Latitudinal gradient, MEND experiment, and BioGen experiment relating species richness and net primary productivity (NPP)

Experiments often find that net primary productivity (NPP) increases with species richness when native species are considered.  However, relationships may be altered by exotic (non-nattive) species, which are hypothesized to reduce richness but increase productivity (invasion-diversity-productivity' paradox).  We compared richness-NPP relationships using a comparison of exotic vs. native-dominated sites across the central USA, and two epxeriments under common enviornments.  ANPP was measured using peak biomasss clipping in all three studies.  In all studies, there was a signfiicantly positive relationship between NPP and richness across native-species doimnated sites and plots, but no relatoinship across exotic-dominated ones.  The results indicatte that relationships between NPP and richness depend on whether natieve or exotic species are dominant, and that exotic species are 'breaking the rules'.