why are there more species on land than in the water?

Firstly: there are many more different environments on land than in the oceans.  This will lead to increased diversity, and subsequently more species.

To a lesser degree/no evidence to back this up :):

a) insects make up a good portion of total number of species, but are virtually absent from marine environments

b) we explore terrestrial environments a lot more easily than we do ocean environments, which will bias the ratio somewhat.

Last edited by Peter Falkingham (5th Jun 2009 12:12:47)

Hi Sara,
Peter is right about the insects.  They skew the numbers in favour of terrestrial (land) animals.  However, it depends on the sort of animal that you look at.  If you look at the chordates (the group that we belong to) about half of the species live on the land, and half in water.  This is about the same for molluscs, the second largest (in terms of species) Phylum after the arthropods, where there are probably slightly more than half of molluscan species in the sea/aquatic environment. 
So, it really depends on the type of animal.  The second thing that we must consider is not necessarily the number of species, but different taxonomic levels: classes, orders, families, genera. There may be a a few families with many species, and many families with a few species, and the number of species, might not actually represent the number of individuals. 
Peter is absolutely right in saying that we have not explored the sea as well as the land; indeed it is said that we know more about the surface of the moon than we do about the seabed.  This means that we are much more conscious of land animals and recognise them more readily.  So, there are certainly more species on land, but this is only because of the insects.  If you look at any other Phylum, I don't know that there are really any more land species than aquatic?

Just to add to Neil's post. I would happily put money on there being more aquatic than terrestrial chordate species out there. Looking just at the vertebrates fishes represent about half the known species but new ones are being described at quite a rate (i.e. much faster than birds, mammals, reptiles or amphibians. You'd have to add at least some of the amphibians to the aquatic category too.

Further to Peter's point that there are many more different environments on land than in the
oceans, I would add that the marine realm is all connected whereas there are distinct isolated landmasses, which increases the amount of diversity gained through endemism.

Secondly, many scientists believe that even with further exploration, we will ultimately find that there are several times more species on land than in the water. Though no one knows why this is some have hypothesized that is it due to the greater potential to capture energy on land then in the water (sunlight only penetrates to a shallow depth and hydrothermal vents do not contribute largely to the oceans' diversity).