Topic 2: Ecosystems and Ecology - ESS

IB Syllabus: First taught 2015 and last exams 2025

Define the term species. (3 marks)

Define the term species. (3 marks)

A group of organisms that share common characteristics that interbreed to produce fertile offspring.

List three biotic interactions occurring between organisms. (3 marks)

1. Predation

2. Competition

3. Parasitism

Define the term limiting factor in the context of environmental systems and ecology. (4 marks)

A limiting factor is any biotic or abiotic factor that restricts the growth of an organism / species / population.

Briefly explain what an S population curve represents. (2 marks)

An S population curve represents logistic growth ending with the population reaching a stable carrying capacity.

Briefly explain what a J population curve represents. (2 marks)

A J population curve represents exponential growth ending with the population overshooting it’s carrying capacity.

Define the term community in the context of environmental systems and ecology. (2 marks)

A group of two or more species living within the same ecosystem.

State the word equation for aerobic respiration. (2 marks)

Glucose + oxygen —> Carbon dioxide + water (+ energy)

State the main energy transformation occuring during respiration (1 mark).

Stored chemical energy into kinetic energy + heat.

State the units for net primary productivity. (1 mark)

Joules per second per metre squared

State the formula for calculating net primary productivity (1 mark)

Gross primary productivity - respiratory losses // GPP - r

Outline what is meant by the term biome. (3 marks)

A collection of ecosystems sharing similar climatic conditions resulting in similar biodiversity.

Insolation is the amount of solar radiation received by given area of land in a given time period. It is one of the main factors governing the distribution of biomes. State the other two. (2 marks)

Precipitation and temperature

The figure below shows an example of a tool that can be used to help ecologists identify the organisms present within an ecosystem. State the name of this tool.

Dichotomous key

Define the term biotic factor.

The living components of an ecosystem.

Define the term abiotic factor.

The physical and chemical components of an ecosystem.

Define the term ecosystem.

The community of interacting organisms and their physical environment.

Define the term habitat.

The environment in which a species normally lives

Generalists

eat a variety of foods (e.g. pig)

Specialists

eat a few / one type of food (e.g. panda)

Define the term ecological niche.

A role taken by a type of organism within its community.

Distinguish between a fundamental niche and a realised niche.

A fundamental niche is the set of resources a population is theoretically capable of using under ideal conditions. Whereas, a realised niche are the resources a population actually uses.

Define the term carrying capacity.

The number of organisms a region can support without environmental degradation.

List the 3 interspecific relationships.

Predator - prey, parasitic, and mutualistic.

Name the 3 different types of ecological pyramids.

Pyramid of numbers, biomass, and productivity.

Pyramid of numbers

numbers of individuals within a population

Units of measure: Pyramid of numbers

Trophic levels

Units of measure: Pyramid of biomass

g/m²

Units of measure: Pyramid of productivity

J/m²/yr

Pyramid of numbers: Advantages

Non-destructive method of data collection, compare changes in ecosystem over time

Pyramid of numbers: Disadvantages

All organisms are difficult to count, does not allow for juveniles

Pyramid of biomass: Advantages

Overcomes the limitations in pyramids of numbers

Pyramid of biomass: Disadvantages

Biomass can change depending on seasons, not all organisms have the same calorific value, ethical issues

Pyramid of productivity: Advantages

The most accurate pyramid, shows energy transfer, ecosystems can be compared

Pyramid of productivity: Disadvantages

Difficult to accurately measure energy, long process

Define the term bioaccumulation.

The build-up of persistent or non-biodegradable pollutants in a single organism over the span of its life, resulting in a higher concentration in older individuals.

Define the term biomagnification.

The increase in either persistent or non-biodegradable pollutant concentration along a food chain.

Distinguish between persistent and non-biodegradable pollutants by naming some examples.

Persistent: rubbish, sewage

Non-biodegradable: DDT, plastic, heavy metal

Distinguish between persistent and non-biodegradable pollutants in relation to rate of degradation.

Persistent: Rapid

Non-biodegradable: Slow

Minamata Bay case study

• 1956

• Chisso Corporation released methylmercury into waste water

• Mercury accumulation in fish and shellfish caused mercury poisoning

DDT case study

• Used widely post-WW2 but banned 1972

• Leach into waterways and accumulated across a span of organisms

• Led to thinning of eggshells, reduced reproductive success, caused bird-of-prey population declines

State the word equation for photosynthesis. (2 marks)

Carbon dioxide + water —> Glucose + oxygen

What happens to some of the solar radiation when it enters Earth’s atmosphere?

It becomes unavailable for ecosystems because it gets absorbed by inorganic matter or is reflected back into the atmosphere.

How much of the sunlight available from the Sun is converted into biomass in ecosystems?

About 51% of available sunlight never reaches producers, meaning little sunlight is converted into biomass.

What are the approximate percentage losses of radiation through reflection and absorption in the Earth’s atmosphere?

Reflection from clouds: 19%

Absorption by clouds: 3%

Reflection from aerosols and particles: 3%

Absorption by molecules and dust: 17%

Reflection from the Earth’s surface: 9%

What is the term used to describe the ability of clouds and reflective surfaces to reflect solar radiation?

Albedo effect

What proportion of solar radiation absorbed by the ground ends up in producers?

Only a small proportion, with around 0.06% of all solar radiation captured by plants.

Why does incoming solar radiation fail to enter chloroplasts in leaves?

Because it is either reflected, transmitted through the leaf, or is the wrong wavelength to be absorbed.

Why is the conversion of light to chemical energy in plants inefficient?

Because only a small percentage of radiation captured by leaves ends up as biomass, and the conversion process itself is inefficient.

Formula for calculating ecological efficiency

(Energy used for new biomass / energy supplied) x 100

Definition of ecological efficiency

Percentage of energy transferred from one trophic level to the next

Reasons for energy losses

Heat, waste products, inedible products, movement

Definition of gross primary productivity

The rate at which plants are able to store chemical energy via photosynthesis.

Definition of net primary productivity

The rate at which energy is stored in plant biomass, allowing for respiratory losses.

Definition of secondary productivity.

The total energy/biomass assimilated by consumers.

How to calculate gross secondary productivity?

Food eaten - faecal loss

How to calculate net secondary productivity? (NSP)

GSP - respiratory losses

Units of GSP and NP

units of biomass / energy per unit area or volume per unit time

Maximum sustainable yield

The maximum amount of a renewable natural resource that can be harvested annually without comprising the long-term productivity of the resource

Name the two biogeochemical cycles.

The carbon cycle and the nitrogen cycle.

Sinks in the carbon cycle

Atmosphere, rocks, oceans, soil, sea life, organisms, fossil fuels.

Flows in the carbon cycle

Photosynthesis, respiration, feeding, decomposition, fossilisation, combustion, and dissolving.

Photosynthesis

Is an endothermic process in which carbon dioxide and water turns into glucose and oxygen. The glucose is used for energy and oxygen will be intaken by other living organisms. Only occurs in plants, specifically the chloroplasts. Converts light energy into stored chemical energy.

Chemical equation for photosynthesis

6CO₂ + 6H₂0 —> C₆H₁₂O₆ + 6O₂

Respiration

Exothermic process that converts glucose and oxygen into carbon dioxide and water. There is anaerobic and aerobic respiration. Occurs in the mitochondria, which is in the cytoplasm. Stored chemical energy changes into kinetic energy and heat.

Chemical equation for respiration

C₆H₁₂O₆ + 6O₂ —> 6CO₂ + 6H₂O

Decomposition

Complex organic matter into simpler inorganic matter. Decomposers: fungi, maggots, bacteria. Types: thermal, chemical, electrolytic, photolytic. AB —> A + B

Combustion

Chemical exothermic reaction between a fuel and an oxidising agent to product energy - usually as heat or light. There is complete and incomplete combustion - complete combustion example = burning of candle. Incomplete combustion example = burning of fossil fuels. Incomplete combustion produces carbon monoxide —> binds with haemoglobin, leads to suffocation.

Human impacts on the carbon cycle

Burning fossil fuels, deforestation, urbanisation, and agriculture.

How does burning fossil fuels affect the carbon cycle?

CO₂ is being returned to the atmosphere faster than it can be absorbed = warmer temperatures = less CO₂ dissolves in oceans = released into the air = causes climate change

How does deforestation affect the carbon cycle?

Mass deforestation = reduces amount of producers to take carbon dioxide out of the atmosphere = more carbon released into atmosphere

How does urbanisation affect the carbon cycle?

Urbanisation = less carbon sinks as they’re replaced with buildings, roads, and infrastructure = reduces amount of carbon that can be sequestered in plants and soil = carbon capacity of the land is decreased

Sinks in the nitrogen cycle

Organisms, soils, fossil fuels, atmosphere, water bodies

Differentiate between the organic and inorganic sinks of the nitrogen cycle.

Organic: organisms, fossil fuels

Inorganic: soil, atmosphere, water bodies

Flows in the nitrogen cycle

Nitrogen fixation by bacteria and lightning, absorption, assimilation, consumption, excretion, decomposition, denitrification

Summary of carbon cycle

Plants take in carbon dioxide from the atmosphere to produce oxygen and glucose —> animals and plants release CO₂ back into the atmosphere through respiration —> when organisms die, decomposers break them down, releasing carbon back into the soil and atmosphere —> burning of fossil fuels and biomass release stored carbon back into the atmosphere as CO₂ —> oceans absorb CO₂ from the atmosphere.

Summary of nitrogen cycle

Nitrogen-fixing bacteria in the soil / root nodules of plants convert atmospheric nitrogen (N₂) into ammonia to be used by plants —> ammonia is converted into nitrites (NO₂^-) and then nitrates (NO₃^-) by nitrifying bacteria which plants can absorb —> plants take up nitrates from the soil and use them to build proteins to be passed along the food chain —> when plants and animals die, decomposers break down organic matter, nitrogen returns to the soil as ammonia —> denitrifying bacteria convert nitrates back into nitrogen gas (N₂) into the atmosphere.

Human impacts on the nitrogen cycle

Fertilisers, burning of fossil fuels, industrial nitrogen fixation, land-use changes, livestock farming, wastewater treatment

How does increased use of fertilisers affect the nitrogen cycle?

Increased use = excess nitrogen can leach into waterways = eutrophication and algal blooms

How does burning of fossil fuels affect the nitrogen cycle?

Burning fossil fuels releases nitrogen oxides into atmosphere = acid rain = increases soil acidity = plants struggle to take up nitrogen

How does land-use changes affect the nitrogen cycle?

Converting natural landscapes into urban areas = removal of nitrogen sinks = more nitrogen is released into waterways and the atmosphere

How does livestock farming affect the nitrogen cycle?

Large amounts of manure and urine = increased nitrogen - if not properly managed = eutrophication