Population Ecology

Chapter 14

What is ecology?

• The study of the interactions between organisms & their environments– can be studied at

many levels• individual/organismal• population• community• ecosystem

A population is . . .• A group of organisms of the same

species living in a specific area • Population ecologists are interested

in how quickly or slowly populations grow– for resource management– for conservation of rare & endangered

species• Population growth influenced by

– birth & death rates– immigration & emigration

Populations can grow quickly . . . for a while

• Exponential growth occurs when each individual produces more than a single offspring to replace itself– the bigger the population, the faster it grows

Exponential growth of rabbits

Why is the planet not overrun with rabbits? (or ants or palm trees or. . .)

• Population growth is limited by the environment; life gets harder as it gets more crowded

• Population density: the # of individuals living in a given area (how crowded it is)

• As population size increases,– competition for resources increases– spread of disease & parasites increases– risk of predation increases

• predator population grows in response to increase in # of prey items

• the more densely packed prey, the more visible

Limitations to population growth

• Density dependent factors– conditions whose growth-limiting effects increase

as population grows – the maximum population size an environment

can sustain, or “carry” is called the carrying capacity

– as population size reaches carrying capacity:• death rates increase• emigration rates increase• birth rates decrease (less food >> poor nutrition >>

reduced fertility)

Logistic growth• When a population

reaches the carrying capacity of the environment (there is now a lack of resources), growth is called logistic– a much better

approximation than exponential growth of how populations grow

Limitations to population growth• Density independent factors

– conditions whose growth-limiting effects are unrelated to population size

– causes increase in death rate or decrease in birth rate because of “bad luck”

• natural disasters, severe weather conditions, oil spills …

Unusual population growth patterns• If an environment is

repeatedly subjected to density independent factors, a population could be in a perpetual state of exponential growth with periodic massive mortality events.

• Population oscillations can occur such as with snowshoe hares & lynx.

These two examples of population bursts & busts are not the norm. The logistic growth pattern describes the general growth of populations better than any other model.

Human population growth• Human population growth is limited by the same

factors as other species• Human populations vary in distribution of space

– dense in cities, more sparse in rural areas• Human populations vary in distribution of age

– there may be more individuals in one age group than another

• Two populations of the same size with different age distributions will have different features– a 10 year old not likely to reproduce; a 30 year old has a

relatively high likelihood of reproducing; an 80 year old has a high likelihood of dying before younger people

Human population growth cont.• A population can be divided

into cohorts– percentages of individuals in

each age grouping• 0-5 years, 6-10 years, etc.

• Cohorts can be “stacked” into age pyramids

• It is helpful for a society to know how many 10, 30, & 80 year olds are in its population– Does the society need to build

more schools? Hospitals? Convalescent homes?

Around the world, countries vary in age pyramids describing their pops.

• Industrialized countries– pyramids more like

rectangle– most growing slowly or

not at all– low birth rates & low

death rates in higher age groups (good health care)

– most of the population is middle age or old

Around the world, countries vary in age pyramids describing their pops.

• Developing countries– pyramids are more

triangular– high birth rates but

also high death rates in older age groups (poor health care)

– most of population is in younger age groups

Age pyramid of the USA• What do you see? Why is this problematic?

Changing demographics• Demographic transition

– when populations move from high birth & high death rates >> high birth & low death rates >> low birth & low death rates

– (moving from developing country to industrialized)

– illustrates how health, wealth & education can lead to reduction in birth rate

Changing demographics, cont.

• ¾ of the world’s population live in developing countries that are transitioning into industrialized– population will

continue to rise quickly & at an unsustainable rate

How high can human population go?• Humans are a very successful species• Human environment also has a carrying capacity

beyond which the population cannot be maintained• Carrying capacity related to an ecological footprint

– the amount of food, water, fuel & space needed per person per country

– ecological footprint varies by country, but as a whole, the worldwide population is living at an unsustainable level

• Carrying capacity hard to determine; we keep moving it

• living in new habitats• increasing agricultural production• circumventing problems of living densely (disease control & waste

management)

Tools that help population ecologists• A life history is the summary of statistics of a

species– age of first reproduction – litter size & frequency – probability of survival at each age– longevity

• Different species reach sexual maturity at different ages & produce different # of offspring with each mating– among mammals, litter size tends to increase as

distance from the equator increases. Why would that be?• Knowing the life history of a species can help

ecologists manage resources & predict population growth

Tools that help population ecologists

• Survivorship curves– graphs that show the

proportion of individuals of a certain age that are alive in a population

– indicate an individual’s likelihood of surviving through an age

– show at quick glance whether most offspring die shortly after birth or if live into adulthood

– curves can change over time