Factors Affecting Biodiversity (OCR A-Level Biology A): Revision Notes

Factors Affecting Biodiversity

Human population growth represents the most significant threat to global biodiversity. The human population has expanded from $1$ billion in $1800$ to over $8$ billion in $2026$. This rapid growth has intensified pressure on natural ecosystems through multiple pathways.

The primary threats to biodiversity arising from human population growth include:

• Destruction of natural habitats and environmental degradation
• Unsustainable exploitation of biological resources
• Modern agricultural practices including monoculture and agrochemical use
• Global climate change

Habitat destruction

Habitat loss occurs when natural environments are destroyed or significantly degraded. Land clearance for agriculture, industry, housing, transport infrastructure, leisure facilities, waste disposal, and water storage removes natural vegetation, eliminating or fragmenting habitats for numerous plant and animal species.

Habitat fragmentation describes the division of continuous habitats into smaller, isolated patches. When populations become divided through fragmentation, they face increased risks of inbreeding and localized extinction events due to reduced genetic diversity and smaller population sizes.

Deforestation

Historical forest loss in the northern hemisphere began approximately $10\,000$ years ago when humans transitioned to agricultural food production. In contrast, extensive southern hemisphere forests in South-East Asia, Africa, Amazonia, and Central America remained largely intact until the twentieth century.

Deforestation has caused severe biodiversity loss in many regions. Madagascar, renowned for its unique endemic species, has lost almost all its natural forest cover. Cleared forest areas are frequently replaced with cattle ranches and oil palm plantations, which support considerably lower biodiversity than natural forests.

Tree roots bind soil particles and absorb rainfall. Forest removal leads to:

• Increased surface water runoff and flooding risk
• Severe land degradation through soil erosion
• Loss of soil nutrients
• Reduced water infiltration

Type of forest	Forest cover in 2000 (km² thousands)	Gross forest cover lost 2000-2005 (km² thousands)
Boreal	$8723$	$351$
Humid tropical	$11\,564$	$272$
Dry tropical	$7135$	$204$
Temperate	$5265$	$184$
Total	32 687	1011

Marine ecosystem destruction

Marine ecosystems face destruction from human activities including:

• Dynamiting coral reefs as a fishing method
• Bottom trawling with nets dragged across the seabed (the North Sea has lost virtually all natural seabed ecosystems)
• Dredging coastal waters and coastal development for industry, housing, and tourism
• Increased sedimentation from tree removal, ploughing, and urban runoff entering coastal waters

Overexploitation

Overexploitation occurs when natural resources are harvested faster than they can regenerate naturally.

Timber resources

Much industrial timber comes from managed fast-growing coniferous forests that are replanted after harvesting. However, slow-growing hardwood species such as teak and mahogany are often felled at rates exceeding natural regeneration capacity, representing unsustainable resource use.

Fish stocks

Several Atlantic cod (Gadus morhua) stocks collapsed during the 1990s due to overfishing, declining by over $95\%$ of their maximum historical biomass. These stocks have failed to recover even with fishing cessation.

The fishing industry has responded to declining large predatory fish populations by targeting smaller species at lower trophic levels. This pattern, particularly pronounced in northern hemisphere fisheries, initially increases catches before eventual decline, demonstrating unsustainable exploitation.

A sustainable resource is one renewed by biological activity at rates sufficient to prevent depletion through human use.

Hunting and collection

Bush meat

In many regions, particularly Africa and South America, people harvest wild animals for protein. Road development into forests for logging has expanded accessible areas for hunting. Species at particular risk include:

• Primates (monkeys and chimpanzees)
• Other mammal species
• Reptiles

Plant collection

Plants face equivalent pressures through removal from natural habitats for sale or consumption. Surveys in East Africa revealed dangerously low wild populations of African violet (Saintpaulia), a common cultivated plant, due to collection from natural habitats.

Agriculture

Monoculture

Monoculture describes growing a single crop species (such as wheat, maize, or soya) across extensive land areas, often for multiple consecutive years. The term also applies to intensive livestock farming and single-species tree plantations (typically conifers for timber and paper pulp).

Arable farmers often cultivate identical crops annually across large fields, enabling specialization, use of large machinery, minimal labour requirements, and high yields per hectare. This intensive farming depends on substantial inputs:

• Chemical fertilisers
• Crop protection chemicals (pesticides)
• Fossil fuels (machinery operation and agrochemical manufacture)

Agricultural intensification impacts

Crop cultivation requires resource provision. Growing identical crops repeatedly rapidly depletes soil mineral reserves. Farmers replenish these through chemical fertiliser application. Crops also require irrigation, increasing water demand.

Competition from weeds reduces crop yields and interferes with mechanised harvesting. Herbicides control weeds competing for resources. Plant pathogens (fungi, bacteria, viruses) threaten crop growth and yields. Fungicides control disease outbreaks.

Monocultures provide unlimited food supplies for pest species, enabling exponential population growth under favourable conditions. Insecticides control insect pests like boll weevils and aphids.

Agrochemical impacts on biodiversity

UK farmland biodiversity loss

Modern UK agriculture has reduced plant, arthropod (insects and spiders), bird, and mammal biodiversity. Contributing factors include:

• Habitat destruction through hedgerow, woodland, and scrub removal to create larger fields, eliminating nesting sites
• Ploughing to field edges, removing plant and insect habitats
• Reduced habitat structural diversity (some farmland bird species prefer tussocky grass not favoured by livestock farmers)
• Improved seed cleaning methods reducing weed seed presence
• Loss of wild food plants for butterflies and hoverflies through herbicide use
• Pasture conversion to arable land reducing soil invertebrate populations, decreasing food for farmland birds and mammals
• Land drainage forcing soil invertebrates deeper, making them less accessible to birds and mammals
• Autumn cereal sowing (starting 1970s) rather than leaving stubble until spring, reducing food availability for birds and mammals

Agricultural pollution

Three main agricultural pollution sources affect biodiversity:

1. Fertilisers
2. Pesticides
3. Intensive livestock waste products

Fertiliser pollution and eutrophication

Fertilisers applied to increase crop yields can pollute water if excess fertiliser is used or application precedes heavy rainfall. Fertiliser draining into rivers and lakes causes eutrophication – enrichment of waters with plant nutrients, primarily nitrate and phosphate ions.

Nitrate represents the main nutrient in arable land runoff. While not typically limiting algal and plant growth in freshwaters (streams and rivers), nitrate limits growth in marine waters, causing marine algal blooms.

Over $400$ ocean areas regularly experience oxygen depletion from algal bloom decomposition, with dead zone numbers increasing.

Pesticide impacts

Pesticides can select for resistant pest and weed strains. Widespread organochlorine pesticide use (such as DDT) caused eggshell thinning and reproductive failure in peregrine falcons (Falco peregrinus) from the 1950s. In India, vulture populations decreased $95\%$ between 1980 and 2010 due to diclofenac poisoning (a veterinary drug for cattle treatment). India's nine vulture species now face severe extinction risk.

Climate change

Throughout Earth's history, climate has changed continuously, with accompanying ecosystem changes and species evolution and extinction. However, these historical changes occurred over relatively long timescales. Current climate change proceeds much faster, likely exceeding species' adaptation rates and potentially causing substantial biodiversity loss.

Global climate change produces two main effects:

1. Modified weather patterns
2. Increased frequency of extreme weather events (hurricanes, typhoons, floods, droughts)

Species distribution changes

With global warming, many species' distributions shift towards poles and higher altitudes. Species entering new ecosystems may be poorly adapted and face competition from better-adapted resident species. As conditions warm, species adapted to current high-altitude and high-latitude conditions may not survive competition from species migrating from warmer regions.

Polar region threats

Global climate change poses specific polar biodiversity threats. Arctic ice loss may eliminate an entire biome. The Arctic ecosystem includes ice-underside algae functioning as producers for this productive ecosystem supporting numerous invertebrates, fish, birds, and marine mammals.

Ocean acidification

Decreased seawater pH (ocean acidification) represents another marine habitat threat. Carbon dioxide's high water solubility means increased atmospheric $\text{CO}_2$ concentrations result in greater dissolved amounts in seawater, decreasing pH.

Ocean acidification represents one threat to coral reef ecosystem survival. Coral reefs, containing approximately $25\%$ of ocean biodiversity, are predicted to disappear by 2050.

Ocean stratification

Ocean water warming may cause stratification, preventing surface water mixing with nutrient-rich deep water. This would limit phytoplankton (the ocean's primary producers) growth, reducing food availability for diverse marine life.