2.1E Population size and growth Notes

1. Populations are regulated by density-dependent factors, which become stronger as population density increases.
2. These factors act as negative feedback mechanisms, returning populations to equilibrium around their carrying capacity (K).
3. In contrast, density-independent factors (e.g., droughts, floods, volcanic eruptions) affect populations regardless of their size, but do not regulate them long-term.

Density-Dependent Factors

They create negative feedback loops, stabilizing populations around the carrying capacity.

Key Density-Dependent Factors

1. Competition for Limited Resources

1. As population density increases, individuals compete for limited resources (e.g., food, water, nesting sites).
2. This reduces individual growth, reproduction, and survival rates.

2. Predation

1. Higher prey density attracts more predators or increases the predator success rate.
2. This acts as a self-regulating mechanism that keeps the prey population in balance.

3. Disease and Pathogen Transmission

1. Close contact in dense populations promotes the rapid spread of infectious diseases.
2. This increases mortality and lowers reproductive success.

Negative Feedback Mechanisms

1. Negative feedback occurs when an increase in population size triggers mechanisms that reduce growth rates and bring the population size back towards equilibrium.
2. These mechanisms ensure that populations do not exceed the carrying capacity of their environment.
3. As populations exceed carrying capacity:
   1. Resource scarcity reduces reproduction rates.
   2. Predation and disease increase mortality.
4. These processes act as stabilizing forces to return the population toward equilibrium (K).

Density-Independent Factors

1. These are abiotic factors that affect populations regardless of density.
2. These include:
   1. Natural disasters (fires, floods, droughts).
   2. Temperature extremes or long-term climate change.
   3. Volcanic eruptions, earthquakes, hurricanes.
3. These can cause sudden population crashes, but do not regulate around K.

Population Growth

1. Population growth follows two primary models:
   1. Exponential growth (J-curve): Growth in the absence of limiting factors.
   2. Logistic growth (S-curve): Growth limited by carrying capacity.
2. These models describe how populations expand and stabilize over time depending on resource availability.

Exponential Growth (J-Curve)

1. Exponential growth occurs when a population grows at a constant rate without any limitations.
2. The population increases rapidly over time, resulting in a J-shaped curve.
3. Growth is unlimited, with no environmental constraints.
4. The rate of change (growth rate) remains constant as long as resources are available.

Phases of Exponential Growth

1. Lag Phase: Slow initial growth due to few reproducing individuals.
2. Exponential Phase: Rapid population increase; birth rates exceed death rates.
3. Crash Phase: Population overshoots K, leading to resource depletion and sudden decline (a “boom and bust” pattern).

Logistic Growth (S-Curve)

1. Logistic growth occurs when a population’s growth rate slows down as it approaches the carrying capacity of the environment.
2. The growth curve starts exponentially but levels off as resources become scarce, leading to an S-shaped curve.
3. Population growth is initially fast (rapid increase), then gradually slows, and eventually stabilizes at the carrying capacity.
4. Density-dependent factors (e.g., food availability, competition, predation) slow the growth rate as the population increases.

Phases of Logistic Growth

1. Lag Phase: Slow growth due to small population size.
2. Exponential Phase: Rapid increase as conditions are favorable.
3. Transitional Phase: Growth slows due to competition and limiting factors.
4. Plateau Phase: Population stabilizes at K, fluctuating slightly with environmental changes.

"Boom and Bust” Cycles

1. Some populations overshoot their carrying capacity, then crash due to starvation or resource depletion.
2. This can lead to population collapse before recovery or extinction.

Limiting Factors on Human Population

1. Human population growth has been shaped by limiting factors, natural constraints that slow or stop growth.
2. However, many of these factors have been reduced or eliminated, leading to rapid population expansion and significant impacts on ecosystems.
3. They can be biotic (living) or abiotic (non-living).
4. Natural Limiting Factors in Early Human Populations include
   1. Food scarcity
   2. Predation
   3. Disease
   4. Water shortage
   5. Lack of shelter or protection

Elimination of Limiting Factors

1. Removal of Natural Predators

1. Humans have eliminated most large predators (e.g., wolves, lions, tigers) that once regulated our numbers.
2. This has disrupted ecological balance, allowing prey species (like deer) to overpopulate in predator-free zones.

2. Technological Advances

1. The agricultural and industrial revolutions increased food production, medicine, and resource extraction.
2. Improved sanitation, vaccines, and healthcare decreased death rates.
3. Transportation networks allowed resource mobility, reducing dependency on local ecosystems.

3. Environmental Degradation

1. Overuse of resources has degraded ecosystems through deforestation, soil erosion, and pollution.
2. Resource exploitation boosts short-term carrying capacity but undermines long-term sustainability.

Consequences for Sustainability

1. Overexploitation of resources such as freshwater, soil, and fossil fuels.
2. Habitat loss and biodiversity decline, reducing ecosystem resilience.
3. Climate change, altering resource availability and habitability.
4. Increased inequality between resource-rich and resource-poor populations.

Human Carrying Capacity

1. Unlike other species, humans have a broad and evolving ecological niche, making carrying capacity variable and uncertain.
2. We continually alter resource needs through innovation, cultural adaptation, and global trade.

Factors Complicating Estimation

1. Resource substitution: When one resource depletes, humans find alternatives (e.g., renewable energy replacing fossil fuels).
2. Unequal consumption: Resource use varies dramatically by lifestyle and region.
3. Technological change: Expands short-term limits but may worsen long-term degradation.
4. Mobility of resources: Humans transport food, energy, and water globally, bypassing local limits.

Human Niche Expansion

1. Humans continually expand their niche through innovation:
   1. Urbanization and industrialization increase energy access.
   2. Biotechnology and renewable energy alter ecosystem dependence.
2. These changes allow humans to occupy nearly every biome, from deserts to polar regions.

Measuring Human Impact: The Ecological Footprint

1. The ecological footprint estimates how much biologically productive land and water area a population requires to produce resources and absorb waste.
2. It inversely relates to carrying capacity:

Higher footprint → Lower carrying capacity.

Overshoot and Consequences

1. Humans are currently living in a state of ecological overshoot, using resources equivalent to 1.7 Earths.
2. This leads to:
   1. Resource depletion (fisheries, forests, fossil fuels).
   2. Loss of biodiversity and ecosystem services.
   3. Climate instability and pollution.