Populations and Sustainability¶

Part of Module 6: Genetics, evolution and ecosystems.

Managing the natural world sustainably requires understanding what determines population size and how ecosystems respond to human pressure. This topic covers the factors that limit population growth, predator-prey dynamics, competition between species, and the principles and practices of conservation. It also extends the evolution content from Module 4 with speciation, artificial selection and the practical tools of genetics (Hardy-Weinberg analysis covered in 6.1.2).

Learning Objectives¶

Population Growth and Carrying Capacity¶

A population is all individuals of the same species living in the same area at the same time. In a resource-unlimited environment, populations grow exponentially — they double in size at each generation. In the real world, resources are finite and populations reach a maximum size called the carrying capacity (K) — the maximum population size that the environment can sustainably support.

The Logistic Growth Curve¶

Real population growth follows an S-shaped (sigmoid) curve:

1. Slow initial growth: Small population; few individuals reproducing
2. Exponential growth: Resources plentiful; birth rate >> death rate
3. Decelerating growth: Resources begin to limit growth as population approaches K
4. Plateau at carrying capacity: Birth rate = death rate; population fluctuates around K

Limiting Factors and Density Dependence¶

Limiting factors are those resources or conditions that restrict population size. They include:

Density-dependent factors have an increasing effect on a population as density rises (e.g. disease spreads more easily; food per individual decreases). These factors are primarily responsible for the plateau at K.

Density-independent factors affect populations regardless of density (e.g. a sudden frost, a volcanic eruption). These can cause sharp reductions in population size but do not account for the stable carrying capacity.

Predator-Prey Relationships¶

Predator and prey populations are coupled: changes in one cause changes in the other. The classic pattern is an oscillating cycle:

1. Prey population increases (abundant food; low predation pressure)
2. Predator population increases in response (more food available)
3. Increased predation reduces prey population
4. Prey population falls; food for predators becomes scarce
5. Predator population falls (starvation, reduced reproduction)
6. With fewer predators, prey population recovers → cycle repeats

The predator cycle lags behind the prey cycle: predator numbers peak after prey numbers peak.

The predator-prey dynamic also exerts reciprocal selection pressures. Prey that evade predators more effectively tend to survive and reproduce, driving evolutionary change in prey. This in turn selects for more efficient predators:

Important note: In practice, real predator-prey dynamics are more complex. Prey also depend on their own food supply; multiple prey species are used; spatial refugia allow prey to escape. The classic oscillating pattern from theoretical models is clearest in simplified laboratory systems or isolated systems (e.g. Canadian lynx and snowshoe hare data from Hudson's Bay Company fur records — though even this dataset shows more complexity than the simple cycle).

Competition¶

Intraspecific competition is density-dependent and is a key mechanism regulating population size at the carrying capacity — as population density rises, competition for food, mates and territory intensifies, reducing birth rate and increasing death rate.

Interspecific competition, if severe enough, can lead to competitive exclusion (one species eliminates the other from the habitat) or to resource partitioning (the two species evolve to use slightly different aspects of the resource, reducing competition and allowing coexistence).

Evolution, Natural Selection and Speciation¶

Natural Selection and Evolution¶

Natural selection is the non-random differential survival and reproduction of individuals with phenotypic variants that make them better adapted to their environment. Over time, allele frequencies in a population change — this is evolution.

Process of evolution by natural selection: 1. Variation exists within a population (due to mutation, meiosis and sexual reproduction) 2. A selection pressure (environmental challenge) means not all individuals survive to reproduce 3. Individuals with advantageous phenotypes (and therefore genotypes) are more likely to survive and reproduce 4. Advantageous alleles are passed on to offspring at greater frequency 5. Over many generations, allele frequencies in the population shift — the population evolves

Types of Natural Selection¶

Factors Affecting Allele Frequency¶

In addition to natural selection, other mechanisms alter allele frequencies:

Speciation¶

Speciation is the process by which new species arise. A species is a group of organisms capable of interbreeding to produce fertile offspring and which shares a common gene pool.

Allopatric speciation (most common): 1. A population is divided by a physical (geographical) barrier (mountain range, sea, river, glacier) 2. The two sub-populations are reproductively isolated — no gene flow between them 3. Each sub-population experiences different selection pressures and/or genetic drift 4. Allele frequencies diverge; different adaptations accumulate 5. Over many generations, the populations become so genetically different that they can no longer interbreed to produce fertile offspring even if the barrier is removed → they are now separate species

Sympatric speciation (less common): - New species arise from the same population in the same geographic location - Mechanisms: polyploidy (chromosomal duplication — common in plants), behavioural isolation (e.g. choice of slightly different microhabitats or mating times)

Artificial Selection¶

Artificial selection (selective breeding) is the deliberate breeding of organisms with desirable traits by humans, mimicking natural selection but applying human-defined selection pressures.

General procedure: 1. Identify individuals in a population that exhibit the desired trait to the greatest degree 2. Use these individuals as breeding stock (parents) 3. Select offspring that most strongly show the desired trait 4. Repeat over many generations

Examples:

Dairy cattle (high milk yield): - Milk yield of cows is measured and recorded - Cows with the highest yields are identified - Best quality bulls (identified from progeny testing — measuring the performance of daughters) are used for breeding - Hormones may be used to superovulate prize cows; eggs are fertilised in vitro and implanted into surrogate cows

Bread wheat: - Modern bread wheat (Triticum aestivum) is hexaploid (2n = 42 chromosomes) - It arose through hybridisation between three diploid ancestral species followed by chromosomal doubling events (polyploidy) - The larger cells resulting from polyploidy produce larger grain - Many thousands of years of artificial selection by farmers have produced high-yielding varieties adapted to different climates

Consequences of artificial selection: - Desired trait becomes more common in the population - Reduced genetic diversity (inbreeding) can make populations vulnerable to disease or environmental change - Unintended traits may be selected alongside the desired trait

Conservation and Sustainability¶

Conservation is the protection and management of ecosystems to ensure natural resources are used sustainably, maintaining biodiversity through active human management. Conservation is a dynamic process: it involves controlling use of resources and their replenishment.

Preservation aims to maintain ecosystems in their natural, undisturbed state by restricting or banning resource extraction and human interference. Unlike conservation, it does not involve active management. Examples include nature reserves and marine conservation zones where human activities are strictly prohibited.

Reclamation is a conservation approach in which ecosystems that have been damaged or destroyed are restored.

The distinction is important: conservation involves active management and allows some human use (e.g. sustainable forestry, regulated fishing); preservation involves limiting human activity (e.g. national parks with no logging or hunting). Reclamation restores already-damaged systems.

Reasons for Conservation¶

Sustainable Management of Woodland¶

Human activities can remove resources from ecosystems. Sustainable management aims to meet current needs without compromising the ability of future generations to meet theirs.

Sustainable Management of Fish Stocks¶

Overfishing occurs when fishing reduces fish populations to a level where they cannot regenerate, potentially disrupting food chains throughout the ecosystem.

Sustainable fishing aims to maintain healthy breeding populations and ensure the long-term viability of fish stocks:

Case Study: The Galápagos Islands¶

The Galápagos Islands (Ecuador) provide a well-studied example of human impact on biodiversity and conservation responses:

Human impacts: - Increased tourism raises demand for water, energy and food; increases waste and pollution - Overfishing (sea cucumbers became threatened by commercial harvesting) - Introduction of non-native species (goats, rats, cats): goats overgraze vegetation, removing food for native herbivores; rats and cats predate tortoise eggs and hatchlings; introduced plants outcompete native species

Conservation measures: - Goat eradication programme: near-total elimination of goats from several islands → rapid recovery of native vegetation → increases in food available for giant tortoises - Giant tortoise captive breeding programme: tortoises incubated and raised in captivity until large enough to withstand predation; then released on their home islands - Control of tourist numbers and areas; designated conservation zones with no fishing or landing - Education and involvement of local communities

The Galápagos programme is widely cited as an effective model because it combined direct species protection (captive breeding), removal of introduced species, and management of human access.

General Principles of Ecosystem Management¶

Effective conservation of ecosystems typically involves a combination of: - Controlling the number of tourists and human visitors - Involving and educating local communities (community-based conservation) - Active management of vegetation (e.g. controlled burning, invasive species removal) - Controlling the introduction of non-native species - International cooperation (e.g. CITES — Convention on International Trade in Endangered Species; Ramsar Convention on wetlands)

Key Terms¶

• Carrying capacity: maximum population size that an environment can sustain over time.
• Limiting factor: resource or condition that restricts population growth.
• Density-dependent factor: factor whose effect becomes stronger as population density increases.
• Predator-prey relationship: interaction in which one organism feeds on another, causing linked population cycles.
• Interspecific competition: competition between individuals of different species.
• Intraspecific competition: competition between individuals of the same species.
• Stabilising selection: natural selection that favours intermediate phenotypes.
• Directional selection: natural selection that favours one extreme phenotype.
• Disruptive selection: natural selection that favours both extreme phenotypes over intermediate ones.
• Speciation: formation of new species from diverging populations.
• Artificial selection: selective breeding by humans to increase the frequency of chosen characteristics.
• Conservation: protection and active management of species or habitats to maintain biodiversity while allowing sustainable use.
• Preservation: protection of ecosystems from human interference with minimal or no active management.
• Reclamation: restoration of ecosystems that have been damaged or destroyed.
• Overfishing: fishing at a rate that reduces populations below the level at which they can recover.
• Sustainability: use and management of resources in ways that maintain ecosystems and future availability.

Connected Pages¶

• 6.3.1 Ecosystems (succession, energy flow, nutrient cycling)
• 6.1.2 Patterns of inheritance (Hardy-Weinberg; allele frequency analysis)
• 4.2.1 Biodiversity (biodiversity indices, conservation agreements)
• 4.2.2 Classification and evolution (natural selection at AS level)
• Mutation, selection and speciation
• Module 6: Genetics, evolution and ecosystems