Biol 1110-Population Dynamics

What is a population?

A group of individuals of the same species living in the same general area. They rely on similar resources, experience similar environmental factors, and are likely to breed with one another.

What is population density, and how is it measured?

The number of individuals per unit area or volume.

Large, easily counted organisms: Count individuals directly within an area.

Herded or densely distributed organisms: Randomly count individuals in sample plots and extrapolate to the larger area.

Scales indicate how much of the population or area was studied.

What is population dispersion, and what are its three patterns?

Dispersion is the spacing pattern of individuals within a population's boundaries.

• Clumped: Individuals form groups.

• Uniform: Individuals are evenly spaced.

• Random: Individuals are unpredictably distributed.

  
  

Dispersion patterns can change when the size of the study area changes.

What causes clumped dispersion? Give examples.

• Plentiful or patchily distributed resources.

• Cooperation, such as hunting.

• Offspring remaining near or attached to parents.

Examples: wolves and sea stars.

What causes uniform dispersion? Give examples.

Repulsion between individuals, caused by aggressive behavior, territoriality, or toxins released into the soil, keeps neighbors evenly spaced.

Examples: fungi, bushes, and penguins.

What causes random dispersion? Give examples.

Rapidly changing factors, such as wind, determine organism locations in a physically homogeneous habitat.

Examples: razor clams and dandelions.

What four factors affect population size?

• Immigration: Individuals enter a population from other areas.

• Emigration: Individuals leave a population for other locations.

• Births: Add individuals.

• Deaths: Remove individuals.

How are per capita birth rate, death rate, and rate of increase calculated?

b = births during a period/population size at beginning

m = deaths during a period/population size at beginning

r = b - m, the per capita rate of increase

A positive r indicates growth; a negative r indicates decline.

What is demography, and why do studies often focus on females?

Demography is the study of population vital statistics and how they change over time. Studies often focus on females because they produce offspring.

What is a cohort?

A group of individuals born at the same time, followed from birth until all members have died.

What is a survivorship curve?

A graph showing the proportion of a cohort still alive at each age.

What characterizes Type I survivorship?

• High survival during early and middle life due to being cared for by parents

• Death rates increase among older individuals.

• Typical of large mammals that invest substantial resources in nurturing offspring.

(concave down shape)

What characterizes Type II survivorship?

• A relatively constant death rate throughout life.

• Survivorship declines at a roughly constant rate.

Example: ground squirrels

(linear shape)

What characterizes Type III survivorship?

• Very high death rates among young individuals, causing a sharp initial decline.

• Survivorship levels off among those that survive early life.

• Typical of organisms producing many offspring with little parental care.

(Shape of 1/x graph)

Examples: many fishes, marine invertebrates, and long-lived plants.

What is exponential population growth, and what equations describe it?

Rapid population growth under favorable conditions, especially when resources are abundant and limiting factors are minimal.

• Differential equation: dN/dt = r_maxN (exponential growth)

• Integrated equation: N= N₀e^{r max (t)}

Where:

• N = population size at time t

• No = initial population size

• r-max = maximum per capita rate of increase

• t= time

What is an example of exponential population growth in nature?

Rats introduced to a new island may reproduce rapidly and establish a large population because they initially encounter abundant resources and few limiting factors.

What is carrying capacity (K)?

The maximum population size an environment can sustainably support given its available resources and conditions.

As resources dwindle, competition increases and population growth slows toward a plateau.

What is logistic population growth, and how does it differ from exponential growth?

Logistic growth accounts for limited resources and carrying capacity.

dN/dt = r_maxN (K-N/K) (logarithmic growth)

• Exponential growth: Resources are effectively unlimited; growth accelerates.

• Logistic growth: Growth slows as N approaches K.

• Populations may oscillate around carrying capacity but generally remain relatively stable near it.

What is semelparity? Give examples.

A reproductive strategy in which an organism reproduces in one major event and typically dies afterward.

Examples: coho salmon and agave plants.

What is iteroparity? Give examples.

A reproductive strategy involving repeated reproductive events throughout an organism's life. The number of offspring produced may vary between events.

Example: large mammals.

What factors influence the evolution of semelparity versus iteroparity?

• The survival rate of offspring.

• The likelihood that adults will survive to reproduce again.

• Environmental predictability and resource availability.

• How organisms allocate energy among growth, survival, and reproduction throughout their lifespan.

Under what conditions is semelparity favored?

Semelparity may be favored in highly variable or unpredictable environments where adult survival to another reproductive event is unlikely and a single major reproductive effort offers the best chance of passing on genes.

Under what conditions is iteroparity favored?

Iteroparity may be favored in more dependable environments where adults are likely to survive and reproduce repeatedly.

It can also be advantageous when competition for resources is intense and spreading reproductive effort across multiple events improves offspring survival.

What is K-selection, and under what conditions does it operate?

Selection favoring traits suited to populations living at densities near carrying capacity K, where resources are limited and competition is strong.

It is also called density-dependent selection because birth or death rates change with population density.

What are examples of density-dependent factors associated with K-selection?

• Competition for resources.

• Predation.

• Toxic waste accumulation.

• Territoriality.

• Disease.

• Increased death rates or decreased birth rates as population density rises.

What is R-selection, and under what conditions does it operate?

Selection favoring traits that maximize reproductive success in uncrowded environments, where population density is well below carrying capacity and competition is limited.

It is also called density-independent selection because the relevant birth or death rates do not change with population density.

What are examples of density-independent factors associated with R-selection?

• Droughts and floods.

• Severe cold or heat.

• Earthquakes.

• Volcanic eruptions.

• Human interference, including human-caused disease.

These factors affect populations regardless of their density.