Describe the main ecological interactions (competition, predation, and symbiosis: mutualism, commensalism, parasitism) and explain how they affect the populations involved (MA STE HS-LS2-2, HS-LS2-6, cause and effect).

What this topic is asking

The Massachusetts STE framework (HS-LS2-2 and HS-LS2-6) asks you to explain how interactions among organisms affect populations and the stability of an ecosystem. On the High School Biology MCAS, this is tested by classifying a described relationship (competition, predation, or a type of symbiosis) and by reading predator-prey graphs. The crosscutting concept is cause and effect: each interaction affects the populations involved in a predictable way.

Competition

Competition can be between members of the same species (for example, two stags competing for mates) or between different species (two bird species eating the same seeds). Because the resource is limited, the more one organism gets, the less the other gets, so competition is a major limiting factor on population size (linking to population dynamics and carrying capacity). Competition is also a selection pressure that drives natural selection.

Predation and predator-prey cycles

Predation is one organism (the predator) killing and eating another (the prey). Predation controls prey populations and provides energy for the predator. A classic MCAS topic is the predator-prey cycle, where the two populations rise and fall in a repeating, linked pattern:

• When prey are plentiful, predators have lots of food, so the predator population rises.
• More predators eat more prey, so the prey population falls.
• With less food, the predator population falls.
• With fewer predators, the prey population rises again, and the cycle repeats.

On a graph, the predator peaks come slightly after the prey peaks, because the predators respond to the change in their food supply. Explaining this feedback loop is a very common question.

Symbiosis: three kinds

• Mutualism: both benefit. For example, a bee gets nectar while pollinating a flower; both the bee and the flower gain.
• Commensalism: one benefits, the other is unaffected. For example, a bird nests in a tree; the bird gains shelter and the tree is neither helped nor harmed.
• Parasitism: one benefits, the other is harmed. For example, a tapeworm lives in a host's gut, taking nutrients and harming the host.

The trick the MCAS uses is to describe a relationship and ask you to classify it. Work out who benefits and who is harmed or unaffected, then match it to the type. This is straightforward once you have the three definitions clear.

Try this

Q1. State the three types of symbiosis and who benefits in each. [2]

• Cue. Mutualism (both benefit), commensalism (one benefits, the other unaffected), parasitism (one benefits, the other is harmed).

Q2. Explain why a predator population usually peaks shortly after its prey population. [2]

• Cue. When prey are plentiful, predators have more food and reproduce more, so the predator numbers rise after the prey, then fall again as they reduce the prey.