Environ 201- chapter 4: species interactions and community ecology. Phytoplankton: microscopic photosynthetic algae, protists, and cyanobacteria that drift near the surface of water bodies and generally form the first trophic level in an aquatic food chain. Zooplankton: tiny aquatic animals that feed on phytoplankton and generally comprise the second trophic level in an aquatic food chain. Competition: a relationship in which multiple organisms seek the same limited resource. Intraspecific competition: competition that takes place among members of the same species. Interspecific competition: competition that takes place among members of two or more different species. Competitive exclusion: an outcome of interspecific competition in which one species excludes another species from resource use entirely. Species coexistence: an outcome of interspecific competition in which no competing species fully excludes others and the species continue to live side by side. Fundamental niche: the full niche of a species. Realized niche: the portion of the fundamental niche that is fully realized (used) by a species.