponential Growth

We will be using these terms in our population growth discussions

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.

No Population Can Grow Indefinitely: J-Curves and S-Curves

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

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

Under Some Circumstances Population Density Affects Population Size

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.

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.

Species Have Different Reproductive Patterns

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