Overview
This lecture covers the concept of carrying capacity in ecosystems, factors that determine it, population dynamics such as overshoot and die-off, and examples showing how resources and predation affect population size.
Defining Carrying Capacity
- Carrying capacity is the maximum population size an environment can sustain indefinitely given available resources.
- It is similar to a maximum occupancy limit in a building—limited by space, food, water, or other necessary resources.
- Limiting factors are resources that restrict population growth, such as food, water, or living space.
Population Dynamics
- Each species has a different carrying capacity based on resource needs and ecosystem availability.
- Populations rarely exist exactly at carrying capacity; instead, they fluctuate above and below it.
- Overshoot occurs when a population exceeds its carrying capacity, usually resulting in a die-off.
- Die-off is a population decline due to resource depletion or other density-dependent factors (e.g., disease, famine).
Examples of Overshoot and Die-Off
- Deer populations rise each spring, leading to overshoot and food shortages, followed by a die-off.
- The St. Paul Island reindeer overshot their carrying capacity by overgrazing winter lichen, resulting in a crash from hundreds to just eight individuals before recovery.
- Reindeer adapted by shifting their diet to grasses, allowing slow population recovery.
Predator-Prey and Carrying Capacity
- The populations of hares (prey) and lynx (predator) influence each other in cyclical patterns.
- When hare populations rise, lynx populations soon grow as well; lynx then overconsume hares, causing both populations to crash.
- Carrying capacity for each species fluctuates depending on food availability and predation.
Key Terms & Definitions
- Carrying Capacity — the maximum number of individuals a species can sustain in an environment.
- Limiting Factor — a resource or condition that restricts population growth.
- Overshoot — when a population temporarily exceeds its carrying capacity.
- Die-Off — a rapid population decline due to lack of resources or other pressures.
- Predator-Prey Cycle — repeating changes in population sizes of predators and their prey.
Action Items / Next Steps
- Practice data analysis by explaining what population graphs illustrate about carrying capacity and environmental issues.
- Prepare to explain the impact of disease on species populations, using the moose and canine virus as an example.