What is it called when the population of one species decreases as the population of another species increases?

The situation described, where the population of one species decreases as the population of another species increases, is known as competition. In ecological terms, competition occurs when two species vie for the same limited resources, such as food, space, or mates. If one species becomes more dominant and is better at securing these resources, the other species may struggle to survive and its population may decline.

This dynamic is fundamental to understanding ecological relationships; it illustrates how inter-species interactions can impact population dynamics. For example, in an ecosystem where two herbivores compete for the same type of vegetation, an increase in one herbivore's population might lead to overconsumption of the plants, leaving fewer resources for the second herbivore, potentially resulting in a decline in its population.

The other terms relate to different interactions: mutualism involves both species benefiting, parasitism involves one species benefiting at the other's expense without necessarily leading to a population increase in the host species, and symbiosis refers more broadly to any close interaction between species, which can include mutualism, commensalism, and parasitism. Understanding these distinctions is key to grasping the complexities of ecological relationships.