All organisms, whether in terrestrial, aquatic or transition zones (e.g wetlands) depend on water. However, the precise ecological response to lack of water during droughts depends both on the magnitude, timing, frequency and duration of a drought and on the tolerance of species to reduced water availability.
In terrestrial ecosystems, droughts reduce plant productivity and species richness, which in turn affects the distribution and abundance of other organisms. In rivers and wetlands, droughts reduce the amount of wetted habitat and connections between habitats, increase water temperature and alter water quality. These changes can stimulate increases in some species, but generally reduce species abundance and diversity and diminish ecological functioning, disrupting food webs and energy flows.
Given time, when wetter conditions return, habitats and organisms may recover fully, because they have evolved drought resilient life history strategies. However, if droughts are severe, prolonged, occur aseasonally, or so frequently that recovery time is insufficient, species can be locally extirpated and habitats irrevocably altered.