SCIENCE

Year 9 Biological Sciences - Term 1, Week 7

Human Impacts on Populations

  • Introduced Species Effects on Populations

  • Environmental Events Affecting Populations

  • Density Dependent and Independent Factors plus Calculating Population Growth Rates

WEEK 7 - LESSON 1: DENSITY DEPENDENT AND INDEPENDENT FACTORS

Learning Outcomes

  • Understand how density-dependent and density-independent factors affect population sizes and ecosystem balance.

Success Criteria

  • I can:

    • Describe the difference between density-dependent and density-independent factors and identify examples of each.

    • Explain how these factors influence population sizes.

    • Use examples to justify how a specific density-dependent or density-independent factor could change the balance of an ecosystem.

What Are Density-Dependent and Density-Independent Factors?

  • Populations change over time due to various limiting factors that determine how many organisms an ecosystem can support.

Types of Factors

  1. Density-Dependent Factors

    • Affect a population more strongly when the population is large or crowded.

    • Depend on the number of individuals in the population.

  2. Density-Independent Factors

    • Affect populations regardless of their size.

    • Usually environmental events or sudden changes to abiotic conditions.

Density-Dependent Factors (Biotic Pressures)

  • Increase in effect as a population becomes denser.

  • Examples:

    • Competition: More individuals lead to greater competition for food, space, water, and mates.

    • Predation: Large prey populations attract more predators.

    • Disease & Parasites: Spread more easily in crowded conditions.

    • Availability of Resources: Overcrowding reduces access to food or shelter.

  • Influence on Ecosystems:

    • Help regulate population sizes naturally.

    • Create balance by preventing unlimited growth.

    • Shift interactions within food webs (increased competition reduces survival).

Density-Independent Factors (Abiotic Pressures)

  • Affect populations regardless of their size.

  • Examples:

    • Bushfires

    • Floods

    • Droughts

    • Heatwaves

    • Temperature extremes

    • Natural disasters (storms, cyclones)

  • Influence on Ecosystems:

    • Cause sudden declines in producers and consumers.

    • May remove entire trophic levels temporarily.

    • Require long recovery periods for ecosystems.

Connection to Food Webs & Trophic Levels

  • When Density-Dependent Factors Act:

    • Higher competition reduces reproduction → fewer individuals in higher trophic levels.

    • Predation pressure controls herbivore populations → protects producer levels.

    • Disease outbreaks reduce one species → freeing food/resources for others.

  • When Density-Independent Factors Act:

    • Severe events affect multiple trophic levels at once.

    • Producer losses reduce energy entering the food web → affecting consumers and predators.

    • Food chains can temporarily collapse.

Population Changes Based on Factors

  1. Competition Example (Kangaroo Population)

    • In a growing population, scarce food during dry seasons leads to:

      • Fewer resources → individuals lose body condition → lower reproduction → population decreases.

      • Decline in herbivores affects predator numbers due to less prey availability.

  2. Heatwave Impact

    • Extreme heat kills both plants and animals, leading to:

      • Producer death → reduced energy → herbivore decline → predator decline → overall energy flow reduction through the ecosystem.

  3. Bushfire Effects

    • Coverage loss from fire diminishes vegetation and habitats:

      • Increased predation risk on small animals due to lost shelter.

      • Reduction in insect populations affects birds and small mammals.

      • Recovery depends on how quickly producers regrow.

  4. Disease Dynamics

    • Contagious parasites spread quickly in close animal populations:

      • Sharp population decline → loss of predators' food supply → increase in prey of that predator → food web imbalance.

What Is Population Growth?

  • Definition: Population growth indicates how the size of a population changes over time.

  • Processes Affecting Growth:

    • Births (increase)

    • Immigration (increase)

    • Deaths (decrease)

    • Emigration (decrease)

  • Growth occurs when more individuals enter than leave; decreases when the opposite happens.

Basic Population Change Formula

  • Population Change = (Births + Immigration) - (Deaths + Emigration)

  • Or, in shorthand: PC = (BR + Im) - (DR + Em)

  • Result interpretation:

    • Positive → population increases

    • Negative → population decreases

    • Zero → population status remains unchanged

Growth Rate (% Change)

  • Growth Rate (%) = rac{ ext{Population Change}}{ ext{Starting Population}} imes 100

Interpreting Growth in Ecosystems

  • Population Growth Connects To:

    • Food availability

    • Competition

    • Predation

    • Energy flow through trophic levels

    • Changes in biotic and abiotic factors

Effects of Rapid Population Growth and Decline

  • When Population Grows Too Quickly:

    • Overuse of resources, reducing producer levels.

    • Later increase in predator populations.

    • Increased competition within species.

  • When Population Declines:

    • Predators may starve or migrate.

    • Other species may flourish due to reduced competition.

WEEK 7 - LESSON 2: HUMAN IMPACT ON POPULATIONS

Learning Outcomes

  • Understand how human activities impact ecosystems by disrupting food webs, trophic levels, and energy flow.

Success Criteria

  • I can:

    • Describe how humans affect ecosystems and explain which biotic or abiotic factors are altered.

    • Explain how human impact alters food webs and energy flow between trophic levels.

    • Justify how a human-caused change to one population affects other organisms in the ecosystem.

How Human Activities Affect Ecosystems

  • Humans influence ecosystems in various ways, affecting:

    • Biotic factors: Animals, plants, bacteria, fungi

    • Abiotic factors: Temperature, water quality, soil nutrients, pH, light, oxygen levels

  • Changes to these factors can disrupt food webs, trophic levels, and energy flow.

Major Types of Human Impacts

  1. Habitat Loss and Land Clearing

    • Caused by farming, housing, mining, and urban expansion.

    • Reduces space, food, and shelter for organisms.

    • Can lead to population declines or local extinctions.

    • Effect on Food Webs: Fewer producers → fewer herbivores → fewer predators, disrupting energy flow.

  2. Pollution

    • Types include:

      • Air pollution: From factories and vehicles.

      • Water pollution: Chemical runoff, plastics.

      • Soil pollution: Pesticides, heavy metals.

    • Abiotic Factors Impacted: Water pH, oxygen levels, soil quality, temperature.

    • Effect on Food Webs: Pollutants kill producers (e.g. algae, plants), reducing the food chain's base.

  3. Climate Change

    • Human activities like burning fossil fuels increase greenhouse gases resulting in:

      • Higher temperatures

      • Increased droughts

      • More extreme weather

      • Ocean warming and acidification.

    • Biotic Impact: Organisms may migrate, decline, or fail to reproduce.

    • Effect on Energy Flow: Decline in producers means less energy enters the ecosystem, affecting all trophic levels.

  4. Competition for Resources

    • Lesser water availability due to usage for irrigation decreases ecosystem water levels.

    • Permanent deforestation affects animal habitats and food source availability.

How Human Impacts Disrupt Food Webs and Energy Flow

  • Changes in human activities often alter the sizes of populations, such as clearing trees that reduce bird habitats, leading to:

    • Reduced biodiversity.

    • Less stable ecosystems.

    • Interrupt impeding energy flow through trophic levels.

Examples Showing Cause and Effect

  1. Example 1: Overfishing

    • Reduces marine predator or prey populations.

    • Alters trophic levels: fewer sharks lead to more smaller fish → algae overgrowth → suffering coral reefs.

  2. Example 2: Pollution in Waterways

    • Nutrient pollution causes algal blooms → reduces oxygen → fish deaths.

    • This collapses multiple levels of the food web.

  3. Example 3: Urban Development

    • Habitat removal leads to reduced species diversity and altered predator-prey relationships.

WEEK 7 - LESSON 3: EFFECT OF INTRODUCED SPECIES ON POPULATIONS

Learning Outcomes

  • Explain how introduced species affect ecosystems by disrupting food webs, trophic levels, and energy flow.

Success Criteria

  • I can:

    • Describe what an introduced species is and identify how it changes biotic or abiotic factors in an ecosystem.

    • Explain how introduced species impact food chains, food webs, and energy flow between trophic levels.

    • Use evidence from examples to justify how an introduced species causes changes to populations and interactions in an ecosystem.

What Is an Introduced Species?

  • Definition: An introduced species is an organism brought into an area outside its natural occurrence, either:

    • Accidental (via cargo, vehicles, soil) or

    • Purposeful (for hunting, pets, pest control).

  • Some introduced species become invasive, characterized by rapid spreading, outcompeting native species, and disrupting ecosystems.

How Introduced Species Affect Food Chains and Food Webs

  • Disrupt Ecosystems By:

    • Preying on native organisms, changing predator-prey interactions.

    • Outcompeting native species for resources.

    • Reducing producer populations through grazing, digging, or shading.

    • Increasing pressure on existing trophic levels.

  • Example Chain Reaction: An introduced predator leading to fewer herbivores → overgrowth or decline of certain plants → insect population changes → impacts higher predators.

How Introduced Species Affect Energy Flow Through Trophic Levels

  • Declines in producers lead to less energy entering ecosystems.

  • Primary consumers may be consumed by introduced predators, reducing energy flow to higher levels.

  • Potential total collapse of some trophic levels, such as the removal of herbivores.

  • Top predators may lose food sources leading to ecosystem instability, with energy flow becoming less efficient and unpredictable.

WEEK 7 - LESSON 3: ENVIRONMENTAL EVENTS AFFECTING POPULATIONS

Learning Outcomes

  • Understand how environmental events (like bushfires, droughts, floods, or heatwaves) affect ecosystems by changing populations, interactions, and energy flow.

Success Criteria

  • I can:

    • Describe different environmental events.

    • Explain how an environmental event impacts food webs, trophic levels, and relationships between organisms.

    • Justify how a specific environmental event leads to changes in population sizes and ecosystem balance.

Limiting Resources

  • Population growth leads to a maximum carrying capacity; when food and shelter become limited, organisms may:

    • Emigrate (leave) or face death, leading to stabilization.

Seasonal Changes

  • Migration occurs in colder weather as animals seek warmer temperatures, impacting population dynamics.

  • Breeding seasons can lead to population increases.

  • Examples:

    • Red-necked stint: Migration from Siberia to Western Australia.

    • Whale Shark: Presence in the Indo-Pacific region among many others.

Natural Disasters

  1. Floods

    • Defined as the overflow of water onto land due to heavy rain, large waves, or dam failures.

    • Event Impact:

      • Immediate growth effects on plants and seed germination.

      • Run-off introduces sediment and chemicals into marine ecosystems, resulting in algal dominance.

      • Breeding conditions improve for fish, offering increased food supply for their predators.

  2. Droughts

    • Defined as prolonged low precipitation levels often influenced by climate changes.

    • Event Impact:

      • Animals migrate from affected areas but may return when conditions improve.

      • Topsoil removal in drought-hit areas leads to nutrient loss affecting various species (plants, animals, and agricultural).

  3. Bushfires

    • Fires alter ecosystem composition through species migration and loss of vegetation.

    • Event Impact:

      • Migration changes species composition in fire-affected areas, where animals may take a long time to return.

      • Changes in vegetation and soil conditions disrupt food sources and shelter, raising predation risks in some cases.