Difficulty: Introductory | Prerequisites: Ecosystems and Nutrient Cycling study notes (companion document).
This set of notes covers how species interact with each other and how those interactions shape biodiversity. It builds on the ecosystem-level material (carbon cycle, nitrogen cycle, feedback loops) from the companion doc by zooming in on the organisms themselves: who competes with whom, how species avoid competition, what defenses look like, and how trophic levels stack up. If you understand what a niche is and how energy flows through a food web, the rest follows logically.
Species compete for resources, and when two species need the exact same things, one will eventually be excluded. To coexist, species partition resources or evolve different traits. Biodiversity hotspots are regions with unusually high diversity that deserve protection, and habitat fragmentation threatens them by creating edge effects and reducing gene flow. Food webs describe who eats whom, organised by trophic level.
Niche
The full range of resources and conditions an organism can use to survive and reproduce. This includes food, habitat, temperature range, and more.
Think of it as an organism's "job description" in its ecosystem: where it lives, what it eats, when it is active.
Competitive exclusion
The principle that two species cannot occupy the exact same niche indefinitely. If two species have complete overlap in resource requirements, one will outcompete and exclude the other.
In simple terms, if two species need exactly the same things, one will win and the other will disappear from that area.
Resource partitioning (niche partitioning)
A shift in resource use by one or both competing species that reduces niche overlap and allows coexistence. Species divide up the available resources rather than fighting over the same ones.
Think of it as roommates splitting the fridge: one takes the top shelf, the other takes the bottom.
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 generations, competing species evolve to become more different from each other in the traits that matter for resource use.
Interspecific competition
Competition between individuals of different species for the same resources.
Intraspecific competition
Competition between individuals of the same species. (Note: the lecture notes say "infraspecific," which is a synonym, but "intraspecific" is the more standard term in most textbooks.)
Mutualism
A species interaction in which both partners benefit. Crucially, mutualism is often conditional: the relationship is maintained only when both parties gain from it.
Think of it as a business partnership. If one side stops benefiting, they stop investing.
Standing defense
A defensive trait that is always present, regardless of whether a predator or threat is nearby. Example: thorns on a cactus.
Inducible defense
A defensive trait that is produced only in response to the presence of a predator or threat. Energetically cheaper than a standing defense when predators are absent, but slower to deploy.
Think of it as the difference between a permanent fence (standing) and an alarm system that triggers a response (inducible).
Biodiversity hotspot
A region with exceptionally high levels of species diversity. These areas are conservation priorities because protecting them preserves a disproportionate amount of global biodiversity.
Vicariance
The division of a species' range into smaller, isolated patches (often by habitat fragmentation). Reduces gene flow, increases inbreeding, and makes populations more vulnerable to genetic drift.
Trophic level
An organism's position in a food web, defined by how many energy transfers separate it from the primary producers. Producers are trophic level 1, primary consumers are level 2, and so on.
A biodiversity hotspot is simply an area with unusually high species diversity. These regions are conservation priorities because protecting a hotspot preserves far more species per unit area than protecting a low-diversity region.
The logic is practical: limited conservation resources should go where they do the most good. A hotspot is "worth protecting" compared with, say, a barren desert with few species.
Hotspots are often threatened by human activity (habitat loss, development, agriculture), which is part of why they receive attention.
Knowing that an area is a hotspot does not tell you why it is diverse. That requires understanding the ecological and evolutionary processes that generated the diversity in the first place.
Competition is one of the main forces shaping community structure. Two forms matter here:
Interspecific competition (between different species) drives processes like competitive exclusion and resource partitioning.
Intraspecific competition (within the same species) regulates population size and can drive variation within a species.
If two species occupy the exact same niche, using exactly the same resources in the same way, they cannot coexist. One will outcompete the other, and the loser goes locally extinct. Complete niche overlap is the key condition: partial overlap allows coexistence.
The usual outcome when two competing species do coexist is that one or both shift their resource use. They divide the niche space so each species uses a somewhat different set of resources or uses them at different times or places.
Example: two bird species feeding on insects in the same tree might partition by foraging at different heights.
Resource partitioning can become encoded genetically over evolutionary time. If natural selection favours individuals whose traits reduce overlap with a competitor, the two species will diverge in those traits. This evolutionary divergence is character displacement.
Character displacement is the evolutionary mechanism; resource partitioning is the ecological outcome. The exam may ask you to distinguish between the two.
Mutualism is a +/+ interaction: both species benefit. The important nuance for this course is that mutualism is often conditional.
A plant will only invest in building root nodule structures for nitrogen-fixing bacteria if nitrogen is scarce. If the soil already has plenty of nitrogen, the cost of maintaining the structures outweighs the benefit, and the plant will not make them.
This conditionality is a frequent exam target. The mutualism is real, but it is not unconditional.
Organisms defend themselves against predators in two broad ways:
Standing defenses are always present. They cost energy to maintain at all times but are available instantly. Examples: thorns, shells, toxins stored permanently in tissues.
Inducible defenses are produced only when a predator is detected. They save energy when predators are absent but take time to deploy. Examples: chemical defenses triggered by predator cues, thickened cell walls in response to herbivory.
The exam focus is on understanding the trade-off: standing defenses have a constant maintenance cost but no delay; inducible defenses have a lower baseline cost but require a trigger and take time.
Facilitation is a +/0 or +/+ interaction in which one species makes conditions more favourable for another. The lecture notes mention this topic briefly. It is worth knowing the concept exists and being able to distinguish it from mutualism (in facilitation, the benefit may be one-directional).
When a continuous habitat is broken into smaller, isolated patches, two problems arise at different scales.
The boundary between a habitat patch and the surrounding altered landscape creates an "edge" with different conditions from the interior.
Moisture loss: edges are drier and lose moisture more quickly. Plants adapted to moist interior conditions struggle.
Exposure: edge organisms face greater exposure to predators, human disturbance, and harsh weather.
Practical consequence: a small patch is nearly all edge. Only large patches retain enough interior habitat to support species that depend on interior conditions.
When a habitat is divided into separate patches, populations become isolated from each other.
Reduced gene flow: individuals cannot move between patches, so alleles stop mixing between populations.
Increased inbreeding: small, isolated populations mate within a shrinking gene pool.
Genetic drift vulnerability: in small populations, random changes in allele frequency have a bigger effect, potentially eliminating beneficial alleles or fixing harmful ones.
Edge effects and vicariance are distinct problems. Edge effects are about conditions at the boundary. Vicariance is about isolation between patches. Both reduce the effective habitat quality, but through different mechanisms.
A food web maps who eats whom. Each organism sits at a trophic level based on how many energy transfers separate it from the primary producers.
Trophic level 1 (primary producers): phytoplankton. They fix energy from sunlight via photosynthesis.
Trophic level 2 (primary consumers): copepods. They eat phytoplankton.
Trophic level 3 (secondary consumers): herring. They eat copepods.
Trophic level 4 (tertiary consumers): salmon. They eat herring.
Trophic level 4 or 5 (quaternary or higher): orcas. Orcas eat salmon (level 4 prey), making them at least level 5 in that chain. Orcas also eat seals, whose own trophic level varies depending on what the seals ate.
An organism's trophic level can shift depending on what it eats. Seals could be tertiary or quaternary consumers depending on their prey.
Orcas can be quaternary or even pentanary (fifth-level) predators, depending on the food chain you trace.
The exam may ask you to assign trophic levels given a food web diagram. Trace the chain from producers upward, counting each transfer.
Students often think competitive exclusion means two species can never coexist. That is only true when there is complete niche overlap. Partial overlap allows coexistence through resource partitioning.
Character displacement and resource partitioning are often confused. Resource partitioning is the ecological pattern (species use different resources). Character displacement is the evolutionary process that produces it (natural selection shifts traits).
Students sometimes assume mutualism is always obligate and permanent. In this course, the emphasis is that mutualism is often conditional: the legume-bacteria example shows a plant that will not invest unless the benefit is there.
Edge effects and vicariance are sometimes treated as the same problem. They are not. Edge effects alter conditions at the boundary of a patch. Vicariance isolates populations genetically. Both result from fragmentation, but through different mechanisms.
⚠️ Be prepared to assign trophic levels from a food web diagram. The marine example (phytoplankton, copepods, herring, salmon, orcas) is a likely exam scenario.
⚠️ Know the distinction between character displacement (evolutionary) and resource partitioning (ecological). An exam question may describe a scenario and ask you to identify which concept applies.
⚠️ Understand the trade-off between standing and inducible defenses: cost, speed, and when each is advantageous.
⚠️ Be able to explain why habitat fragmentation causes problems at two levels: edge effects (altered conditions at boundaries) and vicariance (genetic isolation of populations).
⚠️ Expect a question on mutualism that tests whether you understand it can be conditional. The nitrogen-fixing bacteria example is the go-to.
True or false: competitive exclusion occurs whenever two species compete for any resource. (False. It occurs only when there is complete niche overlap.)
Fill in the blank: the evolutionary process by which competing species diverge in traits is called ________. (Character displacement.)
True or false: an inducible defense is always present. (False. It is produced only in response to a threat.)
Fill in the blank: copepods that eat phytoplankton are ________ consumers. (Primary.)
True or false: vicariance increases gene flow between populations. (False. It decreases gene flow.)
Q: Two species of warbler forage for insects in the same tree. Species A forages near the top, species B near the bottom. What concept does this illustrate, and what is the evolutionary process that could have produced it?
A: This illustrates resource partitioning (or niche partitioning). The evolutionary process that could have produced it is character displacement, in which natural selection favoured individuals whose foraging traits reduced competition with the other species.
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. What trophic level is the salmon?
A: Salmon are tertiary consumers (trophic level 4). Phytoplankton = level 1, copepods = level 2, herring = level 3, salmon = level 4.
Q: A crab species produces a thicker shell only when chemical cues from a predator are present in the water. Is this a standing or inducible defense? What is the trade-off?
A: This is an inducible defense. The trade-off is that the crab saves energy when predators are absent (no shell thickening), but if a predator appears suddenly, the defense takes time to develop and may not protect the crab immediately.
Q: Explain why a legume might not form root nodules even though nitrogen-fixing bacteria are available in the soil.
A: If nitrogen is already abundant in the soil, the plant does not need the bacteria to provide it. Building and maintaining root nodules costs energy (sugars), so the plant will not invest in the mutualism when it can obtain nitrogen directly from the soil without the cost.
Q: How does habitat fragmentation lead to increased inbreeding in a population?
A: Fragmentation isolates populations into smaller patches (vicariance). Individuals in each patch can only mate with others in the same patch, reducing the available gene pool. Over time, this restricted mating leads to increased inbreeding and a loss of genetic diversity.
Competitive exclusion and resource partitioning connect to evolutionary biology: character displacement is natural selection in action, so this material reinforces what you learned about adaptation and fitness earlier in the course.
The mutualism between legumes and nitrogen-fixing bacteria ties directly into the nitrogen cycle and dead zones material in the companion study notes. Understanding the mutualism helps explain how nitrogen enters biological systems, and excess nitrogen from human sources drives eutrophication.
Food webs and trophic levels are the bridge between ecosystem ecology (energy flow, NPP) and community ecology (who interacts with whom). If you understand trophic levels, you can reason about how changes at one level ripple through the system.
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