Lecture 16: Population Ecology PDF

Lecture 16: Population Ecology  Today Overview of populations Population growth – Exponential growth – Logistic growth – Other growth patterns Life history Human population growth  Population Ecology Population – a group of interacting in...

Lecture 16: Population Ecology  Today Overview of populations Population growth – Exponential growth – Logistic growth – Other growth patterns Life history Human population growth  Population Ecology Population – a group of interacting individuals of the same species living in the same area  Population Ecology What determines the distribution and abundance of individuals in a population? Many factors affect population characteristics – Physical environment – Biotic interactions – Species life history  Population Measures Size – number of individuals in a population – Often hard to measure Density – number of individuals per unit area  Population Measures Dispersion – how individuals are spaced within a population Clumped Random Uniform  Which of these do you think is most common in nature? Why? Population Measures Dispersion determined by: Dispersal Interactions among individuals Distribution of resources  Population Measures At large scales, dispersion pattern is almost always clumped  Population Measures Complete counts of populations are difficult Samples of populations used to estimate whole – Plot sampling – Mark-recapture  Population Measures Plot sampling – individuals counted within a set area  What kinds of organisms does this work for?  Population Measures Mark-recapture – animals captured and tagged, released, then recaptured later – Ratio of marked to unmarked individuals used to estimate population size  What kinds of organisms does this work for?  Population Growth Population size changes as a result of: – Birth – Death – Immigration – Emigration Two common population growth models: – Exponential growth (J-shaped curve) – Logistic growth (S-shaped curve)  Exponential Growth Exponential growth – population multiplies by a constant factor per unit time G = population growth G  rN r = per capita growth rate N = population size – r assumed constant over time – Growth depends solely on population size  Life History r-selected species: – Short life span – Rapid development – Early maturation – Low parental investment – High reproductive rate  Exponential Growth Population size increases rapidly Old Population Size:  Exponential Growth Population growth rates vary greatly across species r = 24 r = 1.9 r = 1017 Doubling time = 30 min r = 105,274  Exponential Growth Under ideal conditions, populations grow exponentially Reindeer reintroduced to island of St. Paul, Alaska, 1911 2,046 individuals 25 individuals  Exponential Growth But there are usually limits to population growth – E.g., weather conditions, overuse of resources 2,046 individuals 25 individuals 8 individuals  Exponential Growth Can only occur if: – Birth and death rates are constant – Environment is constant – Resources are unlimited Long periods of exponential growth are unrealistic – Environmental conditions change – Higher density creates competition Dfgvghvhjhbhc  Logistic Growth Logistic growth – population growth slows over time due to limiting factors G = population growth KN r = per capita growth rate G  rN   N = population size  K  K = carrying capacity – Carrying capacity (K) – maximum population size that environment can support  Logistic Growth Growth depends on how close population is to the carrying capacity  Logistic Growth Population increases rapidly, then stabilizes at carrying capacity Acorn barnacle  Logistic Growth Comparison of logistic and exponential growth  Factors Limiting Population Growth Density-independent factors – affect population regardless of its size  Examples? Density-dependent factors – affect population differently depending on population density  Examples? – These cause logistic growth  Factors Limiting Population Growth Density-dependent factors regulate population size – If density increases, birth rates decrease – Competition for resources  Factors Limiting Population Growth Density-dependent factors regulate population size – If density increases, death rates increase  Other Growth Patterns Density-independent factors may limit populations before they reach carrying capacity Exponential Sudden growth decline Seasonal growth in aphids  Other Growth Patterns Many natural populations fluctuate erratically – May reflect environmental changes – E.g., nutrient supplies, temperature Phytoplankton in Lake Erie  Other Growth Patterns Population cycles – Population rises and falls at regular intervals – Many causes, including predator-prey interactions  Life History All organisms grow, reproduce, and die When and how can vary  Life History Life tables describe age-specific survival and reproductive rates Life History  Note age- specific patterns  Life History Survivorship curve – plot of the numbers of individuals surviving to reach different ages  Life History Type I survivorship curve – Most individuals survive to old age Dall mountain sheep Ovis dalli  Life History Type II survivorship curve – Chance of surviving remains constant Song thrush Turdus philomelos  Life History Type III survivorship curve – High death rates for young Desert shrub Cleome droserifolia  Life History Survivorship curves can vary across populations, cohorts, or sexes  What does the graph show? Why might those patterns exist?  Life History Reproductive strategies: – r-selection – live fast, die young r = per capita growth rate High population growth rates Unpredictable environments – K-selection – slow and steady K = carrying capacity High competitive quality Stable environments  Life History K-selected species: – Long life span – Slow development – Late maturation – High parental investment – Low reproductive rate  Age Structure Age structure – proportion of the population in different age groups Afghanistan United States Italy  Age Structure Age structure influences population growth rate  Which population will have a higher growth rate? Why? Human Population Growth  Human Population Growth Human populations have greatly exceeded exponential growth rates – Technological advances Agriculture Medicine Sanitation – Globalization and migration  Human Population Growth World population growth rates now declining Current growth rate = 1.1% per year Predicted population size in 2050 = 9.1 billion  Human Population Growth In 2080, there will be ~10 billion people on Earth Is 10 billion above the carrying capacity for the human population? – Depends on amount of resources used by each person – If everyone used resources at the rate of: U.S. citizens in 2007 K = 1.5 billion people Indian citizens in 2007 K > 13 billion people  Study Questions What is a population? How do we measure populations? What is population dispersion? What are the types of dispersion patterns, and why does each occur? Which is most common? What is exponential growth? How does the population growth rate change over time in exponential growth? What pattern does it produce? Draw the graph. What is logistic growth? How does the population growth rate change over time in logistic growth? What pattern does it produce? Draw the graph. What is a carrying capacity? Under what conditions do exponential growth and logistic growth occur? Which is more common in nature, and why? What are density-independent and density-dependent factors?  Study Questions What is life history? What is a survivorship curve? What are Type I, Type II, and Type III survivorship patterns, and what sorts of organisms show each? What characteristics do r-selected and K-selected species have? What types of species fall into each category? Describe how population age structure relates to population growth.