Introduction to Ecology

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

Population Ecology Goals for the day • 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 of your own design: • Work through the 11 steps of the scientific method • What did you do?

Scientific Method - Steps 1-5 • Observe or suspect pattern • Posit cause or significance of observed difference • Create answerable question to explain pattern • Create testable hypotheses • Null (Ho ) and alternate hypotheses (Ha) • Design experiment

Scientific Method - Steps 6-11 • Collect data (descriptive stage) • Analyze data, primarily using statistics • Evaluate hypotheses, reject Ho? • Make conclusions based on data • Note problems in current work • Predict future directions for research

Population Ecology Goals for the day • 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

What is a Population? • Components? • Definition : • One species • One area • Isolated from other areas • Able to interbreed • Example: Only minimal genetic flow, at most

What is a Metapopulation? • Components? • Definition : • One species • Multiple areas • Isolated from other areas, further away • Able to interbreed • Example: Only minimal genetic flow, at most

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

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 • Exceptions do exist!

Density Dependent Forces • Types? • Examples • Within species • Breeding spaces • Food • Mates • Foraging spots • Between species • Predation • Parasitism • Pollinators • Competition • In Sum: Biotic factors • Exceptions do exist!

Indeterminate Factors • Most influences are 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.

Small Populations • Usually at great risk • Why? • Small population size • Small genetic diversity • Highly susceptible to stochastic forces • Poor competitors with resident biota • Severely limited adaptability

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) • N / t = No + B + I - D – E • Simplify the equation • Assume a closed population • Eliminate migration (I, E) • N / t = No + B - D • Create a growth rate (r) = (B-D)/t • N / t = (r)(No) • This is the basic exponential growth equation

Exponential Population Growth Equation - Implications • N / t = (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?

Carrying Capacity – Unique to Logistic Growth • Definition? • A summary of all factors regulating population sizes • Density dependent • Density independent • Determinate • Stochastic • Site and species specific value

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? • N / t = (r)(No) • Base Expon. Equation • N / t = (r)(No)(1-(N/K)) • Base Logistic equation • (1-(N/K)) is the unoccupied portion of the carrying capacity

Logistic Population Growth Equation - Implications • N / t = (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 geographicscale • From single species to multiple species • Increasing number of ecological factors that may be influential • Decreasing certainty in results Population Community Ecosystem

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

Introduction to Ecology