Introduction to Ecology Midlands State 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 Population interactions

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

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 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

Only minimal genetic flow, at most 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

Population Characteristics Population ecology is the study of interactions within populations (i.e., intraspecific interactions) Recall that populations are groups of interacting conspecifics (e.g., inter-mating) We can characterize individual populations in terms of there… Size (average vs. variation) Density (& impacts on size; density dependence) Patterns of Dispersion Demographics (age structure, sex ratios) Rates of growth (or decline) Limits on population growth Population Characteristics

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

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

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 e.g droughts, floods volcanoes, asteroids, fires etc 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

Population Dispersion

Why Different Types?

Age-Structure Pyramids cohort

(Cohort) Survivorship Curves

Reproduction Age of first reproduction (~sexual maturity) Clutch size Investment in individual progeny Tradeoff between reproduction and survival Number of reproductive episodes per lifetime Upside of Semelparity Upside of Iteroparity Reproduction

Semelparity

Iteroparity

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

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? Invasive alien species water hycinth 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?

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

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

r = 1.0 r = 0.5 Growth Without Limits r = population growth rate

New or Changing Environment (no competition / limits) Imposition of limits dN/dt = r  N  (K-N)/K Impact of Limits New or Changing Environment (no competition / limits)

Environmental Resistance K N N/K = “Environmental Resistance”

dN/dt is maximized when N*r is maximized Maximizing Yield dN/dt is maximized when N*r is maximized

Fitting Curves to Real Pops. Logistic growth model does not consider the effects of predators or interspecific competition, so fails to predict the complexities of the density of many natural populations as a function of time Nevertheless, it serves as good null hypothesis

Oscillations about K Time N K overshoot oscillations

r versus K Selection

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

Boom and then Bust r-like Water flee (Daphnia magna) is adapted to exploit new environment: high growth rate, resistant eggs produced before crash.

Boom and then really Bust Reindeer introduced to Pribilov island. Initial exponential growth, crash, complete extinction. r-like

Boom and sort of Bust K-like? r-like? Predators were removed from Kaibab plateau. Mule deer population size increased from 4,000 to hundred thousand, then dropped and stabilzed at 10,000. K-like? r-like?

Boom but not much Bust r & K-like Sheep introduced to Tasmania: rapid initial growth, overshoot, drop, fluctuation around carrying capacity. r & K-like

Boom & Bust & Boom & Bust & Boom & Bust Hare r tendencies kept under control by 1predation or by their food supply? The familiar 10-11 year hare-lynx cycle might not be true. Biased data. (http://www.behav.org/ecol/wildlife/w_06_populations.htm)

Density-Dependent Limits (to max = K) Competition increases

Density-Independent Factors (e.g., weather) Good Times! (in Australia)

Density-Dependent vs. Independent Limits

Question: Why are humans destroying the earth? destructamundo Question: Why are humans destroying the earth?

Limits: Paul Ehrlich and the Population Bomb Impact = Population * Affluence * Technology Impact = Population * Affluence * Efficiency Consumption per Baby = Resource * Efficiency (or afluence * technology) Consumption Damages Resources (a.k.a., the environment) There are only so many resources to use up!

Human Population Growth

Human Freedom Without Responsibility urban sprawl global warming deforestation desertification overpopulation air polution water polution loss of habitat overconsumption conspicuous consumption loss of farmland overfishing greenhouse effect ozone hole mass extinction greed loss of wetlands NIMBY = “not in my backyard” lack of cooperation out-of-control materialism special interests destruction TEOTWAWKI = “the end of the world as we know it” radical anti-environmentalism might makes right short-term thinking fish kills toxic algal blooms erosion loss of topsoil bigger is better monoculture pesticides the bottom line Who’s going to stop me? It’s a free country I’ve got my rights! = Destructamundo! (destruction of environment)

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 Population Community Ecosystem

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