Competition between species
Many students (and instructors) focus mostly on predator-prey interactions in nature, but there are many other types of interactions. A lot of people are familiar with examples of mutualistic, commensal, or parasitic symbiosis (yes even parasitism is a type of symbiosis) but have more difficulty thinking of examples of competition between species. Here are five local examples that explain a competitive interaction and how it also affects other parts of the ecosystem. Use them for your own edification or to share with students.
There are illustrated cards in the prep room with all of this information that can be used by students as an adaption of Each One Teach One. Use it as an Each Two Teach Two with students paired for the activity - in a field group of 10, each student would represent one part of a competitive relationship (so the 'hemlock' student is paired with the 'alder' student to explain what they are and how they interact, the 'limpet' student is paired with the 'barnacle' student and so on.)