Stability and change

the capacity of an ecosystem to maintain its structure, biodiversity, and functional processes (such as nutrient cycling) over time, even when subjected to disturbances or environmental changes

A stable ecosystem maintains its structure and function over time, despite disturbances.
Example: The Amazon rainforest
= High biodiversity increases resilience
= Efficient nutrient cycling maintains productivity despite poor soils
= Relatively stable climate (temperature and rainfall) supports continuous growth

= Continuous energy input (usually sunlight)
= Efficient nutrient cycling with minimal losses
= High genetic and species diversity
= Environmental conditions within tolerance limits

Human-induced: deforestation, pollution, overfishing, climate change
Natural: wildfires, floods, droughts, hurricanes

A tipping point is the threshold at which a small change causes a rapid and often irreversible shift to a new ecosystem state.

In the Amazon rainforest:

- Trees contribute to rainfall via transpiration
- Deforestation reduces transpiration → reduced rainfall
- Leads to drier conditions, increasing tree mortality
- Positive feedback loop accelerates change
- May shift ecosystem to savannarassland biome
- Results in loss of biodiversity and carbon storage

A mesocosm is a controlled, enclosed experimental system that simulates natural environmental conditions.

Energy source (light for photosynthesis)
Nutrient recycling (decomposers present)
Balanced community of producers, consumers, decomposers
Stable environmental conditions (temperature, water)

Continuous energy input
- Usually light (sunlight or artificial) for photosynthesis by producers
Nutrient recycling
- Presence of decomposers (bacteria/fungi) to break down waste and return nutrients to the system
- System must be closed to matter (no nutrient loss)
Balanced community structure
-Producers (e.g. plants/algae) to capture energy
-Consumers (e.g. small invertebrates) to transfer energy
-Detritivores/decomposers to recycle nutrients
Stable abiotic conditions
-Appropriate water, oxygen, and CO₂ levels
-Temperature kept relatively constant
Closed system for matter, open for energy
-Matter is recycled internally, while energy enters (light) and exits (heat)

Benefits:
Variables can be controlled and manipulated
Easier to replicate experiments
Allow study of ecosystem processes in simplified systems

Limitations:
Reduced biodiversity compared to natural ecosystems
May not accurately represent complex interactions
Edge effects and small scale limit realism

A keystone species has a disproportionately large effect on ecosystem structure and biodiversity relative to its abundance.

Resource harvesting is the removal of natural resources from an ecosystem for human use.

Sustainable harvesting is the use of resources at a rate that does not exceed their natural regeneration, ensuring long-term availability.

Marine (fish populations):
= Harvest must not exceed reproductive rate
= Overfishing reduces breeding population → population decline
= Requires monitoring and quotas

Terrestrial (forests):
= Trees harvested ≤ trees regrowing
= Ensures long-term forest stability
= Prevents habitat loss and soil degradation

MSY = maximum rate of harvesting without reducing population size
Occurs at half the carrying capacity (K/2) on a logistic growth curve
At this point, population growth rate is highest

Evaluation:
Sustainable if harvesting = growth rate
Risky due to:
= Environmental variability
= Inaccurate data
= Ignoring population structure
= Often safer to harvest below MSY

Soil erosion reduces fertility
Leaching removes nutrients
Harvesting crops removes nutrients → depletion
Agrochemical runoff pollutes ecosystems
Carbon emissions contribute to climate change

Causes:
= Fertilizers (nitrates, phosphates)
= Sewage and waste runoff

Effects:
= Algal blooms block light
= Plants die → decomposers increase
= Oxygen depletion (hypoxia)
= Death of aquatic organisms

Biomagnification is the increase in concentration of toxins at higher trophic levels in a food chain.

Mercury (example)
Cause:
= ndustrial pollution releases mercury into water
= Mercury is converted to methylmercury (toxic form)
= It is absorbed by plankton and passed up the food chain

Effect:
= Concentration increases at each trophic level
= Top predators (e.g. large fish, humans) accumulate the highest levels
=Causes neurological damage and developmental issues

Cause:
= Widespread use of DDT pesticide in agriculture
= Enters ecosystems and is not easily broken down (persistent)
= Accumulates in organisms and passes through food chains

Effect:
= Biomagnifies in top predators (birds of prey)
= Causes eggshell thinning → eggs break during incubation
= Leads to population declines (e.g. peregrine falcon, bald eagle)
= Resulted in bansestrictions on DDT use

A biodegradable substance can be broken down by decomposers into simpler substances.

Macroplastics: large visible plastic items

Microplastics: particles <5 mm in size

cause:
Formed by breakdown of larger plastics
Ingested by marine organisms

effect:
Cause toxicity, physical damage, and bioaccumulation
Can enter food chains and affect higher trophic levels

Rewilding is the restoration of ecosystems by reducing human impact and allowing natural processes to recover.

Removal of human pressures (e.g., farming)
Reintroduction or protection of native species
Controlemoval of invasive species
Allow natural succession and ecosystem recovery

At Hinewai Reserve:

Land was cleared for farming
Farming stopped and land purchased for conservation
Grazing animals (goats/deer) controlled
Native vegetation naturally regenerated
Demonstrates passive rewilding success

((Orginal area - now are)/Orginal area) * 100