Welcome to Communities (A2 Biology Unit 1)
Welcome to one of the most dynamic topics in your A2 1 module: Communities. In this chapter, we explore how different living organisms interact with one another, how biological communities develop and change over time through succession, and how ecologists measure biodiversity and distribution in the field.
Don't worry if ecological terminology feels overwhelming at first. We will break down every concept into step-by-step, digestible parts with clear definitions, analogies, and exam tips specifically tailored to the CCEA specification.
---1. Ecological Hierarchy & Fundamental Definitions
Before diving into complex interactions, it is essential to have crystal-clear definitions of ecological terms. Examiners frequently test these as short-answer questions, and using exact biological phrasing will secure you full marks.
• Community: All the interacting populations of different species living in the same habitat or area at the same time.
• Habitat: The specific geographic place or physical environment where an organism, population, or community lives.
• Ecosystem: A dynamic system made up of a community of living organisms (biotic components) interacting with each other and with the non-living chemical and physical environment (abiotic components).
• Ecological Niche: The precise role and position a species occupies within an ecosystem. This includes its spatial habitat, how it uses available resources, and all of its interactions with other living organisms.
The Competitive Exclusion Principle
What happens when two different species try to occupy the exact same niche? According to the Competitive Exclusion Principle, two species with identical ecological niches cannot coexist indefinitely if resources are limiting. One species will inevitably outcompete the other for food, light, or nesting sites, leading either to the decline/extinction of the weaker competitor or an evolutionary shift to a slightly different niche.
Quick Review Box: Avoiding Examiner Traps
Common Mistake: Mixing up a population and a community.
Memory Trick: A Population is Purely one species (e.g., all red squirrels in a wood). A Community is a Collection of all different species living together (squirrels, oak trees, fungi, birds, and beetles).
2. Ecological Succession: How Communities Change Over Time
Ecosystems are not static; they change continuously. Succession is the progressive, directional, and predictable change in community composition and structure in a given area over ecological time.
The entire sequence of intermediate stages and communities that replace one another during succession—from the very first colonisers to the final stable stage—is known as a sere (or seral stages).
Primary Succession (Starting from Scratch)
Primary succession begins on newly formed, completely barren ground where there is no pre-existing soil or organic matter. Examples include bare rock exposed by a retreating glacier, cooled volcanic lava flows, or newly formed sand dunes (a psammosere).
Step-by-Step Process of Primary Succession:
Step 1: Colonisation by Pioneer Species
The first organisms to colonise the harsh, barren area are called pioneer species (e.g., lichens and mosses). Pioneer species possess specialised adaptations to survive extreme conditions:
• High tolerance to extreme abiotic conditions (such as desiccation, high wind, and wide temperature swings).
• Ability to fix atmospheric nitrogen or survive in nutrient-poor substrates.
• Rapid reproduction with small, wind-dispersed spores or seeds.
Step 2: Weathering and Soil Formation
Pioneer species physically and chemically weather the rock surface. As these pioneers die and decompose, their dead organic matter (humus) accumulates. This builds a thin layer of primitive soil, which dramatically improves water retention and mineral nutrient availability.
Step 3: Secondary Colonisation and Interspecific Competition
Because the abiotic environment is now less hostile, seeds of small plants like grasses and herbs can germinate and establish. As these new species grow taller and spread, they outcompete the initial pioneer species for light, water, and space.
Step 4: Increasing Complexity
The cycle repeats: deeper soil supports shrubs, which in turn are eventually outcompeted by larger trees. At each seral stage, species richness, total biomass, and soil depth increase.
Secondary Succession (Regrowth after Disturbance)
Secondary succession occurs on land where an existing community has been disrupted or cleared, but soil and seed banks/spores remain intact. Examples include abandoned farmland, land cleared by deforestation, or forest recovering after a wildfire.
Why is secondary succession much faster than primary succession?
Because fertile soil, organic humus, moisture, and viable seeds/propagules are already present. The community does not need to wait for the slow process of rock weathering and initial soil formation.
The Climax Community
Succession reaches an end-point known as the climax community. This is the final, stable, self-sustaining community that exists in dynamic equilibrium with the local climate and environment.
In temperate climates like the UK and Ireland, the natural climatic climax community is typically dominated by large, long-lived deciduous woodland (e.g., oak and ash forests). Characteristics include:
• High biodiversity and complex food webs.
• High total biomass.
• Organisms adapted to stable, highly competitive conditions rather than extreme abiotic stress.
Plagioclimax (Deflected Succession)
Sometimes, human activity interrupts natural succession, preventing a community from ever reaching its climatic climax. A stable community maintained in this way is called a plagioclimax (or deflected succession). Examples include:
• Regular livestock grazing on grassland (grazing animals eat young tree saplings before woodland can establish).
• Controlled burning of heather moorland.
• Regular mowing of lawns and sports pitches.
• Agricultural plowing and crop management.
Key Takeaway for Exam Success:
Never say: "Pioneers change the environment so they can help new plants grow."
Always say: "Pioneer species alter the abiotic conditions (e.g., by adding humus and retaining moisture), making the environment less hostile for subsequent species, which then outcompete earlier colonisers."
3. Interactions Within Communities
Populations within a community do not live in isolation; they interact continuously in several key ways:
1. Competition
• Intraspecific Competition: Competition between individuals of the same species for identical, limiting resources (such as mates, nesting territory, food, or light).
• Interspecific Competition: Competition between individuals of different species for overlapping resources (such as two bird species hunting the same caterpillar species).
2. Predation
Predation involves a predator hunting, killing, and consuming prey. In stable ecosystems, predator and prey populations often undergo cyclical oscillations:
• An increase in prey numbers provides more food, leading to a subsequent rise in the predator population.
• As predator numbers peak, they consume more prey, causing the prey population to crash.
• A shortage of food then causes the predator population to decline, allowing the prey population to recover and restart the cycle.
Note: Cyclical peaks in predator numbers always trail behind (lag behind) the peaks in prey numbers.
3. Symbiosis
Symbiotic relationships involve close, long-term physical associations between two different species:
• Mutualism: A relationship in which both species benefit.
Example: Lichens (a mutualistic partnership between a fungus providing structure/moisture and an alga/cyanobacterium providing photosynthetic carbohydrates) or mycorrhizal fungi associating with plant roots to exchange minerals for sugars.
• Commensalism: A relationship where one species benefits while the other is unaffected (neither harmed nor helped).
• Parasitism: A relationship where one organism (the parasite) benefits at the expense of another organism (the host), living on or inside the host and causing it harm.
4. Sampling and Quantitative Investigation of Communities
Ecologists cannot count every single organism in an entire habitat. Instead, representative sampling techniques are used to collect unbiased, statistically valid data.
Sampling Strategies: Random vs Systematic
Random Sampling:
• Used in habitats that are uniform or homogeneous (e.g., an open, flat meadow).
• Performed by laying out two tape measures at right angles to form a coordinate grid and using a random number generator to select quadrat coordinates. This eliminates investigator bias.
Systematic Sampling:
• Used when there is an obvious environmental gradient or transition across a habitat (e.g., changes in zonation from low tide to high tide on a rocky shore, or changes in vegetation moving inland across sand dunes).
• Relies on a transect line laid along the environmental gradient.
Types of Transects
• Line Transect: A tape or rope laid across the gradient. The presence of species touching the line at regular intervals is recorded (gives qualitative presence/absence data).
• Belt Transect: Quadrats are placed along the transect line to record quantitative abundance data alongside abiotic measurements (such as soil pH, light intensity, or moisture):
- Continuous Belt Transect: Quadrats are placed continuously side-by-side along the entire line.
- Interrupted Belt Transect: Quadrats are placed at regular, fixed intervals along the line (e.g., every 5 metres).
Quantitative Measures of Abundance
When sampling plants or slow-moving/sessile animals with quadrats, ecologists record abundance using three standard metrics:
• Density: The actual number of individual organisms per unit area (e.g., individuals per square metre, \(\text{m}^{-2}\)).
• Percentage Cover: The estimated percentage of ground area enclosed by the quadrat that is covered by a particular species. This is ideal for plants like grasses or mosses where individual stems cannot easily be separated.
• Percentage Frequency: The proportion of sampled quadrats in which a given species is recorded (e.g., if daisies appear in 15 out of 20 quadrats, their frequency is \(75\%\)).
Quantifying Biodiversity: Simpson’s Index of Diversity
When comparing different communities, simply counting the number of species present (species richness) is not enough. A community where one species makes up \(99\%\) of individuals is far less diverse than a community where five species are evenly distributed.
Ecologists use Simpson’s Index of Diversity (\(D\)) to measure biodiversity because it accounts for both richness (number of species) and evenness (relative abundance of each species).
You may encounter Simpson’s Index in either of these standard mathematical forms:
\(D = 1 - \sum \left(\frac{n}{N}\right)^2\)
or
\(D = \frac{\sum n(n - 1)}{N(N - 1)}\)
Where:
• \(n\) = total number of individuals of a particular single species.
• \(N\) = total number of individuals of all species combined.
• \(\sum\) = "sum of".
Interpreting Simpson's Index:
A higher calculated value of \(D\) represents greater biodiversity and greater species evenness. Communities with high diversity values are generally more stable, resilient to environmental change, and characterised by complex, robust food webs.
5. Quick Summary Checklist
Before moving on to past paper practice, make sure you can confidently answer the following questions:
1. Can you define community, ecosystem, and ecological niche accurately?
2. What is the fundamental difference between primary and secondary succession? (Hint: presence of soil and seed banks).
3. Why does an interrupted belt transect represent systematic rather than random sampling?
4. Can you describe how a pioneer species facilitates the arrival of subsequent seral species?
5. What are the key ecological features of a climax community and a plagioclimax?