Sea area is much larger than the land surface, but in addition we need to consider the volume of the oceans.
Various sources estimate the volume of the water in all the oceans of the world at something over 1.3 billion cubic kilometres. They cover 71 percent of the earth’s surface - giving them 99 percent of the living-space.
Sea Surface Area and Volume
However one looks at it the seas and oceans of this planet provide a larger space for living things than the land. Just considering the area this is obvious, but with an average ocean depth of 4-5km the difference is staggering. Marine organism have been found at all depths, but land dwellers have only a relatively small - soil up to tree tops - volume to inhabit (birds and many insects do fly much higher, but they cannot be said to truly live in the air).
Sea Life and Land Life
Life probably evolved first in the sea, so there has been much more time for organisms to diverge and adapt to their environment. This adds a further dimension to the differences between sea and land – it adds in time. The oceans were seething with life long before plants and animals crept ashore, and all land plants and animals have marine ancestors. Most living things have not yet made it ashore, so we find much more diversity in the sea than on land.
The Human Perspective
We have been aware of land animals and plants for a very long time, and while this must also be true for many coastal and sea-shore organisms, our contact with most ocean life is very recent. We can easily explore the land surface of the earth, but when we enter the sea we move into an unfamiliar and hostile environment. We still know more about the surface of the moon than we do about the ocean floor. The extent of our knowledge about marine life could be expressed as equivalent to that of a person who has only wandered around in his or her own back yard and takes that as representative of all life on land.
Sea Life and Currents
A huge difference between life on land and life in the sea is caused by the fact that the environment for many marine creatures and plants is always on the move. On land we are used to studying particular geographical locations, but this approach is not helpful if everything is constantly in motion – planktonic organisms cannot be said to live in any one ‘place’, they are constantly at the mercy of the currents.
Marine Biology and Oceanography
Marine Biologists need to be fully aware of the physical aspects of the marine environment (oceanography) before they can begin to understand marine life. Light levels and nutrient availability will influence the plankton, that will affect the fish, and their density will be important for the larger marine predators. All will be influenced by the ocean currents which are themselves determined by salinity and temperature differences as well as the winds.
Primary source: Estimates of the Volume of the Earth’s Oceans – with references.
Other articles by John Blatchford
