How do ecosystems change over time and recover after a disturbance?
Predict the impact of environmental change on the stability of an ecosystem, including ecological succession (primary and secondary) and the effects of natural and human-induced disturbances (GSE SB5.d).
A Georgia Milestones Biology EOC answer on environmental change: ecological succession (primary versus secondary), pioneer and climax communities, and how natural and human-induced disturbances affect the stability of an ecosystem.
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What this topic is asking
Standard SB5.d asks you to predict the impact of environmental change on an ecosystem's stability. For the Georgia Milestones Biology EOC you must explain ecological succession (the difference between primary and secondary), know pioneer and climax communities, and reason about how natural and human-induced disturbances affect stability. Items often describe a disturbance and ask which type of succession follows or how the ecosystem will respond.
Ecological succession
Succession describes how an ecosystem changes and recovers. It comes in two types, and telling them apart is the most-tested point.
Primary versus secondary succession
In both cases, succession moves through stages: pioneer species, then a sequence of communities, toward a relatively stable climax community (such as a mature forest) that persists until the next major disturbance.
Disturbance and stability
A disturbance is an event that changes an ecosystem. Natural disturbances include fires, floods, storms, and volcanic eruptions; human-induced disturbances include deforestation, pollution, and introducing invasive species. Disturbances can reduce stability by removing species and breaking food-web links.
How well an ecosystem recovers depends on:
- the severity of the disturbance (a small fire versus complete clearing),
- whether the soil survives (secondary succession recovers faster than primary), and
- the ecosystem's biodiversity (more diverse systems tend to recover more readily, linking to the biodiversity-and-stability topic).
Try this
Q1. State the key difference between primary and secondary succession. [2 points]
- Cue. Primary succession starts where there is no soil (bare rock) and is slow; secondary succession starts where the soil remains after a disturbance and is faster.
Q2. Name the first organisms to colonize in primary succession. [1 point]
- Cue. Pioneer species, such as lichens and mosses, which can grow on bare rock and help form soil.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Milestones (style)1 marksA forest is destroyed by a fire, but the soil remains. The community that regrows over time is an example of: (A) primary succession (B) secondary succession (C) a climax community forming on bare rock (D) carrying capacityShow worked answer →
A 1-point selected-response item on succession.
The correct answer is B. Secondary succession occurs when a disturbance (such as a fire, flood, or farming) clears an area but the soil remains, so the community regrows relatively quickly from the surviving soil, seeds, and roots. Primary succession (A and C) starts where there is no soil at all (bare rock, such as after a volcanic eruption or a retreating glacier) and takes much longer because soil must form first. The presence of soil after the fire is the clue for secondary succession.
Milestones (style)2 marksExplain the difference between primary and secondary succession, and name the first organisms to colonize in primary succession.Show worked answer →
A 2-point comparison item.
Primary succession begins in an area with no soil (bare rock, such as after a volcanic eruption or glacier retreat), so it starts from scratch and takes a very long time because soil must form first. Secondary succession begins where a disturbance has cleared an existing community but the soil remains (after a fire or abandoned farmland), so it proceeds faster because the soil, seeds, and roots survive. The first organisms to colonize in primary succession are pioneer species, such as lichens and mosses, which can grow on bare rock and help break it down into soil. Full points need the soil-versus-no-soil difference and pioneer species (lichens/mosses) named.
Related dot points
- Evaluate the factors that affect biodiversity and the stability of ecosystems, including keystone species, the effects of removing species, and symbiotic relationships (GSE SB5.c).
A Georgia Milestones Biology EOC answer on biodiversity and stability: why diverse ecosystems are more stable, the role of keystone species, the effects of removing a species, and the three types of symbiosis (mutualism, commensalism, parasitism).
- Analyze data on population growth, including exponential and logistic growth, carrying capacity, and limiting factors (density-dependent and density-independent) (GSE SB5.a).
A Georgia Milestones Biology EOC answer on population growth: exponential versus logistic growth, carrying capacity, density-dependent and density-independent limiting factors, and how to read a population growth curve.
- Evaluate the impact of human activities on ecosystems (habitat destruction, pollution, invasive species, climate change) and design solutions to reduce that impact (GSE SB5.c, SB5.e).
A Georgia Milestones Biology EOC answer on human impact: habitat destruction, pollution, invasive species, and climate change, their effects on biodiversity and ecosystem stability, and conservation solutions to reduce the impact.
- Analyze the flow of energy through ecosystems using food chains, food webs, and energy pyramids, including the roles of producers, consumers, and decomposers and the ten percent rule (GSE SB5.b).
A Georgia Milestones Biology EOC answer on energy flow: producers, consumers, and decomposers, trophic levels, food chains and food webs, the ten percent rule, and why energy pyramids narrow toward the top.
- Analyze the cycling of matter through ecosystems, including the carbon, nitrogen, and water cycles, and the roles of photosynthesis, respiration, and decomposers (GSE SB5.b).
A Georgia Milestones Biology EOC answer on the cycling of matter: the carbon cycle (photosynthesis and respiration), the nitrogen cycle (fixation by bacteria), and the water cycle, and how decomposers recycle nutrients, contrasted with the one-way flow of energy.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)