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.

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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.

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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)"]

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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
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.

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.

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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
Temperature	Affects enzyme activity, metabolic rate, and distribution of ectotherms and endotherms
Light intensity	Affects rate of photosynthesis; determines plant distribution and productivity
Water availability	Essential for all life; limits terrestrial organism distribution
pH	Affects enzyme activity; determines species composition in aquatic and soil environments
Mineral ion concentration	Nutrients such as nitrate and phosphate limit plant growth
Wind	Affects transpiration, seed dispersal, and physical exposure
Oxygen concentration	Critical for aerobic respiration in aquatic organisms
Carbon dioxide concentration	Affects rate of photosynthesis
Soil type and depth	Determines water retention, mineral availability, and root anchorage
Salinity	Determines which organisms can survive in aquatic habitats (freshwater vs marine)

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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.

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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
Fundamental niche	The full range of conditions and resources in which a species could survive and reproduce in the absence of competition
Realised niche	The actual conditions and resources a species uses, which is typically narrower than the fundamental niche due to interspecific competition

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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

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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.