CQ1 - How do environmental pressures promote a change in species diversity and abundance
Ecosystems and Selection Pressure
An ecosystem consists of all living organisms (biotic factors) and nonliving components (abiotic factors) in a given area.
The distribution of Australian ecosystems is influenced by variations in abiotic (e.g., climate, soil) and biotic factors (e.g., species interactions).
Selection Pressure
Selection pressures are biotic and abiotic factors that influence an organism’s behavior, instincts, and reproduction.
Natural selection occurs when selection pressures favor individuals with advantageous traits, increasing their survival and reproductive success.
Environmental changes can lead to competition for limited resources, favoring organisms with beneficial adaptations.
Abiotic Factors are non-living environmental components that impact survival. Examples:
Light
Essential for photosynthesis, impacting plant growth, pigment production, and expansion.
Influences animal behavior (growth, reproduction, metabolism, migration).
Temperature
Varies across land, ocean, and altitude.
Dormancy (e.g., bud dormancy in plants, metabolic slowdown in animals) occurs in response to temperature changes.
Weather
Short-term variations in wind, temperature, radiation, moisture, and pressure affect organism growth, behavior, and reproduction.
Water
Availability depends on rainfall, fresh vs. saltwater sources, and atmospheric moisture.
In aquatic environments, tidal changes and salinity impact survival.
Shelter
Provides protection from weather and predators.
Topography
Shape of land affects water runoff, soil type, and microclimates.
Aspect (sunlight exposure) and altitude influence temperature, air pressure, and oxygen availability.
Soil is essential for nutrient cycling, water retention, and structural support in ecosystems.
Abiotic Factors (Continued)
Light: essential for photosynthesis, influences distribution and productivity; in oceans, light only reaches the photic zone (upper layers), where photosynthetic phytoplankton thrive and serve as producers.
Temperature: drives seasonal patterns, dormancy, and metabolic rates.
Weather: short-term variability shapes behavior and reproduction.
Water: availability governs habitat suitability and organism limits.
Shelter: microhabitat availability affects survival, refuge from predators and weather.
Topography: creates microclimates and influences water flow and soil properties.
Soil: nutrient cycling, water retention, structural support for plants and roots.
Biotic Factors
Biotic factors influence organism survival through interactions within the ecosystem.
Examples:
Food availability & abundance
Competition (for food, mates, territory)
Predators & prey relationships
Mating opportunities
Disease-causing organisms
Organisms rely on interactions within the ecosystem, shaping survival and evolutionary outcomes.
Relationships within environments
The biosphere is the part of Earth that supports all living things.
An organism’s environment can positively or negatively impact its survival.
Ecology is the study of how organisms interact with each other and their environment, including how these interactions affect distribution and abundance.
Ecology considers both:
Biotic factors (living)
Abiotic factors (non-living)
Not Evenly Distributed Across Ecosystems
Not evenly distributed across ecosystems. Example:
Sunlight: water filters light; in oceans, light only reaches the photic zone (upper layers), where photosynthetic phytoplankton thrive and serve as producers.
Community Interactions (Types of Relationships)
Predation
One organism (predator) kills and eats another (prey).
Predators may actively hunt or use traps.
Factors influencing predation:
Number of predators/prey
Prey’s food availability
Birth/death rates
Sex ratio (males to females)
Ecosystem size
Shelter availability
Movement between ecosystems
Competition
Occurs when organisms compete for shared resources (food, mates, shelter).
Can be:
Direct (physical fights, aggression)
Indirect (vocal signals, scent marking)
Allelopathy: plants release chemicals that affect other plants (can be helpful or harmful).
Consequences of competition:
Affects reproduction and survival
Leads to population fluctuations
Symbiosis
Symbiosis = two organisms living closely together, benefiting at least one.
Types:
Mutualism: both organisms benefit. E.g. clownfish and anemone, emus and peaches
Commensalism: one benefits, the other is unaffected. E.g. birds nesting in tree hollows
Parasitism: one benefits, the other is harmed. Parasites usually don’t kill hosts to maintain their food source.
Effects of parasites: shortened lifespan, impaired function
Consequences of symbiosis:
Can increase biodiversity
May result in new species through genetic integration
Ecological Niches
A niche is an organism’s role in the ecosystem, including:
When it is active
What it eats
Where it lives
Its interactions with other species
Example: Koalas – active times, type of leaves eaten, tree preferences.
Resource partitioning: different species using different parts of the same resource to reduce competition.
E.g. Birds hunting at different canopy heights in woodlands.
Food Chains
A food chain shows the feeding relationships between organisms in an ecosystem.
Arrows in a food chain show the transfer of energy from one organism to another.
Organisms in a food chain are classified as:
Producers: Make their own food using photosynthesis (e.g. phytoplankton)
Consumers: Eat other organisms to gain energy (e.g. zooplankton, fish)
Example food chain: \text{Phytoplankton} \rightarrow \text{Zooplankton} \rightarrow \text{Small Fish} \rightarrow \text{Large Fish}
Trophic Interactions
Trophic levels = positions in the food chain based on how organisms get energy.
Organisms are classified as:
Autotrophs: Make their own food (e.g. plants, phytoplankton)
Heterotrophs: Eat other organisms for nutrients (e.g. animals, fungi)
Decomposers: Break down dead organisms into inorganic nutrients that can be recycled by producers (e.g. bacteria, fungi)
Biomass: the total mass of organisms at a given trophic level.
Energy is lost as you move up the food chain, so:
More biomass is needed at the bottom (e.g. 1000 units of phytoplankton)
Less biomass at the top (e.g. one large fish), because energy is used for life processes or lost as heat.
Energy efficiency is low between trophic levels – large predators must eat many prey to meet energy needs.
Food Webs
A food web is a network of interconnected food chains.
Most organisms are part of multiple food chains, creating a complex food web.
Complex Food Webs and Population Dynamics
Complex food webs = stable ecosystems
If one species is lost, others may fill its role.
In simple food chains, losing one species can be disastrous due to a lack of alternatives.
Changes to population distribution of species
Distribution = where a species is found in an ecosystem.
No species is evenly spread across the environment.
Organisms live where:
Abiotic and biotic factors suit them
Their survival needs are met
They can avoid predators and thrive
Abundance of Species
Abundance = how many individuals of a species live in an area.
Abundance varies by location and time.
Abundance increases when:
Birth rate > death rate
Resources are plentiful
Low predation and disease
Changes in animal abundance:
Increases: births and immigration
Decreases: deaths and emigration
Changes in plant abundance:
Increases: seed/spore germination
Decreases: death or consumption
Often shown using graphs
Factors Affecting Distribution & Abundance
Abiotic Factors (non-living)
Light availability
Wind strength & rainfall
Temperature (seasonal/daily)
Topography (shape of the land)
Tidal strength
Water availability, salinity, and pH
Space & shelter
Biotic Factors (living)
Seasonal food availability
Number of competitors
Number of mates
Number of predators
Pathogens (diseases)
Population Estimates
Why estimate?
Counting every organism is impractical, time-consuming, and damaging to the environment.
Scientists use sampling techniques to estimate abundance and distribution.
Capture-Recapture Method
Best for mobile animals
Steps:
Capture animals and tag them
Release them
After time, recapture another sample
Count how many tagged individuals are recaptured
Formula for abundance:
\text{Abundance} = \frac{\text{Number tagged} \times \text{Number recaptured}}{\text{Average number of tagged recaptured}}
Quadrats
Best for stationary organisms (e.g. plants, fungi)
A quadrat is a square, rectangular, or circular frame of a set size.
Used to estimate population density or % cover in selected areas
Can be:
Randomly placed
Placed at intervals along a transect or grid
Transects
Used for non-random sampling
A transect is a straight line across an area.
Record organisms only along the line
Useful for observing the relationship between organisms and abiotic factors
E.g. tracking frog distribution across a rainforest
Extinction
What is Extinction?
Extinction = when no individuals of a species remain alive.
Once extinct, a species is gone forever from Earth.
Types of Extinction:
Background Extinction
Happens gradually over time
Caused by natural selection and environmental changes
Common and ongoing process in nature
Mass Extinction
A sudden and widespread loss of many species
Occurs in a relatively short time
Can drastically change ecosystems
What is Mass Extinction?
A sharp decrease in the number of species in a short period.
Involves the rapid loss of biodiversity on a global scale.
Causes of Mass Extinctions:
Asteroids (e.g. the event that wiped out the dinosaurs)
Comets
Global warming (rapid climate changes)
Ice ages (extreme cooling and glaciation)
Key Takeaways for Exam Preparation
Ecosystems are governed by a balance between abiotic and biotic factors.
Selection pressures drive adaptation and natural selection.
Abiotic factors shape where organisms live and how they survive; sunlight, temperature, weather, water, shelter, topography, and soil are central.
Biotic factors include food, competition, predation, mating opportunities, and diseases; interactions among organisms determine distributions and abundances.
Community interactions (predation, competition, symbiosis) drive energy flow and biodiversity.
Niches, resource partitioning, and food chains/webs explain how energy and resources move through ecosystems.
Trophic levels and biomass explain why bottom levels contain more mass than top levels; energy transfer is inefficient.
Population distribution and abundance vary in space and time; sampling methods (capture-recapture, quadrats, transects) estimate populations without exhaustive counting.
Extinction occurs naturally (background) and in mass events; mass extinctions have major global ecosystem impacts and diverse causes.
Notation and Formulas to Remember
Abundance in capture-recapture:
\text{Abundance} = \frac{\text{Number tagged} \times \text{Number recaptured}}{\text{Average number of tagged recaptured}}Example food chain: \text{Phytoplankton} \rightarrow \text{Zooplankton} \rightarrow \text{Small Fish} \rightarrow \text{Large Fish}
No explicit numerical constants are given beyond the formula above; remember the general concepts: energy transfer efficiency, biomass distribution, and the role of producers, consumers, and decomposers.