Unit AS 2: Organisms and Biodiversity — Human Impact on Biodiversity

Welcome to this revision chapter! In this section, we explore how human activities affect the living world around us. We will look at why biodiversity matters, how modern practices threaten species and habitats, and the practical steps taken to conserve them. Don't worry if some ecological terms feel tricky at first—we will break down each mechanism step by step so you can secure top marks in your CCEA AS 2 exam.

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1. Core Concepts & Key Definitions

Before looking at human damage, let's make sure our foundational definitions are precise. CCEA examiners look for exact wording here!

What is Biodiversity?

Biodiversity is the variety of life in an area. It is not just a simple count of species; it incorporates two vital components:

  • Species Richness: The number of different species present in a community.
  • Species Evenness: The relative abundance of each individual species in that community (how close in numbers each species is).

Examiner Tip: Never describe a habitat as having "high biodiversity" purely because of a high species count. A woodland with 10 species where 95% of individuals belong to just one species has high richness but poor evenness, meaning lower overall biodiversity than a balanced woodland.

Extinction and Monoculture

  • Extinction: The permanent loss of a species. Human activity has accelerated extinction rates far above natural baseline (background) levels.
  • Monoculture: The agricultural or forestry practice of cultivating a single crop or tree species over a large area. This drastically lowers both species and genetic diversity.

Quick Review: Biodiversity = Species Richness (number of species) + Species Evenness (relative abundance of each species).

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2. Primary Threats to Biodiversity

A. Habitat Destruction & Fragmentation

Mature ecosystems like ancient woodlands and hedgerows are frequently cleared for intensive agriculture, road building, and urban expansion.

  • Habitat Destruction: Complete removal of native ecosystems, leading directly to the death or displacement of populations.
  • Habitat Fragmentation: The breaking up of large, continuous natural habitats into small, isolated patches or "islands."

Why is fragmentation so dangerous genetically?
When populations are isolated in small patches:

  1. Migration between groups is restricted.
  2. The gene pool shrinks, reducing genetic diversity.
  3. Organisms are forced to inbreed, making the population vulnerable to genetic drift and unable to adapt to environmental changes.
  4. This sharply increases the risk of localized extinction.

B. Modern Agriculture & Forestry

1. Removal of Hedgerows
Hedgerows are vital micro-habitats and biological corridors. Removing them to enlarge fields removes food sources (berries, nectar), destroys nesting and hibernation sites for birds and mammals, and reduces floral diversity.

2. Overgrazing
When livestock graze excessively on land, delicate native wildflowers and herbs are eaten before they can seed. This allows tough, unpalatable weeds or single aggressive grass species to dominate, leading to soil compaction, erosion, and a crash in soil biodiversity.

3. Chemical Use: Pesticides vs. Herbicides
Examiner Warning: Avoid simply writing "chemicals pollute the land." Be specific:

  • Insecticides / Pesticides: Non-specific sprays kill non-target organisms, including essential pollinators (bees) and beneficial natural predators (ladybirds). They can also accumulate up food chains (bioaccumulation).
  • Herbicides: Weedkillers eradicate wild plants (flora), which removes the primary producers that support entire food webs (insects, birds, small mammals).

4. Commercial Monoculture Forestry
Planting commercial monocultures of non-native conifer trees (such as Sitka spruce) planted very close together blocks sunlight from reaching the ground. This produces an extremely dark forest floor with acidic needles, preventing an understory of native herbs, shrubs, and associated insects from developing. Planting monoculture conifers does not increase biodiversity compared to native deciduous woodlands!

C. Overexploitation, Invasive Species & Climate Change

  • Overexploitation: Overfishing and overhunting at rates that exceed the maximum sustainable yield, depleting populations faster than they can reproduce.
  • Invasive Alien Species: Non-native species introduced by human activity that outcompete native species for ecological niches and resources, introduce novel diseases/pathogens, or aggressively predate native wildlife.
  • Climate Change: Rising global temperatures and shifting weather patterns alter species distributions, change the timing of biological events (phenology, like flowering and migration dates), and cause direct habitat loss (such as coral reef bleaching and melting polar ice).

Key Takeaway: Intensive farming and forestry lower biodiversity through habitat removal, genetic isolation, chemical disruption of food webs, and dense monocultures.

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3. Pollution & Eutrophication (Step-by-Step)

One of the most frequently tested topics in CCEA AS 2 is eutrophication caused by synthetic fertilizer runoff (nitrates and phosphates) or organic slurry/sewage entering freshwater lakes and rivers.

The Step-by-Step Mechanism:

  1. Nutrient Runoff: High concentrations of soluble nitrates and phosphates leach from agricultural fields into waterways.
  2. Algal Bloom: The rapid increase in nutrients causes an exponential growth of algae at the water surface (an "algal bloom").
  3. Light Blockage: The dense blanket of surface algae blocks sunlight from penetrating deeper into the water column.
  4. Death of Submerged Plants: Deprived of sunlight, submerged aquatic plants can no longer photosynthesize and die.
  5. Decomposition by Aerobic Bacteria: Decomposers (aerobic saprobic bacteria) feed on the abundant dead plant and algal material.
  6. Oxygen Depletion: The bacterial population multiplies rapidly and consumes dissolved oxygen during aerobic respiration, causing a huge increase in Biological Oxygen Demand (BOD).
  7. Anoxia & Fish Kill: The water becomes severely depleted of oxygen (anoxic). Fish and aquatic invertebrates asphyxiate and die, leading to a collapse in aquatic biodiversity.

Common Exam Pitfall: Never say "algae use up all the oxygen when they grow." Living algae produce oxygen via photosynthesis during daylight. It is the aerobic bacteria decomposing the dead algae and plants that consume the dissolved oxygen!

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4. Conservation and Management Strategies

Conservation aims to protect biodiversity, maintain genetic diversity, and ensure ecosystems remain resilient. These strategies are divided into two main categories:

A. In Situ Conservation (On-Site)

Protecting species within their natural, native habitats.

  • Nature Reserves and ASSIs / SSSIs: Designating Areas of Special Scientific Interest (ASSIs) or Sites of Special Scientific Interest (SSSIs) restricts damaging activities like drainage, development, or excessive grazing.
  • Marine Protected Areas: Restricting commercial trawling and fishing to allow marine ecosystems to recover.
  • Wildlife Corridors: Preserving and replanting hedgerows and field margins to connect fragmented habitats, allowing gene flow and safe migration.

B. Ex Situ Conservation (Off-Site)

Protecting endangered species outside their natural habitats.

  • Botanic Gardens & Seed Banks: Storing seeds of endangered plants in cool, dry conditions to preserve genetic diversity and safeguard against extinction in the wild.
  • Captive Breeding Programmes: Breeding endangered animals in zoos and wildlife parks with careful studbook management to prevent inbreeding, with the ultimate goal of reintroducing them to safe habitats.

C. Sustainable Agricultural & Forest Management

Agri-environment schemes encourage farmers and foresters to use methods that safeguard biodiversity:

  • Buffer Strips & Field Margins: Leaving uncultivated grassy margins along riverbanks and field edges to filter chemical runoff and provide food and nesting habitats.
  • Crop Rotation: Alternating crops to break pest cycles naturally and reduce the need for synthetic pesticides and chemical fertilizers.
  • Selective Logging & Native Reforestation: Felling individual trees rather than clear-felling entire forests, and planting mixed native deciduous broadleaf trees (e.g., oak, ash, birch) rather than dense conifer monocultures.
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Quick Summary Checklist for AS 2 Revision

Before walking into your exam, ensure you can:

  • Define biodiversity using both species richness and species evenness.
  • Explain how habitat fragmentation reduces genetic diversity and leads to inbreeding.
  • Describe the ecological consequences of hedgerow removal, overgrazing, and monocultures.
  • Distinguish between the biological effects of insecticides and herbicides.
  • Detail the 7-step eutrophication sequence, specifically naming aerobic decomposers/bacteria as the cause of oxygen depletion.
  • Compare in situ (e.g., ASSIs, nature reserves) and ex situ (e.g., seed banks, captive breeding) conservation methods.