Biodiversity is the abundance of life within a given area. Marine biodiversity is the variety of life found in the ocean. It includes all species from the tiny phytoplankton to the mighty blue whale (Balaenoptera musculus). Places with a higher-than-average number of different species are called biodiversity hotspots.

High biodiversity in Earth’s oceans is critical to a healthy planet. Biodiverse marine ecosystems in the ocean produce half of the oxygen we breathe and are a massive carbon sink. Coral reefs and mangrove forests protect coastlines from storms. 44% of the world’s human settlements living within 150 kilometers (93 miles) of the ocean receive vital ecosystem services from the ocean as well. In addition to protection, ocean life provides food and income for millions of people through fishing and aquaculture.

However, biodiversity—both marine and terrestrial—is in decline. The World Wildlife Fund estimates we experienced an average of 68% decline in global populations of mammals, fish, birds, and amphibians from 1970 to 2020. Things are so bad, some scientists suspect we are experiencing the sixth great extinction.

Humans reduce marine biodiversity in many different ways, including overfishing, catching species we do not need (bycatch), polluting the ocean, introducing invasive species, and our contributions to climate change. Each species serves a purpose in its ecosystem. Removing one can have significant impacts, such as causing environmental changes or the disappearance of other species that were dependent on the function the species served. Removal of a keystone species, for example, will have a chain reaction on that species' environment. Higher biodiversity may make ecosystems more resilient to change, something particularly important today as climate change alters the temperature and chemistry of the ocean.

You can make a difference to protect marine biodiversity. You can choose sustainable seafood options, reduce your use of single-use plastic and recycle when you can, reduce your carbon footprint, volunteer with organizations that pick up litter, teach others about the ocean, or advocate for the creation of marine protected areas.

The data in this map layer was created by the Map of Life and depicts global patterns of species richness and rarity for marine mammals and fish. Species richness is the number of species in a place. Species rarity is an estimation of the number of rare species in a given area. This data is organized into cells. Each cell represents 55 kilometers (34 miles) by 55 kilometers (34 miles). Within each cell, the percentile (1-100 percent, low to high) for species richness and rarity are shown.