Ecology - Ch53

Definitions

• Population - a group of individuals in a single species that live in the same area

• (Population) Density - number of individuals per unit of area/volume

• (Population) Dispersion - the spacing pattern among individuals within a population

• Immigration - influx of individuals from other areas

• Demography - a study of the key characteristics of a population (births, deaths, migrations, etc.) and how they change over time

• Life History - traits that impact an organisms reproduction/survival

• Semelparity - A single reproduction before death

• Iteroparity - Organisms that undergo multiple reproductive cycles in its lifetime

• Carrying Capacity - The maximum population size that an environment can sustain

• Density Independent - Birth/Death rate do not change with population density

• Density Dependent - A death/birth rate that changes with population density

Notes

33.1-2

• Population Density (PDen)

  • Can be determined through counting, estimating through an indicator (i.e. number of nests, droppings), calculating average density of a smaller plot and multiplying across habitat

    • Also may use mark-recapture method to estimate size of population

      • Marking a sample of a population & releasing, wait for reintegration with rest of population, sample again.

        • Number of marked animals in second sample = x

        • Total number of animals in second sample = n

        • Number of individuals marked during first sample = s

        • Estimated population size =N

      • x/n = s/N, N = sn/x

  • PDen fluctuates over time (deaths, births, etc.)

    • Increases: Immigration and births

    • Decreases: Emigration and deaths

• Population Dispersion (PDisp)

  • Common Patterns: clumped, uniform, random

    • Clumped: Resources gathered in an area. i.e.) starfish

    • Uniform: Even spacing due to territorial instincts. i.e.) nesting penguins

    • Random: Even distribution of resources. i.e.) dandelions

• Demographics

  • Summarized using a life-table - summarizes reproductive rates and survival of individuals at specific age ranges. Typically done using cohorts (using a sample of individuals that are all the same age until all of them are dead)

  • Typically focus on females (males don’t produce new offspring)

  • Survivorship curve: A representation of the survival rate from a life table. Most species have curves between 3 types or show more complex patterns

    • Type 1: relatively flat curve - represents fewer deaths in early/middle life then sharply drops as age increases.

      • Species that represent this curve: humans

    • Type 3: a sharp drop at the beginning symbolizing high death rates in young individuals then flattens as population ages

      • Usually associated with with very large number of offspring but provide little care (fish, some plants)

    • Type 2: a constant death rate as the population ages (constant slope)

      • Usually seen in rodents, lizards

• Exponential Model

  • Calculating changes in population size:

    • ∆population size = births + immigrants - deaths - emigrants

      • N = population size, t = time, B = births, D = deaths

      • Formula simplified to not account for immigration and emigration

    • ∆N/∆t = B - D

      • R = difference in births and deaths

    • ∆N/∆t = R

  • On a per capita basis:

    • Per capita change in population size = r_∆t

    • r_∆t * N → used to determine how many individuals are added/subtracted from a population each year

    • ∴ R = r_∆t * N

      • ∆N/∆t = r_∆t * N → used for calculations over a specific period of time (ie a year)

    • Changes at each instant in time:

      • dN/dt = rN

        • r = per capita change in a population at each instant in time

• Exponential Growth

  • When a population experiences ideal conditions to increase in size by a constant proportion at each instant in time

    • dN/dt = rN

    • Graph looks like physic d-t graph when accelerating

33.3-6

• Carrying Capacity (symbol K)

  • Varies depending over space, time w/ limited resources

  • i.e.) bats with a higher K have abundant flying insects/roosting sites

  • Limiting Factors:

    • Energy

    • Shelter

    • Refuge from predators

    • Nutrient availability

    • Water

    • Sustainable nesting sights

• Logistic Growth Model (LGM) /Logistic Population Growth (LPG)

  • Adjustment of exponential growth model so that r decreases as N increases

  • LPG: per capita rate of pop. growth approaches 0 as population size nears carrying capacity

    • K - N = number of addition indv. that the environment can support

    • (K - N)/K = fraction of K that is still available for population growth

    • dN/dt = rN * ((K - N)/2) = population growth rate

      • if N < K, (K - N)/K is close to 1

        • Per capita rate of population growth will be close to, but, less than r (r = the rate of increase seen in exponential population growth)

      • if N is large + resources are limited, (K - N)/K is close to 0

        • Per capita rate of pop growth is small

      • if N = K, no growth

  • LPG graph crease S-shaped curve

  • 

• Life History

  • The traits that impact an organism’s schedule of reproduction/survival. Evolutionary outcomes reflected in development, physiology, and behavior

  • 3 Main Components:

    • When reproduction begins (first age of reproduction/maturity)

      • i.e.) loggerhead turtle ~ 30yo to lay eggs for first time. coho salmon ~ 3-4yo

    • How often reproduction occurs

      • Semelparity vs Iteroparity

    • How many offspring produced per reproductive episode

      • How many are going to survive? Selective pressures influence number/size of offspring. If small chances of offspring surviving, many produced (plants)

  • Semelparity Advantages/Disadvantages:

    • Organisms that experience a single reproductive cycle in their lifetime

    • Release multiple eggs/potential offspring all at once

      • i.e. agave - b/c it grows in areas w/o lots of rain/nutrients, agave saves nutrients in its tissues until a rainy year where it sends out lots of seeds and dies

        • Adaptation due to harsh environment

  • Iteroparity Advantages/Disadvantages:

    • Organisms with multiple reproductive cycles

    • Longer reproductive timeline (multiple chances over long period of time for offspring)

      • i.e. loggerhead turtles being able to lay eggs for ~30 years after initial eggs laid

• European Kestrels Investigation

  • Study of effects of parental caregiving in kestrels

  • Researchers transferred chicks among nests to produce reduced broods (3-4 vs 5-6) or enlarged broods (7-8). Measured percentage of male and female parent birds that were alive after a winter

    • Found there was a lower survival rate of kestrels in lager broods due to the parent birds having to care for more birds negatively impacting survival of parent birds

• Exponential Growth Phase - Behaviors and Characteristics

  • Environmental factors that keep populations from indefinitely growing

    • Competition for Resources

      • Increasing density = increase competition for resources = reduced reproductive rates

      • i.e.) Not squishing crops together/fertilizing to increase nutrient availability

    • Disease

      • High transmission rate of disease increase disease’s impact on population (if density dependent)

      • i.e.) Influenza spreading through population of densely populated cities

    • Territoriality

      • When space becomes a resource that individuals have to compete for

      • i.e.) cheetahs marking territory

    • Intrinsic Factors

      • Physiological factors can regulate pop. size

      • i.e.) reproductive rates in white-footed mice can drop when high population density

    • Toxic Wastes

      • i.e.)

Summary