A small-scale test of the species-energy hypothesis in a southern California estuary

Theoretical predictions of a direct positive association between productivity and diversity (P/D) have received considerable empirical support in marine systems, although this relationship tends to break down at smaller scales (< 25 km). Another theory commonly advanced in terrestrial systems, the species-energy hypothesis (SEH), relates productivity (= energy) to diversity indirectly through an increase in population abundances, which then reduces the likelihood of localized extinction and therefore enhances richness. The SEH has been supported on both global and local scales but has not been experimentally tested in estuarine systems or at the scale of a patch. We tested whether the predictions of the SEH (increased abundance and diversity) or the P/D hypothesis (increased diversity, no predictions for abundance) described the relationship between macroalgal productivity and the abundance and diversity of infauna and site-attached epifauna (hereafter macrofauna) and the microphytobenthos (MPB) in an estuarine benthic community. We conducted a single-factor experiment comparing the effects over time of three treatments comprised of two levels of macroalgae as an added energy source and a plastic mimic as a control that provided structure and habitat but no energy. Contrary to predictions of both the SEH and the P/D hypothesis, there were no measurable differences in taxonomic richness among treatments. However, in partial support of the SEH, we found that total macrofaunal abundance was 50% higher in both macroalgal treatments compared to the mimic and these were largely comprised of detritivores such as spionid polychaetes and oligochaetes. Phaeopigments increased in macroalgal plots but not the mimic, suggesting that added productivity decomposed, supplying more food for detritivores. As this taxonomic group is present in all plots, although in differing abundances, increasing energy flow through this pathway did not enhance taxonomic richness. Macroalgae stimulated the MPB (measured as chl a) more than did the mimic, and elevated chl b in macroalgal treatments indicated the increase was due to sporeling green macroalgae. However, benthic diatoms and dinoflagellates (estimated from chl c) were not affected by treatment. Thus, we found that at the patch level, neither the SEH nor the P/D hypothesis predicted patterns of diversity for estuarine macrofauna; rather, in this system, increased productivity simply supported higher abundances within the same taxonomic groups that occurred at all productivity levels.