IB Biology Study Notes: Ecological Niches

Welcome to the World of Ecological Niches!

Hey future Biologists! This chapter, Ecological Niches, is super important because it helps us understand *how* organisms survive and interact within their environment. It’s not just about where an organism lives; it's about what it does, who it interacts with, and how it manages all the resources available.

Understanding niches is foundational to studying population dynamics, competition, and biodiversity—key components of ecology. Let’s dive in and break down these concepts!

Section 1: Defining the Niche – Your Organism's Job Description

1.1 Habitat vs. Niche: Making the Crucial Distinction

Students sometimes confuse these two terms. Think of it this way:

  • Habitat: This is simply the address where an organism lives. It's a specific physical place.
    Example: The rainforest canopy, a shallow pond, or the bark of an oak tree.
  • Ecological Niche: This is the organism's role or job description in the environment. It defines how the organism interacts with its surroundings.

The ecological niche encompasses three main components:

  1. Spatial Requirements: The physical space needed (e.g., nesting sites, foraging territory).
  2. Nutritional Requirements (Resources): What the organism consumes and needs to survive (e.g., light, water, specific types of prey).
  3. Interactions: How the organism modifies or is affected by its environment (both living and non-living factors).

Key Point: A niche is a description of all the biotic (living) and abiotic (non-living) conditions and resources required for a species to survive and reproduce.

Analogy: If a lion’s habitat is the African savanna, its niche is that of a top predator, controlling zebra populations, requiring specific watering holes, and interacting with scavengers like vultures.

Quick Review: The Niche Checklist

A species’ niche includes:

  • Temperature range it can tolerate.
  • Type and size of food it consumes.
  • Time of day it hunts or feeds.
  • Moisture and pH levels it requires.
  • Predators, parasites, and competitors it interacts with.

Section 2: Fundamental vs. Realized Niche

Don't worry if this seems tricky at first—this is a high-yield concept in IB Biology! We differentiate between the ideal life a species *could* live and the limited life it *actually* lives.

2.1 The Fundamental Niche (The Ideal)

The fundamental niche represents the largest possible area and range of resources that a species can use in the absence of competition or predation from other species.

It describes the theoretical conditions under which an organism can thrive, based solely on its physiological adaptations and tolerance limits (e.g., how much heat it can stand, what specific chemical compounds it needs).

Mnemonic: Think Fundamental means Full potential or Free from negative interactions.

2.2 The Realized Niche (The Reality Check)

The realized niche is the actual space and resources that a species occupies and uses in the presence of limiting factors, such as competition, predation, and resource scarcity.

In the real world, species rarely, if ever, achieve their fundamental niche because other organisms limit their access to space or food.

Mnemonic: Think Realized means Restricted or Reality.

Example: A barnacle species might be able to live anywhere from the high-tide line to the low-tide line (its fundamental niche). However, a faster-growing competitor might push it out of the lower, wetter areas. Therefore, the barnacle species only lives near the high-tide line (its realized niche).

Common Mistake Alert!

Always remember that the Realized Niche is always smaller than or equal to the Fundamental Niche. It can never be larger, as competition and predation always restrict survival, they don't expand it.

Section 3: Competition and the Competitive Exclusion Principle

3.1 The Principle Defined

A core concept arising from niche theory is the Competitive Exclusion Principle (also known as Gause's Law, after the scientist who formulated it).

Definition: Two species cannot occupy the exact same ecological niche in the same habitat indefinitely without one driving the other to local extinction.

If two species have completely overlapping niches (meaning they require the identical resources and conditions), they will compete intensely. Eventually, the slightly more efficient competitor will outcompete the other species, leading to the exclusion of the less successful species from that area.

3.2 Evidence from Gause’s Experiments

Russian ecologist G.F. Gause demonstrated this principle using two species of microscopic protozoans, Paramecium aurelia and Paramecium caudatum.

  1. When grown separately in controlled conditions with plenty of food, both species populations grew rapidly and stabilized (they achieved their fundamental niche).
  2. When grown together in the same flask, both initially competed for the limited food source (bacteria).
  3. Over time, P. aurelia, which had a slightly higher growth rate, outcompeted P. caudatum for food. The population of P. caudatum declined until it was completely excluded (driven extinct) from the flask.

Key Takeaway: Total niche overlap leads to competitive exclusion.

Section 4: Coexistence: Avoiding Exclusion

If competition is so fierce, how can so many different species exist in one ecosystem, like a rainforest or coral reef? They must find ways to reduce niche overlap!

4.1 Resource Partitioning

Resource Partitioning is the mechanism by which species avoid competitive exclusion by specializing in slightly different resources or behaviors.

This process minimizes the overlap between their realized niches, allowing them to coexist.

How do species partition resources? They divide them up based on:

  • Time: Two rodent species might eat the same seeds, but one is nocturnal (active at night) and the other is diurnal (active during the day).
  • Space/Location: Different species of insectivorous bats might hunt in different parts of the forest (e.g., high canopy vs. forest floor).
  • Food Type/Size: Different finches on the Galápagos Islands have evolved beak shapes specialized for seeds of different sizes. They are all seed-eaters, but they minimize competition by targeting different resources.

Classic Example: MacArthur's Warblers. Robert MacArthur studied five species of warblers (small birds) that all fed on insects in the same spruce trees. He found they coexisted because each species fed in a specific, distinct zone of the tree, thereby partitioning the resource (insects) and minimizing direct competition.

Did You Know?

Species that show similar traits but live in different geographical locations are called ecological equivalents. For example, the niche of the wolf in North America is ecologically equivalent to the niche of the wild dog in Africa—both are apex mammalian predators hunting large herbivores. They occupy the same niche but live far apart.

Chapter Summary: Key Takeaways

1. Definition: The ecological niche is the total range of conditions and resources required by a species to survive and reproduce (its role/job). The habitat is just its physical location (its address).
2. Fundamental Niche (Ideal): The niche a species could occupy without limiting factors (competition/predation). (F = Full potential)
3. Realized Niche (Actual): The smaller niche a species actually occupies due to competition and other limiting factors. (R = Reality/Restricted)
4. Competitive Exclusion: If two species try to occupy the exact same niche, one will outcompete the other.
5. Coexistence: Species avoid exclusion through Resource Partitioning—specializing to use different parts of the overall resource pool.