Population Growth: Density dependent and independent factors

Population Growth: Density-dependent and independent factors

1. Introduction Populations = groups of same species in a region. Population size changes through births, deaths, immigration, emigration. Growth is not infinite → shaped by natural limits

2. Why Study Population Growth? Predicts ecosystem stability Helps manage wildlife & fisheries Guides human resource planning Provides conservation insights

3. Growth Patterns Exponential growth:  J-curve, rapid expansion Logistic growth:  S-curve, stabilizes at carrying capacity (K) Key idea: Limiting factors turn exponential growth into logistic

4. Limiting Factors Environmental elements that control population growth Two categories: Density-dependent  → internal biological regulation Density-independent  → external environmental disturbances

5. Density-Dependent Factors Effects  increase in strength as population density rises Driven by organism interactions (biotic) Mechanisms: Competition Predation Disease Parasitism Stress from overcrowding

6. Intraspecific : within same species (e.g., oak saplings competing for sunlight). Interspecific : between species (wolves vs. coyotes for prey). Results: fewer resources → lower birth rate, higher mortality. Example: Reindeer introduction on St. Matthew Island → population boom, then crash due to food scarcity Competition

7. Predator-prey cycles regulate populations. High prey density → predator numbers rise → prey declines → predator numbers decline. Example: Lynx-hare cycles (Canadian boreal forest, 10-year pattern). Keeps populations in balance; prevents one taking over ecosystem. Predation

8. Disease Spreads faster in dense populations due to close contact. High density = higher transmission rates Example: Avian flu outbreaks in poultry farms. In humans: 1918 Flu Pandemic, COVID-19—cities hit harder than rural areas

9. Parasites thrive in dense populations, easier host-to-host spread. Weakens individuals, reduces reproduction, or increases mortality. Example: Tick infestations in overpopulated deer herds. Often interacts with disease dynamics (parasites can carry pathogens) Parasitism

10. Stress from Overcrowding Overcrowding triggers stress responses: hormones rise, immunity weakens, aggression increases. Leads to lower reproduction and survival Example: Lab rat colonies at high density → increased fighting, decreased fertility. Also seen in wild species: birds abandon nests if colonies too dense

11. Summary of Density-Dependent Mechanisms Competition:  struggle for limited resources Predation:  predator-prey cycles regulate numbers Disease:  faster transmission in dense groups Parasitism:  easier host-to-host spread Overcrowding stress:  physiological/behavioral responses lower survival

12. Density-Independent Factors Affect populations regardless of density. Typically, abiotic or catastrophic events. Can cause sudden declines or even wipe out populations

13. Examples of Density-Independent Factors Climate events: drought, flood, hurricanes. Natural disasters: volcanic eruptions, earthquakes, wildfires. Human impacts: deforestation, pollution, nuclear accidents.

14. Case Study – Mount St. Helens (1980) Volcanic eruption killed thousands of animals instantly. Elk, salmon, birds died regardless of density. Recovery depended on colonization and habitat regrowth.

15. Data Examples Hurricane Katrina: massive loss of Gulf Coast fish & bird populations. Australian Bushfires 2019–20: millions of mammals, reptiles, birds lost. All independent of population density—whether 10 animals or 10,000 present, impact was similar

16. Density-Dependent vs. Density-Independent Feature Density-Dependent Density-Independent Nature Biotic (organisms affect each other) Abiotic / external events Effect strength Increases with population density Same effect on all densities Typical factors Predation, disease, competition Weather, disasters, human activity

17. Human Populations Density-dependent: crowding → faster disease spread, competition for water and housing. Density-independent: tsunamis, droughts, wildfires affect communities globally. World population ~8 billion (2023), still showing exponential trends but pressures are rising

18. Conservation & Management Wildlife management considers both factor types. Example: Fishing → population decline worsened by climate shifts. Protecting endangered species = understanding both predictable (density effects) and unpredictable (climate disasters) factors

19. Conclusion Population growth is never unlimited Density-dependent = predictable, self-regulating mechanisms within ecosystems Density-independent = unpredictable events, big environmental resets Together, they shape the intricate dance of survival and stability in ecosystems (Closing remark: “Population growth isn’t just numbers—it’s a story of limits, balances, and surprises from both nature and ourselves.”)