Welcome to Human Impact on Biodiversity
Welcome to this essential study guide for AS 2: Organisms and Biodiversity. As human populations expand, our demand for food, land, timber, and water places immense pressure on the natural world. In this chapter, we explore how human activities alter ecosystems, how we can monitor environmental quality using biological tools, and the vital conservation strategies used to protect species and habitats.
Don't worry if the technical terms seem overwhelming at first! We will break down every process step by step, highlight common exam traps reported by examiners, and provide simple memory aids to help you secure top marks.
1. Threats to Biodiversity from Human Exploitation
Biodiversity refers to the variety of living organisms in a given area. Human population growth drives intensive resource extraction, which threatens biodiversity through several direct mechanisms:
A. Habitat Destruction and Fragmentation
Deforestation: Clearing land for agriculture, urban development, infrastructure, and timber directly removes primary producers (keystone vegetation). This eliminates food resources, breeding sites, and structural niches for countless animal species.
Habitat Fragmentation: When large, continuous ecosystems are divided into smaller, isolated patches (by roads, housing developments, or farmland), several critical ecological issues arise:
• Edge Effects: The boundary (edge) of a forest or woodland experiences different microclimates (more wind, higher light, fluctuating temperatures) compared to the deep interior. Fragmentation increases the proportion of edge habitat, disrupting interior species.
• Reduced Gene Flow & Genetic Drift: Small, isolated sub-populations cannot easily interbreed. This shrinks the gene pool, increases inbreeding, and leaves populations vulnerable to genetic drift and local extinction.
B. Modern Intensive Agriculture
Modern farming aims to maximize crop yield, but intensive practices often come at the expense of species richness:
• Monoculture: Growing vast fields of a single crop species dramatically reduces plant diversity. This eliminates diverse food webs and structural microhabitats, leaving only a tiny number of ecological niches for primary and secondary consumers.
• Hedgerow Removal: Farmers often remove hedgerows to combine small fields and accommodate large modern machinery. Hedgerows are crucial biological corridors and nesting habitats; removing them destroys shelter and isolates wildlife populations.
• Agrochemicals (Pesticides and Herbicides): Broad-spectrum pesticides kill non-target insects, including essential pollinators (such as bees) and natural pest predators. Herbicides kill wild arable plants, eliminating food plants for birds and insects. Furthermore, persistent synthetic chemicals can accumulate in fatty tissues (bioaccumulation) and increase in concentration at higher trophic levels (biomagnification).
• Inorganic Fertilisers: High applications of synthetic nitrates and phosphates easily wash into waterways, acting as a primary driver of aquatic eutrophication.
• Overgrazing: High densities of livestock compact the soil, preventing water infiltration and damaging plant roots. Overgrazing also prevents tree saplings from regenerating and allows unpalatable, aggressive weed species to outcompete delicate native wildflowers.
C. Overexploitation: Overfishing and Hunting
Overfishing: Harvesting commercial marine fish at rates faster than populations can naturally reproduce depletes fish stocks, reduces reproductive biomass, and disrupts marine food webs (trophic cascades).
Bycatch: Non-selective fishing techniques (such as deep-sea trawling or drift nets) catch and kill non-target marine animals, including marine mammals (dolphins), seabirds, and juvenile fish, causing severe ecological damage.
Key Takeaway: Habitat loss, monocultures, chemical inputs, and overexploitation reduce species richness and genetic diversity, leaving ecosystems fragile and vulnerable to environmental change.
2. Aquatic Pollution and Eutrophication
Eutrophication is one of the most frequently examined multi-step biological sequences. It is the process whereby an aquatic ecosystem becomes artificially enriched with excess mineral nutrients, leading to ecological collapse.
The 6-Step Sequence of Eutrophication
Step 1: Nutrient Influx (Leaching and Runoff)
Excess soluble inorganic ions, particularly nitrates (\(\text{NO}_3^-\)) and phosphates (\(\text{PO}_4^{3-}\)) from synthetic fertilisers, or organic nutrients from agricultural slurry and untreated sewage effluent, leach through the soil and wash into slow-moving rivers, lakes, or loughs.
Step 2: The Algal Bloom
Nitrate and phosphate act as limiting factors for microscopic surface algae and phytoplankton. The sudden nutrient surplus triggers rapid population growth, forming a dense, green, opaque mat on the water's surface known as an algal bloom.
Step 3: Death of Submerged Plants
The thick algal bloom blocks sunlight from penetrating down through the water column. Submerged macrophytes (deep aquatic plants) cannot absorb photosynthetically active radiation; their rate of photosynthesis drops below their respiration rate, and they die.
Step 4: Proliferation of Decomposers
The massive quantity of dead submerged plant biomass and decaying algae provides a vast food supply for saprobic (decomposer) aerobic bacteria. Bacterial populations multiply exponentially.
Step 5: Increased BOD and Severe Hypoxia / Anoxia
As the saprobic bacteria decompose the organic matter, they carry out rapid aerobic cellular respiration, consuming large amounts of dissolved oxygen (\(\text{O}_2\)) from the water.
• This sharply increases the Biochemical Oxygen Demand (BOD).
• The dissolved oxygen concentration plummets, creating severe hypoxia (low oxygen) or total anoxia (no oxygen).
Step 6: Death of Aerobic Aquatic Organisms
Fish, macroinvertebrates, and other active aquatic animals cannot obtain sufficient oxygen across their gills. They asphyxiate and die, drastically reducing aquatic biodiversity.
Understanding Biochemical Oxygen Demand (BOD)
Official Definition: Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen consumed by aerobic microorganisms when decomposing organic matter in a water sample over a set period (standardised at 5 days at 20°C in darkness).
Analogy: Think of BOD as the "appetite for oxygen" in the water. High organic waste = huge bacterial population = massive oxygen consumption = very high BOD.
Examiner Warning: Common Mistakes to Avoid
• Mistake 1: Stating that living algae directly "suffocate" the fish. Correction: Living algae release oxygen during the day; it is the aerobic decomposer bacteria feeding on dead biomass that consume the dissolved oxygen.
• Mistake 2: Stating that lack of sunlight directly kills the fish. Correction: Lack of sunlight kills submerged plants, which leads to bacterial decomposition and subsequent oxygen depletion.
Key Takeaway: Fertiliser Runoff \(\rightarrow\) Algal Bloom \(\rightarrow\) Light Blocked \(\rightarrow\) Submerged Plants Die \(\rightarrow\) Aerobic Bacteria Decompose Biomass \(\rightarrow\) Dissolved Oxygen Depleted (High BOD) \(\rightarrow\) Fish Die of Hypoxia.
3. Environmental Monitoring: Biotic and Abiotic Indicators
To assess how severely an aquatic or terrestrial ecosystem has been impacted by human pollution, biologists use indicator organisms alongside quantitative mathematical calculations.
A. Biological Indicator Species (Freshwater Macroinvertebrates)
Different freshwater invertebrate species have precise tolerances to dissolved oxygen levels and organic pollution. By sampling the macroinvertebrate community, scientists can deduce the pollution status of a waterway:
1. Clean, Highly Oxygenated Water (Unpolluted):
• Stonefly nymphs
• Mayfly larvae
• Caddisfly larvae
These organisms require high dissolved oxygen concentrations and are very sensitive to organic pollution.
2. Moderate Pollution / Mildly Reduced Oxygen:
• Freshwater shrimp (Gammarus)
• Water louse (Asellus)
These species tolerate mild organic contamination and intermediate oxygen levels.
3. Severely Polluted / Low Oxygen (Hypoxic):
• Tubifex worms (sludgeworms)
• Chironomid midge larvae (bloodworms)
• Rat-tailed maggots
Note: Bloodworms and Tubifex worms contain specialized forms of haemoglobin with an exceptionally high affinity for oxygen, enabling them to survive in oxygen-depleted sediment where predators cannot live.
Examiner Warning:
Do not assume a water body is "healthy" just because it contains thousands of bloodworms or Tubifex worms. A massive population of only bloodworms indicates low species richness and severe organic pollution!
B. Simpson’s Index of Diversity (\(D\))
Simpson's Index is a mathematical tool used to measure species diversity. It accounts for both species richness (the number of different species present) and species evenness (the relative abundance of each species).
The standard formulas used in CCEA Biology are:
\(D = 1 - \sum \left(\frac{n}{N}\right)^2\) or \(D = \frac{N(N-1)}{\sum n(n-1)}\)
Where:
• \(n\) = Total number of individuals of a particular species.
• \(N\) = Total number of individuals of all species combined (\(N = \sum n\)).
• \(\sum\) = "The sum of".
Interpreting Simpson's Index Values:
• Values range from 0 (no diversity / complete monoculture) to 1 (infinite diversity).
• High \(D\) value (close to 1): Represents a diverse, stable, and complex community with many ecological niches. An environmental change (e.g., disease outbreak or drought) is unlikely to destroy the entire community.
• Low \(D\) value (close to 0): Represents a stressed, polluted, or heavily managed habitat (such as a monoculture crop or polluted river) dominated by one or two species. Small environmental changes can cause ecosystem collapse.
Key Takeaway: Indicator species provide biological evidence of water quality (e.g., stoneflies = clean; bloodworms = polluted). Simpson’s Index gives a numerical score of biodiversity that reflects both richness and abundance.
4. Conservation and Habitat Management Strategies
Conservation aims to maintain and enhance biodiversity while managing resources sustainably for future generations.
A. Designation of Protected Areas
Statutory protection gives legal safeguarding against destructive human developments:
• ASSI (Areas of Special Scientific Interest): Sites of national importance in Northern Ireland designated under wildlife legislation because of their unique flora, fauna, geological, or physiographical features.
• SAC (Special Areas of Conservation): High-level protected sites designated under conservation directives to protect rare, vulnerable, or threatened natural habitats and wild species.
B. Sustainable Agriculture and Agri-Environment Schemes
Governments and conservation bodies incentivize farmers to adopt wildlife-friendly practices:
• Field Margins and Buffer Strips: Leaving uncultivated, grassy strips along field edges and riverbanks traps sediment and filters agrochemical runoff before it enters streams, while providing habitat corridors for small mammals and predatory insects.
• Hedgerow Maintenance: Retaining and replanting native hedgerow shrubs maintains continuous wildlife corridors and bird nesting sites.
• Crop Rotation: Alternating crop types breaks pest life cycles, reduces the need for synthetic chemical pesticides, and prevents soil nutrient exhaustion.
• Regulated Slurry Application: Restricting the timing and quantity of slurry spreading (e.g., avoiding winter spreading on waterlogged ground) prevents runoff into drainage networks.
C. In Situ vs. Ex Situ Conservation
Conservation strategies are divided into two fundamental approaches:
1. In Situ Conservation (Conserving species within their natural habitat):
• Examples: Nature reserves, national parks, habitat restoration projects, controlled rotational grazing (to maintain species-rich grasslands), and sustainable coppicing in woodlands.
• Advantages: Conserves the species in its natural ecological context; protects whole communities and natural interactions; allows ongoing evolutionary adaptation.
2. Ex Situ Conservation (Conserving species outside their natural habitat):
• Examples: Seed banks (storing dormant seeds in dry, sub-zero conditions to preserve genetic diversity), botanical gardens, and captive breeding programs in zoos accompanied by planned reintroduction schemes.
• Advantages: Protects endangered individuals from predation, poaching, and catastrophic local disasters; allows scientific research and artificial insemination.
Key Takeaway: Effective conservation combines legal protection (ASSI, SAC), sustainable farming practices (buffer strips, crop rotation), and a balanced mix of in situ habitat management and ex situ species preservation.
5. Quick Review & Summary Checklist
Before heading into your AS 2 exam, make sure you can confidently answer each of these review points:
• Habitat Fragmentation: Can you explain how edge effects and genetic drift reduce species survival in fragmented landscapes?
• Intensive Agriculture: Can you outline the specific impacts of monoculture, hedgerow removal, overgrazing, and pesticide usage?
• The Eutrophication Cascade: Can you write out the 6-step eutrophication sequence, correctly naming aerobic decomposer bacteria as the cause of oxygen depletion?
• BOD Definition: Can you define Biochemical Oxygen Demand, including the standard conditions (5 days at 20°C in darkness)?
• Indicator Organisms: Do you know which macroinvertebrates indicate clean water (stoneflies, mayflies) versus polluted water (Tubifex, bloodworms)?
• Simpson’s Index: Can you calculate \(D\) using \(D = 1 - \sum \left(\frac{n}{N}\right)^2\) or \(D = \frac{N(N-1)}{\sum n(n-1)}\) and interpret what a high vs. low value means?
• Conservation: Can you explain the difference between in situ and ex situ conservation, and identify the roles of ASSIs, SACs, and buffer strips?