05 Ecological Relationships

How to describe ecological interactions

Organisms in a community interact as they obtain food, space, shelter, and other resources. These interactions can affect survival, reproduction, and population size. Ecologists describe the effects on two interacting species with signs: $(+)$ means a benefit, $(- )$ means harm, and $(0)$ means no significant effect. A pair such as $(+,-)$ indicates that one species benefits while the other is harmed.

Competition and shared resources

Competition occurs when organisms use the same limited resource, such as food, water, light, nesting sites, or territory. Because each competitor has less access to that resource, competition generally harms both: $(-,-)$. It can happen among members of the same species or between different species.

Plants growing close together may compete for sunlight and soil nutrients. When species rely on exactly the same limited resources, they may not coexist indefinitely in the same place. They can sometimes reduce competition through resource partitioning: using different foods, areas, or feeding times. For example, two bird species might feed on insects in different parts of the same tree.

Takeaway: Competition is about limited resources; using resources differently can ease it.

Predation and herbivory

In predation, a predator captures and eats prey. The predator benefits and the prey is harmed: $(+,-)$. If prey becomes abundant, predators may have more food and increase in number; greater predation may then contribute to a decline in prey. Other factors also influence populations, so their numbers do not always follow a simple cycle.

Herbivory is feeding on plants or algae. The herbivore gains food while the plant or alga is harmed, although it is not always killed. Plants may limit damage with defenses such as thorns or chemical substances.

Takeaway: Predation and herbivory both involve one organism gaining food while another is harmed, but herbivory specifically involves plants or algae.

Symbiosis: three kinds of relationship

Symbiosis is a close, sustained interaction between organisms of different species. Its effects depend on the relationship:

• Mutualism $(+,+)$: both species benefit. In lichens, a fungus provides structure and protection, while photosynthetic algae or bacteria provide sugars.

• Commensalism $(+,0)$: one species benefits while the other is not significantly affected. A bird nesting in a tree gains shelter, while the tree may be unaffected.

• Parasitism $(+,-)$: a parasite benefits by living in or on a host, which is harmed. A tapeworm obtains nutrients from its host.

The effects of an interaction can depend on conditions. It may also be difficult to determine whether one species is truly unaffected.

Takeaway: The sign pairs summarize who benefits or is harmed, but the effects may depend on circumstances.

From food chains to food webs

A food chain shows one pathway by which energy and nutrients pass as organisms eat one another. It commonly begins with a producer, such as a plant or alga, followed by consumers. For example, a plant may be eaten by a grasshopper, which is then eaten by a frog.

A food web connects many food chains and better represents varied feeding relationships. An organism may eat more than one kind of food and may be eaten by multiple predators. In a meadow, grass may feed both rabbits and grasshoppers; frogs may eat grasshoppers, while foxes may eat rabbits.

In a food chain or food web, arrows point from the food organism to the organism that consumes it. Thus, the arrow shows the direction of energy transfer.

Trophic levels, energy, and ecosystem change

Organisms can be grouped by their feeding position, or trophic level. Producers form the base of many food webs. Herbivores are primary consumers, and predators that eat consumers occupy higher levels. Decomposers, including many fungi and bacteria, break down dead organisms and waste, returning nutrients to the environment.

Energy flows through food webs, and some is lost as heat at each transfer. Matter, including nutrients, is recycled. Because species are connected, a change in one population can affect others indirectly. If a predator declines, some prey populations may increase, potentially changing the plants or other organisms those prey consume. The outcome depends on the food web's structure and the strength of its interactions; not every change causes an ecosystem to collapse.

Takeaway: Food webs connect species, energy moves through them, and nutrients are recycled. Changes can spread through the web, but their effects depend on the connections involved.