Why Does Species Number Decrease with Smaller Body Mass?

[farful]

# of species vs body mass

Here are some graphs:

http://en.wikipedia.org/wiki/File:Intercontinental_land_mammals.png
http://en.wikipedia.org/wiki/File:North_american_land_mammals_graph.png
http://en.wikipedia.org/wiki/File:May_slope_-2_line.png

It's obvious why the number of species that have smaller body sizes are greater in number.
However, once we get into organisms with mass smaller than 100g, it seems to decrease. Why? One would assume it'd keep increasing, no?
Furthermore, would the trend continue for microbials?

Edit: Thanks DaveC, links have been fixed

 

[Ygggdrasil]

There may be a problem with missing species. Most microbes are impossible to culture in lab (or at least we haven't figured out how to yet), so there are likely a great number of undiscovered microbial species.

 

[farful]

There may be a problem with missing species. Most microbes are impossible to culture in lab (or at least we haven't figured out how to yet), so there are likely a great number of undiscovered microbial species.

The graph is for animals, not microbes, so I can't imagine there being missing species.

Also, with the advent of metagenomics, not being able to culture microbes is not a problem to get a distribution of species in an environment.

 

[Andy Resnick]

I couldn't see the graphs... does it mention the wide range of the size of dogs?

 

[DaveC426913]

Edit: Thanks DaveC, links have been fixed

Uh, that's DaveC426913. I hate nicknames...