Population Size

Population Ecology – Grade 11 Study Notes

Introduction
- Population ecology is the ecological study of the dynamics of populations of a single species.
- A population is not static; its size and structure change over time.
- Key goal: understand why and how population size fluctuates and what regulates it.
Terminology (Key Concepts)
- Population Ecology (term)
- Definition: ecological study of the dynamics of populations of a single species.
- Use: helps explain why population size never remains constant.
- Species
- Definition: a group of organisms that share similar characteristics and have the ability to interbreed randomly to produce viable and fertile offspring.
- Notes: viable = alive; fertile = capable of producing offspring.
- Population
- Definition: a group of organisms of a similar species that share a habitat at the same time and have the ability to interbreed randomly to produce viable and fertile offspring.
- Example nuance: individuals from different geographic areas may belong to different populations.
- Community
- Definition: a number of different populations occupying the same habitat at a particular time; populations are interdependent and share resources; they may compete.
- Ecosystem
- Definition: communities that interact with each other and with abiotic factors in a particular area (e.g., animals visiting a watering hole).
Population Parameters (4 main factors that increase or decrease population size)
- Natality (+): birth rate; contributes to population growth.
- Mortality (−): death rate; reduces population size.
- Immigration (+): one-way inward movement of individuals into a population; increases size.
- Emigration (−): one-way outward movement out of a population; decreases size.
Determination of Population Size (Methods)
- Direct technique (census)
- Involves physically counting all individuals of a population.
- Methods: direct observation, aerial photography.
- Suitable for sessile organisms (e.g., plants) and slow-moving animals (e.g., barnacles); can be used for fast-moving animals (e.g., bats at roosts, territorial birds) in specific contexts; effective for counting large animals (e.g., elephants) via aerial photography (one-day habitat survey).
- Indirect technique (estimation methods)
- More common; provides estimates rather than exact counts.
- Two main examples: Mark-recapture technique and Quadrat sampling.
Indirect Technique 1: Mark-Recapture Technique
- Purpose: determine population demographics and, with multiple captures over time, infer population size/movements.
- Procedure:
- A group of animals is captured and tagged (marked).
- They are released back into the population to mix with others.
- After some time, a second sample is captured.
- Count how many of the second sample are marked.
- Use data to estimate total population size.
- Formula (as given in transcript):
- $N = \frac{M imes S}{T}$
  - N = estimate of total population size
  - M = number of animals originally marked
  - S = total number of animals captured in the second sample
  - T = number of marked animals recaptured in the second sample
- Example from transcript:
- M = 10 (marked initially)
- S = 20 (second sample size)
- T = 6 (marked in second sample)
- N = (10 × 20) / 6 = 33.3 wildebeest (approximate)
- How to increase reliability
- Repeat the procedure (more data) to calculate an average.
- Sample randomly to avoid bias.
- Allow tagged individuals time to mix with untagged individuals (not too short, not too long).
- Validity considerations
- Tag/mark should not affect behavior or movement.
- Marking should not injure or scare away individuals.
- Marks must be clearly visible.
- Population must be closed (no births/deaths during sampling interval).
- Sufficient time for mixing between samples.
Indirect Technique 2: Quadrat Sampling
- Used for plants or slow-moving/sessile animals.
- Quadrat: a square frame of known size placed randomly in the habitat.
- Procedure:
- Know the size of the habitat (area).
- Place the quadrat randomly within the habitat.
- Count all organisms/plants within the quadrat.
- Repeat several times.
- Formula (as given):
- $N = \frac{ ext{ave. number in sample} imes ext{SA of demarcated area}}{ ext{SA of quadrat}}$
  - ave. number in sample = average count per quadrat sample
  - SA = surface area
- Increase reliability by:
- Random sampling
- Repeating the procedure and taking the average
- Accurate knowledge of the surface area of the quadrat and demarcated area
- Example from transcript:
- Habitat area = 500 m^2
- Quadrat area = 5 m^2
- In a random plot, observed 4 weeds
- N = (4 × 500) / 5 = 400 weeds on the school ground
Population Growth Forms
- Population growth form describes the pattern of change in population size.
- Two main forms:
- Geometric (J-shaped) Growth Form (exponential-like growth under unlimited resources)
  - Conditions: unlimited resources; maximum growth rate under ideal conditions.
  - Graph: J-shaped; rapid growth after a slow start.
- Logistic (S-shaped) Growth Form
  - Graph: S-shaped; four phases: Lag, Accelerating growth, Decelerating growth, Equilibrium.
  - Phases described:
  - Lag phase (A): initial slow growth as populations acclimate, individuals mature sexually, and offspring production begins.
  - Accelerating growth phase (B): rapid growth; little to no environmental resistance; natality > mortality.
  - Decelerating growth phase (C): growth slows due to increasing environmental resistance; natality still higher than mortality but approaching balance.
  - Equilibrium phase (D): carrying capacity reached; natality ≈ mortality; population stabilizes.
Carrying Capacity
- Definition: the maximum number of individuals an environment can sustain under prevailing conditions.
- Important note from transcript: carrying capacity is not constant; it can vary with conditions.
- Terminology:
- Carrying capacity (K)
- Population size tends to fluctuate around K due to environmental resistance.
Population Fluctuation and Environmental Resistance
- Population size fluctuates seasonally and annually based on resource availability.
- Seasonal