Community
A community is all the populations of different species living and interacting in the same area.
Definition
A community is all the different kinds of living things that live together in one place, the plants, animals, fungi, and microbes but not the water, rocks, or air, so a pond community is only its living parts, and adding the nonliving surroundings gives the whole ecosystem. More formally, a community is the group of all populations of different species that live and interact in a particular area, shaped by interactions such as competition, predation, herbivory, parasitism, and mutualism, and by succession over time, sitting in the hierarchy between population and ecosystem; some species matter far more than their numbers suggest, since a keystone species such as the sea otter can hold a whole community together. At the structural level, a community is described by species richness, relative abundance (evenness), trophic organization, and interaction networks, shaped by regional species pools filtered through dispersal, abiotic tolerance, and biotic interactions, with keystone species exerting disproportionate effects, foundation species creating habitat, and trophic cascades propagating a top predator's effects downward; diversity indices such as Shannon's $H' = -\sum p_i \ln p_i$ summarize richness and evenness together.
Example
After a forest fire in the North Carolina mountains, grasses and wildflowers appear first, then shrubs and pines, and after $100$ years or more the oak and hickory community returns, each stage a different community occupying the same place. Robert Paine removed the starfish Pisaster from a rocky intertidal community and watched species richness fall from $15$ to $8$ as mussels, freed from predation, crowded out everything else, the experiment that defined the keystone concept, and a community of $4$ equally abundant species has $H' = \ln 4 \approx 1.39$, compared with about $0.17$ if one species holds $97$ percent of the abundance.
Key Insight
Communities are networks, not lists, since the identity of the links, who eats whom and who depends on whom, predicts what happens when a species is lost far better than the species count alone.