Population Ecology

By the end of this class you should understand:

• The language of ecology and how to describe a population

• The different types of growth and survivorship curves

• How to identify limiting factors of a population

• How microevolution and niche competition can lead to speciation

Ecology

• Ecology is the study of how organisms behave and interact with each other– Population Ecology is the study of a single

population of animals (all the same species)– Community Ecology is the study of interactions of

populations• Population ecology often closely studies the

size of a population over time– More sex and violence!

Key Population Terms

• Population Size– Number of individuals in a

given population• Population density– How closely packed these

individuals are on average• Population distribution– Whether the individuals are

evenly spread out or clumped in some way

Population Size

• The number of individuals in a population depends on how you define “population”– May be all animals in a particular region– May be the entire species– May be more specific, such as all

female or all with a particular gene• Usually a population includes all

organisms that regularly interbreed– Geographically isolated groups are

usually different populations

Population Size Change• A population’s numbers can change

due to any number of factors– Death of organisms– New organisms being born– Arrival (immigration) and/or leaving

(emigration) of organisms (especially animals)

• If a population’s numbers do not change over time it is never because none of these are happening– This is a stable population and almost

never happens

Why No Stable Population?

• Remember the struggle for existence!– Even if a population is isolated, by natural

selection only the horniest of organisms have reproduced over the years, so they will always be trying to make babies

– Even if there are only limited resources, the impulse to make babies cannot be denied

– Also, there are always predators and disease• A population will only be stable if the birth

and death rates perfectly cancel out– They might on one given year but not constantly

No Predators Or Disease!

• When there is no emigration and no death, a population will experience exponential growth– This means a population will double

in size every generation• Generation time varies wildly by

species– Bacteria: 20 minutes– Humans: 20 years– All species will have the same curve

though

Exponential Growth• Exponential growth cannot be

sustained without unlimited resources– A resource is anything organisms

need to survive• Some resources are in much larger

supply than others and it varies by environment– In the ocean: sunlight is a limited

resource and water is a plentiful resource

– In the desert: sunlight is a plentiful resource and water is a limited resource

Infinite Resources

• It is obviously not possible to have infinite resources– Eventually space and air would become limited

resources

• That means whichever resource runs out first will become a limiting factor– Normally predators, parasites and natural

disasters are also limiting factors by causing death

Limiting Factors

• The limiting factors of a population create a certain theoretical maximum of a population size that would be stable– This limit is called the carrying

capacity• The carrying capacity is dictated by

predators, limited resources, etc. and is not fixed but an average– Environments can be created or

damaged and carrying capacities can be changed!

Environmental Change• During the time of the bible, the

Levant (in the middle east) was a very fertile region that was the birthplace of civilization

• After centuries of overfarming the plains and clear-cutting the forests, it is now a desert– NOT due to climate change!

• Elephants also instigate major changes in local environments, even without climate change

Let’s make a list!

• What are some limiting factors on populations?– Note some of these are density-dependent (more

problematic as organisms are more crowded)– Others are density-independent (no change in

danger as population density increases)

Carrying Capacity

• When a population hits carrying capacity it can hit it in one of two ways:– It can level out at the carrying

capacity (logistic growth)– It can overshoot the capacity and

then crash (windfall pattern)

• If there are many strong limiting factors then you usually see logistic growth

Windfall Pattern• True story: an empty island off the

coast of Alaska had a substantial buildup of moss when some humans left a few reindeer on this island– No predators or disease = lots of

babies!• The population underwent

exponential growth until the moss was all gone, then crashed during a harsh winter– Eventually all the reindeer died

Windfall Population Pattern

Why Is This Relevant?• All populations have carrying

capacities– Including the human population!

• A pre-industrial society has limiting factors like disease and lack of food– Industrial revolutions are

awesome for sanitation and farming

• A post-industrial society has a low birth rate because babies are expensive

Human Population Growth• The world didn’t

hit 1 billion people until around 1800

• Hit 2 billion around 1927

• 4 billion by 1974• 7 billion back in

2012• There are

several projections…

Reproductive Strategies

• Different organisms have different strategies for survival of the species:– Some make tons of babies and

hope they survive– Some put a lot of energy into

keeping each offspring alive

• What are some organisms that are on each extreme?

Survivorship Curve

• Closely related to the reproductive strategy is the survivorship curve– Describes the percentage of a group

of organisms that are alive after a given percent of their maximum lifespan

• Organisms that produce many babies have type III survivorship while organisms that produce few babies but protect them have type I

Survivorship Curves

Microevolution• Microevolution (change in allele

frequencies) can occur due to limiting factors– Something that preferentially kills

large organisms will favor small organisms, etc.

• A good example is guppies in streams that are hunted by other fish– When hunted by larger predators,

smaller guppies result– When hunted by small predators,

larger guppies result

Genetics Over Time

• When small guppies (from streams where they are hunted by large fish) are grown in lab aquariums they are still small– They are expressing more alleles that make them

smaller as adults– Also usually reproductively active sooner

• These microevolutionary changes add up over time– The different pressures can also result in speciation

Advantages of Sizes

• Larger organisms:– Are less vulnerable to smaller predators– May be able to win competition for food and

mates against smaller organisms

• Smaller organisms:– Can be passed over by large predators– Need less food and resources– May become reproductively active sooner

Human Size

• Do you suppose humans have been getting bigger or smaller over the last few hundred years?

• Why?

Adaptive Radiation

• If there is a lot of pressure from competition for resources, the population may undergo speciation to take advantage of new resources– The African seedcrackers show this!

• The sum total of resources a population needs can be thought of as its niche– Species that lose their niche must find a new way

to get their resources quickly or else die out

Genus Panthera• Only a few million years ago,

there was some unnamed species of large cat– Its range covered Africa, Asia and

the Americas• One population became

geographically isolated and became the modern-day Jaguar

• Lions, Tigers and Leopards all still have overlapping ranges (and will very occasionally still interbreed and produce sterile offspring)– So how did that happen?

99 Problems But a Niche Ain’t One• During some of the turmoil of the

ice ages, new niches opened up– WANTED: Large predator that can

use teamwork to bring down large prey; must live in open spaces where pure surprise is difficult

– WANTED: Large predator that hunts without teamwork; must live in jungles where there is more camouflage

– WANTED: Less large predator that can catch and live off smaller prey anywhere

Niche Overlap

• If two species need the same resource, they are in competition for that resource

• The organisms that are in the fiercest competition will probably die off first

• In the case of Lion vs. Leopard, the smallest leopards and largest lions competed with each other the least– Hence, speciation!

More on Monday!

• See you in lab!