Populations, Sustainability and Conservation

Spec Mapping — OCR H420 Module 6.3.2 — Populations and sustainability, content statements covering population growth (exponential and logistic models, carrying capacity), density-dependent and density-independent limiting factors, predator-prey dynamics (Lotka-Volterra oscillations), interspecific and intraspecific competition, sustainable resource management (timber, fisheries), and in-situ and ex-situ conservation strategies with named case studies (refer to the official OCR H420 specification document for exact wording). This final lesson of the course synthesises the entire ecology arc.

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 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.

The mathematical framework for population dynamics comes from two near-contemporary scientists working independently. Alfred Lotka in the United States (1925) and Vito Volterra in Italy (1926) — the latter motivated by the Italian fisheries during World War I, when a reduction in fishing was paradoxically accompanied by a rise in predatory fish proportional to prey — independently derived the differential-equation model of predator-prey oscillation now universally called the Lotka-Volterra equations. Paraphrased: their insight was that prey and predator populations are coupled through reciprocal mortality / reproduction effects, generating sustained out-of-phase oscillations that need not converge on a single equilibrium. This was the first quantitative theory of ecological dynamics and remains the textbook starting point. Modern population biology, exemplified by Charles Krebs's long-running work on snowshoe hare and lynx in the Yukon, has refined and extended the Lotka-Volterra framework while preserving its core insight that ecological communities are dynamic, oscillating systems rather than steady-state equilibria. The population-ecology paradigm is also associated with Paul R. Ehrlich (Stanford), whose work on butterfly populations and human-population ecology (not to be confused with the immunologist Paul Ehrlich of side-chain theory fame) has shaped conservation biology since the 1960s.

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.

---

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.

Mathematical models

Exponential (J-curve) growth assumes unlimited resources:

$\frac{dN}{dt} = rN$dtdN​=rN

where $N$N is population size, $t$t is time, and $r$r is the intrinsic per-capita growth rate (births − deaths per capita per time). The solution is $N_t = N_0 e^{rt}$Nt​=N0​ert — unrestricted exponential growth, unrealistic over long time scales.

Logistic (S-curve) growth introduces a carrying capacity $K$K:

$\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$dtdN​=rN(KK−N​)

When $N$N is small, the bracketed term is ~1 and growth is exponential; as $N$N approaches $K$K, the bracket approaches zero, and growth slows asymptotically. This is the foundational model of density-dependent population regulation in ecology.

Factors Limiting Population Size

• Abiotic — temperature, water, light, space, pH, oxygen. Typically density-independent.
• Biotic — food supply, predators, parasites, disease, competition. Typically density-dependent.

Factors can be density-dependent (their effect increases as population density rises, e.g. disease transmission, intraspecific competition, parasite load) or density-independent (weather, fire, flood — affect populations regardless of size). Density-dependent factors stabilise populations near $K$K; density-independent factors cause fluctuations around it.

r-selected vs K-selected strategies

Ecologists distinguish two evolutionary strategies along the r/K continuum:

Trait	r-selected	K-selected
Body size	Small	Large
Lifespan	Short	Long
Offspring per reproduction	Many	Few
Parental investment	Low	High
Reproduction rate	Fast	Slow
Habitat stability	Unstable / unpredictable	Stable / predictable
Examples	Insects, weeds, bacteria, rodents	Elephants, whales, oaks, humans
Dominant limiting factor	Density-independent	Density-dependent

The r/K framework is a useful first approximation, though modern ecology recognises it as a continuum rather than a binary classification, with most species falling somewhere between the extremes.

---

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 — Charles Krebs's long-running Kluane Lake experiment in the Yukon has shown that the lynx-hare cycle is also modulated by plant-defence chemistry as hares overgraze willow and birch.

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

---

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.

---

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.

---

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:

• Quotas — each country or vessel has a maximum permitted catch, set below the maximum sustainable yield (MSY).
• Minimum net mesh sizes — let small fish escape to grow and breed.
• No-take zones / marine protected areas — allow populations to recover.
• Closed seasons — no fishing during breeding.
• Aquaculture — farming fish (salmon, trout, tilapia, carp) instead of catching wild ones.
• International agreements — because fish move across borders.

The North Sea cod collapsed in the 1990s when catches exceeded sustainable limits. Strict EU quotas, smaller fleets and net changes have led to a slow recovery. The Grand Banks cod fishery off Newfoundland collapsed in 1992 and has never fully recovered — a cautionary tale.

---

Case Studies

OCR expects you to know at least one case study from each of four contexts.

Masai Mara (Kenya)

The Masai Mara National Reserve in Kenya is part of the wider Serengeti ecosystem. It hosts the great wildebeest migration — about 1.5 million wildebeest, zebra and gazelle moving between the Serengeti and the Mara each year in pursuit of rainfall and fresh grass.

Conservation issues:

• Human encroachment — the Maasai people traditionally herd cattle in the region. Population pressure and loss of grazing land have caused conflict.
• Ecotourism — provides essential income (Masai Mara earns about US$30 million/year from tourism) but can damage habitat through off-road driving and disturbance of animals.
• Poaching — rhinos and elephants for horn and ivory.
• Climate change — altered rainfall patterns threaten migration.

Conservation strategies:

• Revenue-sharing with local communities to align their interests with conservation.
• Community conservancies — land leased from Maasai for wildlife, with income to landowners.
• Anti-poaching patrols and SMART ranger technology.
• Breeding programmes for rhinos.
• Controlled burning to maintain grassland.

Galápagos Islands (Ecuador)

Darwin's living laboratory. Iconic species include giant tortoises, marine iguanas, flightless cormorants and 13 species of finch.

Conservation issues:

• Introduced species — goats, rats, cats, dogs, pigs devastate native species. Goats strip vegetation; rats eat tortoise eggs.
• Tourism — over 275,000 visitors per year; brings money but also disturbance and invasive species.
• Fishing pressure — illegal shark fin harvesting.
• Population growth — permanent population has grown from ~1,000 in 1950 to ~30,000 today.

Conservation strategies:

• 97% of the islands' land area is national park; access is restricted.
• Eradication campaigns for invasive species (goats eradicated from Pinta and Santiago by shooting and sterile "Judas goats" that led hunters to hidden herds).
• Captive breeding and reintroduction of tortoises.
• Strict visitor rules — all tourists must be accompanied by licensed guides.
• Marine reserve covering 198,000 km² of surrounding ocean.

Antarctica

Governed by the Antarctic Treaty (1959), which designates Antarctica as a scientific preserve, bans military activity and prohibits mining until 2048.

Conservation issues:

• Climate change — the Antarctic Peninsula is warming faster than almost anywhere on Earth.
• Krill harvesting — commercial fishing for krill threatens the food web (krill underpin penguins, seals and whales).
• Tourism — now over 100,000 visitors per year. Brings disturbance and biosecurity risks.
• Plastic pollution — even remote Antarctic beaches contain microplastics.

Conservation strategies: