APES Video Notes 3.4 - Carrying Capacity

Main ideas and concepts (carrying capacity)

• Carrying capacity (K) is the maximum population size that an ecosystem can support for a given species, based on available resources.
• It works like an occupancy limit for a building or restaurant: population growth is limited by limiting factors (e.g., seats, staff, or space).
• Limiting resources commonly include:
  • Food
  • Water
  • Habitat/space (e.g., nesting sites, hunting areas)

Species-specific carrying capacity

• Carrying capacity is species-specific: different species have different needs.
• For example, if one species (e.g., deer) has a certain K, another species (e.g., wolves) may have a different—often lower—K because of different habitat and food requirements.
• Populations typically fluctuate around K, rather than staying exactly at it.

Overshoot and die-off (what happens when populations exceed K)

• When a population grows above carrying capacity, it is called overshoot.
• Overshoot is usually followed by a die-off, often because resources are depleted (e.g., starvation or insufficient habitat).

Severity of die-offs

• Moderate die-off: population drops back below K.
• Catastrophic die-off (major crash):
  • Happens when resources are severely depleted
  • Or when high population density triggers severe disease outbreaks
  • Severe shortages can lead to famine, causing a massive decline

Methodology / instruction emphasized (science practice)

The video emphasizes an APES-style science practice focused on:

• Data analysis
  • Explain what the data illustrates about an environmental issue

How this applies to carrying capacity

Using population graphs/data, students should describe:

• Where overshoot occurs
• How and why die-offs happen
• How predator–prey interactions shift limiting factors over time

A key application mentioned near the end is explaining the impact that a “canine virus” had on the moose population.

Examples used to illustrate carrying capacity dynamics

1) Deer population cycle (overshoot → die-off)

• Deer mate in the fall and give birth in spring.
• Spring births cause a sudden increase in fawns.
• The ecosystem can’t support the increased deer number because:
  • Vegetation can’t keep up with the increased grazing demand.
• Many deer struggle to find food and die, bringing the population back below carrying capacity.

2) Reindeer on St. Paul Island (severe overshoot → crash → adaptation)

• 1910: A small herd of 25 reindeer is introduced to St. Paul Island.
  • Population grows gradually from ~1910 to 1930.
• ~1930 to 1937: Population grows exponentially (very rapid increase).
• Carrying capacity:
  • Not directly shown, but implied through significant overshoot.

Consequences

• Overgrazing of lichen, an important winter food.
• Lichen regrows slowly, creating a long-term food bottleneck after depletion.

Peak and crash

• Estimated peak density (1938): about three times their usual maximum density.
• 1950: Severe crash down to 8 individuals.

Adaptation and recovery

• Reindeer became more generalist and began eating/digging up grasses in winter.
• This supports gradual recovery to about 400 reindeer today.

3) Lynx–hare predator–prey cycle (carrying capacity driven by changing food availability)

• The cycle demonstrates how populations can influence each other predictably.
• The video describes a repeating sequence (mapped to the graph):
  1. Hare increase dramatically while lynx are low.
  2. Lynx increase rapidly because hare (their main food) is abundant.
  3. Lynx over-consume hare faster than hare can reproduce → hare crash.
  4. Lynx crash after hare declines due to scarce food.
  5. With fewer lynx, hare increases again, restarting the cycle.

Key interpretation

• Carrying capacity for lynx decreases when hare availability drops.

Estimated carrying capacity values mentioned

• Hare carrying capacity ~ 60,000
• Lynx carrying capacity ~ 30,000

Speakers / sources featured

• Mr. Smith (referenced as “it’s Mr. Smith”)