AS Level Marine Science (9693) Study Notes: Biodiversity

Welcome to the fascinating world of Marine Biodiversity! This chapter is essential because it moves beyond simply identifying organisms (classification) and helps us understand how they interact, why some marine areas are richer in life than others, and how we measure that richness. Understanding biodiversity is crucial for marine conservation efforts globally.

Don't worry if the formulas look complicated at first; we will break them down into simple steps. The key is understanding why we use these tools!

Section 1: Key Ecological Concepts (Syllabus 4.4.1 & 4.4.2)

Before diving into diversity, we need to clarify the fundamental terms used to describe life and its environment:

1.1 Core Ecological Definitions

  • Species: A group of organisms that can interbreed and produce fertile offspring.
  • Population: All the individuals of a same species living in the same area at the same time. (Example: The population of clownfish living on one specific anemone.)
  • Community: All the different populations (of different species) living and interacting in the same area. (Example: All the fish, corals, algae, and invertebrates living on a single patch reef.)
  • Ecosystem: The community (biotic components) plus the non-living (abiotic) environment they interact with. (Example: The coral reef community + the surrounding water, rocks, temperature, and light.)
  • Habitat: The physical place where an organism lives. (Example: The sandy seabed, or the holdfast of a kelp plant.)
  • Niche: The specific role an organism plays within its ecosystem, including all its interactions with biotic and abiotic factors (what it eats, where it lives, when it reproduces).

Quick Tip: Think of Habitat as the organism’s address, and Niche as its profession.

1.2 Factors Affecting Marine Life Distribution

Organisms are distributed where they are best adapted to survive. These influences are split into two groups:

Biotic Factors (Living Influences)

These involve interactions between organisms:

  • Intra-specific competition: Competition within the same species (e.g., two male seals fighting for territory).
  • Inter-specific competition: Competition between different species (e.g., different species of barnacles competing for space on a rock).
  • Symbioses: Close relationships between two species (e.g., mutualism, parasitism).
  • Predation: One organism (predator) consumes another (prey).
  • Disease: Pathogens affecting populations.
Abiotic Factors (Non-Living Influences)

These are physical and chemical conditions in the environment:

  • Salinity (salt content).
  • Temperature (affects metabolism and solubility of gases).
  • pH (acidity/alkalinity).
  • Oxygen concentration (low solubility in water means this is often a limiting factor).
  • Carbon dioxide concentration.
  • Light availability (crucial for producers like phytoplankton).
  • Turbidity (how cloudy the water is).
  • Wave/tide action (physical force and exposure).
  • Nutrient availability (e.g., nitrate, phosphate).
  • Exposure to air (critical in the littoral zone).

Key Takeaway: The distribution and abundance of marine life are governed by the interplay of these biotic and abiotic factors. For instance, high light availability (abiotic) supports producers, which in turn fuels high levels of predation (biotic).

Section 2: The Three Levels of Biodiversity (Syllabus 4.3.1)

Biodiversity is defined as the measure of the range of different species and ecosystems, as well as the genetic diversity within a species. We assess it at three distinct levels:

1. Genetic Diversity

  • What it is: The variation in the genes among individuals of the same species.
  • Why it matters: High genetic diversity means a population has a better chance of adapting to environmental changes (like disease or temperature rise) because some individuals may possess the genes needed for survival.

2. Species Diversity

  • What it is: The measure of both the number of species present (species richness) and their relative abundance (how evenly distributed they are).
  • Why it matters: This is the most common measure of biodiversity. An area with many different species that are all equally common is considered more diverse than an area with many species, but where 99% of the individuals belong to just one species.

3. Ecological Diversity (Ecosystem Diversity)

  • What it is: The variation in the different ecosystems present on a regional or global level.
  • Why it matters: This variation encompasses different marine habitats, such as deep-sea vents, coral reefs, mangrove forests, and open ocean pelagic zones. Maintaining this range ensures that all unique ecological processes are protected.

Key Takeaway: Biodiversity is a complex concept. When scientists talk about "saving biodiversity," they are thinking about genetic variation, species numbers, and the variety of ecosystems.

Section 3: Importance and Benefits of Marine Biodiversity (Syllabus 4.3.2)

High biodiversity is strongly linked to ecosystem stability and provides crucial ecosystem services to humans and the planet.

Here are five key benefits:

  1. Maintaining Stable Ecosystems:

    A diverse community (many different species) maintains complex interactions. If one species fails (e.g., due to disease), other species can often step in and fill its role, preventing the entire ecosystem from collapsing. Analogy: A diverse stock portfolio is less likely to crash if one industry fails.

  2. Protection of the Physical Environment:

    Biodiverse habitats act as natural barriers. Example: Healthy coral reefs and mangrove forests dissipate wave energy, protecting coastlines from erosion and storm surges.

  3. Climate Control:

    Marine life plays a vital role in regulating global climate. Example: Phytoplankton absorb enormous amounts of \(CO_2\) from the atmosphere during photosynthesis and release \(O_2\). This helps buffer the effects of climate change.

  4. Providing Food Sources:

    Biodiversity ensures a wide range of commercially and ecologically important food resources, including algae, crustaceans, and fish.

  5. Providing a Source of Medicines:

    Many marine organisms produce unique chemical compounds used in medicine. Example: The protein Keyhole Limpet Hemocyanin (KLH), derived from a type of marine mollusc, is used in cancer drug research and vaccine development.

Key Takeaway: Marine biodiversity provides essential stability and services—from protecting our shores to breathing life into the atmosphere and providing medical breakthroughs.

Section 4: Estimating Population Size – Mark-Release-Recapture (Syllabus 4.4.3 & 4.4.4)

It's usually impossible to count every single individual of a mobile species (like fish or crabs). For estimating the population size (\(N\)) of mobile animals, we use the mark-release-recapture method, often analyzed using the Lincoln Index.

The Mark-Release-Recapture Method

  1. First Sample (\(n_1\)): Capture a specific number of individuals, record this number (\(n_1\)), mark them in a harmless way (e.g., paint spot), and then release them back into the habitat.
  2. Time Delay: Allow enough time for the marked individuals to randomly mix back into the general population.
  3. Second Sample (\(n_2\)): Capture a second sample of individuals (\(n_2\)) and count how many of this second sample are marked (\(m_2\)).

The Lincoln Index Formula

The estimated population size (\(N\)) is calculated assuming that the proportion of marked animals in the second sample is the same as the proportion of marked animals in the total population.

\[N = \frac{n_1 \times n_2}{m_2}\]

Where:

  • \(N\) = estimate of population size.
  • \(n_1\) = number of individuals captured in the first sample (and marked).
  • \(n_2\) = total number of individuals captured in the second sample.
  • \(m_2\) = number of marked individuals recaptured in the second sample.

Limitations of the Lincoln Index

This method only works well if several key assumptions are met. The limitations occur when these assumptions are violated:

  • The population is closed (no significant births, deaths, immigration, or emigration during the study).
  • The mark is not lost or removed.
  • The mark does not affect the survival (predation risk) or behavior (e.g., movement or mating) of the animal.
  • The marked individuals have had enough time to randomly mix back into the population.

Common Mistake to Avoid: If the organisms are highly territorial or clumped, they might not mix randomly, leading to an inaccurate estimate!

Key Takeaway: The Lincoln Index provides a mathematical estimate of a mobile population based on the ratio of marked to unmarked individuals, but its accuracy depends heavily on realistic assumptions about animal behavior.

Section 5: Sampling Distribution and Abundance (Syllabus 4.4.5 & 4.4.6)

When studying stationary or slow-moving organisms (like barnacles or kelp) in the littoral zone, ecologists use specific sampling strategies.

Random vs. Systematic Sampling

  • Random Sampling:

    Used to get an unbiased estimate of abundance or density across a uniform habitat. Locations are chosen randomly (often using random number generators for coordinates).

    Advantage: Reduces researcher bias.

    Disadvantage: May miss rare species or patterns if the area is large.

  • Systematic Sampling (Transects):

    Used when there is a clear environmental gradient (e.g., change in water level down a rocky shore). Samples are taken at fixed intervals along a line.

    Advantage: Excellent for studying the relationship between an abiotic factor (like exposure to air) and species distribution.

    Disadvantage: Can introduce bias if the initial line placement is not representative.

Sampling Tools

  • Frame Quadrats: Square frames of known area (e.g., 0.5 m²) used to count organisms or estimate percentage cover within that specific area. Used for both random and systematic sampling.
  • Line Transects: A line (tape measure) stretched across the gradient. Organisms touching the line are recorded at regular intervals. Simple measure of *presence/absence*.
  • Belt Transects: A strip of known width (often created by placing quadrats continuously or at intervals along a line transect). Provides data on both presence and abundance.

Ethical Note: When sampling organisms, safety and ethical treatment must always be prioritized. This includes minimizing disturbance and ensuring researchers are safe, especially in tidal zones.

Key Takeaway: The choice between random and systematic sampling, and the use of transects and quadrats, depends entirely on the type of data you want to collect and whether you are investigating a gradient.

Section 6: Measuring Species Diversity – Simpson’s Index (Syllabus 4.4.7)

The Simpson’s Index of Diversity (\(D\)) is a powerful tool because it considers both the number of species (richness) AND their relative abundance (evenness). A higher \(D\) value indicates higher diversity.

The Simpson’s Index Formula

The equation calculates the probability that two individuals randomly selected from a habitat belong to the *same* species. We then subtract this probability from 1 to find the diversity.

\[D = 1 - \sum \left(\frac{n}{N}\right)^2\]

Where:

  • \(\sum\) = sum of (total)
  • \(n\) = number of individuals of each different species (the count for Species A, Species B, etc.)
  • \(N\) = the total number of individuals of all the species.

Interpreting the Value of D

  • A value of D close to 1 indicates high species diversity. (The ecosystem is healthy, stable, and less dominated by one species).
  • A value of D close to 0 indicates low species diversity. (The ecosystem is dominated by a few species, making it less stable).

Did you know? Coral reefs typically have very high D values, while polluted estuaries might have very low D values.

Key Takeaway: Simpson's Index is the standard way to quantify species diversity, reflecting both how many species there are and how equally common they are.

Section 7: Analyzing Relationships – Spearman’s Rank Correlation (Syllabus 4.4.8)

After collecting data on abundance and abiotic factors (e.g., number of limpets vs. rock height), we use statistical tests like Spearman’s Rank Correlation (\(r_s\)) to determine if there is a relationship (correlation) between these two variables.

What Spearman’s Rank Measures

The \(r_s\) value measures the strength and direction of a relationship between two ranked variables.

\[r_s = 1 - \left(\frac{6 \times \sum D^2}{n^3 - n}\right)\]

Where:

  • \(\sum D^2\) = sum of the squared difference in rank between each pair of measurements.
  • \(n\) = number of pairs of items in the sample.

Interpreting the \(r_s\) Value

The \(r_s\) value will always fall between -1 and +1:

  • \(r_s = +1\): Perfect positive correlation. As one factor increases, the other factor increases perfectly. (e.g., Abundance increases perfectly as temperature increases).
  • \(r_s = -1\): Perfect negative correlation. As one factor increases, the other factor decreases perfectly. (e.g., Abundance decreases perfectly as wave action increases).
  • \(r_s = 0\): No correlation. The two factors are unrelated.

CRITICAL POINT: Correlation Does Not Imply Causation!

Just because you find a strong correlation (e.g., \(r_s = 0.9\)) between the number of sea stars and the local water depth, it does not automatically mean that depth *causes* the sea stars to be there. There could be a third, hidden variable (like food availability) that is influenced by depth and is the true cause.

Key Takeaway: Spearman’s Rank gives us a numerical value for how strongly two factors are linked, but scientific explanation and further evidence are required to prove a causal relationship.