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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 Growth Without Limits r = 1.0 r = 0.5 r = population growth rate What if it stops growing? Logistic growth, compared to exponential growth K = carrying capacity Maximizing Yield dN/dt is maximized when N*r is maximized Impact of Limits Imposition of limits dN/dt = r N (K-N)/K 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 r-like Reindeer introduced to Pribilov island. Initial exponential growth, crash, then complete extinction. 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. Boom but not much Bust r & K-like Sheep introduced to Tasmania: rapid initial growth, overshoot, drop, fluctuation around carrying capacity. 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 swarming adults feeding 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) ©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! Bacterial pathogens (destruction of the body environment) Human Freedom Without Responsibility lack of cooperation destruction TEOTWAWKI = “the end of the world as we know it” air polution Who’s going to stop me? the bottom line greed It’s a free country NIMBY pesticides = “not in my backyard” loss of habitat toxic algal blooms radical anti-environmentalism overfishing I’ve got my rights! loss of farmland short-term thinking bigger is better might makes right global warming deforestation overconsumption fish kills loss of wetlands out-of-control materialism loss of topsoil urban sprawl overpopulation = Destructamundo! (destruction of environment) conspicuous consumption special interests greenhouse effect monoculture desertification erosion ozone hole water polution mass extinction lack of cooperation