ch. 19
DESCRIPTION
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 Properties. Population Spacing. Dispersal patterns within a population. clumped. random. uniform. Clumped – clustered together - PowerPoint PPT PresentationTRANSCRIPT
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