AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/1/1-1)
Unit 1 · Topic 1.1
1.1 Introduction to Ecosystems
Species in an ecosystem affect each other by eating, competing and living closely together. Knowing the difference between predation, competition and the three kinds of symbiosis, plus how species split up resources, lets you explain why communities look the way they do.
Key terms
- predator-prey relationship
- symbiosis
- mutualism
- commensalism
- parasitism
- resource partitioning
What an ecosystem is
An ecosystem is all the living things in an area plus the nonliving parts they depend on, like sunlight, water, air, soil and temperature. Ecologists call the living parts biotic factors and the nonliving parts abiotic factors.
Within an ecosystem, a population is all the members of one species in one place, and a community is all the populations of different species living together. Most of this topic is about how species in a community interact. Each kind of interaction can be described by who gains (+), who is harmed (−) and who is unaffected (0).
The main types of interactions
Symbiosis means a close, long-term relationship between two different species. Mutualism, commensalism and parasitism are all types of symbiosis. Predation is not usually called symbiosis, because the predator kills and eats the prey rather than living with it.
A parasite usually harms its host without killing it quickly, since it depends on the host to stay alive. A predator kills its prey right away. Both relationships are + and −, so read the scenario carefully to tell them apart.
| Interaction | Effect on each species | Example |
|---|---|---|
| Predation | + for predator, − for prey | A lynx eating a snowshoe hare |
| Herbivory | + for herbivore, − for plant | A deer browsing on shrubs |
| Competition | − for both | Lions and hyenas competing for the same prey |
| Mutualism | + for both | Bees get nectar; flowers get pollinated |
| Commensalism | + for one, 0 for the other | Barnacles riding on a whale |
| Parasitism | + for parasite, − for host | A tick feeding on a deer |
Predators and prey
Predator and prey populations often rise and fall in linked cycles. When prey are plentiful, predators have lots of food and their numbers grow. More predators eat more prey, so the prey population drops. With less food, the predator population then drops too, which lets the prey recover. On a graph, the predator peaks come shortly after the prey peaks.
The classic example is the Canada lynx and snowshoe hare, whose numbers swing up and down on a cycle of roughly 10 years. Predators also help a prey population by removing weak or sick individuals.
Competition and resource partitioning
Competition happens when two organisms need the same limited resource, like food, water, light, nesting sites or space. Competition between members of the same species is intraspecific; between different species it's interspecific. Competition hurts both sides because each gets less of the resource.
If two species need exactly the same resources in the same place, one usually outcompetes the other. This idea is called the competitive exclusion principle. Species avoid it through resource partitioning, which means dividing a resource so each species uses a different part of it.
- Temporal partitioning (by time): hawks and owls both hunt small mammals, but hawks hunt by day and owls hunt at night.
- Spatial partitioning (by place): several warbler species feed on insects in the same spruce trees but at different heights and parts of the tree.
- Morphological partitioning (by body features): bird species with different beak sizes eat different sizes of seeds, so they compete less.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying relationships
Classify each relationship and give the effect on each species. (a) Clownfish live among the stinging tentacles of a sea anemone; the anemone protects the clownfish from predators, and the clownfish chases away fish that eat anemones. (b) Cattle egrets follow grazing cattle and eat the insects the cattle stir up from the grass. (c) A tapeworm lives in a dog's intestine and absorbs its food.
Show the solutionHide the solution
- Step 1: For each pair, ask who gains, who loses and who is unaffected.
- Step 2: (a) Both species gain: the clownfish gets shelter and the anemone gets protection. That's + / +, so it's mutualism.
- Step 3: (b) The egret gains food. The cattle are neither helped nor harmed in a meaningful way. That's + / 0, so it's commensalism.
- Step 4: (c) The tapeworm gains food and the dog loses nutrients but usually isn't killed quickly. That's + / −, living inside the host, so it's parasitism, not predation.
Answer: (a) Mutualism (+/+); (b) commensalism (+/0); (c) parasitism (+/−).
- Example 2
Explaining coexistence
Two species of lizard live on the same small island and both eat insects. One species hunts on tree trunks and the other hunts on the ground. Explain how this lets both species survive on the island.
Show the solutionHide the solution
- Step 1: Name the problem: both lizards need the same limited resource (insects), so they could compete strongly enough for one to push the other out.
- Step 2: Name the solution: they split the resource by place, which is spatial resource partitioning.
- Step 3: Connect to the outcome: because each species hunts in a different area, they compete less directly, so each has enough food and both populations can persist.
Answer: Spatial resource partitioning: hunting in different places reduces competition for insects, so both species can coexist.
Common mistakes
- Calling predation a type of symbiosis. Symbiosis means two species living closely together over time; mutualism, commensalism and parasitism are the three types.
- Mixing up commensalism and mutualism. If one species is unaffected (0), it's commensalism, even if the other benefits a lot.
- Saying that in resource partitioning the species ‘share’ the resource equally. They divide it by time, place or body features so they use different parts of it.
- Drawing predator and prey peaks at the same time. Predator numbers peak shortly after prey numbers, because predators need time to respond to more food.
On the exam
- Expect short scenarios where you must name the relationship. Always state the effect on both species (+, − or 0) to justify your answer.
- On free-response questions, if asked how two similar species coexist, name the specific type of resource partitioning (temporal, spatial or morphological) and explain how it reduces competition.
Connected topics
Videos
Check yourself
5 questions on 1.1 Introduction to Ecosystems. Pick an answer to see if you got it, and why.
Cattle egrets follow grazing cattle and eat the insects that the cattle stir up from the grass. The cattle are neither helped nor harmed. Which type of interaction does this describe?
Five species of warbler eat insects in the same spruce forest. Each species feeds mostly in a different part of the trees, such as the treetops, the outer branches or the lower trunk. What is the most likely effect of this behavior?
Bullhorn acacia trees in Central America have large hollow thorns. Colonies of acacia ants live inside the thorns and feed on sugary nectar and protein-rich tips that the trees produce. The ants sting and bite insects that land on the tree and clip away vines that grow on it.
In a field study, researchers removed all ants from 20 acacia trees and left the ants on 20 similar trees nearby. After one year, trees without ants had lost about 45% of their leaf area to insects and grew about 0.3 m taller. Trees with ants had lost about 8% of their leaf area and grew about 1.2 m taller.
Described field study with hypothetical results
Which type of interaction between the ants and the acacias is best supported by the passage?
What is the independent variable in this study?
Which additional result would best support the claim that the ants, not just the trees, benefit from the relationship?
0 of 5 answered