6.2 Community Ecology and Species Interactions
Key Takeaways
- The six core species interactions classified by sign are competition (-/-), predation, herbivory, and parasitism (+/-), mutualism (+/+), and commensalism (+/0)
- Gause's competitive exclusion principle states that two species occupying the same niche cannot coexist indefinitely; one excludes the other or they partition the niche
- The fundamental niche is the full range a species could occupy without competitors; the realized niche is the smaller portion actually occupied due to competition and predation
- Keystone species (sea otters, wolves) have disproportionate effects relative to their abundance; foundation species (corals, kelp) are abundant and physically structure the community
- The 10% rule means only about 10% of energy passes between trophic levels, limiting most food chains to 4-5 levels and concentrating non-metabolized toxins through biomagnification
Why Community Ecology Matters for the Praxis Biology Exam
A community is an assemblage of populations of different species living together and interacting in the same area. The Praxis Biology exam regularly tests species interactions, niche theory, trophic structure, and the 10% rule for energy transfer. Expect at least one item asking you to classify a relationship (mutualism vs commensalism vs parasitism) and at least one item asking you to identify the trophic level of a labeled organism in a food web.
Species Interactions: The Six Major Types
Interactions are usually classified by their effect on each partner: positive (+), negative (-), or neutral (0).
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Competition | - | - | Two warbler species feeding in the same tree |
| Predation | + | - | Lynx hunting snowshoe hare |
| Herbivory | + | - | Caterpillars eating leaves |
| Parasitism | + | - | Tapeworm in a mammal gut |
| Mutualism | + | + | Mycorrhizal fungi and plant roots |
| Commensalism | + | 0 | Barnacles attached to a whale |
Predation, herbivory, and parasitism share the (+/-) sign but differ in how the interaction unfolds. Parasites typically do not kill the host quickly because their fitness depends on keeping the host alive long enough to reproduce.
Competition: Intraspecific vs Interspecific
- Intraspecific competition occurs within a single species. It is one of the strongest density-dependent regulators because individuals share identical resource needs.
- Interspecific competition occurs between different species and is what drives niche partitioning.
The Competitive Exclusion Principle (Gause)
In 1934, G.F. Gause demonstrated that two species of Paramecium competing for the same food in the same culture could not coexist - one always drove the other to extinction. The competitive exclusion principle (also called Gause's principle) states that two species cannot occupy exactly the same niche in the same habitat indefinitely. One outcome must occur:
- One species outcompetes and excludes the other, or
- The species partition the niche through resource partitioning or character displacement.
Robert MacArthur's famous study of five warbler species in spruce trees showed each species feeding in a different vertical zone of the same tree - a textbook example of resource partitioning.
The Ecological Niche
The niche is the role a species plays - its resource use, habitat, activity patterns, and interactions. Two flavors matter:
- Fundamental niche - the full range of conditions and resources a species could use in the absence of competitors and predators.
- Realized niche - the actual, smaller portion of the fundamental niche the species occupies when competition and predation are present.
The realized niche is almost always smaller than the fundamental niche. Joseph Connell's classic barnacle experiments on Scottish rocks demonstrated this clearly.
Special Ecological Roles
| Role | Definition | Example |
|---|---|---|
| Keystone species | Disproportionately large impact relative to abundance | Sea otters controlling sea urchins in kelp forests; wolves in Yellowstone |
| Foundation species | Abundant species that physically structures the community | Reef-building corals, kelp, oak trees |
| Ecosystem engineer | Species that physically modifies habitat | Beavers building dams, earthworms aerating soil |
| Dominant species | Most abundant or with highest biomass | Sugar maples in a northern hardwood forest |
The exam often contrasts keystone (low abundance, high impact) with dominant (high abundance, high impact) and foundation (high abundance, structural).
Trophic Levels and Energy Flow
Energy enters communities through primary producers and flows upward.
- Primary producers (autotrophs) - plants, algae, cyanobacteria. Convert sunlight or chemical energy into organic matter.
- Primary consumers (herbivores) - eat producers. Grasshoppers, deer, zooplankton.
- Secondary consumers (carnivores or omnivores) - eat herbivores. Frogs, small fish.
- Tertiary consumers - top predators eating secondary consumers. Hawks, sharks.
- Decomposers - bacteria and fungi that break down dead organic matter and return nutrients to the soil.
Detritivores (earthworms, dung beetles) consume detritus and overlap functionally with decomposers, but the exam usually reserves "decomposer" for microbes.
Food Chains vs Food Webs
A food chain is a single linear sequence (grass to grasshopper to frog to hawk). A food web is a network of interconnected chains and is far more realistic, because most organisms eat - and are eaten by - many species. The Praxis may present a food web and ask you to identify all trophic levels of a particular consumer (many organisms occupy more than one level).
The Energy Pyramid and the 10% Rule
According to U.S. EPA and USGS educational materials, only about 10% of the energy at one trophic level is transferred to the next. The rest is lost as heat (cellular respiration), used in growth and reproduction, or remains in undigested waste. This rule has three exam-relevant consequences:
- Energy pyramids are always upright (energy decreases at each step).
- The number of trophic levels in most ecosystems is limited to 4-5 because too little energy remains beyond that.
- Toxins that are not metabolized (mercury, DDT) become more concentrated at higher levels - biomagnification.
Cattle egrets follow grazing cattle and catch insects flushed from the grass. The egrets clearly benefit, while the cattle are neither helped nor harmed. Which ecological interaction does this represent?
An ecosystem's primary producers fix 10,000 kilocalories of energy. According to the 10% rule of ecological efficiency, approximately how much energy is available to support tertiary consumers (the third consumer trophic level above producers)?