Relationship Between Organisms — Reading Comprehension Lesson

Reading Passage

Relationship Between Organisms

No organism lives in isolation. Every living thing interacts with other organisms in its environment in ways that affect its survival and reproduction. Ecologists classify these interactions based on whether each species benefits, is harmed, or is unaffected. The main categories are predation, competition, mutualism, commensalism, and parasitism. Together, these relationships form the web of interactions that holds ecosystems together.

Predation is a relationship in which one organism — the predator — kills and eats another — the prey. Predation is essential to ecosystem health: predators control prey populations, preventing any single species from overrunning its habitat. The relationship drives evolutionary arms races — prey evolve better defenses (camouflage, speed, toxins, armor) while predators evolve better hunting abilities. Classic predator-prey pairs include wolves and deer, hawks and mice, and sea otters and sea urchins. When sea otters were hunted to near-extinction, sea urchin populations exploded and devastated kelp forests.

Mutualism is a relationship in which both organisms benefit. One of the most important examples is the relationship between flowering plants and their pollinators. Bees, butterflies, hummingbirds, and bats visit flowers to collect nectar or pollen for food; in the process, they carry pollen from flower to flower, enabling the plants to reproduce. Another example is the relationship between clownfish and sea anemones: the clownfish gets shelter among the anemone’s stinging tentacles (to which it is immune), and chases away fish that would eat the anemone. Mycorrhizal fungi and plant roots have a mutualistic relationship in which fungi help plants absorb water and minerals while receiving sugars from the plant.

Parasitism is a relationship in which one organism, the parasite, benefits at the expense of another, the host. Unlike predators, parasites usually do not kill their hosts immediately — a dead host is no longer useful to a parasite. Ticks, fleas, tapeworms, and many viruses and bacteria are parasites. Commensalism describes a relationship in which one organism benefits and the other is neither helped nor harmed. Barnacles attaching to a whale’s skin are often cited as an example: the barnacles get transport to food-rich waters, while the whale is largely unaffected. In practice, it is often difficult to prove that the host is truly unaffected.

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