Ecosystems push back against change — until they can't. The mathematics of resilience and tipping points.
Every sub-topic below feeds at least one of these questions.
What features of ecosystems allow stability over unlimited time periods?
What changes caused by humans threaten the stability of ecosystems?
The required syllabus content for D4.2, in order. Each card is one lesson-sized checkpoint.
Illustrate ecosystem stability with evidence of forest, desert or other ecosystems that have shown continuity over long periods.
Requirements for stability in ecosystems
Deforestation of Amazon rainforest as an example of a possible tipping point in ecosystem stability
Use of a model to investigate the effect of variables on ecosystem stability
Role of keystone species in the stability of ecosystems
Assessing sustainability of resource harvesting from natural ecosystems
Factors affecting the sustainability of agriculture
Eutrophication of aquatic and marine ecosystems due to leaching
Biomagnification of pollutants in natural ecosystems
Effects of microplastic and macroplastic pollution of the oceans
Restoration of natural processes in ecosystems by rewilding
Stable ecosystems resist or recover from disturbances. Two distinct properties: resistance (not changing) and resilience (recovering after change).
A resistant ecosystem absorbs disturbances without major shift. Examples: deep forests with intact canopy can absorb small fires; old-growth ecosystems with many species can absorb loss of one without collapse.
A resilient ecosystem may be disrupted by a disturbance but returns to its previous state. Examples: many grasslands recover quickly after fire; tidal communities recover after storms.
Sources of stability include high biodiversity (functional redundancy — multiple species can fill a role); intact food webs; stable abiotic conditions.
Most ecosystem change is gradual — but some changes are abrupt and effectively irreversible. A small additional push beyond a threshold can flip the system to a new state.
Examples:
Some species have effects on their ecosystems disproportionate to their abundance. Remove them and the ecosystem reorganises dramatically.
After disturbance, communities re-form through a predictable sequence of species — pioneer to climax.
Starts on lifeless substrate — fresh lava, glacial moraine, bare sand. Pioneer species (lichens, mosses) colonise first; they build soil. Then grasses, shrubs, trees follow over centuries.
Soil is still present (fire, abandoned farmland, flood). Recovery is much faster — decades rather than centuries — because soil and dormant seeds are already in place.
Eventually a climax community forms — relatively stable, dominated by long-lived species adapted to the local climate (oak forest in temperate Europe, tropical rainforest in equatorial zones, tundra near the poles).
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Ecological succession and its causes
Changes occurring during primary succession
Cyclical succession in ecosystems
Climax communities and arrested succession
If you can't define one of these in a sentence, that's where to revise next.
“What is the distinction between artificial and natural processes?”
“Over what timescales do things change in different biological systems?”