📌Definition Table

Term	Definition
Ecological Niche	The role and position of a species within its environment, including interactions with biotic and abiotic factors.
Fundamental Niche	The full range of environmental conditions and resources a species could theoretically use without competition or predation.
Realised Niche	The actual conditions and resources a species occupies due to competition, predation, and other limiting factors.
Competitive Exclusion	Principle stating that two species competing for the same resource cannot coexist indefinitely under constant environmental conditions.
Resource Partitioning	Differentiation of ecological niches to reduce competition and allow coexistence of similar species.

📌Introduction

An ecological niche defines how a species interacts with its environment and other organisms, encompassing its habitat, resource use, and role in energy and nutrient cycles. The distinction between a fundamental niche and a realised niche highlights the gap between theoretical potential and ecological reality. Studying niches is crucial for understanding biodiversity patterns, predicting species distributions, and managing ecosystems under environmental change.

❤️ CAS Link: Organise a local biodiversity survey and map the niches of key species in a nearby park, linking species presence to specific habitat features and resources.

📌 Fundamental vs. Realised Niches

• Fundamental Niche represents the total possible conditions a species can tolerate, determined by its physiological limits and resource needs.
• Realised Niche is narrower due to interspecific competition, predation, parasitism, and human disturbance.
• Example: Barnacle species — Chthamalus occupies higher shore (fundamental niche broader but restricted by competition), while Balanus dominates lower shore.
• Abiotic factors (temperature, salinity, pH) primarily shape the fundamental niche, while biotic factors constrain the realised niche.
• Niche modelling uses environmental data to predict potential distributions, crucial for conservation planning.

🧠 Examiner Tip: In application questions, always mention both abiotic and biotic influences when explaining why a realised niche is smaller than a fundamental niche.

📌 Competitive Exclusion Principle

• States that two species with identical niches cannot coexist in the same habitat long-term.
• Leads to local extinction of one species or evolutionary divergence into different niches.
• Example: Paramecium aurelia outcompetes Paramecium caudatum in lab experiments when grown together.
• Encourages natural selection for traits that minimise direct competition.
• Forms the basis for predicting invasive species impacts on native communities.

🌍 Real-World Connection: The decline of native red squirrels in the UK is partly due to competitive exclusion by the invasive grey squirrel, which is more efficient at exploiting certain food resources.

📌 Resource Partitioning

• Occurs when species divide resources by specialising in different aspects of the niche.
• Can be spatial (different habitats), temporal (different activity times), or dietary (different prey or plant species).
• Example: Anole lizards in the Caribbean occupy distinct vertical zones of forest to avoid competition.
• Reduces direct competition, allowing coexistence of ecologically similar species.
• Often associated with morphological adaptations, such as beak size in Darwin’s finches.

📝 Paper 2: Data Response Tip: When interpreting niche diagrams, clearly label the fundamental and realised niches, and explain limiting factors in context of the given data.