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Population Ecology Professor Andrea Garrison Biology 3A Illustrations ©2014 Cengage Learning unless otherwise noted Population Dynamics • Patterns of change seen in a population over time and from place to place – Why does a population grow exponentially in some areas and remain stable in others – How do interactions between biotic and abiotic factors influence population characteristics – How do populations respond over generations to their interactions with the biotic and abiotic environment Population Ecology 2 Population Dynamics • Many population characteristics help ecologists understand the dynamics of a given population Population Ecology 3 Population Characteristics • Geographic range – Often determined by climate • Many species limited by temperature and/or rainfall • Genetic variability often plays large role in determining the extremes of climate a species can survive – Only a few individuals live at the extreme boundaries of the range Population Biology; picture: http://www.allaboutbirds.org/guide/California_Towhee/id 4 Population Characteristics • Habitat – Every species is found in specific habitat(s) which provide it with required resources (food, nesting space, etc) – Habitats differ for each species – California Towhee requires chaparral and other tangled, shrubby, dry habitats within its range Population Ecology; photo by Jamie Chavez, https://creativecommons.org/licenses/by-nc/2.0/ 5 Population Characteristics • Population size = #individuals in the population • Population density = #individuals/volume (or area) of habitat – Measure of resources available for each individual – Use to monitor and manage populations (esp. endangered species) • Less dense populations have more resources • Less dense populations may have trouble finding a mate Population Ecology 6 Population Characteristics • Spatial dispersion (distribution) – Random dispersion pattern • Unpredictable distribution • Environmental conditions don’t vary within habitat • Individual not attracted to or repelled by other individuals of same species • Some spiders, rainforest trees, burrowing clams Population Ecology 7 Population Characteristics • Spatial dispersion (distribution) – Clumped dispersion pattern • Most common dispersion pattern • Individuals grouped together • Resources often patchy – EX: Plant growth in area around cowpats • Social animals cluster • Plants and animals often cluster because of reproductive patterns – Asexually reproducing plants and animals create clones » EX: sea anemones create cluster of clones – Reproduction via seeds, eggs, larvae that lack dispersal mechanisms » Young grow near parents Population Ecology 8 Clumped dispersion Anemones closed, bits of shell on outside Anemones open under water 2 anemone clones w/ anemone-free zone due to fighting Population Ecology 9 Population Characteristics • Spatial dispersion (distribution) – Uniform dispersion pattern • Individuals of a species may repel each other – EX: creosote bushes produce toxins so less competition for water » Territorial behavior to defend space/resources produce uniform dispersion pattern Population Ecology 10 Population Characteristics • Population growth rate – Determined by age structure, generation time and sex ratio Population Ecology 11 Population Characteristics • Age structure – Break down of population into age groups based on reproductive capabilities Population Ecology 12 Population Characteristics • Generation time – Time required to reach sexual maturity • Sex ratio – Relative proportions of males and females – #females usually has bigger impact than #males • Females actually produce the offspring • Males often mate with >1 female • If pairs bond for life, then #males is important Population Ecology 13 Population Dynamics • The demography of a population helps ecologists understand the dynamics of a given population • Demography = statistical study of the processes that change a population’s size and density over time • Demographic analysis allows prediction of future population growth patterns – Allows federal and local govts to anticipate needs for school, water and sewage lines, etc – Allows conservation ecologists to develop plans to protect endangered species Population Ecology 14 Demography • Age structure, generation time and sex ratio contribute to demographic understanding of a population Population Ecology 15 Demography • Life tables summarize demographic characteristics of a population – Similar to mortality charts for life insurance companies – Look at several aspects of population • Age structure of population (#individuals within each age group) • What percentage of each age group in a population survives to the next age group • Average #offspring produced by each female in age group Population Ecology 16 Life Table Population Ecology 17 Demography • Survivorship Curves reflect info in life tables – Survivorship data roughly match one of three types of curves Population Ecology 18 Demography • Type I curves – Most individuals survive until late in life – High mortality late in life – EX: Dall’s sheep have only 1-2 offspring and nurture them through high-risk first year Population Ecology 19 Demography • Type II curves – Mortality relatively constant throughout life • Predation • Disease • Starvation – Common among lizards, songbirds, small mammals Population Ecology 20 Demography • Type III curves – High juvenile mortality – Lower mortality once individuals reach critical age/size – Common among organisms that broadcast gametes, seeds (plants, marine inverts, fish) • Plants produce many seeds • Only few survive to germinate • Less than ½ of those become established – Good location – Sturdy enough to avoid predation Population Ecology 21 Population Dynamics • Life history characteristics govern fluctuations in population size over time – Characteristics vary for different organisms • r-selected species are opportunists – Take advantage of a situation when it arise – Type III survivorship • K-selected species equilibrium based – Require more stable environment – Types I and II survivorship Population Ecology 22 Population Dynamics • Population growth models help ecologists understand the demography of any given population Population Ecology 23 Population Growth Models • Geometric growth – Occurs among organisms that reproduce via binary fission • Bacteria, protists – Parent cell divides in two, no longer exists Population Ecology 24 Population Growth Models • Exponential growth – Occurs among organisms that reproduce offspring, then live alongside their offspring for period of time • Most plants and animals • We will track growth characteristics of these organisms Population Ecology 25 Population Growth • Births increase population size • Deaths decrease population size • Population growth = births – deaths N = population size t = time B = births D = deaths • ∆N/∆t = B – D over the time span Population Biology 26 Population Growth • ∆N/∆t = B – D over the time span • Ecologists use per capita numbers – Averaged over entire population – Give an average number per individual – B/N = b; D/N = d • Therefore, per capita ∆N/∆t = b – d • Per capita ∆N/∆t = r (per capita growth rate) • r = (b – d) Population Biology 27 Population Growth • Per capita growth rate r = b – d If b > d, then r positive and population is growing If b < d, r negative and population declining If b = d, r = 0 and population stable Population Biology 28 Population Growth • r=b–d r N = 0.4 X 1000 = 400 new mice • Population of mice N = 1000 500 births this month 100 deaths this month b = 500/1000 = 0.5 d = 100/1000 = 0.1 r = (0.5 – 0.1) = 0.4 = per capita growth rate N = 1000 + 400 = 1400 mice at end of month The 2nd month 0.4 X 1400 = 560 new mice N = 1400 + 560 = 1960 mice at end of month Population Biology 29 Population Growth • Intrinsic rate of increase = rmax = rate at which a population will increase if there are no limits on its growth rate – Important aspect of a population – Varies with reproductive strategy • r-selected species have higher intrinsic rates of increase Population Biology 30 Exponential Growth Pattern over time • Exponential growth – Early periods of population growth – Plenty of resources – Rmax is achieved Population Biology; Illustration ©2010 Pearson Education, Inc. 31 Population Growth Pattern over time • Logistic growth – Later stages of population growth – Resources become limiting – growth slows as it approaches maximum population the environment can sustain (Rmax is no longer achieved) Population Biology; Illustration ©2010 Pearson Education, Inc. 32 Population Growth • Carrying capacity = maximum population the environment can sustain indefinitely Population Biology; Illustration ©2010 Pearson Education, Inc. 33 Population Growth • Some populations overshoot the carrying capacity more rapidly than others Population Biology; Illustration ©2010 Pearson Education, Inc. 34 Population Growth • When population overshoots, it will usually decline and hover around carrying capacity Population Biology; modified by A. Garrison 35 Population Growth • Carrying capacity is determined by densitydependent limiting factors – Resources whose availability decreases as pop density increases • Food, water, space – Impact shown at A -- rate of pop increase slows until pop size is stable Population Biology; Illustration ©2010 Pearson Education, Inc. 36 Density-Dependent Regulation of Population Growth Population Biology; Illustration ©2010 Pearson Education, Inc. 37 Density-Dependent Regulation of Population Growth • Density-dependent factors may also increase death rates – Disease transmission – Accumulation of wastes – Predation due to greater availability • All these factors (& others) cause slowing and leveling off of population growth Population Biology 38 Density-Independent Factors • Distribution is determined by densityindependent factors – Factors unrelated to population density which limit the presence of the species – Weather, soil type, other necessary biotic or abiotic factors Population Biology 39 Density-Independent Factors • Geographic range – Often determined by climate • Genetic variability often plays large role in determining the extremes of climate a species can survive – Only a few individuals live at the extreme boundaries of the range • Global climate change is increasing impact of density-independent factor – Ranges changing for many species – Altitude limitations changing within geographic ranges • Moving upwards as climate warms Population Biology 40 Density-Independent Factors • Distribution within the geographic range – Abiotic factors • You don’t find worms living on rocks! Or fish living on land! – Soil factors (another abiotic factor) • Nutrients present in soil play large factor in determining location of plant species within their range – Deciduous plant require more nutrients than evergreens – Biotic factors • Other species may play role in distribution of a species within its range – Herbivores cannot live where their plant food source doesn’t grow – Predators cannot live where their prey are not found – Plants cannot live where their pollinators are not found Population Biology 41 Population Cycles • Some species show cyclical patterns of population increases and decreases – Arctic rodents have 4 year cycle – Snowshoe hares, ruffed grouse, lynx have 10 years cycles • Uncertain of reasons • Hypotheses – Intrinsic control • Physiological, hormonal changes foster dispersal – ↑ aggression – ↓reproduction – Extrinsic control • Cycling of predators or prey – Dense population may exhaust food supply » ↑ mortality, ↓ population size – Supplementing food supply doesn’t balance this, however Population Ecology 42 Human Population Population Ecology 43 Human Population • Just starting to slow down it’s increase • Hasn’t reached carrying capacity – Why not? Population Ecology 44 Human Population • Human population also expanded its range from tropics and subtropics throughout most of the earth • Every population has it’s own niche – All resources used by that population and the manner in which they are used • Technology allows human population to constantly change it’s niche – Agriculture: used food sources differently – Used different food sources – Improved health care ↓ effects of density-dependent limiting factors – Allows humans population to change its carrying capacity • At least for a while Population Ecology 45 THE END Population Ecology 46 Predator-Prey Interactions • Density-dependent growth factors – Predator-prey model – Predator pop changes lag behind prey population changes Population Ecology 47 Predator-Prey Interactions • In reality, the Canada lynx population doesn’t lag behind – Further studies show that population cycles of hare are related to complex interactions between the hare, its food sources and its predators Population Ecology 48