keystone species

A species on which other species in an ecosystem largely depend, such that if it were removed the ecosystem would change drastically.

Citation: 
http://www.oxforddictionaries.com/definition/english/keystone-species
English

keystone species[photo]

Figure 2: Reduction of diversity and food-web complexity upon removal of sea otters, a keystone predator, on sub-tidal communities. Arrows indicate direction and quantity of nutrient and energy flow; dash-lined arrows indicate minimal energy/nutrient flow. Light grey organisms indicate small/unstable populations.[fn][[nid:932]][/fn]
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keystone species [cite]

1. Credit: Brumbaugh, DR, T Agardy, and F Staub. Introduction to Marine Conservation Biology: Trophic Interactions in a Marine Community (Exercise solutions). Network for Conservation Educators and Practitioners, American Museum of Natural History.
2. Kelp forest by Kip Evans - Published by National Oceanic and Atmospheric Administration at http://www.photolib.noaa.gov/nurp/nur03505.htm Licensed under Public domain via Wikimedia Commons -
3. Credit: Ocean Defenders Alliance