The community is a subset of the species pool created by the action of one or more environmental filters. We propose (Proposition 1) that in any habitat, the power of a filter can be measured as the proportion of species that it removes from the species pool. Filters can be abiotic or biotic in origin. Drought, frosts, hypoxia and wildfires are important abiotic (physical) filters, while predation (including herbivory) and competition are important biotic filters. We also propose (Proposition 2) that in any habitat only a small number of filters is likely important. Moreover (Proposition 3), in any region a large number of species are likely controlled by the same filters. These three propositions provide guidance for setting priorities in research and habitat management. One of the most important challenges in community ecology is to identify and rank the filters that control species composition in specified landscapes.