# Species and communities

> IB Biology SL · IB Biology SL 2025
> Source: https://www.owlsprep.com/study/ib-biology-sl-u3-species-and-communities/

This sub-topic introduces core ecological concepts that underpin all of IB Biology ecology. You will learn to classify organisms and interactions, building the foundation for analyzing energy flow, nutrient cycling, and ecosystem change.

**Prerequisites:** [Basic classification of living organisms](https://www.owlsprep.com/study/ib-biology-sl-u2-characteristics-of-life/)

## Learning objectives

- Define core levels of ecological organization from species to ecosystem
- Classify organisms by their mode of nutrition (autotroph vs heterotroph)
- Identify and categorize common types of species interactions
- Interpret food webs and assign trophic levels to organisms

## Core Levels of Ecological Organization

**Species** — A group of organisms that can potentially interbreed to produce fertile, viable offspring, and are reproductively isolated from other such groups

*Example:* Horses and donkeys are separate species because their hybrid offspring (mules) are infertile

Ecological organization builds from the species level up to entire ecosystems. Key additional terms you must memorize are:

- **Habitat**: The environment where a species normally lives, or the location of an individual organism
- **Population**: A group of organisms of the *same species* living in the same area at the same time
- **Community**: All populations of *different species* living and interacting in the same area at the same time
- **Ecosystem**: A community plus all the abiotic (non-living) factors that interact with it

**Worked example:** A researcher studies 42 maple trees of the species *Acer saccharum* growing in a Vermont forest. What level of ecological organization does this group represent?

1. First, check how many species are in the group: all individuals are the same species (*Acer saccharum*), and all live in the same geographic area at the same time.
2. Eliminate other options: A species refers to all members of the group globally, not just this local group. A community would require multiple different species. An ecosystem includes abiotic factors, which are not part of this group.
3. Conclusion: This group is a population of sugar maple trees.

> **Exam tip:** IB examiners regularly test your ability to distinguish between these levels, so always check whether the group is single-species or multi-species first.

## Classifying Organisms by Nutrition Mode

**Autotroph** — An organism that synthesizes organic molecules from inorganic sources (carbon dioxide, water, minerals) via photosynthesis or chemosynthesis

*Example:* Flowering plants, cyanobacteria, chemosynthetic archaea

Heterotrophs cannot produce their own organic molecules, so they obtain organic compounds from other organisms. Heterotrophs are further divided into three subgroups based on how they get their nutrition:

- **Consumers**: Feed on living or recently killed organic matter from other organisms
- **Detritivores**: Ingest and digest dead organic detritus (dead organisms, waste) internally
- **Saprotrophs**: Secrete digestive enzymes onto dead organic matter, then absorb the digested nutrients (also called decomposers)

**Worked example:** An earthworm ingests decaying leaf litter from soil, digests it internally, and absorbs nutrients. How would you classify the earthworm?

1. Earthworms cannot photosynthesize, so they are heterotrophs, not autotrophs.
2. Check how they obtain nutrition: they ingest dead organic matter (leaf litter) and digest it internally. This matches the definition of a detritivore.
3. Conclusion: The earthworm is a detritivorous heterotroph.

**Check your understanding**

Test your classification understanding

1. Which of the following describes a saprotroph?

   - A: Produces organic molecules from inorganic carbon
   - B: Ingests dead organic matter for digestion
   - C: Secretes enzymes to break down dead organic matter and absorbs nutrients
   - D: Feeds on living host tissue and harms the host

   *Why:* Saprotrophs (most fungi and many bacteria) digest externally, unlike detritivores that ingest detritus first. Option B describes detritivores, A describes autotrophs, D describes parasites.

## Species Interactions in Communities

All species in a community interact with one another, and interactions are categorized by their effect on the fitness of each species involved. The most common interactions tested in IB Biology are summarized below:

| Interaction Type | Effect on Species A | Effect on Species B | Example |
| --- | --- | --- | --- |
| Mutualism | + | + | Bees pollinating flowering plants |
| Predation | + | - | Wolves hunting moose |
| Herbivory | + | - | Deer grazing on shrubs |
| Parasitism | + | - | Ticks feeding on human blood |
| Interspecific Competition | - | - | Two tree species competing for sunlight |

**Worked example:** Clownfish live among the tentacles of sea anemones. Clownfish get protection from predators, and they remove parasites from the anemone. What type of interaction is this?

1. Identify the effect on each species: Clownfish gain protection from predators, so their fitness increases (+). Anemones have harmful parasites removed, so their fitness also increases (+).
2. Match to the interaction category: an interaction where both species benefit is defined as mutualism.
3. Conclusion: This is a mutualistic interaction between clownfish and sea anemones.

## Food Webs and Trophic Levels

**Trophic Level** — The position an organism occupies in a food chain, representing the number of energy transfer steps from the original producer in the ecosystem

A food chain shows a linear sequence of feeding relationships, while a food web shows all interconnected feeding relationships in a community. Most organisms feed at multiple trophic levels depending on what they eat, so you cannot always assign a single fixed level to an organism.

**Worked example:** In a wetland food web: algae are eaten by minnows, which are eaten by herons and large bass. Large bass also eat herons. What trophic levels do large bass occupy?

1. First, map all feeding pathways for large bass: 1. Algae (1) → Minnow (2) → Bass  2. Algae (1) → Minnow (2) → Heron (3) → Bass.
2. Count the trophic level for each pathway: In the first pathway, bass are at trophic level 3. In the second pathway, bass are at trophic level 4.
3. Conclusion: Large bass occupy both the third and fourth trophic levels in this food web.

> **Exam tip:** Always check all feeding connections for an organism before assigning a trophic level. Most exam questions are designed to test that you know organisms can occupy multiple levels.

## Common pitfalls

- **Wrong:** Calling a group of multiple species in the same area a population
  - Why it fails: Population only refers to individuals of the same species, multiple species form a community
  - Correct: Use 'population' for single-species groups, 'community' for all interacting species in an area
- **Wrong:** Classifying decomposer fungi as detritivores
  - Why it fails: Detritivores ingest detritus internally, while fungi secrete enzymes to digest externally
  - Correct: Classify decomposer fungi as saprotrophic heterotrophs
- **Wrong:** Including abiotic factors as part of a community
  - Why it fails: Communities only include living (biotic) components of an ecosystem
  - Correct: Reserve 'ecosystem' to refer to the combination of a community plus its abiotic environment
- **Wrong:** Assigning a single fixed trophic level to all organisms in a food web
  - Why it fails: Most heterotrophs feed at multiple trophic levels depending on what they eat
  - Correct: Check all feeding pathways an organism is part of to identify all trophic levels it occupies
- **Wrong:** Confusing parasitism and predation
  - Why it fails: Parasites typically live on or in the host and do not usually kill the host immediately, unlike predators that kill and eat prey immediately
  - Correct: Parasites benefit while harming but not immediately killing their host, predators kill and consume their prey

## Cheatsheet

| Term | Key Definition |
| --- | --- |
| Species | Interbreed to produce fertile offspring |
| Population | Same species, same area, same time |
| Community | All different species, same area |
| Ecosystem | Community + abiotic environment |
| Autotroph | Producer: makes own organic molecules |
| Saprotroph | Decomposer: external digestion of detritus |
| Mutualism | +/+ species interaction |
| Predation/Parasitism | +/- species interaction |
| Competition | -/- species interaction |

## What's next

The core concepts of species and communities you learned here are the foundation for all further ecological study in IB Biology. Next, you will build on these ideas to explore how energy flows through ecosystems from producers to top consumers, and how carbon and other nutrients cycle between biotic and abiotic components of the biosphere. These concepts also underpin later topics on ecological succession, conservation of biodiversity, and human impacts on ecosystems. Mastery of these core definitions and classifications will help you correctly answer more complex exam questions about larger-scale ecological processes.

- [Ecosystems and carbon cycling](https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystems-and-carbon-cycling/)
- [Classification and Biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-classification-and-biodiversity/)
- [Conservation of biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-conservation-of-biodiversity/)

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