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Basics in population ecology

Basics in population ecology. It is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change. Charles Darwin. Our program Simple growth processes Outbreaks Age structured populations Harvesting and viability analysis

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Basics in population ecology

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  1. Basics in population ecology It is not the strongest of the species that survives, nor the most intelligent, but the one most responsive to change. Charles Darwin.

  2. Our program • Simple growthprocesses • Outbreaks • Age structuredpopulations • Harvestingand viabilityanalysis • Competition , predation and parasitism • Populations in space: Metapopulation and spatial dynamics • Populations in space: Metapopulation and spatial dynamics

  3. Literature

  4. What is a population? A population is a group of potentially interbreeding individuals of the same species living in the same area at the same time and sharing a common gene pool. Population ecology is a sub-field of ecology that deals with the dynamics of species populations and how these populations interact with the environment. It is the study of how the population sizes of species living together in groups change over time and space. Carabus coriaceus in a forest Carabidae in a forest Basic characteristics of populations: Absolute density (individuals per unit area) Relative density (Proportion of individuals with respect to some standard) Abundance (size; total number of individuals) Age structure (triggered by natality and age dependent mortality) Dispersal (spatial dynamics)

  5. Main axiom of population ecology: Organisms in a population are ecologically equivalent. Ecological equivalency means: Organisms undergo the same life-cycle Organisms in a particular stage of the life-cycle are involved in the same set of ecological processes The rates of these processes (or the probabilities of ecological events) are basically the same if organisms are put into the same environment (however some individual variation may be allowed)

  6. Sometimesspecies of differentspeciesinterbred. These do not form a population per definition In Sulawesisevenspecies of macaques (Macacaspp.) interbreedwheretheirhomerangesoverlap. Interbreedinis the cause of endangerment of Macacanigra. Adapted from Riley (2010) The endemicseven: fourdecades of researchonthSulawesiMacaques. Evol. Anthr. 19: 22.

  7. Spatially separated individuals do not form true populations Raven (Corvus corax) A species occurring on four islands that are isolated is divided into four independently evolving populations. Ravens in different continents do not form a single population. There is no (or only limited) gene flow. Due to limited gene flow populations on two islands might be considerd as foring a single genet ically structured populations

  8. Temporary separated individuals do not form populations Mikiola fagi Omphale lugens Macrotera arcuata Number of bees hatching from eggs N N Spring Summer Spring Summer 2 3 0 1 Summer Eggs Hatching year Spring and summer generations have only limited overlap and thus form partly separated populations. Overlaying is connected with host change. M. fagi is univoltine. Overlaying is astrategy to reduce risk due to unfavourable conditions. If overlaying is genetically fixed the genotypes of the three hatching cohorts never meet.

  9. Life cycles Man is iteroparous North atlantic salmon is semelparous Iteroparous populations are of age structured with each age cohorte having a different reproductive output. • Important questions: • What is the population rate of growth or decline? • To what factor is the population growth • rate most responsive? • Will the population eventually go extinct? • What happened to the population in the • past?

  10. Differences in life history Semelparous species reproduce only once and can be described by simple growth models Egg Egg Larva 1 Juvenile Iteroparous species reproduce at least two times and might form age structured populations Larva n Adults 1 Fertility = number of eggs per female Adult Adult n Fertility = number of eggs per female Some species have age cohorts after the reproductive phase Senex Why grandparents?

  11. Somebasicdefinitions Femalesonly Total fertility rate (TFR) isthe total number of children a femalewould bear during her lifetime. Gross Reproduction Rate (GRR) isthe potentialaveragenumber of femaleoffspringper female. Net Reproduction Rate (NRR) is the observedaveragenumber of femaleoffspringper female. NRR is always lower than GRR. When NRR is less than one, each generation is smaller than the previous one. When NRR is greater than 1 each generation is larger than the one before. In semelparousspeciesagespecificfertility(ASF) is the averagenumber of offspring per female of a certainageclass. Males and females Population growth is the change in populationsizeover time. Growthcan be negative. Population growthrateis the multiplicationfactorthatdescribes the magnitude of populationgrowth. Growthrateisalwayspositive.

  12. Fertilityversus populationgrowthrate Bacterialgrowth Animalgrowth Males Females R describes the populationgrowthrate R describes the net reproductionrate R is the averagenumber of daughters of eachfemale in the population In demographicanalysisonlyfemalesarecounted. The number of females in reproductiveageiscalled the effectivepopulationsize. Netrefers to the number of daughters, whichreachreproductiveage.

  13. Birth and death dynamics Discrete population growth Natality A population growth process considers four basic variables (BIDE model) B: number of births D: number of deathsI: number of immigrations E: number of emigration N Emigration Immigration Mortality I, E = 0 ) - The population increases if Rt > 1. R: fundamental net population growth rate The population decreases if Rt < 1. - The population increases if rt > 0. The population decreases if rt < 0. r: intrinsic rate of population change

  14. Simple population growth processes Discrete growth model • The growth model has only one free parameter: • R: fundamental net growth rate • The model is simple. • The model parameter has a clear and logical ecological interpretation. • The parameter r can be estimated from field data. Change equation Difference equation Recurrence functions Ratio equation

  15. Recurrence functions Leonardo Pisano (Fibonacci; 1170-1250) developed this model to describe the growth of rabbit populations. Fibonacci series 1=1+0 2=1+1 3=2+1 5=3+2 8=5+3 13=8+5  This is the first model in population ecology. Assume a couple of immortal rabbits that five birth to a second couple every month. Start 1 1. month 1 2. month 2 3. month 3 4. month 5

  16. N The discrete form of the exponential growth model Exponental growth is a very fast increase in population size. R: fundamental net population growth rate Basic reproductive rate N0 Intrinsic rate of increase per unit of time t Whooping crane (Grus americana) population in North America after protection in 1940 Scots pine (Pinussylvestris) population in Great Britain after introduction (7500 BC) www.whoopingcrane.com

  17. The Human population growth Human growth was hyperexponential until about 1970. Net growth rate was not constant but increase until about 1970 Since 1970 net growth rate declined

  18. Continuous population growth Exponential growth model If r > 0: population increases If r < 0: population decreases Intrinsic rate of increase In the lack of resource limitation a population will exponentially grow. In this case population grows is density independent. N ln N ln N0 a tan a = (r-1) a tan a = (r-1)t N0 t0 t t

  19. Logistic growth Discrete logistic growth N K The Pearl – Verhulst model of logistic population growth K/2 t0 t1/2 t Continuous logistic growth Solution to thisdifferentialequation

  20. The logistic growth model has only two free parameters: • r: net reproductive rate • K: the carrying capacity. • The model is simple. • The model parameters have a clear and logical ecological interpretations. • The parameters can be estimated from field data. • The model does not refer to a specific group of species, but applies to all populations from Bacteria to vertebrates amd plants. • The model is based on realistic assumptions about population growth. • The model is sufficiently precise. • Constraints: • The model refers to homogeneous environments. • Reproductive rates are supposed to be constant. • Carrying capacity is supposed to be constant. • Generations do not overtlap. Limitation: The model is symmetrical around the point of inflection.

  21. The discrete version of logistic growth The logistic growth function is a discrete recursive model r = -0.05 K = 500 r = 0.1 K = 500

  22. r = 1 K = 500 r = 2.099 K = 500 Density dependent populationregulation Stablecycling

  23. r = 1.95 K = 500 r = 2.70 K = 500 Pseudochaos r = 2.87 K = 500 r = 2.85 K = 500

  24. r = 3.01 K = 500 • High reproductive rates imply: • high population fluctuations • pseudochatotic population size • no density dependent population regulation Local extinction Pseudochaos does not mean that population size is unpredictable. Very simple determinstic processes might cause pseudochaos. r-strategists often have pseudochaotic population fluctuations. A random walk is a pure stochastic process that causes unpredictable population sizes.

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