Population Dynamics and Ecological Interactions

Population Growth

Population Density

• Population density (Dp) is the number of individuals in a given volume or area.
• Dp = N/A or Dp = N/V
• Example 1: 500 mosquitoes in 450ml of water.
• Example 2: 50 buffalo on 10 hectares of land.

Distribution Patterns

• Three types of distribution patterns exist in populations.
  1. Uniform Distribution:
     • Individuals are evenly spaced due to competition for resources.
     • Distance between individuals is maximized.
     • Often seen in territorial species (e.g., eagles) or human-managed populations (e.g., orchards).
  2. Random Distribution:
     • Unpredictable spacing.
     • Least common in nature.
     • Occurs when an individual's position is independent of others.
     • Typically found in consistent environmental conditions and resource availability.
  3. Clumped Distribution:
     • Most common type of dispersion.
     • Distance between individuals is minimized.
     • Organisms clump around resources.
     • Can be due to social factors or predator avoidance (e.g., humans).

Population Distribution Summary

• Clumped: Most common, for safety in numbers, social interaction, mating, caring for young, and resources being clumped.
• Uniform: Less common, often due to scarcity of resources.
• Random: Quite rare; distinguishing between truly random and largely clumpy can be difficult.

Population Growth

• Four processes change the number of individuals in a population (\Delta N).
• \Delta N = [b + i] - [d + e]
  • \Delta N = change in population size
  • b = births
  • i = immigration
  • d = deaths
  • e = emigration

Population Change Example

• A 10-year study of buffalo in the Northwest Territories:
  • Natality (births): 40 per year
  • Mortality (deaths): 30 per year
  • Immigration: 3 per year
  • Emigration: 8 per year
• The population was increasing because (40 + 3) > (30 + 8)

Rate of Population Growth

• Speed of population change is important.
• Growth rate (gr): change in the number of individuals over a specific time frame.
• gr = \frac{\Delta N}{\Delta t}
• Growth rate calculation does not consider the initial population size.

Growth Rate Example

• Banff Springs snails:
  • January 1997 population ≈ 3800
  • Two years later population ≈ 1800

Per Capita Growth Rate

• Per capita growth rate (cgr) compares populations of different sizes.
• Change in population size is a rate per individual.
• Takes into account the original population size.
• cgr = \frac{\Delta N}{N}

Per Capita Growth Rate Example

• Small town of 2500 people:
  • 60 births, 25 deaths, no immigration or emigration in 3 years.
• Calculate the per capita growth rate during this interval.

Factors Affecting Population Growth

• Biotic and Abiotic factors limit population growth
  • Biotic limiting factors: Disease (parasites), food availability, predation, competition.
  • Abiotic limiting factors: Climate, water availability, sunlight, natural disasters.

Biotic Potential

• Biotic potential (r): highest possible per capita growth rate with unlimited resources and ideal conditions.
• Determined by:
  • Number of offspring per reproductive cycle
  • Number of offspring surviving to reproduce
  • Age of reproductive maturity
  • Lifespan of individuals and number of reproductive cycles

Exponential Growth

• Population growing at its biotic potential experiences exponential growth.
• Graph shows a J-shaped curve.
• Example: Housefly lays over 100 eggs and reproduces after 1 month. After 7 generations, 1 fly can result in 15 billion flies.

Carrying Capacity

• Carrying capacity (K): maximum population size an environment can sustain.
• Maximum number of individuals that can live in an environment without depleting resources or harming their habitat/themselves.
• Limited by nutrients, space, etc.

Logistic Growth Pattern

• As a population increases, limiting factors slow growth rate.
• Demonstrated by an S-shaped curve.

Density Dependent vs. Density Independent Factors

• Density-dependent factors (BIOTIC):
  • Arise from population density (limited resources, food, space).
  • E.g., big populations are more affected by parasites and disease, predator-prey interactions.
• Density-independent factors (ABIOTIC):
  • Affect members regardless of size/density (flood, fires, temperature).

Life Strategies: r-selected vs. K-selected Species

• K-selected species:
  • Population sizes fluctuate at or near their carrying capacity (K).
  • Environments are usually at or near equilibrium for a long time.
  • Few offspring per reproductive cycle.
  • One or both parents care for offspring.
  • Offspring take a relatively long time to mature and reach reproductive age.
  • Tend to have larger bodies.
• r-selected species:
  • Reproduce close to their biotic potential (maximum reproductive capacity).
  • Usually live in highly variable environments.
  • Species have short lifespans.
  • Reproduce at an early age.
  • Produce large numbers of offspring.
  • Provide little to no parental care.

Interactions and Ecological Communities

• Interactions within and between populations cause population changes.
• Ecological community: association of interacting populations in a defined area.
• Individuals compete for limited resources, leading to a selective advantage for some.

Intraspecific Competition

• Competition for limited resources among members of the same species.
• E.g., competition for water, nutrients, mates, shelter.
• This is a density-dependent factor.
• To avoid competing with offspring, some parents disperse their offspring, or organisms go through different life phases.

Interspecific Competition

• Competition between members of different species in the same community.
• No two species can share the exact same niche.
• When populations share overlapping niches, they compete for limited resources.
• Often when a non-native species is introduced, it competes with native species.

Producer-Consumer Interactions

• The producer-consumer relationship puts selective pressure on both partners.
• Example: Lynx and Hare population cycles.

Defenses Against Consumers

• Protective coloration: Black, yellow, and red are warning colors.

Batesian Mimicry

• A harmless species evolves to imitate the warning signals of a harmful species, directed at a common predator.
• E.g., The syrphid fly has the same warning coloration as the yellow-jacket wasp but has no stinger.

Mullerian Mimicry

• Two or more poisonous species, that may or may not be closely related and share one or more common predators, mimic each other's warning signals.

Symbiotic Relationships

• Mutualism: Both partners benefit.
• Commensalism: One partner benefits, the other is neither harmed nor benefits.
• Parasitism: One partner benefits at the expense of the host.

Succession: Changes of a Community Over Time

• Succession: slow, orderly, progressive replacement of one community by another during vegetation development.
• Driven by abiotic factors (e.g., climate) and biotic factors (e.g., competition).

Primary Succession

• Occurs where no soil is present (e.g., barren rock).
• Starts with pioneer species: small, opportunistic species that grow in harsh conditions.
• As they die, soil builds up, allowing other organisms to survive.
• Larger species grow and take over.

Climax Community

• Eventually, a climax community is formed, which is generally stable unless a major change occurs.

Secondary Succession

• Occurs after an ecological disturbance: partial or complete community destruction (e.g., after a forest fire or flood).
• Soil is still present, containing nutrients and roots.

Ecological Disturbances

• Ecological disturbances can be beneficial.
• E.g., some plants produce seeds that only germinate after extreme heat (fire), such as sequoia and lodgepole pine.

Sustainability

• Sustainability: living in a way that meets our needs without compromising the health/resources of future generations.

Human Population Growth

• Age pyramids: graphical illustration examining age structure and proportion of males and females, used to assess a population's potential for growth.
• Wide-based pyramid (more births than deaths) indicates rapid growth (e.g., Kenya).
• Rectangle-shaped pyramid represents a stable population (births = deaths) (e.g., United States/Canada).
• Inverted triangle indicates a declining population (narrow base) (births < deaths) (e.g., Italy).

World Population

• Current world population: around 7.2 billion.
• Question: When will we reach our carrying capacity?