Welcome to Biodiversity!
Hello and welcome to your study notes for Biodiversity, a core topic in AS 2: Organisms and Biodiversity. Don't worry if ecology sometimes feels full of definitions and calculations—we are going to break everything down into clear, manageable steps with real-world examples, handy memory tricks, and simple explanations.
By the end of this chapter, you will understand what biodiversity is, how biologists measure it using sampling and maths, why biodiversity is under threat, and how we can protect it for the future.
1. What is Biodiversity?
At its simplest, biodiversity is the variety of life in a given area. It isn't just about counting how many different animals live in a forest; it includes all living things (plants, animals, fungi, and micro-organisms) and the genetic variety within them.
The Three Levels of Biodiversity
Biodiversity is studied at three distinct levels:
1. Habitat (or Ecosystem) Diversity: The range of different habitats found within an area (for example, a countryside landscape containing woodland, streams, ponds, and hedgerows has high habitat diversity).
2. Species Diversity: The variety of different species living in a habitat. This consists of two important components:
• Species Richness: The total number of different species present in a community.
• Species Evenness: The relative abundance (proportions) of each species present.
3. Genetic Diversity: The variety of alleles (gene variants) present within a single species or population.
Everyday Analogy: Imagine two fruit bowls. Bowl A contains \(10\) apples, \(10\) bananas, and \(10\) oranges. Bowl B contains \(28\) apples, \(1\) banana, and \(1\) orange. Both bowls have the exact same species richness (3 species), but Bowl A has much higher species evenness because the numbers of each fruit are balanced.
Did you know? A community dominated by just one single species is much more fragile. If a disease wipes out that dominant species, the whole community can collapse!
Key Takeaway: High species diversity requires both high species richness (lots of different species) and high species evenness (balanced population sizes among species).
2. Measuring Biodiversity: Simpson's Diversity Index
To compare different habitats objectively, biologists use a mathematical tool called Simpson's Index of Diversity (\(D\)).
The Formula
\(D = 1 - \frac{\sum n(n - 1)}{N(N - 1)}\)
Where:
• \(N\) = Total number of organisms of all species combined
• \(n\) = Total number of organisms of a particular species
• \(\sum\) = Sum of (add them all together)
Understanding the Value of \(D\)
• The value of \(D\) always falls between \(0\) and \(1\).
• A value close to \(1\) indicates high biodiversity: the habitat is diverse, stable, has complex food webs, and is resilient to environmental changes.
• A value close to \(0\) indicates low biodiversity: the habitat is dominated by one or two species, has simple food webs, and is vulnerable to changes or disease outbreaks.
Step-by-Step Calculation Example
Let's calculate \(D\) for a small woodland sample:
• Oak trees: \(n = 10\)
• Birch trees: \(n = 5\)
• Beech trees: \(n = 5\)
Step 1: Calculate total \(N\):
\(N = 10 + 5 + 5 = 20\)
Step 2: Calculate \(N(N - 1)\):
\(N(N - 1) = 20 \times 19 = 380\)
Step 3: Calculate \(n(n - 1)\) for each individual species:
• Oak: \(10 \times (10 - 1) = 10 \times 9 = 90\)
• Birch: \(5 \times (5 - 1) = 5 \times 4 = 20\)
• Beech: \(5 \times (5 - 1) = 5 \times 4 = 20\)
Step 4: Sum all the \(n(n - 1)\) values:
\(\sum n(n - 1) = 90 + 20 + 20 = 130\)
Step 5: Put it into the formula:
\(D = 1 - \frac{130}{380} = 1 - 0.342 = 0.658\)
This woodland has a diversity index of \(0.658\), indicating relatively good biodiversity!
Common Mistake to Avoid: Don't forget the "\(1 -\)" at the very beginning of the formula! Without it, you are calculating the probability of picking two organisms of the same species, rather than measuring diversity.
3. Sampling Techniques in Ecology
We cannot count every single organism in an ecosystem—it would take too long and could damage the habitat. Instead, we take representative samples.
Avoiding Bias: Random Sampling
To ensure sampling is reliable and unbiased, we use random sampling:
1. Lay out two perpendicular tape measures at right angles to create a coordinate grid over the study area.
2. Use a random number generator to obtain pairs of coordinates.
3. Place your sampling apparatus (e.g., a quadrat) at these exact coordinates.
4. Repeat many times to obtain a large, representative sample size.
Ecological Sampling Equipment
• Frame Quadrats: Square frames (often \(0.5\text{ m} \times 0.5\text{ m}\)) placed on the ground to sample stationary or slow-moving organisms (like plants or barnacles). We can record species presence/absence, percentage cover, or individual counts.
• Point Quadrats: A horizontal frame holding long vertical pins. As each pin is lowered, every plant species it touches is recorded. This provides an objective estimate of percentage cover.
• Line and Belt Transects: Used when there is an environmental gradient (e.g., from an open field into dense shade, or up a rocky shore):
- Line Transect: A tape measure is laid across the area; organisms touching the tape at set intervals are recorded.
- Belt Transect: Quadrats are placed alongside the tape continuously or at regular intervals (interrupted belt transect).
• Sweep Nets and Pitfall Traps: Used for mobile animals. Sweep nets collect insects from tall grasses, while pitfall traps (small containers sunk into the soil) collect small, ground-dwelling invertebrates.
Key Takeaway: Reliable ecological sampling requires random positioning to remove investigator bias and a sufficiently large sample size to give an accurate representation of the community.
4. Threats to Biodiversity: Human Impact
Human activities have significantly accelerated the rate of species extinction and habitat loss. The main drivers include:
1. Agriculture and Intensive Farming:
• Monoculture: Growing large fields of a single crop reduces habitat and food variety for native organisms, drastically lowering species diversity.
• Removal of Hedgerows: Done to create larger fields for machinery, destroying vital wildlife corridors and nesting habitats.
• Pesticides and Herbicides: Chemicals reduce pest populations and wild weeds, removing food sources for birds and beneficial insects.
• Fertiliser Runoff (Eutrophication): Excess nitrates and phosphates wash into waterways, causing algal blooms, light blockage, deoxygenation, and death of aquatic organisms.
2. Deforestation:
Clearing natural forests for timber, agriculture, or cattle ranching directly destroys habitats, leading to immediate loss of plant species and displacement of animal populations.
3. Climate Change:
Rising global temperatures and extreme weather events alter habitats faster than organisms can adapt or migrate, leading to range shifts and local extinctions.
4. Pollution and Overexploitation:
Industrial pollution, plastic waste, overfishing, and overhunting deplete wild populations beyond their capacity to recover.
5. Why Should We Maintain Biodiversity?
Conserving biodiversity is not just about saving charismatic animals; it is essential for the health of our planet and our economy. The reasons fall into three main categories:
1. Ecological Reasons
• Ecosystem Stability: Diverse communities have complex food webs; if one species declines, others can take its place, preventing total ecosystem collapse.
• Keystone Species: Some species have a disproportionately large impact on their environment relative to their abundance (e.g., bees as pollinators, sea otters controlling urchin populations). Losing them disrupts the entire system.
• Ecosystem Services: Natural habitats provide soil formation, water purification, flood protection, and climate regulation.
2. Economic and Agricultural Reasons
• Medicines: Many modern drugs originate from wild plant and fungal compounds. Undiscovered species may hold treatments for future diseases.
• Crop Breeding: Wild relatives of commercial crops possess alleles for drought tolerance, pest resistance, and disease resistance. Conserving wild varieties provides genetic material for future crop improvement.
• Ecotourism: Natural, biodiverse landscapes generate vital income for local communities.
3. Ethical and Aesthetic Reasons
• Moral Responsibility: Many believe humans have a duty of stewardship to preserve species for future generations.
• Mental Well-being: Natural, species-rich environments enrich human life, offering recreation, inspiration, and psychological benefits.
Memory Trick: Remember the "3 Es" for why we maintain biodiversity: Ecological balance, Economic benefits, and Ethical duties.
6. Conservation Strategies
Conservation aims to protect biodiversity while allowing sustainable human development. Strategies are divided into two complementary approaches:
In Situ Conservation (On-Site)
Protecting species within their natural habitat.
• Examples: National Parks, Nature Reserves, Marine Protected Areas, and Areas of Special Scientific Interest (ASSIs / SSSIs).
• Advantages:
- Species remain adapted to their natural environment and ecological niches.
- Entire natural ecosystems and interdependent species are preserved together.
- It is often more cost-effective than managing captive populations.
• Disadvantages: Difficult to control external threats like climate change, poaching, or invasive species.
Ex Situ Conservation (Off-Site)
Protecting species outside their natural habitat.
• Examples: Botanic gardens, seed banks, and zoos with captive breeding programmes.
• Seed Banks: Seeds from wild plants are collected, dried, and stored at sub-zero temperatures (around \(-20^\circ\text{C}\)). This keeps them dormant and viable for decades while saving space.
• Captive Breeding Programmes: Endangered animals are bred in controlled zoo environments with studbooks to maintain genetic diversity, with the long-term goal of reintroducing them into the wild.
• Advantages: Complete protection from predators, poaching, and food shortages; enables controlled breeding.
• Disadvantages: High cost; animals may lose natural survival behaviours; reproductive difficulties in captivity; limited genetic pool if founder populations are small.
Quick Review Box:
• In situ: In the natural home (e.g., nature reserve). Best for whole communities!
• Ex situ: Exit the natural home (e.g., seed bank, zoo). Best as a safety net for critically endangered species!
Chapter Summary
• Biodiversity encompasses genetic diversity, species diversity (richness and evenness), and habitat diversity.
• Simpson's Diversity Index (\(D = 1 - \frac{\sum n(n - 1)}{N(N - 1)}\)) gives an objective value between \(0\) and \(1\); higher values indicate greater diversity and ecosystem resilience.
• Ecological sampling must be random to eliminate bias and large enough to be reliable.
• Human threats include intensive farming (monocultures, hedgerow removal), deforestation, pollution, and climate change.
• Maintaining biodiversity is vital for ecological stability, economic resources (medicines, crop genes), and ethical stewardship.
• Conservation combines in situ (protecting habitats) and ex situ (seed banks, captive breeding) methods.