Welcome to Biodiversity (AS 2: Organisms and Biodiversity)
Hello and welcome! In this chapter, we explore biodiversity—the incredible variety of life found all across our planet. We will look at what biodiversity actually means, how biologists measure it using mathematical indices, how we classify and name all living things, and how human activities impact natural communities.
Don't worry if mathematical formulas or long scientific terms seem intimidating at first. We will break everything down into clear, step-by-step chunks with simple analogies and worked examples!
---1. What is Biodiversity?
Biodiversity is defined as the variety of life on Earth. It encompasses all species of plants, animals, and microorganisms, the genetic information they contain, and the ecosystems they form.
To study biodiversity properly, scientists examine it at three hierarchical levels:
1. Genetic Diversity: The variation of alleles and genes within a single species or population.
Analogy: Think of a box of crayons of the same color but with many different shades. High genetic diversity means a species has lots of different genetic traits, making it more adaptable to diseases or environmental changes.
2. Species Diversity: The number of different species (known as species richness) and the relative abundance of each species (known as species evenness) living within a community or ecosystem.
3. Ecosystem / Habitat Diversity: The range and variety of different habitats or ecosystems within a defined geographical area (for example, having woodlands, wetlands, and grasslands all in one region).
Species Richness vs. Species Evenness
It is very common to mix these two ideas up, so let's look at them carefully:
• Species Richness: The total count or number of different species present in a particular community or area.
• Species Evenness: A measure of the relative abundance of the individuals of each different species present in a community.
Why does evenness matter?
Imagine two small woodlands, each containing \(100\) individual trees across \(4\) different species:
• Woodland A: \(97\) Oak trees, \(1\) Birch tree, \(1\) Ash tree, \(1\) Beech tree.
• Woodland B: \(25\) Oak trees, \(25\) Birch trees, \(25\) Ash trees, \(25\) Beech trees.
Both woodlands have the exact same species richness (4 species). However, Woodland B has much higher species evenness. Woodland A is overwhelmingly dominated by just one species. Therefore, Woodland B has higher overall biodiversity and is generally more stable and resilient if an environmental change or disease strikes.
Key Takeaway: Biodiversity includes genetic, species, and ecosystem variety. High biodiversity requires not just many species (richness), but also a balanced abundance among them (evenness).
---2. Measuring Biodiversity: Simpson’s Index of Diversity
Biologists use mathematical tools to assign a numerical value to biodiversity. For your CCEA examination, you must know and use Simpson’s Index (\(D\)).
The Formula
\(D = \frac{\sum n(n - 1)}{N(N - 1)}\)
Where:
• \(N\) = Total number of organisms of all species collected/sampled.
• \(n\) = Number of individuals of a particular/each single species.
• \(\sum\) = "The sum of" (add up the calculated values for every species).
Crucial Rule for CCEA Students: Interpreting \(D\)
Warning: In the standard CCEA formula, the calculated value of \(D\) ranges between \(0\) and \(1\).
• \(D = 0\) represents infinite / maximum diversity.
• \(D = 1\) represents zero diversity (only one species is present).
• Golden Rule: The lower the calculated value of \(D\), the higher the biodiversity! Conversely, if the numerical value of \(D\) increases, it indicates a decrease or reduction in biodiversity.
Step-by-Step Worked Example
Let's calculate Simpson's Index for a pond sample with the following organisms:
• Species A (Mayfly nymph): \(n = 10\)
• Species B (Water beetle): \(n = 5\)
• Species C (Dragonfly nymph): \(n = 5\)
Step 1: Calculate the total number of individuals (\(N\)):
\(N = 10 + 5 + 5 = 20\)
Step 2: Calculate \(N(N - 1)\) for the denominator:
\(N - 1 = 20 - 1 = 19\)
\(N(N - 1) = 20 \times 19 = 380\)
Step 3: Set up a table to find \(n(n - 1)\) for each species and sum them up (\(\sum\)):
• For Species A: \(n = 10 \implies 10 \times (10 - 1) = 10 \times 9 = 90\)
• For Species B: \(n = 5 \implies 5 \times (5 - 1) = 5 \times 4 = 20\)
• For Species C: \(n = 5 \implies 5 \times (5 - 1) = 5 \times 4 = 20\)
• Sum of \(n(n - 1)\) = \(\sum n(n - 1) = 90 + 20 + 20 = 130\)
Step 4: Divide the numerator by the denominator:
\(D = \frac{130}{380} \approx 0.342\)
Common Pitfalls to Avoid:
• Do not square \(N\); always multiply \(N\) by \((N - 1)\).
• Remember to calculate \(n(n - 1)\) for each individual species before adding them together.
• Never say "a higher \(D\) value means higher biodiversity"—in CCEA, a smaller value of \(D\) means greater biodiversity.
3. Taxonomy and Classification
Taxonomy is the science of naming, describing, and classifying organisms into groups based on shared characteristics and evolutionary relationships.
The Hierarchy of Taxa
Living organisms are classified into hierarchical groups called taxa (singular: taxon). As you move down from Domain to Species, the groups become smaller, and the organisms within them share more specific traits.
The hierarchy in order is:
Domain \(\rightarrow\) Kingdom \(\rightarrow\) Phylum \(\rightarrow\) Class \(\rightarrow\) Order \(\rightarrow\) Family \(\rightarrow\) Genus \(\rightarrow\) Species
Memory Trick: To remember this order, use the mnemonic:
"Dear King Philip Came Over For Good Soup"
The Binomial System
Devised by Carl Linnaeus, the binomial system gives every organism a two-part scientific Latin name:
• The first part is the Genus (always written with a Capital letter).
• The second part is the species (always written in lowercase).
• When printed, scientific names are in italics (e.g., Homo sapiens). When handwritten in an exam, you must underline them (e.g., Homo sapiens).
The Three Domains
Based on molecular phylogenetics (specifically looking at rRNA sequences and membrane lipid biochemistry), Carl Woese proposed dividing all life into three domains:
1. Archaea: Extremophile prokaryotes that live in harsh environments (e.g., high temperatures or extreme salinity).
2. Bacteria (Eubacteria): "True" bacteria.
3. Eukarya: All organisms whose cells contain a true nucleus and membrane-bound organelles.
The Five Kingdoms
Within the classification system, organisms are divided into five distinct kingdoms:
1. Prokaryotae (Monera)
• Unicellular organisms.
• Lack a true membrane-bound nucleus and membrane-bound organelles.
• Possess circular DNA not associated with histone proteins.
• Have smaller \(70\text{S}\) ribosomes.
• Cell wall made of peptidoglycan (murein).
2. Protoctista
• Eukaryotic organisms (possess a true nucleus and membrane-bound organelles).
• Mostly unicellular, though some exist as simple multicellular forms (e.g., algae and protozoans).
3. Fungi
• Eukaryotic organisms.
• Heterotrophic nutrition: mostly saprophytic (absorb nutrients from dead decaying matter) or parasitic.
• Cell walls made of chitin (never cellulose!).
• Body structure often made of microscopic thread-like filaments called hyphae (forming a network called a mycelium).
• Reproduce via spores.
4. Plantae
• Multicellular eukaryotes.
• Autotrophic nutrition (produce their own organic food via photosynthesis).
• Cell walls made of cellulose.
• Cells contain chloroplasts and large permanent vacuoles.
5. Animalia
• Multicellular eukaryotes.
• Heterotrophic nutrition (ingestive: ingest food and digest it internally).
• Cells lack cell walls.
• Possess nervous coordination and the ability to move at some life stage.
Key Takeaway: Classification arranges organisms hierarchically from Domain down to Species. Organisms are grouped into 3 Domains and 5 Kingdoms based on cellular structure and biochemistry.
---4. Human Impact on Biodiversity and Conservation
Human activities pose major threats to global biodiversity, but active conservation strategies aim to protect and restore species and their habitats.
Major Threats to Biodiversity
• Habitat Destruction and Fragmentation: Clearing land for roads, urban development, and housing splits large habitats into small, isolated patches.
• Intensive Agriculture: Creating monocultures (growing single crops over vast areas), removing hedgerows to enlarge fields, and using heavy synthetic agrochemicals (fertilisers and pesticides).
• Deforestation: Removing forests destroys habitats, reduces species richness, and contributes to climate change.
• Overfishing and Overexploitation: Harvesting populations faster than they can naturally replenish.
• Pollution and Climate Change: Acid rain, plastic waste, eutrophication of waterways, and rising global temperatures alter ecosystems.
• Invasive Alien Species: Introducing non-native species that outcompete indigenous wildlife for food and space.
Conservation Strategies
Conservation aims to preserve genes, species, and ecosystems through two primary approaches:
1. In situ Conservation (Within the natural habitat):
Protecting species directly in their native environments.
• Examples: Establishing National Parks, Nature Reserves, Areas of Special Scientific Interest (ASSIs), and Sites of Special Scientific Interest (SSSIs).
• Advantages: Animals and plants live naturally, maintain natural behaviors and food webs, and entire ecosystems are protected simultaneously.
2. Ex situ Conservation (Outside the natural habitat):
Removing individuals or genetic material from threatened areas to protect them in controlled environments.
• Examples: Botanical gardens, seed banks (storing seeds at low temperatures and low humidity to preserve genetic diversity), and captive breeding programmes in zoos.
• Advantages: Protects endangered individuals from immediate threats like predation, poaching, and habitat loss, allowing numbers to build up before potential reintroduction.
Key Takeaway: Human activities like intensive farming and deforestation reduce biodiversity. Conservation strategies protect biodiversity both inside their natural habitats (in situ) and in managed facilities (ex situ).
---Quick Revision Checklist
Before your exam, make sure you can confidently:
• Define biodiversity, species richness, and species evenness.
• State the 3 levels of biodiversity: genetic, species, and ecosystem.
• Calculate Simpson's Index using \(D = \frac{\sum n(n - 1)}{N(N - 1)}\) and remember that a lower value of \(D\) means higher diversity.
• Recite the classification hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Correctly format binomial names: Genus species.
• State key diagnostic features of the 3 Domains and the 5 Kingdoms.
• Distinguish between in situ (e.g., ASSIs, nature reserves) and ex situ (e.g., seed banks, zoos) conservation methods.