Population Ecology Population – n. a group of individuals of a single species that occupies the same general area.

How are Populations Measured and Distributed? Is it possible to count EVERY individual in a population? Scientists perform “mark and recapture” experiments to estimate population sizes Ex. “Something’s Fishy” Randomly No pattern at all Clumped Groups of pop. concentrations Uniform Evenly spread out or spaced in the environment

Population Dispersion

Why Different Types?

How Do Populations Grow? There are several mathematical models… Exponential growth model – the rate of expansion of a population under “ideal” conditions Population-limiting factors – hunting, amount of space suitable for breeding, restricted population growth, food availability, etc. Logistic growth model – idealized population; growth slowed by limiting factors as the population size increases Carrying capacity – the maximum population size that an environment can support at a particular time, without degradation of the habitat

Exponential growth of bacteria

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

What if it stops growing What if it stops growing? Logistic growth, compared to exponential growth K = carrying capacity

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

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)

Growth of a population of fur seals (logistic growth model)

What does the logistic growth model suggest about real populations in nature? A population’s growth rate will be small when the population size is either small or large and highest when the population is at an intermediate level relative to the carrying capacity. Limiting factors make the birth rate decrease, the death rate increase, or both Eventually the population will stabilize at the carrying capacity, when the birth rate equals the death rate (These are mathematical models and no wild population fits either model perfectly!)

Some factors that limit population growth As density of song sparrows increase, the number of eggs laid decreases because of food shortages Plants grown under crowded conditions tend to be smaller and less likely to survive Disease transmission or accumulation of toxic waste products can increase mortality

Continued…… A predator may capture more of a particular kind of prey as the prey becomes abundant White-footed mice stop reproducing at a colony size of 30-40 even when food and shelter are provided. Stress? The graph shows aphids which feed on the phloem sap of plants; increase in population in the summer and then die-off in the fall and winter

Continued…. Some populations remain fairly stable in size close to carrying capacity Most populations fluctuate as seen at the left This graph shows song sparrow populations, with periodic catastrophic reductions due to severe winter weather

Boom and bust cycles Hare cycles may be caused by increasing food shortages during winter caused by overgrazing They may be due to predator-prey interactions Cycles could be affected by a combination of food resource limitation and excessive predation Predators reproduce more slowly than their prey so they always lag behind prey in population growth.

Why Does Population Size Change? Density Independent Forces Forces that are at work irrespective of the population density Doesn’t matter how many individuals there are in an area: ALL are affected the same way Density Dependent Forces Forces that vacillate depending on the population density May be caused by either low or high densities

Density Independent Forces Examples Climate Topography Latitude Altitude Rainfall Sunlight In Sum: Abiotic factors Exceptions do exist!

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

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

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

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 (ex. passenger Pigeon), series of single sex born (ex. Alligators).

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 don’t we 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, then 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 predation 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)

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 time-point called?

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 becomes approx. equal to K, population stops increasing Logistic is a special case of Exponential, when K = infinity

Large Variation in Pop. Size

Life tables – compiled by life insurance agents

Survivorship curves Type I curve – parents produce few offspring and give them good care Type III curve – high death rates for infants then a period when death rates are lower for those who survive to a certain age

Exponential growth of the human population Throughout human history parents had many children but only two on average survived to adulthood Estimates that by 2025 the world will have to double food production, 2/3 of the available fresh water on earth will be in use, 60,000 plant species will be lost to support the population Issues: overgrazing, rivers running dry, decrease in groundwater, energy?

Human Population Growth

Population Density

Human carrying capacity estimates Ecological footprint (food, fuel & water consumption, housing size, and waste production) Calculates current demand on world resources by each country, in hectares of land per person World ecological capacity is approx. 1.7 ha per person alive in 1997

How to achieve population stability? Zero population growth – when birth rates equal death rates Two ways to reach ZPG. High birth and death rates or low birth and death rates. Demographic transition is moving from the first to the second. Most developed countries have made the transition See the demographic transition in Mexico at the left.

Question: Why do locusts destroy crops? nymph aggregating nymphs adults feeding swarming destructamundo

Limits: Locust Freedom Without Responsibility I’ve got my rights! It’s a free country! Who’s going to stop me? = Destroyed Crops (destruction of environment)

Question: Why do some microbes make us sick? ©Phage et al. Question: Why do some microbes make us sick?

Limits: Freedom Without Responsibility It’s a free country Who’s going to stop me? I’ve got my rights! = Disease! (destruction of the body environment) Bacterial pathogens

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)