Difficulty: Introductory | Prerequisites: Basic understanding of natural selection and ecosystem structure.
This set of notes covers how species interact with each other and with their environment, what biodiversity means and why certain areas have more of it, and how food webs organise the flow of energy through ecosystems. You should already be comfortable with the idea that organisms compete for limited resources and that traits are selected for over time. These topics tend to show up together on the exam because they form a connected picture: species compete, partition resources, sit at different trophic levels, and all of it plays out in habitats that are increasingly fragmented by human activity.
Species interact through competition, mutualism, and predation, and these interactions shape how biodiversity is distributed. Competition can lead to exclusion or resource partitioning. Food webs describe who eats whom and at what trophic level. Habitat fragmentation creates edge effects and genetic isolation that threaten species survival.
Niche
The full range of resources and conditions that an organism can use to survive and reproduce.
Think of it as a species' "job description" within an ecosystem: what it eats, where it lives, when it is active.
Competitive exclusion
The principle that two species cannot permanently occupy the exact same niche. If two species require identical resources with complete overlap, one will outcompete and eliminate the other.
In simple terms, two species doing the exact same thing in the exact same place cannot coexist indefinitely.
Resource partitioning (niche partitioning)
A change in resource use by one or both competing species that reduces overlap and allows coexistence.
Think of it as species dividing up the "workspace" so they are no longer in direct competition.
Character displacement
An evolutionary change in traits that facilitates resource partitioning. Natural selection favours individuals whose traits reduce competition with the other species.
In simple terms, over time, competing species evolve to become more different from each other where they overlap.
Interspecific competition
Competition between individuals of different species for the same resources.
Intraspecific competition
Competition between individuals of the same species for the same resources. Note: the source material uses "infraspecific," which is a synonym.
Mutualism
A species interaction in which both organisms benefit. Mutualism is often conditional: organisms invest in the relationship only when the benefit exceeds the cost.
In simple terms, both sides gain something, but only when it is worth the investment.
Standing defence
A defence that is always present, regardless of whether a predator or threat is nearby. Examples include thorns, thick bark, or toxic compounds produced continuously.
Inducible defence
A defence that is produced only in response to a predator or threat. The organism saves energy by not maintaining the defence when it is not needed.
Biodiversity hotspot
A region with exceptionally high levels of species diversity, often prioritised for conservation efforts.
Vicariance
The splitting of a population's habitat into smaller, isolated patches. Results in decreased gene flow, increased inbreeding, and greater vulnerability to genetic drift.
Trophic level
An organism's position in a food web, defined by how many energy transfers separate it from primary producers. Producers are the first trophic level, primary consumers the second, and so on.
A biodiversity hotspot is an area with unusually high species diversity. These regions are conservation priorities because protecting them preserves a disproportionately large share of the world's species.
The concept is straightforward: some areas are more biologically rich than others, and limited conservation resources should be directed where they will protect the most diversity.
Competitive exclusion is the baseline expectation: if two species occupy exactly the same niche with complete resource overlap, one will drive the other to extinction
This is a theoretical extreme. In practice, most competing species differ in at least some aspect of their resource use
Resource partitioning is how competing species avoid exclusion
Species shift their resource use so that overlap is reduced
This can involve using different food sources, foraging at different times, or occupying different microhabitats within the same area
Character displacement is the evolutionary mechanism behind resource partitioning
Natural selection favours traits that reduce competition with the other species
Over generations, the two species become more different from each other in the area where they overlap
Classic example: Darwin's finches developing different beak sizes to exploit different seed sizes
Interspecific vs. intraspecific competition
Interspecific: between different species
Intraspecific: between members of the same species
Both are important, but competitive exclusion applies only to interspecific competition
Mutualism is conditional. Organisms invest in mutualistic relationships only when the benefit outweighs the cost.
A plant will form root nodule structures for nitrogen-fixing bacteria only when soil nitrogen is scarce
When nitrogen is plentiful, the plant does not bother: the cost of maintaining the structures exceeds the benefit
This cost-benefit logic applies broadly across mutualistic interactions
The distinction is about timing and cost.
Standing defences are always present
Examples: thorns, shells, continuous toxin production
Cost: the organism pays the energetic price of maintaining the defence at all times, even when no threat is present
Benefit: immediate protection with no delay
Inducible defences are produced only in response to a predator or threat
Examples: chemical defences triggered by detection of a predator, thickening of cell walls in response to herbivory
Cost: there is a time lag between detecting the threat and deploying the defence
Benefit: the organism saves energy when the threat is absent
The exam is unlikely to ask you to classify a specific defence as standing or inducible (the source notes say this distinction is hard to pin down in practice). Focus instead on understanding the trade-offs: why would an organism evolve one strategy over the other?
Facilitation is a species interaction where one species makes conditions more favourable for another, without the direct reciprocal exchange that defines mutualism. The source material flags this topic but does not elaborate, so be aware of the term and its basic meaning.
Trophic levels describe the position of an organism in a food web based on how it obtains energy. Each step up the food web represents one energy transfer.
Primary producers (trophic level 1): organisms that produce their own food through photosynthesis
Example: phytoplankton
Primary consumers (trophic level 2): herbivores that eat producers
Example: copepods (which eat phytoplankton)
Secondary consumers (trophic level 3): organisms that eat primary consumers
Example: herring (which eat copepods)
Tertiary consumers (trophic level 4): organisms that eat secondary consumers
Example: salmon (which eat herring)
Quaternary / higher consumers (trophic level 5+): top predators
Example: orcas eat salmon, placing them at the quaternary or pentanary level depending on the chain
A few things to keep straight:
An organism's trophic level can vary depending on which food chain you trace. Seals, for instance, could be tertiary or quaternary consumers depending on what they ate.
Food webs are more realistic than food chains because most organisms eat (and are eaten by) multiple species.
Energy is lost at each trophic level (roughly 90% is lost as heat through respiration), which is why top predators are rarer and food chains rarely exceed five levels.
When a continuous habitat is broken into smaller patches (fragmented), the edges of each patch experience different conditions from the interior.
Edge effects
Edges are drier and lose moisture faster than the interior
Plants adapted to moist, stable interior conditions struggle at the edges
Edges are more exposed to predators, human activity, and adverse weather
The smaller the patch, the higher the ratio of edge to interior, so more of the habitat is degraded
Vicariance occurs when a previously continuous population is split into smaller, isolated patches.
Genetic consequences
Decreased gene flow between patches: individuals can no longer freely move and breed across the whole population
Increased inbreeding within each patch, as the pool of potential mates shrinks
Increased vulnerability to genetic drift: in small populations, random changes in allele frequencies have a larger effect
These genetic effects compound over time. A fragmented population is less genetically diverse and less able to adapt to changing conditions than a large, connected one.
Competitive exclusion and resource partitioning explain patterns visible in any garden or park. Warblers famously partition the canopy of spruce trees by foraging at different heights and on different parts of branches, which is a textbook case of niche partitioning allowing coexistence.
Habitat fragmentation is one of the primary drivers of species loss worldwide. Roads, agriculture, and urban development break continuous habitats into isolated patches, producing the edge effects and genetic isolation described above. Wildlife corridors, strips of habitat connecting patches, are a practical conservation tool designed to counteract vicariance by restoring gene flow.
Students often confuse interspecific and intraspecific competition. "Inter" means between different species. "Intra" means within the same species. A helpful mnemonic: "inter" as in "international" (between nations).
Students sometimes think competitive exclusion means both species go extinct. Only the weaker competitor is eliminated; the stronger competitor persists.
Students assume that organisms at higher trophic levels are "better" or "more evolved." Trophic position has nothing to do with evolutionary advancement. It simply describes where energy enters an organism's diet.
Students frequently misidentify an organism's trophic level by counting from the wrong starting point. Primary producers are always trophic level 1. Primary consumers eat producers and are level 2. Count up from there.
⚠️ Know the definitions and distinctions: competitive exclusion vs. resource partitioning vs. character displacement. These three form a logical chain and the exam may test whether you understand how they connect.
⚠️ Be able to assign trophic levels in a food web. Practice tracing chains: phytoplankton (producer) to copepod (primary consumer) to herring (secondary) to salmon (tertiary) to orca (quaternary or higher).
⚠️ Understand why an organism can occupy different trophic levels depending on which chain you trace.
⚠️ Know the difference between standing and inducible defences in terms of costs and benefits, even if you cannot always classify a specific example.
⚠️ Be able to explain how habitat fragmentation leads to edge effects and how vicariance reduces genetic diversity.
⚠️ Understand that mutualism is conditional and be able to give the nitrogen-fixing bacteria example.
True or false: Two species with identical niches can coexist indefinitely.
Fill in the blank: The evolutionary process that leads to resource partitioning is called ______.
True or false: Orcas are always quaternary consumers.
Fill in the blank: Habitat fragmentation increases ______ and decreases ______.
True or false: An inducible defence saves energy when no predator is present.
Answers: 1. False (competitive exclusion principle). 2. Character displacement. 3. False (their trophic level depends on the food chain traced; they can be quaternary or higher). 4. Inbreeding; gene flow. 5. True.
Q: Two bird species eat the same type of insect in the same forest. One species begins foraging higher in the canopy while the other stays near the ground. What process does this describe, and what evolutionary mechanism could drive it?
A: This describes resource partitioning (also called niche partitioning). The evolutionary mechanism that could drive this shift is character displacement, where natural selection favours traits in each species that reduce competition with the other.
Q: In a marine food web, phytoplankton are eaten by copepods, copepods are eaten by herring, herring are eaten by salmon, and salmon are eaten by orcas. Assign a trophic level to each organism.
A: Phytoplankton: trophic level 1 (primary producer). Copepods: trophic level 2 (primary consumer). Herring: trophic level 3 (secondary consumer). Salmon: trophic level 4 (tertiary consumer). Orcas: trophic level 5 (quaternary consumer).
Q: A large forest is divided by a new highway. Describe two genetic consequences for the animal populations on either side.
A: Gene flow between the two populations decreases because individuals can no longer move freely between patches. Over time, inbreeding increases within each smaller population because the pool of potential mates is reduced. Both effects reduce genetic diversity and make the populations more vulnerable to genetic drift.
Q: A plant species produces toxic compounds in its leaves at all times, regardless of herbivore presence. Is this a standing or inducible defence, and what is the trade-off?
A: This is a standing defence. The trade-off is that the plant pays the energetic cost of producing toxins continuously, even when no herbivore is feeding on it. The benefit is immediate protection with no time lag.
Q: Why might a legume growing in nitrogen-rich soil fail to develop root nodules for nitrogen-fixing bacteria?
A: The mutualism is conditional. Forming and maintaining root nodules costs energy. When soil nitrogen is already abundant, the plant can obtain nitrogen without the bacteria, so the cost of the mutualism exceeds the benefit. The plant does not invest in nodule formation.
This material connects to the ecosystems and nutrient cycling notes (the companion doc). The mutualism between legumes and nitrogen-fixing bacteria appears in both contexts: here as an example of conditional mutualism, there as part of the nitrogen cycle. Dead zones, covered in the companion notes, are a consequence of biodiversity loss driven by nutrient loading, which ties directly to trophic-level disruption.
Habitat fragmentation and vicariance also connect to population genetics concepts from earlier in the course, specifically genetic drift, gene flow, and the founder effect in small populations.
Biodiversity, biodiversity hotspot, species interactions, competition, competitive exclusion, niche, ecological niche, resource partitioning, niche partitioning, character displacement, interspecific competition, intraspecific competition, mutualism, conditional mutualism, standing defence, inducible defence, facilitation, food web, food chain, trophic level, primary producer, primary consumer, secondary consumer, tertiary consumer, quaternary consumer, phytoplankton, copepods, herring, salmon, orca, habitat fragmentation, edge effects, edge habitat, vicariance, gene flow, inbreeding, genetic drift, BIOL180, Exam 4, Introduction to Biology, UW Biology