Populations and sustainability — OCR A-Level Biology
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Populations and sustainability explained
Populations and sustainability covers 5 specification points, including (a) the factors that determine size of a… and (b) interactions between populations.
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Open a point below for its explanation and revision guidance.
(a) the factors that determine size of a population
Population size is the number of individuals of one species in a defined area. It is set by births and immigration adding individuals and by deaths and emigration removing them, so change in population = (births + immigration) − (deaths + emigration). These rates are themselves controlled by density-dependent factors, whose effect grows as numbers rise (competition for food, space or light; predation; disease; accumulation of toxic waste), and density-independent factors, whose effect does not depend on numbers (temperature extremes, drought, fire, pollution events). Carrying capacity is the maximum population an ecosystem can sustain; when numbers exceed it, mortality rises and the population falls back. Reproductive potential, generation time and the availability of resources also shape how quickly a population grows.
(b) interactions between populations
A population is all the organisms of one species in a habitat. Interactions between populations shape their sizes: predation, competition and mutualism each change birth and death rates. In predation, a rise in prey numbers supports more predators, whose increased numbers then reduce the prey, so the two populations oscillate out of phase. In interspecific competition, two species needing the same resource both grow more slowly, and the better-adapted species may exclude the other through competitive exclusion. Mutualism benefits both populations, for example nitrogen-fixing bacteria in root nodules supplying fixed nitrogen while receiving carbohydrates. Learners should predict how a change in one population alters another, and distinguish these interactions from abiotic factors such as temperature.
(c) the reasons for, and differences between, conservation and preservation
Conservation and preservation are both ways of protecting ecosystems but differ in intent. Conservation is the active, sustainable management of a resource so humans can keep using it: for example, rotational coppicing of woodland maintains habitat while yielding timber, and controlled fishing quotas keep a stock harvestable. Preservation is protection with minimal human interference, keeping an area in its existing state: for example, a nature reserve closed to logging and replanting. The reasons include maintaining biodiversity, protecting ecosystem services such as flood control and carbon storage, safeguarding genetic resources and meeting ethical or legal duties. Learners should classify a management example as conservation or preservation and justify the choice by whether sustainable use is intended.
(d) how the management of an ecosystem can provide resources in a sustainable way
Sustainable management means harvesting resources at a rate the ecosystem can replace, so the resource and the ecosystem's functions persist for future use. Learners should connect management techniques to the specific resource taken and to the ecological processes that keep it renewable. For example, coppicing woodland lets trees regrow from cut stumps, giving timber and fuel repeatedly while roots hold soil and support habitat. Rotational grazing prevents overgrazing so grass regrows and soil is not compacted. Controlled fishing with quotas and minimum mesh sizes lets breeding stock survive and replenish populations. In each case the management limits extraction, protects the regenerative capacity of the population or system, and maintains biodiversity and nutrient cycling. A good answer names the resource, the management method and the mechanism that makes it sustainable.
(e) the management of environmental resources and the effects of human activities.
This specification point asks you to connect two ideas: how environmental resources are managed, and how human activities affect ecosystems. Environmental resources include the land, water, air, minerals, forests and fisheries that humans use. Management means controlling how these are exploited so that use is sustainable — for example, setting fishing quotas, designating protected areas, rotating timber extraction, or regulating water abstraction. Human activities such as deforestation, overfishing, pollution, urbanisation and intensive agriculture can reduce biodiversity, degrade habitats and deplete resources. In an MCQ, expect to distinguish sustainable management from unsustainable exploitation, and to identify the specific effect of a named human activity on a named resource or population.
Your focus
- Explain how births, deaths, immigration and emigration change population size.
- Distinguish density-dependent from density-independent factors affecting a population.
- Relate carrying capacity and resource availability to sustainable population management.
Show all 15 objectives
- Describe how predation links the sizes of two populations over time.
- Explain how interspecific competition for a limiting resource affects two populations.
- Give an example of mutualism and state the benefit to each population.
- Define conservation and preservation and state the key difference between them.
- Explain at least two reasons for protecting ecosystems.
- Classify a management example as conservation or preservation and justify the choice.
- Explain how a named management technique allows a resource to be harvested sustainably.
- Link extraction rate to the regenerative capacity of a population or system.
- Distinguish sustainable management from over-exploitation in a given scenario.
- Identify examples of environmental resources and how they can be managed sustainably.
- Describe the effects of named human activities on resources, habitats or populations.
- Select the option that correctly links a management strategy or human activity to its effect.
Populations and sustainability exam tips
Marking Points
- Identifies births and immigration as additions and deaths and emigration as losses to population size.
- Distinguishes density-dependent factors (competition, predation, disease) from density-independent factors (temperature, drought, fire).
- Explains carrying capacity as the maximum sustainable population set by available resources.
- Links resource availability and reproductive potential to the rate at which a population can grow.
- Relates population factors to sustainability, for example keeping harvesting within carrying capacity.
- Defines a population as all organisms of one species in a habitat, and identifies an interaction as an effect of one population on another.
- Explains predation as a coupled cycle: more prey raises predator numbers, which then lowers prey numbers, so the peaks are out of phase.
- Explains interspecific competition as two species sharing a limiting resource, reducing both populations and potentially causing competitive exclusion.
- Describes mutualism as an interaction benefiting both populations, illustrated by nitrogen-fixing bacteria gaining carbohydrates while the plant gains fixed nitrogen.
- Distinguishes biotic interactions between populations from abiotic factors that also affect population size.
- Defines conservation as active, sustainable management allowing continued human use of a resource.
- Defines preservation as protection with minimal human interference, keeping an area in its existing state.
- Distinguishes the two by intent: conservation permits sustainable exploitation, whereas preservation excludes it.
- Gives reasons such as maintaining biodiversity, protecting ecosystem services, safeguarding genetic resources and ethical or legal duty.
- Classifies a given example, such as coppicing or a closed reserve, and justifies the classification.
- Identifies the resource being harvested (for example timber, fish, grazing, water) and links it to a named ecosystem.
- Describes a specific management technique (coppicing, rotational grazing, quotas, mesh-size limits, replanting) rather than a vague claim of 'looking after' the ecosystem.
- Explains the mechanism that makes it sustainable: extraction stays below the rate of replacement or regeneration.
- Links management to maintaining ecosystem function, such as soil stability, breeding populations, biodiversity or nutrient cycling.
- Distinguishes sustainable use from over-exploitation, where removal exceeds replacement and the resource declines.
- Recognises that environmental resources include land, water, air, minerals, forests and fisheries used by humans.
- Identifies management strategies such as quotas, protected areas, rotation of extraction or regulation of abstraction that aim to make use sustainable.
- Distinguishes sustainable management from unsustainable exploitation in a given scenario.
- Links a named human activity (for example deforestation, overfishing, pollution, urbanisation or intensive agriculture) to a specific effect on a resource, habitat or population.
- Recognises that human activities can reduce biodiversity and deplete resources, and that management aims to limit these effects.
Examiner Tips
- 💡Write the population change equation explicitly before substituting any figures.
- 💡Classify each named factor as density-dependent or density-independent and say why.
- 💡Use the term carrying capacity precisely when explaining why a population levels off.
- 💡Read the graph or scenario and state which population changes first, then trace the effect on the other.
- 💡Use the terms interspecific competition and competitive exclusion precisely rather than 'fighting'.
- 💡When asked to predict, give the direction of change and the mechanism linking the two populations.
- 💡Anchor each definition to the key distinction: sustainable use versus minimal human interference.
- 💡When classifying an example, state the intended use and then name the approach.
- 💡Give at least two distinct reasons rather than repeating biodiversity in different words.
- 💡Read the stem to see which resource and ecosystem are named, then select the option that links method to regeneration.
- 💡Eliminate options that describe over-exploitation or give no mechanism for replacement.
- 💡Check that the chosen option names both the resource and how its supply is maintained.
- 💡Read the stem carefully to see whether it asks about management, about effects, or about both, and eliminate options that answer the wrong clause.
- 💡Match the named human activity in the option to a plausible, specific effect rather than accepting a vague 'damages the environment'.
- 💡Reject options that describe exploitation without any control when the question asks about management.
Common Mistakes
- Treating emigration as an addition to a population; correction: emigration removes individuals, immigration adds them.
- Assuming all limiting factors depend on density; correction: some, such as drought or fire, act regardless of population size.
- Confusing carrying capacity with unlimited growth; correction: carrying capacity is the ceiling set by resources, beyond which mortality increases.
- Treating a population as all species in a habitat; correct this by defining it as one species only.
- Assuming predator and prey peaks coincide; correct this by explaining the predator peak lags behind the prey peak.
- Confusing mutualism with parasitism; correct this by stating that mutualism benefits both populations.
- Treating conservation and preservation as synonyms; correct this by contrasting sustainable use with minimal interference.
- Assuming preservation means doing nothing at all; correct this by noting that protection may still require excluding damaging activity.
- Listing only 'saving animals' as a reason; correct this by including biodiversity, ecosystem services and ethical or legal reasons.
- Treating 'sustainable' as simply 'good for the environment' without explaining the rate of replacement; correction: state that removal must not exceed the rate at which the resource regenerates.
- Naming a management method without linking it to a resource or mechanism; correction: pair each method with the resource it protects and how regeneration is maintained.
- Assuming any harvesting is unsustainable; correction: controlled, limited harvesting can be sustainable if breeding stock or regrowth is preserved.
- Treating 'management' as simply using a resource: correction — management means controlling exploitation so it remains sustainable, not just extracting it.
- Assuming any human activity is automatically harmful: correction — the effect depends on the activity and whether it is managed; some activities can be managed sustainably.
- Confusing a resource with a habitat: correction — a resource is something humans use (e.g. a fishery), while a habitat is where organisms live; the two can overlap but are not identical.
- Ignoring the second clause about effects of human activities: correction — the statement requires both management of resources and the effects of human activities, so check the option addresses the relevant clause.