What is Biodiversity?
Biodiversity is now commonly defined as the variety of life in genes, species and habitats. According to the definition of the Convention on Biological Diversity, biodiversity is the variability among living organisms from all sources, including inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.
The three domains of life, bacteria, archaea and eukarya are present in the marine environment. In addition there are viruses. About 230,000 species of marine plants and animals have been scientifically described and a few thousand bacteria and archaea. This known biodiversity only represents a small fraction of the number of species existing, except for the macrophytes and seagrasses which are living in coastal environments and, in general, for the pelagic environment.
Species diversity in the oceanic pelagic environment is extremely low. The number of species in the upper 200 meters (m) of the pelagic oceanic environment is well known for four groups of animals, the Euphausicacea, Chaetognatha, Pteropoda and Copepoda, which dominate the biomass everywhere. There are only 80 species of euphausiids, 50 of chaetognaths, about 40 of pteropods and less than 2000 for the most diverse group, the calanoid copepods. These data are based on more than 20,000 net tows and, although new species will certainly continue to be discovered, it is obvious that pelagic biodiversity is of another order than both terrestrial and marine benthic diversity.
This low number of (animal) species is in striking contrast with the diversity of animals in sediments. About 200,000 species are currently known from benthic environments. Most of them have been described from coral reefs, and only about 60,000 are known from soft bottom habitats that cover most of the Earth’s surface. Benthic species from the temperate shallow waters of Europe are reasonably well known, especially in the larger macro- and megafauna. The smaller meiofauna (mm-sized animals) is less well described and, as an example, a survey of the benthos in the North Sea in 1986 yielded about 40% of benthic copepod species new to science.
For both animals and microbes, the exploration of environments that are difficult to access, such as the deep-sea floor or marine caves, and the application of new technologies, are constantly yielding new species and higher taxonomic categories, even up to phylum level. Especially the availability of rapid sequencing technologies has shown that variability in the microbial domain, including the small eukaryotes, is extremely high and that tens of thousands of ‘species’ may co-occur in a single liter of sea water.
Life originated in the sea and is much older in the sea than on land. As a consequence, animal and plant diversity at higher taxonomic levels are much greater in the sea where there are 14 endemic (unique) animal phyla whereas only 1 phylum is endemic to land. For plants the situation seems to be different—almost all algal groups have representatives in both fresh and marine waters and higher plants are nearly exclusively terrestrial. There is also a remarkable diversity of life-history strategies in marine organisms. The sum total of genetic resources and physiological diversity in the sea is therefore expected to be much more diverse than on land.
Habitat diversity and the number of marine habitats is difficult to define. Studies of zonation have typically demonstrated the existence of very narrow zones in intertidal areas, where direct observation is possible, and broader and broader zones as one goes deeper. However, it is recognized that this is due to our limited possibilities of observation and with increasing technological capability, finer discontinuities are revealed even in the water column. Besides zonation bands, a number of very specific habitats often linked to tectonic activities have been discovered over the last decades, starting with the hydrothermal vents in 1977 and followed in later years by cold seeps of gases and fluids, carbonate mounds, mud volcanoes, etc. Multibeam sonar has allowed much more detailed analysis of the sea floor showing fine-grained features in sediments that were previously thought to be rather uniform, or the very complex topology of marine canyons in the continental slope. With increasing potential of observation, the number of marine habitats on many different scales will certainly increase, and, as these habitats often contain species which are specifically adapted to their environmental conditions, so will species diversity.
Source: Heip, Carlos H.R. (Lead Author); Jean-Pierre Gattuso (Topic Editor). 2006. "Marine biodiversity." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [Published December 11, 2006; Retrieved April 16, 2007]. http://www.eoearth.org/article/Marine_biodiversity.