Re: Why are C3 species so abundent? (as rubisco is inefficient)

This is a great question that probably does not have an absolute answer.  
I will try to give you a brief overview of carbon assimilation schemes.  
The predominance of the C3 PCR cycle is most likely due to temperature and 
to some extent CO2 levels.  The fact that Rubisco is inefficient is really 
not an important issue because there are few environmental factors that 
are limiting for C3 PCR.  In other regions where there are limiting 
factors alternative PCR cycles have developed.  C4 PCR developed in the 
tropics as a method of concentrating CO2 levels for assimilation.  As the 
temperature rise the concentration ratio of [CO2]/[O2] decreases.  CAM 
developed in arid regions of the world as a safeguard against dehydration 
of the plant.

Evolution of photorespiration is interesting.  When [OC2] are high, 
oxygenation reactions are suppressed.  However, if [CO2] fall oxygenation 
occurs.  One hypothesis proposes that under conditions of high light and 
low CO2 (perhaps when stomata are closed due to water stress) 
photorespiration occurs to prevent damage to the photosynthetic pathways 
by actually providing CO2 for the light reaction allowing excess ATP and 
reducing power to dissipate.