Which term best describes the boundaries between different habitats where population changes occur?

The term that best describes the boundaries between different habitats where population changes occur is "ecotones." An ecotone is a transitional area between two different ecosystems or habitats, such as where a forest meets a grassland or a freshwater body meets a saltwater body. These regions often have a rich diversity of species and a unique set of environmental conditions compared to the adjacent ecosystems. Because of their transitional nature, ecotones can significantly influence the interactions between species and the processes of population change as organisms move between habitats.

In contrast, biomes refer to large ecological areas that share similar climate, plants, and animals, but do not necessarily focus on the boundaries between specific habitats. Niches pertain to the specific role or function of an organism within its environment, rather than the interactions at habitat boundaries. Microhabitats are smaller, localized areas within a habitat that provide specific conditions suitable for certain species, but do not capture the broader boundary role of ecotones in population changes. Thus, "ecotones" accurately reflects the concept of transitional spaces where population dynamics are particularly dynamic.