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.
- 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).
- 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.
- 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: 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.
- 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?