D4.2 Stability & Change

Define ecosystem stability

An ecosystem’s ability to maintain structure and function over time, despite environmental changes

Define resistance (in context of ecology)

The ability of an ecosystem to withstand a disturbance without significant change

Define resilience (in context of ecology)

The ability of an ecosystem to recover and return to its original state after a disturbance

Describe how different ecosystems show stability [3]

Ecosystem	Evidence of stability
Tropical rainforests	Fossil records show rainforests in similar form for over 100 million years
Deserts	Exist in some form for millions of years, w/ specialised plants & animals adapted to arid conditions
Coral reefs	Persisted for millions of years, showing resilience & recovery after storms/bleaching events

Describe the significance of requirements to ecosystem stability [4]

Requirement	Significance to stability
Energy supply	Mainly through solar energy, which are captured by photosynthetic plants This energy flows through the ecosystem via food chains/webs
Recycling of nutrients	Nutrients like carbon, nitrogen, & phosphorous must be continuously recycled to support plant growth  Decomposers (bacteria, fungi) break down dead matter, returning nutrients to soil & water
Genetic diversity	High genetic diversity within species allow populations to build resilience & even resistance toward environmental changes (e.g. diseases)
Climatic variables maintained within tolerance levels	Extreme changes can stress or kill species, disrupting ecosystem stability

Describe the natural conditions of the Amazon rainforest [2]

• The Amazon rainforest generates huge amounts of atmospheric water vapour via transpiration 

• Water vapour helps cool the air, drive airflows, & sustain rainfall levels

Explain how deforestation drives the Amazon rainforest to a tipping point [4]

Deforestation creates disturbance which may cause the rainforest to reach a tipping point:

1. Cutting down trees reduces transpiration

2. This lowers rainfall & raise temperatures → eventually undergoing irreversible changes (e.g. land drying out) 

3. As a result, parts of the rainforest would convert to another ecosystem type, especially grassland 

What is the formula of percentage change?

Percentage change =

(Final area - Current area) / Original area x 100

What is a mesocosm, and its most successful forms?

• A mesocosm is a controlled experimental ecosystem model used to study ecological processes in a natural environment 

• Aquatic or microbial ecosystems are often more successful than terrestrial because ease of maintaining balance, faster life cycle for quick observations, ease of replication

Describe components required for a mesocosm

1. Autotrophs (producers) - Plants or algae capture light, produce oxygen & organic matter

2. Consumers - Herbivores & carnivores feed through the food web

3. Decomposers - Bacteria & fungi break down waste & recycle nutrients

4. Abiotic factors - Soil, water, light, & temperature dictate the physical conditions organisms live in

Compare the types of mesocosms [4]

1) Open mesocosm - 

• They exchange matter with surroundings

• Suits testing a specific variable, e.g. pollution

2) Sealed mesocosm -

• They allow energy such as sunlight pass in but stop matter from transferring

• Suits longitudinal closed-system studies

Define keystone species

Species that have a disproportionate impact on their ecosystem relative to their abundance

Describe the roles of keystone species, and consequence if removed [4]

1) Regulates populations of other species

2) Maintains balance by controlling dominant species (that would otherwise outcompete everything else) 

3) Support ecosystem functions (inc. nutrient cycling, habitat formation, & energy flow)

→ Hence, if keystone species are removed, ecosystems risk drastic changes in community structure (e.g. overpopulation, extinction) & even collapse

What is sustainability in resource harvesting? [3]

• Sustainability means harvesting resources at a rate lower than their natural replacement 

• This ensures resources are available for future generations

  • Hence, preventing ecosystem degradation & species decline 

[Example] Brazil nuts (Bertholletia exclesa) & their sustainable harvesting [2]

• Bertholletia excelsa trees in the Amazon rainforest are lost to logging 

  • Sustainable harvesting practice: Control logging by leaving some nuts to germinate & grow into trees → protecting Agoutis (large rodent) populations which feed on nuts 

[Example] Atlantic cod (Gadus morhua) and their sustainable harvesting [3]

• Atlantic cod populations collapsed due to overfishing

  • Sustainable harvesting practices: 

1. Use surveys to estimate population sizes & reproductive rates 

2. Fishing quotas limit catch so it aligns with recovery rates 

3. Create exclusion zones to allow undisturbed breeding 

What is sustainability in agriculture?

It refers to producing food without damaging the environment or depleting resources, enabling long-term farming

Describe how different factors affect sustainability [9]

Factor affecting sustainability	Explanation
Soil erosion	Wind, water runoff, & poor farming practices can cause loss of topsoil  This reduces soil fertility → lowering crop yields & land degradation
Leaching of nutrients	Nutrients wash away from soil into water bodies (e.g. nitrogen & phosphorus)  This causes soil nutrient loss & eutrophication
Supply of fertilisers	Fertilisers replace nutrients lost from soil Overuse leads to pollution & can harm beneficial soil organisms
Pollution due to agrochemicals	Pesticides & herbicides can contaminate soil, water, & harm non-target species  This can affect human health & biodiversity
Carbon footprint	Agriculture contributes to greenhouse gas emissions (methane, nitrous oxide) through fuel use for machinery, fertiliser production, & livestock digestion

Define eutrophication

The excessive growth of algae & aquatic plants caused by high nutrient levels

Describe the process of eutrophication [2]

1. Fertilisers applied to farmland can wash away (leach) into rivers, lakes, & oceans  

2. Increased nutrients, especially nitrates & phosphates stimulate rapid algae grow

Describe the process in which eutrophication affects marine ecosystems [4]

1. Algal blooms block sunlight, killing underwater photosynthetic plants 

2. When plants/algae die, decomposers consume oxygen to break them down, increasing Biochemical Oxygen Demand (BOD) 

3. High BOD = less oxygen is available for fish & other aquatic animals

4. This creates a hypoxic (low oxygen) environment which suffocates & kills fish → loss of biodiversity 

Define biomagnification

The increase in toxin concentration in the tissues of organisms at higher trophic levels

Define bioaccumulation

The increase in toxin concentration within a single organism over its lifetime

Describe the process of biomagnification [4]

1. Pollutants like DDT and mercury, enter the ecosystem at low levels (e.g. in water or soil)

2. Producers (plants & algae) absorb or accumulate toxins 

3. Primary consumers eat producers & toxin concentration increases in their tissues

4. Predators have the highest toxin levels as they consume numerous contaminated prey, risking health effects (e.g. developmental & reproductive issues)

What are the different plastic types that pollute oceans?

• Microplastics are tiny plastic particles formed from breakdown of larger plastic or microbeads in products

• Macroplastics are large plastic debris (e.g. bags, bottles, fishing nets)

Explain the effect of plastic pollution, and give examples [2×3]

Type of plastic	Effect on marine life	Examples
Macroplastics	Entanglement lead to injury or suffocation Ingestion causes internal injury or starvation	Turtles trapped in plastic bags
Microplastics	Ingestion by small marine animals introduce plastic to food chains  They emit toxic chemicals which can be absorbed from the environment	Fish & plankton consuming microplastics

Describe different methods of rewilding [5]

1) Reintroduction of apex predators & keystone species

• Apex predators control prey populations, while keystone species support ecological balance 

2) Re-establishment of habitat connectivity

• Connecting isolated habitats across large areas help species migrate, breed, & survive

• This prevents genetic isolation & support ecosystem resilience 

3) Minimising human impact 

• Ecological management reduces human disturbances like farming, logging, & pollution

• This encourages natural regeneration & species recovery

[CS] Rewilding effort - Hinewai Reserve, New Zealand

• A nature reserve where farming was stopped to allow natural regeneration

• 1250 hectares of farmland allowed to be returned to native forest 

• Result: Native forests & wildlife have recovered w/o active planting or predator control

Define ecological succession

The gradual, natural change in the species composition of an ecosystem over time

State abiotic and biotic causes of succession [6]

1) Abiotic changes -

• Soil formation/erosion

• Light availability

• Water availability

• Temperature 

2) Biotic changes - 

• Arrival or removal of species (e.g. invasive species)

• Competition between species

Describe what is primary ecological succession [2]

• A type of ecological succession (change in species composition) that begins on bare, lifeless land 

• Over time, the ecosystem will develop from simple to complex

Describe the changes during primary succession [5]

Aspect	Change
Plant size	Small pioneer species (mosses, lichens) grow first, gradually replaced by larger plants (shrubs & trees)
Primary production	Amount of energy captured by plants increases as biomass builds up
Species diversity	Starts low, increases as more species colonise & survive
Food web complexity	Simple food chains develop into complex food webs w/ multiple trophic levels
Nutrient cycling	Initially limited, improves as plants & decomposers add organic matter to soil

[Example] Primary succession - Volcanic Island formation [4]

1. Bare rock is colonised by lichens & mosses, which break down rock into soil

2. Soil builds up, allowing grasses & herbs to grow

3. Over decades, shrubs & trees establish, which support animals 

4. Nutrient cycling improves as dead organic matter decomposes

Describe features of cyclical succession, and their causes? [3]

• Cyclical succession refers to ecosystems where communities cycle repeatedly through different stages 

• The ecosystem does not settle into one climax community but experiences regular, predictable changes over time
  
  

• This is caused by periodic disturbances or natural cycles → resetting ecosystems to an earlier stage 

[Example] Cyclical succession - Heathland ecosystem

• Heathlands cycle between the following stages:

1. Grass-dominated 

2. Shrub-dominated 

3. Tree-dominated 

• Natural fires or grazing can reset succession to earlier stages → maintaining biodiversity & ecosystem functions

What is a climax community?

• A climax community is the final, stable stage of ecological succession

• It depends on local environment conditions (e.g. climate, soil) → remaining relatively unchanged unless disturbed 

Define arrested succession, and give examples [3]

• Definition: When human activities prevent an ecosystem from reaching its climax community, keeping it in an earlier stage

Human influence	Effect on succession
Grazing by farm livestock	Continuous grazing prevents growth of shrubs & trees  → maintaining early succession stages (e.g. grassland) instead of forest climax communities
Drainage of wetlands	Draining wet areas stop wetland plants from establishing → preventing natural succession to wetland climax communities