Population Ecology - Chapter 53

“Either a species learns t control its own population, or something like disease, famine, and war will take care of the issue” Who said/wrote this?

Chuck Palahniuk

How biotic and abiotic factors influence size, distribution, density, and composition of populations

Population Ecology

What is population?

A group of individuals of a single species living in the same area at the same time

Define Density

Number of individuals per unit area or volume

Define Distribution

The way in which individuals are spaced within a population

• There are 3 patterns of distribution

Within a population’s geographic range, what creates contrasting patterns of dispersion?

The availability of resources, environmental conditions, and interactions among individuals impacting local densities which can differ substantially

What are the three Patterns of Dispersion?

1. Clump Distribution

2. Random Distribution

3. Uniform Distribution

Individuals that are found in groups or patches within their particular habitat. The presence of other individuals increases their fitness. Usually prey, few predators (wolves) This is known as…

Clump Distributions

When does Random Distribution usually occur?

When there is an absence of strong attractions or repulsion’s from among individuals

Define Random Distribution

Individuals are spread out in the environment irregularly; the position of one individual is independent of another.

What results from direct negative interaction between individuals?

Uniform Distribution and Territoriality

Uniform Distribution is…

a distribution pattern where individuals are evenly spaced in an environment due to competition or territorial behavior.

-Rare in nature

-Presence of one hinders another

-Distance between individuals is maximized

Territoriality

a behavior exhibited by individuals to defend and maintain exclusive use of a specific area, often to secure resources such as food, mates, or nesting sites. Boundaries are created

What are ways to calculate population distribution?

1.Census

2.Survey

What does the Census do?

Counts every individual in the population

*The MOST accurate but usually not feasible/practical

What does a Survey do?

Estimates population size by sampling a subset of individuals instead of counting them all, then extrapolating to the geographic range.

*Can be modified based on pop size and geographic range THOUGH some info about the population is needed beforehand, usually feasible/practical

Survey is most effective when

Uniform Distributed or Random Distribution is occuring

Survey NOT effective when…

Clump Distribution is occurring

Survey Types

1.Area/Volume (quadrant) based survey

2.Line Survey (transect), counting as you move along a line - immobile and mobile organisms, not effective if geographic range is large

3.Mark and Recapture survey - VERY effective for birds and large mammals (mobile populations)

• Photography

• Drones

• Physical capture

• Nests, burrows, or fecal droppings as indicator(s)

How to calculate population size from Mark and Recapture Density?

Example of how to calculate density

N = (S*n)/x

N = (328)(200)/64

N = 1025 individuals

Density is a static property. True or False?

False; density is constantly changing based on individuals being added or removed from the pop.

Immigration

Influx of new individuals from another pop

Example: southern border in U.S.

Emigration is the movement of individuals where?

Movement of individuals out of a pop.

Birth…

adds individuals to the population (all forms of reproduction)

Death…

removes individuals from a population

Define Demography

The study of the vital statistics of a population and how they change over time

• Often done using life tables

What are Life Tables and what do they do?

A age-specific summary of the survival pattern of a population

• Developed in the 1950s for insurance companies

• Uses cohort: group of individuals of the same age

• Can be dynamic (horizontal) or static (vertical)

• Provide “snapshot” of a pop at ALL life stages at same time

• There are 3 main types

????Types of Life Tables

1. Cohort Table - follows groups of same aged individuals from birth until they all die

2. Static Life tables - made from data collected from all ages at one time

TWO types of tables are theoretically identical if

• the environment is not changing

• The population is at equilibrium (B=D, I=E)

Static Life Table

A type of life table that summarizes the mortality and survival rates of a population from data collected at one specific point in time.

• Static/stationary (vertical)

• A specific point in time of all individuals regardless of age

Cohort Table

• Dynamic (horizontal)

• Follows one (1) group of same-age individuals

  • Single breeding season in throughout their lives

• Much harder to do

• Takes more time and money

• Takes into account birth until death

The first column (x) specifies the age class while the second column (N_x) is the number of individuals at start of each age class

As (x) increases, (N_x) is equal to or less than the previous (N_x) value

Demographic types

1. Semelparity

2. Iteroparity

Semelparity

Have only ONE reproductive event in their lifetime

*Semel meanings “once” in Latin

Iteroparity

Capable of multiple reproductive events

*Itero meaning “repeat” in Latin

Life Table variables/demographics for semelparous organism

x —> age or stage of individuals in the population

n_x—> # of individuals at start of each age class

I_x—> proportion surviving to each age class in comparison to the starting # of individuals at age class 0

Fx—> number of offspring produced by each female at each age class

b_x—> average number of offspring per individual in a specific age class

Ixbx —># of offspring produced per original individual at each age

• Ixbx accounts for all ages; you can calculate the net reproductive rate (R₀)

T —> generation time or the time it takes one cohort to replace itself in the population

r —> per capita rate of increase

How to calculate The Net Reproductive Rate (R₀) aka the population’s replacement rate

Sum of lxbx, which is the # of offspring produced per original individual at each age

1.043; population is growing

If R₀ = less than 1; population is declining

If R₀ = 1; population is replacing itself

If R₀ = greater than 1; population is growing

What does T represent and how is it calculated 

T represents the generation time

In this case, T = 7 months

x*lx*bx divided by sum of Ix*bx (R ₀)

What is r and how is it calculated?

Per capita rate of increase

In(R₀) divided by T

If r = less than 0; the population is decreasing in size

If r = 0; population size will remain constant

If r = greater than 0; population is increasing in size

Solve

What is a Survivorship Curve?

a graphical way of representing the data presented in a life table

• plot of N_x vs x (on log scale)

• Is extrapolated to begin with cohort of convenient size (1000 individuals)

• Has 3 classifications

What are the three general idealized types/classifications of Survivorship Curves?

1. Type I, Type II, Type III

Type 1 represents

Low death rates during early and middle life

Increase in death rates among older age groups

-Animals that provide parental care

-Examples include humans and many large mammals.

Type 2 represents

A constant death rate over the organism’s life span

-Birds

-Small mammals

-Crustaceans that molt/shed

Type 3 represents

High death rates for the young

Lower death rate for survivors

-Larvae

How to calculate rate of increase per capita (change in population)?

Births

+

Immigrants entering population

-

Deaths

-

Emigrants leaving population

If immigration and emigration are ignored, how is a population’s growth calculated?

Births minus Deaths

How can births and deaths be expressed as the average number of births and deaths per individual over a specified time frame

B = bN = where b annual per capita birth rate

D = men = where m annual per capita death rate

Calculate

B = bN = (annual per capita birth rate) multiplied by population size

(0.034) * 500 =

17 births per year

Zero population growth occurs when r is equal to?

zero, indicating no net population increase or decrease.

When all members of a population have access to unlimited resources and are free to reproduce at their physiological capacity, that population will experience what?

Exponential Population Growth

*This is the maximum per capita rate of increase

Examples of species whose population size grows exponentially (increasing at a constant rate)?

• Species introduced to new environments 

• Rebounding species from catastrophic number loss

  • example: elephant population

When there is continuous exponential growth, how is the population size it calculated at any given time (t) ?

Nt = N0ert

N_t = population size at time t

N₀ = the original population size

R = Per capita rate of growth

Example of how to solve for population size at the given time

Solve this example

Calculate r first → brith rate - death rate

???????

Solve this

(17×15) / 8 =31.875 = ~32 individuals

Is greater than zero (0)????

Solve:

Nt = N0e^rt

10,000 = 24 e^r(t)

r = 0.116 so,

(In[10,000/24]) / 0.116 = t

~52 weeks = t

Solve for r to plug into the equation above:

Nt = n₀^{e^rt}

_{is the intrinsic growth rate, indicating how quickly a population can grow.} 

2 = 1e^r(6 weeks)

In (2) =In (e^6r)

0.693 = 6r

r=0.116

True or false? Resources rarely remain abundant enough for exponential growth?

True

Define carrying capacity (K)

• Always changing - Can increase or decrease depending on availability of resources

• The closer a population get to reaching the max capacity (density) = mortality (death) increases, birth rates decline

• R will decline until it reaches zero

Switch from exponential to logistical occurs at what point on a Logistical Model?

Half of the exponential growth?????

Logistic Population Growth model expressed mathematically

• Best used when the population is beyond half of the carrying capacity

• The logistical model assumes that populations adjust instantly to changes in resources.

• In some species, there is a delay in this adjustment.

• This delay can cause a population to grow beyond the environment's carrying capacity for a short period.

• The images show examples of species that might be studied in this context, including a water flea, a sea urchin, beetles, and a human crowd.

N small —> N/K is LOW

N large —> N/K is HIGH

Solve this

Change in population / Change in time = r x N x [1-N/K]

= 0.2311 × 900,000 x [1- (900,000/1,072,764)]

= 33,496 deer added to the population (THIS IS POSITIVE # = increase in population)

Pop size in 2007 = 933,496

Solve

*NOTE: the population in 2024 (124) is greater than the carrying capacity (~104) meaning there will be a DECLINE in the population size

2024 —> Change in population / Change in time = r x N x [1-N/K]

= (0.6) x (124) x [1- (124/104)]

= about negative 14 (decline)

124 -14 =110

2025–> Change in population / Change in time = r x N x [1-N/K]

= (0.6) × (110) x [1- (110/104)]

= - 3.8

= about negative 4 (decline)

110 - 4 = 106

226–> Change in population / Change in time = r x N x [1-N/K]

= (0.6) x (106) x [1- (106/104)]

= about negative 1

*NOTE: Decline in population becomes small every year (year one–> ~ -14, year two–> ~ -4, year three–> ~ -1

R is measured in “per year”

What model do species in laboratories fit (Beetles, crustaceans, paramecium, etc) ? Exponential or Logistical?

Logistical Model

Define K-selection

Density-dependent selection, selection for traits that are sensitive to population densities

Lots of parental involvement

Examples: Humans, large mammals usually

Typically follow the Logistical growth model

When is K-selection most favorable?

In populations living at a density near the limit imposed by resources (carrying capacity = K)

Define r-selection

Density-INDEPENDENT selection, selects for life history traits that maximize reproduction

Little parental involvement

Typically follow the exponential growth curve 

Fast growth, mature quickly

Examples: weeds, many plants

When is r-selection most favorable?

Occurs in environments in which population densities are well below carrying capacity or face little competition

Define Life History

The traits that affect an organism’s schedule of reproduction and survival

• 3 Main Variables that impact an organism’s schedule

• Often dependent on semelparity vs iteroparity

What are the 3 main variables that impact an organism’s schedule of reproduction and survival?

1. The age at which reproduction begins

2. How often the organism reproduces

3. How many offspring are produced per reproductive episode

What environmental conditions and organisms favor semelparity and iteroparity?

Highly variable or unpredictable environment likely favor semelparity (Have only ONE reproductive event in their lifetime) —> R selected organisms

Dependable environments favor Iteroparity (Capable of multiple reproductive events)  —> K selected organisms

Are there trade-offs between survival and reproduction?

Yes

Define Allee Effect

a population's growth rate decreases at low population densities (small pop size) due to difficulties in finding mates or cooperative behaviors.

-mate limitation

-cooperative defense

-cooperative feeding

-environmental conditioning

Allen Effect impacts which distribution type?

Organisms that are Clumped Distribution

Many factors that regulate population growth are density dependent. True or false?

True

Density-independent populations

birth rate and death rate do not change with population density

• Some physical factor which kills similar portions of the population regardless of its density

Density-dependent populations

birth rates fall and/or death rate rates increase with population density

• Limiting resource, behavioral changes, biotic control

Can you have a mixture of both?

Yes

Population Dynamics focuses on what?

Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

Long-term population dynamic studies have challenged the hypothesis that…

populations of large mammals are relatively stable over time. The scientists found that large mammals ACTUALLY exhibit significant fluctuations in response to environmental changes and human impacts.

Define Boom-and-Bust Cycles

Fluctuations in population size characterized by rapid growth (boom) followed by dramatic declines (bust), often due to resource availability and environmental changes.

• Predator and Prey relationship

Exampls: Zooplankton(predator) and phytoplankton(prey)—>the more zooplankton the more fish

Define Metapopulations

are groups of populations linked by immigration and emigration

• Usually local populations are surrounded by both suitable and unsuitable habitats

• Local Population becomes extinct but can be recolonized by another patch in a suitable habitat