Ecosystems and Communities

This lesson covers the key ecological terms and concepts of ecosystems, communities, and levels of ecological organisation, as required by the Edexcel A-Level Biology specification (9BI0), Topic 10 -- Ecosystems. You need to understand the distinction between biotic and abiotic factors, and how organisms interact within their habitats and ecological niches.

---

Key Ecological Terminology

Term	Definition
Species	A group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other species
Population	All the organisms of one species living in a particular area at a particular time
Community	All the populations of different species living and interacting in a particular area at a particular time
Habitat	The place where an organism lives; characterised by its physical (abiotic) and biological (biotic) conditions
Ecosystem	A community of organisms and their abiotic environment interacting as a functional unit through nutrient cycling and energy flow
Niche	The role of an organism within its ecosystem, including its habitat, feeding relationships, interactions with other species, and its contribution to energy flow and nutrient cycling
Biome	A large-scale ecosystem characterised by its climate and dominant vegetation type (e.g. tropical rainforest, tundra, desert)

Exam Tip: The ecological niche is not just where an organism lives (that is its habitat). The niche includes everything about the organism's role: what it eats, what eats it, when it is active, how it reproduces, and how it interacts with abiotic factors. The competitive exclusion principle states that two species cannot occupy the same niche indefinitely -- one will outcompete the other.

---

Levels of Ecological Organisation

flowchart TB
    A["Individual\nOrganism"] --> B["Population\n(all individuals of\none species in an area)"]
    B --> C["Community\n(all populations of\ndifferent species\nin an area)"]
    C --> D["Ecosystem\n(community +\nabiotic environment)"]
    D --> E["Biome\n(large-scale ecosystem\ndefined by climate)"]
    E --> F["Biosphere\n(all ecosystems\non Earth)"]

Understanding these levels is crucial for interpreting ecological data. For instance, measuring the population of bluebells in a woodland requires you to define the area (the woodland), the species (Hyacinthoides non-scripta), and to distinguish between the population of bluebells and the wider plant community that also includes oak trees, brambles, and ferns.

---

Biotic Factors

Biotic factors are living (biological) components of an ecosystem that influence the distribution and abundance of organisms:

Biotic Factor	Description	Example
Predation	Organisms being consumed by predators	Fox population controlling rabbit numbers
Competition	Organisms competing for the same limited resources	Interspecific (between species) or intraspecific (within a species)
Disease	Pathogens reducing population size	Myxomatosis in rabbits; ash dieback (Hymenoscyphus fraxineus) in UK woodlands
Food availability	Quantity and quality of food resources	Abundance of prey for a predator
Symbiotic relationships	Mutualism, parasitism, commensalism	Nitrogen-fixing bacteria in legume root nodules (mutualism)

Intraspecific vs Interspecific Competition

Type	Definition	Resources Competed For
Intraspecific competition	Competition between members of the same species	Food, territory, mates, nesting sites
Interspecific competition	Competition between members of different species	Food, light, water, space

Intraspecific competition is often more intense because individuals of the same species have identical resource requirements. A classic UK example is red squirrels (Sciurus vulgaris) being outcompeted by introduced grey squirrels (Sciurus carolinensis) -- an interspecific competition that has driven red squirrels to the point of regional extinction across much of England and Wales.

Exam Tip: Intraspecific competition is a key density-dependent factor that regulates population size. As population density increases, competition for resources intensifies, reducing birth rates and increasing death rates. This links directly to the population dynamics topic -- make sure you can explain both concepts together.

---

Abiotic Factors

Abiotic factors are non-living (physical and chemical) components of an ecosystem that influence the distribution and abundance of organisms:

Abiotic Factor	Effect on Organisms	Measurement Method
Temperature	Affects enzyme activity, metabolic rate, and distribution of ectotherms and endotherms	Thermometer / data logger
Light intensity	Affects rate of photosynthesis; determines plant distribution and productivity	Light meter
Water availability	Essential for all life; limits terrestrial organism distribution	Soil moisture probe / rain gauge
pH	Affects enzyme activity; determines species composition in aquatic and soil environments	pH meter / universal indicator
Mineral ion concentration	Nutrients such as nitrate and phosphate limit plant growth	Colorimetric testing
Wind	Affects transpiration, seed dispersal, and physical exposure	Anemometer
Oxygen concentration	Critical for aerobic respiration in aquatic organisms	Dissolved oxygen probe
Carbon dioxide concentration	Affects rate of photosynthesis	CO2 sensor
Soil type and depth	Determines water retention, mineral availability, and root anchorage	Soil auger / texture analysis
Salinity	Determines which organisms can survive in aquatic habitats (freshwater vs marine)	Conductivity meter

Interaction Between Biotic and Abiotic Factors

In practice, biotic and abiotic factors interact in complex ways. For example, in a UK oak woodland:

• Light intensity (abiotic) determines which plants grow in the understorey
• The canopy cover (biotic -- created by the oak trees) reduces light reaching the ground
• This affects the species composition of the ground flora (e.g. shade-tolerant species like dog's mercury dominate under dense canopy)
• Grazing by deer (biotic) prevents regeneration of tree seedlings, changing the community structure

---

The Ecosystem Concept

An ecosystem is a dynamic system in which:

1. Energy flows through the system (from sunlight through producers, consumers, and decomposers) -- energy flow is one-way (it is lost as heat at each trophic level)
2. Nutrients (matter) cycle within the ecosystem -- elements such as carbon and nitrogen are continually recycled between living organisms and the abiotic environment

The key point is that energy flows but nutrients cycle. This distinction is fundamental to understanding ecosystems and links directly to the carbon cycle, nitrogen cycle, and energy transfer lessons in this course.

flowchart LR
    subgraph "Energy Flow (one-way)"
        S["Sunlight"] --> P["Producers"]
        P --> C1["Primary\nconsumers"]
        C1 --> C2["Secondary\nconsumers"]
        C2 --> C3["Tertiary\nconsumers"]
    end
    P -.->|"Heat lost\nvia respiration"| H["Heat"]
    C1 -.-> H
    C2 -.-> H
    C3 -.-> H
    subgraph "Nutrient Cycling (recycled)"
        N1["Nutrients in\nliving organisms"] -->|"Death /\nexcretion"| N2["Nutrients in\ndead matter / soil"]
        N2 -->|"Decomposition\nand uptake"| N1
    end

---

The Niche Concept

Every species occupies a unique ecological niche -- its specific role in the ecosystem. The niche includes:

• Habitat requirements -- where the organism lives (e.g. forest canopy, soil surface)
• Diet -- what the organism eats or how it obtains energy
• Activity pattern -- when it is active (diurnal, nocturnal, seasonal)
• Reproductive behaviour -- how and when it reproduces
• Interactions -- relationships with other species (predation, competition, mutualism)
• Abiotic tolerances -- range of temperature, pH, humidity, etc. that it can tolerate

Fundamental vs Realised Niche

Niche Type	Definition	Example
Fundamental niche	The full range of conditions and resources in which a species could survive and reproduce in the absence of competition	Red squirrels could occupy all deciduous and coniferous woodland in the UK
Realised niche	The actual conditions and resources a species uses, which is typically narrower than the fundamental niche due to interspecific competition	Red squirrels are now largely restricted to coniferous forests in northern England and Scotland, where grey squirrels are less competitive

Common Misconception

Many students confuse habitat and niche. The habitat is a physical place (e.g. "an oak woodland"). The niche is the organism's functional role within that habitat -- what it eats, when it is active, how it reproduces, and all its interactions with other species and the abiotic environment. Two species can share the same habitat but occupy different niches (e.g. a robin feeding on ground invertebrates by day and a tawny owl hunting small mammals at night in the same woodland).

---

Worked Example: Identifying Biotic and Abiotic Factors

Question: A student investigates the distribution of a species of grass in a sand dune ecosystem. Suggest two biotic factors and two abiotic factors that might affect the distribution of this grass.

Answer:

Biotic factors:

1. Interspecific competition -- other plant species may outcompete the grass for light, water, or mineral ions
2. Grazing -- herbivores (such as rabbits) may selectively graze on the grass, reducing its abundance in some areas

Abiotic factors:

1. Soil water availability -- sand dunes near the shore are drier; the grass may be more abundant in dune slacks where water is more available
2. Wind exposure -- strong winds near the shore increase transpiration and cause physical damage, limiting grass growth

---

Worked Example: Applying the Competitive Exclusion Principle

Question: Two species of barnacle, Chthamalus stellatus and Semibalanus balanoides, are found on a rocky shore. When both are present, Chthamalus is restricted to the upper shore and Semibalanus occupies the lower shore. When Semibalanus is experimentally removed, Chthamalus colonises the lower shore as well. Explain these observations using the concepts of fundamental and realised niche.

Answer:

The fundamental niche of Chthamalus includes both the upper and lower shore (as shown by its colonisation of the lower shore when Semibalanus is removed). However, its realised niche is restricted to the upper shore due to interspecific competition with Semibalanus. In the lower shore zone, Semibalanus outcompetes Chthamalus for space (by growing over and smothering it). This is an example of the competitive exclusion principle: two species cannot coexist in the same niche indefinitely, so Chthamalus is excluded from the lower shore by the superior competitor.

---

Diversity and Ecosystem Stability

Ecosystems with higher biodiversity tend to be more stable and resilient. This is because:

• More species means more complex food webs, so if one species declines, others can fill its role
• Greater genetic diversity within species allows populations to adapt to changing conditions
• Diverse ecosystems are more productive (more efficient use of available resources)

This is why conservation of biodiversity is so important -- it maintains the stability of ecosystems on which humans depend for ecosystem services (clean water, pollination, climate regulation, food production).

---

Summary

• An ecosystem comprises a community of organisms and its abiotic environment, linked by energy flow and nutrient cycling.
• Biotic factors (living) include predation, competition, disease, and food availability.
• Abiotic factors (non-living) include temperature, light, water, pH, and soil type.
• The ecological niche is the role of an organism in its ecosystem, not just its habitat.
• Energy flows through ecosystems (one-way) while nutrients cycle (recycled).
• Intraspecific competition (within a species) and interspecific competition (between species) both regulate populations.
• The competitive exclusion principle limits the coexistence of species with identical niches.
• Ecosystems with higher biodiversity tend to be more stable and resilient.