Notes

Outline

Introduction to Ecology CERC Certificate Program Columbia University Session 2 – Population Ecology

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth Before all that however…

An Exercise… From a study reported in the Tuesday New York Times’ Science Times section: Work through the 11 steps of the scientific method What did you do?

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth

Important Applications of Population Ecology - Local Growth rates of introduced species Population Viability Analyses of endangered species What is the minimum number of individuals needed to ensure a 90% chance of survival for 100 years Population genetics of endangered species Rosenbaum’s work with whales

Important Applications of Population Ecology - Regional Metapopulation analyses Tracing the survival of all component populations More in a bit Captive Breeding projects at zoos Applied metapopulation analyses

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth

What is a Population? Components? Definition : One species One area Isolated from other areas Able to interbreed Example:

What is a Population? Components? Definition : One species One area Isolated from other areas Able to interbreed Example:

Characteristics of a Population What features can we measure of a population? Features: Size Age structure Effective population size Birth rate Death rate Immigration Emigration

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth

Why Does Population Size Change? Density Independent Forces Forces that are at work irrespective of the population density Density Dependent Forces Forces that vacillate depending on the population density

Density Independent Forces Types? Examples Climate Topography Latitude Altitude Rainfall Sunlight In Sum: Abiotic factors

Density Dependent Forces Types? Examples Within species Breeding spaces Food Mates Foraging spots Between species Predation Parasitism Pollinators In Sum: Biotic factors

Indeterminate Factors Previous influences have been pretty constant and Deterministic Opposite of deterministic factors is Stochastic forces Examples Environmental: Droughts, floods, asteroids, volcanoes, fires, etc. Demographic: Crash in effective population size, series of single sex born, etc.

Carrying Capacity Definition? A summary of all factors regulating population sizes Density dependent Density independent Determinate Stochastic Site and species specific value

Density (In)Dependent Forces Special applications: Independent: Dinosaurs in the K-T boundary: dominance to extinction Dependent: Why killing coyotes causes trouble for their prey

Small Populations Usually at great risk Why?

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth

Types of Population Growth Exponential Unlimited, rapid growth Often called Malthusian Growth without bounds Logistic Growth within natural limits What sets that limit? What is the limit? More in a moment…

Exponential Population Growth Examples of this? Think close to home Often an unnatural occurrence Conditions under which this occurs naturally Introduced species Nutritionally enriched environments Cultural innovations?

Exponential Population Growth Equation Derivation Which measured population growth components can change? They are: Birth Death Immigration Emigration Relationship between these? No + B + I - D – E

Exponential Population Growth Equation Derivation The equation for population change over a unit t (time) DN / Dt = No + B + I - D – E Simplify the equation Assume a closed population Eliminate migration (I, E) DN / Dt = No + B - D Create a growth rate (r) = B-D DN / Dt = (r)(No) This is the basic exponential growth equation

Exponential Population Growth Equation - Implications DN / Dt = (r)(No) What can be experimentally changed here and how does our close-to-home example apply? Only r can change r in humans has been continually increasing with technology When r = 0, the population growth has stopped What is this timepoint called?

Population Ecology Why is this field important? What is a population? Why does a population change in size? Unlimited, exponential population growth Logistic population growth Exponential vs. Logistic growth

Logistic Population Growth What is added in this form of population growth? The Carrying Capacity is added What is it and what determines it? Typically summarized as K How would we modify the exponential population growth equation to reflect this?

Logistic Population Growth Equation Derivation Add the Carrying Capacity (K) – how? DN / Dt = (r)(No) Base Expon. Equation DN / Dt = (r)(No)(1-(N/K)) Base Logistic equation

Logistic Population Growth Equation - Implications DN / Dt = (r)(No)(1-(N/K)) Base Logistic equation Implications: As N ~ K, population increase stops Logistic is a special case of Exponential, when K = infinity

Growth Matters! How many humans can we expect? May be unlimited? What about implications of Ecological Footprint exercise? Currently 6 billion people Hotly contested

Growth Matters! r-selected species Why most weeds are weedy Edge species are typically r-selected Invasive species are often r-selected

Growth Matters! K-selected species Why we don’t get many species of oaks in most young forests? Climax communities Susceptible to habitat fragmentation

Assignment for Next week: Population Growth Models Learn more about the consequences of these models Instructions are all online, and available here Turn in at beginning of class next week We will discuss it then

Proximate Ecological Fields - Revisited Trends down pyramid: Increase in geographic scale From single species to multiple species Increasing number of ecological factors that may  be influential Decreasing certainty in results

Next Week: The Tour of Ecology Continues Population ecology Community ecology Next week’s emphasis Ecosystem ecology Conservation Issues