Describe the interactions between species, including predation, competition, and the three forms of symbiosis (mutualism, commensalism, and parasitism) (Ohio's Learning Standards for Science, Biology, B.DI.2).

What this topic is asking

Ohio standard B.DI.2 (Ecosystems) treats the community as a web of interactions between species, and the model curriculum expects students to recognize predation, competition, and symbiosis. The Ohio Biology EOC turns this into items where you classify a relationship from a scenario by working out who benefits and who is harmed. The crosscutting idea is systems and system models plus cause and effect: each interaction shapes the populations involved. This connects to population dynamics (interactions are limiting factors) and to the niche concept.

Predation

In predation, one organism, the predator, kills and eats another, the prey. Predators (a wolf, a hawk, a frog) gain energy and nutrients; prey (a rabbit, a mouse, an insect) are killed. Herbivory, where an animal eats a plant, is usually grouped here too: the herbivore benefits and the plant is harmed (though often not killed). Predation is a major control on prey populations, and predator and prey numbers often rise and fall together over time, each affecting the other.

Competition

Competition happens when two or more organisms need the same limited resource, such as food, water, space, light, or mates. Because the resource is limited, competition harms at least one of the competitors (it gets less of the resource). There are two kinds:

• Intraspecific competition, between members of the same species (two oak trees competing for light).
• Interspecific competition, between different species (two bird species competing for the same nest holes).

If two species compete for exactly the same resources (the same niche), the better competitor tends to win, and the other may decline or shift to a different resource. This links back to why species with different niches can coexist.

Symbiosis and its three types

Symbiosis is a close, long-term relationship between two different species living together. The three types are defined entirely by who benefits (+), who is unaffected (0), and who is harmed (-):

• Mutualism: both species benefit. A bee gets nectar while pollinating the flower; gut bacteria get a home while helping their host digest food.
• Commensalism: one benefits and the other is unaffected (neither helped nor harmed). A barnacle gets carried to food on a whale that is not affected.
• Parasitism: one (the parasite) benefits and the other (the host) is harmed. A tapeworm takes nutrients from its host; unlike a predator, a parasite usually does not kill its host outright.

How interactions shape communities

These interactions are not just labels; they control population sizes and the structure of the community. Predation and competition act as density-dependent limiting factors (stronger when populations are crowded), helping hold populations near the carrying capacity. Mutualisms can make species depend on each other, so the loss of one harms the other. Tracing who is affected lets you predict knock-on effects when one population changes.

Try this

Q1. State the difference between mutualism and commensalism. [2]

• Cue. In mutualism both species benefit; in commensalism one species benefits and the other is unaffected.

Q2. A tick feeds on the blood of a deer, harming the deer. Name this type of relationship and state who benefits and who is harmed. [2]

• Cue. Parasitism: the tick (parasite) benefits by gaining food (blood); the deer (host) is harmed.