Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 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 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 • • • • Predation Parasitism Pollinators Competition – Between species • 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 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 Growth Without Limits r = 1.0 r = 0.5 r = population growth rate Impact of Limits Imposition of limits dN/dt = r N (K-N)/K New or Changing Environment (no competition / limits) Environmental Resistance K N N/K = “Environmental Resistance” 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 overshoot K N oscillations Time 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 rselected 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 r-like Reindeer introduced to Pribilov island. Initial exponential growth, crash, 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 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 destructamundo destructamundo destructamundo 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 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! out-of-control materialism loss of farmland short-term thinking bigger is better might makes right global warming deforestation loss of topsoil overconsumption overpopulation = Destructamundo! (destruction of environment) conspicuous consumption special interests urban sprawl fish kills loss of wetlands greenhouse effect monoculture desertification erosion ozone hole water polution mass extinction lack of cooperation Proximate Ecological Fields - Revisited • Trends down pyramid: – Increase in geographic scale Population – From single species to multiple species Community – Increasing number of ecological factors that may be influential Ecosystem – Decreasing certainty in results Next Week: The Tour of Ecology Continues • Population ecology • Community ecology – Next week’s emphasis • Ecosystem ecology • Conservation Issues