Comparing the realized niche of tree frogs with their fundamental niche makes it clear that interactions with other species play key roles in the distribution and abundance of populations. Virtually every organism is involved in interactions with other species—think of plants and their pollinators or predators and their prey. These interactions often narrow the geographic distributions of species within the larger boundaries of their physical requirements, and so help shape their realized niches.

All interactions combine costs (often in resources) and benefits (usually increases in reproduction). Some interactions are direct, where species physically interact (such as plants and their pollinators). Others are indirect, where they influence one another through competition for a shared resource, such as food. In many interactions, the benefits are one sided: One organism actually consumes another or competes with it for food or some other resource. In this section, we focus on such one-sided interactions, in which at least one participant loses more than it gains.