Why does greater biodiversity make an ecosystem more stable?
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).
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What this topic is asking
Standard SB5.c asks you to evaluate factors that affect biodiversity and the stability of ecosystems. For the Georgia Milestones Biology EOC you must explain why greater biodiversity brings stability, the role of a keystone species, the effects of removing a species, and the three kinds of symbiotic relationship. Items often ask why diversity matters or to classify a relationship between two species.
Biodiversity and stability
Keystone species and removing a species
A keystone species is one whose effect on the ecosystem is disproportionately large relative to its abundance. Removing it can cause dramatic, cascading change, because many other species depend on it directly or indirectly (for example, a top predator that keeps a prey population in check; without it, the prey may overgraze and collapse the system).
More generally, removing any species affects the food web. Because organisms are connected, the loss of one species can change the populations of its predators, prey, and competitors, sometimes far beyond the obvious link. This is why food webs (not just single chains) matter for predicting the effects of a change.
Symbiosis: how species interact
Symbiosis is a close, long-term relationship between two species. The three types are classified by the outcome for each partner:
- Mutualism: both species benefit (a bee gets nectar while pollinating a flower; both gain).
- Commensalism: one benefits and the other is unaffected (a bird nests in a tree without harming it).
- Parasitism: one benefits (the parasite) and the other is harmed (the host) (a tick feeding on a dog).
The test for classifying is to ask the outcome for each partner: both gain, one gains and one is neutral, or one gains and one is harmed.
Try this
Q1. Explain why a more biodiverse ecosystem is generally more stable. [2 points]
- Cue. Many species and overlapping food-web links provide redundancy, so the loss of one species is less likely to collapse the system; low-diversity systems are more fragile.
Q2. Classify a relationship in which one organism benefits and the other is harmed. [1 point]
- Cue. Parasitism (the parasite benefits, the host is harmed).
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 marksWhy is an ecosystem with high biodiversity generally more stable than one with low biodiversity? (A) It has fewer organisms to feed. (B) Many species and food-web links mean the loss of one species is less likely to collapse the system. (C) High biodiversity removes all competition. (D) It has no limiting factors.Show worked answer →
A 1-point selected-response item on biodiversity and stability.
The correct answer is B. A diverse ecosystem has many species and many overlapping food-web connections, so if one species declines, others can fill similar roles and the system is less likely to collapse. Low-diversity systems are more fragile because each species is more critical. A and D are false, and C is wrong because biodiversity does not remove competition. Redundancy and many links are why diversity brings stability.
Milestones (style)2 marksClassify each relationship as mutualism, commensalism, or parasitism: a tick feeding on a dog; a bee pollinating a flower while getting nectar; a bird nesting in a tree without harming it. Explain the difference.Show worked answer →
A 2-point classification item on symbiosis.
A tick feeding on a dog is parasitism: the tick (parasite) benefits and the dog (host) is harmed. A bee getting nectar while pollinating a flower is mutualism: both the bee (gets food) and the flower (gets pollinated) benefit. A bird nesting in a tree without harming it is commensalism: the bird benefits and the tree is neither helped nor harmed. The difference is the outcome for each partner: both benefit (mutualism), one benefits and the other is unaffected (commensalism), or one benefits and the other is harmed (parasitism). Full points need the correct classification of all three and the benefit-harm difference.
Related dot points
- 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 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.
- 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.
- 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 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)