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

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 billion by 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

More on Monday! See you in lab!