How do species interact within a community?
Topic 8.5 Community Ecology: explain the types of interactions between species in a community and their effects on the species involved.
A focused answer to AP Biology Topic 8.5, covering competition, predation, the niche, symbiosis (mutualism, commensalism, parasitism), keystone species and trophic relationships, with a worked interaction example.
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What this topic is asking
The College Board (Topic 8.5) wants you to explain the interactions between species in a community, competition, predation, the niche, symbiosis (mutualism, commensalism, parasitism), and keystone species, and their effects on the species involved.
Competition and the niche
Predation and symbiosis
Keystone species
These interactions also drive evolution. Predator and prey, or host and parasite, exert selective pressure on each other, so each adapts in response to the other in a process called coevolution (for example, faster prey selecting for faster predators). Competition likewise drives resource partitioning, where competing species evolve to use slightly different resources or niches, which reduces direct competition and lets them coexist.
Try this
Q1. State the effect on each species in mutualism, commensalism and parasitism. [3 points]
- Cue. Mutualism: both benefit (+/+). Commensalism: one benefits, the other is unaffected (+/0). Parasitism: parasite benefits, host is harmed (+/-).
Q2. Explain why removing a keystone species can change a whole community. [2 points]
- Cue. A keystone species has an effect far larger than its abundance; for example, a keystone predator controls a dominant prey, so its removal lets that prey take over and reduces diversity.
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 2019 (style)4 marksSection II (long FRQ excerpt). (a) Distinguish between mutualism, commensalism and parasitism, giving the effect on each species. (b) Explain how the removal of a keystone predator could change the structure of a community.Show worked answer →
A 4-point distinguish-and-explain FRQ on species interactions.
(a) Distinguish (3 points): (1 point) mutualism benefits both species (+/+); (1 point) commensalism benefits one and does not affect the other (+/0); (1 point) parasitism benefits one (the parasite) and harms the other (the host) (+/-).
(b) Explain (1 point): removing a keystone predator can let its prey increase greatly, outcompeting or overgrazing other species, so overall diversity falls and the community structure changes.
Markers reward the correct effects for each symbiosis type and explaining the disproportionate effect of removing a keystone species.
AP 2017 (style)1 marksSection I (multiple choice). A relationship in which one species benefits and the other is harmed is called: (A) mutualism. (B) commensalism. (C) parasitism. (D) competition.Show worked answer →
The correct answer is (C).
In parasitism one species (the parasite) benefits while the other (the host) is harmed (+/-). Mutualism (A) benefits both (+/+); commensalism (B) benefits one with no effect on the other (+/0); competition (D) harms both (-/-).
Related dot points
- Topic 8.3 Population Ecology: explain exponential and logistic growth, carrying capacity, and the factors that regulate population size.
A focused answer to AP Biology Topic 8.3, covering exponential and logistic growth, carrying capacity, growth rate calculations, and the factors that shape population size, with a worked growth-rate calculation.
- Topic 8.2 Energy Flow Through Ecosystems: explain how energy flows through trophic levels and why energy is lost between levels.
A focused answer to AP Biology Topic 8.2, covering trophic levels, food chains and webs, the 10 percent rule, energy pyramids, productivity, and why energy decreases up the chain, with a worked energy-transfer calculation.
- Topic 8.6 Biodiversity: explain how biodiversity contributes to ecosystem stability and resilience.
A focused answer to AP Biology Topic 8.6, covering species and genetic diversity, how diversity supports ecosystem stability and resilience, the effects of low diversity, and a worked example using a diversity comparison.
- Topic 8.4 Effect of Density of Populations: distinguish density-dependent from density-independent factors and explain how each limits population size.
A focused answer to AP Biology Topic 8.4, covering density-dependent factors (competition, predation, disease) and density-independent factors (weather, disasters), how each regulates populations, and K-selected versus r-selected strategies, with a worked example.
- Topic 8.7 Disruptions to Ecosystems: explain how natural and human-caused disruptions affect ecosystems and how ecosystems respond.
A focused answer to AP Biology Topic 8.7, covering natural and human disturbances, invasive species, habitat loss, climate change, ecological succession, and how ecosystems respond and recover, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)