Describing PopulationsPopulation Ecology

POPULATION•Individuals of the same species living in a

particular area•Population size, density, distribution and

age structure can help the ecologist understand how a population may grow or decline

POPULATION SIZE•The overall health of a population can

often be monitored by tracking how its size changes

•POPULATION SIZE– Describes the number of individual organisms present in a given population at a given time

•Population size may increase, decrease, undergo cyclic change, or remain the same over time.

The Decline of the Passenger Pigeon• Shows extremes of population size• Once most abundant bird in North America• Huge flocks darkened skies.• In 1800’s 2 billion formed a near solid mass

240 miles long and took 5 hours to fly overhead…sounded like a tornado!

• Passenger pigeons nested and bred in forests.• Forests cut down and hunted. Shipped to market as food

1898 Photograph of a Passenger Pigeon.The last wild Passenger Pigeon was shot by a 14-year-old boy in Ohio in 1900, while the last known individual of the passenger pigeon species, named "Martha" after Martha Washington, died at 1 p.m. on the 1st of September 1914 in the Cincinnati Zoological Garden. She was 29.

Passenger Pigeons, Continued

• By end of 1800s the population was so small the pigeons could not form needed colonies to breed.

• Last passenger pigeon on Earth died in the Cincinatti Zoo in 1914

DETERMINING POPULATION SIZE•Were 2 billion pigeons actually “counted?”•Probably not—sometimes not possible•Population size is estimated using

sampling techniques•We will be using a sampling technique

later today in our lab!

POPULATION DENSITY•Measure of “how crowded” an area is•Number of individuals within a population

per unit time•Example: 1500 golden toads counted in 1987

within 4 square kilometers•Population density = 1500toads/4 square km = 375 toads/square km•In general, larger organisms like lions have

lower population densities because they require more resources and room to survive

Different Densities•High population density can make it

easier for organisms to group together and find mates but can also lead to conflict/competition

•Overcrowded = more vulnerable to predators

= increase transmission of disease•Low density = may benefit from more

space and resources, but harder to mate

DENSITY AND THE HARLEQUIN FROG•Lived in very specific locations called “splash

zones” in Costa Rica•Spash zone= area alongside rivers and streams

that receive spray from waterfalls and rapids•1980s and ‘90s Montverde region of Costa

Rica got warmer and drier water flow decreased and streams dried up

•Few splash zones left•Frog densities up to 4.4x higher than normal

around remaining splash zones

DENSITY AND THE HARLEQUIN FROG•Overcrowding= disease transmission

= predator attack = assault from parasitic

flies•Harlequin frog disappeared from

Monteverde region but a few exist elsewhere

POPULATION DISTRIBUTION•How individuals are arranged in an area•Sometimes called population

DISPERSION•Three types: random, uniform and

clumped

RANDOM DISTRIBUTION

•Individuals are arranged within a space in no particular pattern

•Found where resources needed are found throughout an area

UNIFORM DISTRIBUTION•Individuals evenly spaced throughout an

area•When individuals hold territory or

compete for space

CLUMPED DISTRIBUTION

•Individual organisms arrange themselves according to the availability of resources needed to survive

•Most common in nature•Examples: Desert animals live in patches

around isolated sources of water•Harlequin Frogs around splash zones•Humans by urban centers•Ants living in colonies

AGE STRUCTURE •Describes relative numbers of organisms

of each age in a population (also known as age distribution)

•Age pyramids/age structure diagrams– visual tools used to show age structure of population

•Can be useful for predicting population growth

SEX RATIOS•Proportion of males to females•Age structure diagrams can give this

information•50:50 ratio ideal, otherwise harder for

any individual to reproduce and pass on genes