Ch. 19

Populations

Recall: A population is…• a group of organisms of the same

species living in the same area at the same time

Population PropertiesPopulation Properties

Descriptions

Size # of individuals; counted directly or estimated using population sampling

Density how crowded the population is; expressed as # of individuals per unit of area or volume

Exp. 30 people/km2

Dispersion Distribution over space; 3 types:

clumped = clustered together

uniform = separated by fairly consistent distance

random = each individual’s location is independent of others

Population Spacing• Dispersal patterns within a population

uniform

random

clumped

• Clumped – clustered together• Often occurs when resources are clustered or due to

social behavior

Clumped Pattern (most common)

• Uniform – separately by a fairly constant distance

• May be due to social behavior

May result from direct interactions between individuals in the population territoriality

Uniform

• Random – each individual’s location is independent of other individuals

• Few populations; most show degree of clumping or uniformity

Getting population size:• Counted directly – for immobile

organisms like trees• Estimated – for abundant or

widespread organisms– population sampling

• Exp. If a small patch of land contains 25 oak trees, an area 10 times larger would likely contain 10x the number of oak trees

– mark and recapture- marking a number of individuals in a population, returning them, and recapturing some of them as a basis for estimating the size of the population at the time of marking and release.

All populations are dynamic• All populations change in size and

composition over time!

Four things affect population dynamincs:

• Birth rate• Death rate• Life expectancy• Age structure

Birth and Deaths

• Birth Rate is the number of births in population in a certain time period; adds to population

• Death Rate is the number of deaths in a population in a certain amount of time; subtracts form population

• Life expectancy – how long individuals are expected to live; longer life expectancies add to population

• Age structure – distribution of individuals among different age groups; if higher percentage of young individuals there’s potential for rapid pop. growth

Graphs of Age Structure and Effect on Population Growth

High percentage of young individuals of reproductive age – rapid growth

Small percentage of young individuals of reproductive age – negative growth

Growth rate• amount by which a population’s size

changes in time

What does growth rate

depend upon?– adding & removing

individuals from a population• birth• death• immigration• emigration

Population Equation• growth rate = birth rate – death rate• If birth rate > death rate, population size

increases.• If birth rate < death rate, population size

decreases

Exponential Growth• Pattern of increase in # due to steady

growth rate• J shape curve• Ideal conditions

Exponential growth does not apply to most populations because of limiting factors.

As the population grows, competition for shrinking supply of resources intensifies. Each individual gets a smaller share.

Individuals’ ability to grow, reproduce, and fight off disease decreases. As a result, the population’s birth rate declines and death rate increases.

Logistic Model • Builds on exponential model but accounts

for limiting factors• Limiting factor – any factor that restrains

population growth– food– space– waste accumulation– competition– predators– disease

Logistic Model• Population grows, approaches

carrying capacity and slows, stops when it reaches the carrying capacity

• S shape

Carrying capacity = number of individuals the environment can support over a long period of time

Real populations may fit this pattern for some period of time but rarely remain stable; carrying capacity also can fluctuate.

More on limiting factors:Density- independent vs. Density

dependentDensity-

independentDensity-

dependent

Description Do not depend on population size

Depend on population size

Examples natural disasters like fires or floods, temperature, sunlight, rainfall

food supply, space/nesting sites, competition, predation

Changes in Carrying Capacity

• Population cycles– predator – prey

interactions

Populations rarely remain stable and can fluctuate!

Why do we worry about small populations?

• small populations are more vulnerable to extinction– disturbances can kill entire population– less organisms to survive and reproduce– less genetic variability

• Inbreeding in small populations may be a problem– decreases genetic variability even more– population less likely to adapt

Human population growth• Developments that have contributed to

the increase in human population growth:– Agriculture – stabilized and increased

food supply– Hygiene– Sanitation– Diet – Medical care– Improved economic conditions