How do interactions between organisms shape the structure of an ecosystem?
Topic 1.1 Introduction to Ecosystems: explain how species interactions, including predation, symbiosis and competition, shape ecosystems and influence the survival of organisms.
A focused answer to APES Topic 1.1, covering ecosystems, predator-prey relationships, the three symbioses (mutualism, commensalism, parasitism), competition and resource partitioning, with a worked FRQ on interpreting interaction data.
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What this topic is asking
The College Board (Topic 1.1) wants you to explain how species interactions organize an ecosystem. You must classify and describe predator-prey relationships, the three forms of symbiosis (mutualism, commensalism, parasitism), and competition (both between and within species), and explain how resource partitioning lets competing species coexist. The thread running through all of it is that the availability of resources shapes how organisms interact and survive.
What an ecosystem is
The organisms in an ecosystem do not live in isolation. They eat one another, share or compete for resources, and form close partnerships. These interactions, more than any single species, determine the structure and stability of the ecosystem.
Predator-prey relationships
In predation one organism (the predator) kills and consumes another (the prey). Predation controls prey populations and shapes both species through natural selection: prey evolve defenses (camouflage, speed, toxins) and predators evolve better hunting traits. Predator and prey populations often cycle together out of phase, with a rise in prey followed by a rise in predators, then a fall in prey, then a fall in predators. Herbivory, where an animal eats a plant, is a related consumer interaction.
Symbiosis
- Mutualism: bees pollinate flowers while gaining nectar; mycorrhizal fungi and plant roots exchange nutrients.
- Commensalism: barnacles riding on a whale gain transport and feeding opportunities while the whale is unaffected.
- Parasitism: a tick feeding on a host gains nutrition while harming the host.
The availability of resources can shift these relationships. A partnership that is mutualistic when resources are scarce may become less important, or even parasitic, when conditions change.
Competition and resource partitioning
Competition happens when organisms use the same limited resource, such as food, water, light or space. It occurs between species (interspecific) and within a species (intraspecific). Competition is costly because it reduces the resources available to each competitor.
Resource partitioning is the way competing species reduce this cost: they use the shared resource in different ways, in different places, or at different times, so their use overlaps less. Classic examples are warblers feeding in different parts of the same tree, or species active at different times of day. Partitioning lets species coexist that would otherwise exclude one another.
When two species compete for exactly the same resources in exactly the same way, the competitive exclusion principle predicts that one will eventually outcompete and displace the other. Resource partitioning is the evolutionary escape from this outcome, and it explains why diverse communities can pack many similar species into the same habitat. Within a single species, intraspecific competition is often the most intense competition of all, because individuals of the same species need identical resources; this is a major force limiting population growth as a population approaches the carrying capacity of its environment.
Try this
Q1. Identify the type of symbiosis where one species benefits and the other is harmed. [1 point]
- Cue. Parasitism (+/-).
Q2. Explain why intraspecific competition is often more intense than interspecific competition. [2 points]
- Cue. Individuals of the same species need identical resources (the same food, mates and habitat), so their resource overlap is complete, whereas different species can partition resources.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). Ecologists studied two warbler species feeding in the same spruce trees. Species A fed mainly in the upper canopy; species B fed mainly on the lower branches. (a) Identify the type of species interaction occurring between the two warblers. (b) Describe how resource partitioning reduces the negative effect of this interaction. (c) Explain one way symbiosis differs from this interaction. (d) Predict what would happen to species B if species A were removed from the ecosystem.Show worked answer →
A 4-point FRQ on species interactions and resource partitioning.
(a) Identify (1 point): interspecific competition, because both species use the same food resource (insects) in the same trees.
(b) Describe (1 point): resource partitioning means the two species use the resource in different ways (different parts of the tree), so they do not directly compete for the exact same prey, reducing the intensity of competition and allowing coexistence.
(c) Explain (1 point): symbiosis is a close, long-term relationship between two species (such as mutualism, commensalism or parasitism), whereas competition is an interaction over a shared limited resource rather than a close partnership.
(d) Predict (1 point): species B would likely expand into the upper canopy (competitive release), increasing its realized niche and possibly its population, because the competing species is no longer using that resource.
Markers reward naming competition, linking partitioning to reduced overlap and coexistence, a correct contrast with symbiosis, and a logical prediction of competitive release.
AP 2020 (style)1 marksSection I (multiple choice). A bird removes ticks from the back of a buffalo, gaining food while the buffalo loses parasites. Which interaction best describes this relationship? (A) Commensalism (B) Mutualism (C) Parasitism (D) Predation. Justify your choice.Show worked answer →
A 1-point MCQ on classifying symbiosis. The answer is (B).
Both species benefit: the bird gains food and the buffalo is rid of parasites, which is mutualism. Commensalism (A) would mean one benefits and the other is unaffected; parasitism (C) means one benefits at the other's expense; predation (D) is one organism killing and eating another. The trap is choosing commensalism by overlooking that the buffalo also benefits.
Related dot points
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
- Topic 1.3 Aquatic Biomes: describe the major freshwater and marine biomes and explain how abiotic factors such as salinity, depth, light, temperature and nutrients shape them.
A focused answer to APES Topic 1.3, covering freshwater and marine biomes, salinity, the photic and aphotic zones, estuaries, coral reefs and wetlands, and the abiotic factors that control aquatic productivity, with a worked dissolved-oxygen question.
- Topic 1.9 Trophic Levels: describe the trophic levels of an ecosystem and explain the roles of producers, consumers and decomposers in transferring energy and matter.
A focused answer to APES Topic 1.9, covering producers, primary, secondary and tertiary consumers, decomposers and detritivores, autotrophs and heterotrophs, and how energy and matter move through trophic levels, with a worked classification question.
- Topic 1.11 Food Chains and Food Webs: describe how food chains and food webs represent the flow of energy and matter, and predict the effects of changes to a food web.
A focused answer to APES Topic 1.11, covering food chains and food webs, how energy and matter flow through them, keystone species, trophic cascades, and predicting the effects of removing a species, with a worked food-web disruption question.
- Topic 2.1 Introduction to Biodiversity: describe the three levels of biodiversity and explain how genetic and species diversity contribute to ecosystem resilience.
A focused answer to APES Topic 2.1, covering genetic, species and habitat diversity, species richness and evenness, the value of genetic diversity, bottlenecks and resilience, with a worked diversity-comparison question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)