Populations, Sustainability and Conservation

The final topic of Module 6.3 brings together population dynamics, the management of ecosystems for sustainable use, and the conservation of biodiversity. OCR A-Level Biology A specification 6.3.2 (a)–(f) requires you to understand predator–prey and competitive interactions, to distinguish conservation from preservation, to evaluate sustainable management strategies, and to know specific case studies in detail.

Key Definitions:

• Population — all individuals of one species in a defined area at the same time.
• Carrying capacity (K) — the maximum population size an environment can sustain indefinitely.
• Interspecific competition — competition between individuals of different species.
• Intraspecific competition — competition between individuals of the same species.
• Conservation — the active management of ecosystems to maintain biodiversity while allowing sustainable human use.
• Preservation — protecting an ecosystem from any human interference.
• Sustainability — meeting present needs without compromising the ability of future generations to meet theirs.

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Population Growth

In an environment with unlimited resources a population grows exponentially. In the real world, limited resources, predators, disease and weather cause growth to slow and eventually stop. The population growth curve has four phases:

1. Lag phase — slow initial growth as individuals establish themselves.
2. Exponential phase — rapid growth; birth rate far exceeds death rate.
3. Stationary phase — birth rate equals death rate; population stable at or near carrying capacity (K).
4. Decline — if conditions deteriorate.

Factors Limiting Population Size

• Abiotic — temperature, water, light, space, pH, oxygen.
• Biotic — food supply, predators, parasites, disease, competition.

Factors can be density-dependent (their effect increases as population density rises, e.g. disease, competition) or density-independent (weather, fire, flood).

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Predator–Prey Relationships

Predator and prey populations often oscillate out of phase — the prey population rises, the predator population rises in response, prey numbers fall as predation increases, then predator numbers fall, allowing prey to recover, and the cycle repeats.

Classic Example: Canada Lynx and Snowshoe Hare

Hudson's Bay Company fur trading records going back 200 years show a regular 10-year oscillation between Canada lynx and snowshoe hare populations. Hare numbers peak first; lynx numbers peak 1–2 years later. The lynx population crashes when hares become scarce; hares recover as lynx pressure eases; and the cycle repeats.

This pattern is explained by simple mathematical models (Lotka–Volterra equations) but in reality is also driven by food availability, disease, and weather.

Exam Tip: Always state that prey numbers change first, followed by predator numbers — not the other way round.

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Competition

Interspecific Competition

Competition between different species. Close competitors cannot coexist indefinitely (the competitive exclusion principle). Examples:

• Red and grey squirrels in the UK — grey squirrels (introduced from North America in 1876) out-compete native reds because they digest acorns more efficiently and carry squirrelpox virus, which kills reds. Red squirrels now survive only in Scotland, northern England and a few island refuges.
• Galápagos finches — each species has a distinct beak adapted to different foods, avoiding competition (niche differentiation).
• Chthamalus and Balanus barnacles — on Scottish shores, Balanus is confined to the lower shore by Chthamalus's tolerance of dry conditions, while Chthamalus is confined to the upper shore by competition from Balanus. A classic experiment by Joseph Connell (1961) proved the pattern is maintained by competition, not just physiology.

Intraspecific Competition

Competition between members of the same species. It is the most intense form of competition because all individuals need exactly the same resources. Intraspecific competition drives natural selection and keeps populations at or below carrying capacity. Examples:

• Seedling thinning — many germinate, only a few mature.
• Territorial behaviour in birds and mammals.
• Fighting between males for mates.
• Scramble competition for food in crowded populations.

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Conservation vs Preservation

OCR wants you to distinguish these carefully.

Feature	Conservation	Preservation
Approach	Active management	Non-intervention
Human use	Permitted (sustainably)	Excluded
Goal	Maintain biodiversity while allowing use	Keep ecosystem untouched
Example	Sustainable forestry in Norway	Core zones of national parks

Conservation is generally more practical in a crowded world, because excluding humans entirely is rarely possible or fair to local people.

Why Conserve?

• Ethical — other species have a right to exist.
• Aesthetic — nature provides beauty and inspiration.
• Economic — fisheries, timber, tourism, pharmaceuticals.
• Medical — ~25% of modern drugs come from plants.
• Ecological — ecosystem services (pollination, water purification, climate regulation).
• Agricultural — wild relatives of crops carry useful alleles.
• Future potential — undiscovered species may be valuable.

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Sustainable Resource Management

Sustainable Timber Production

Timber is a renewable resource only if it is harvested sustainably. Three main strategies:

1. Coppicing — cutting deciduous trees (hazel, ash, willow) near ground level. New shoots grow from the stool, producing a flush of straight, thin stems that can be harvested on a 5–20 year cycle. Coppiced woodlands are among the richest UK habitats for butterflies, wildflowers and songbirds, because light reaches the ground.
2. Rotational forestry — large areas divided into blocks; only one block is felled each year, rotating through the whole forest on a cycle (e.g. 80 years for softwood, 150+ years for hardwoods).
3. Selective felling — only mature trees are removed, leaving younger trees to continue growing. Mimics natural gap dynamics. Maintains forest cover and biodiversity better than clear-felling.

After harvesting, replanting with a mix of species avoids monoculture. Protection from pests and fire is essential.

Sustainable Fishing

Fish stocks worldwide have been depleted by over-fishing. Sustainable fishing requires: