Print PageGlobal biodiversity losses have motivated researchers to elucidate the relationships between species biodiversity and stability. One mechanism offered to explain the link between diversity and stability is the insurance hypothesis, which argues that diverse communities are buffered against changes because individual species have different responses to perturbations. We aimed to test the insurance hypothesis by perturbing microalgal monocultures and polycultures with a rotifer predator, Brachionus sp. We hypothesized that Brachionus would consume small, poorly-defended cells, which would allow for the growth of larger, better-defended cells in polycultures. As such, polycultures would be more productive and stable than monocultures. Our experimental design included three richness treatments (2, 4, and 6 species) plus a monoculture control. Polycultures were assembled by randomly selecting species from two functional pools: small, poorly defended cells (Dunalliella tertiolecta, Nannochloropsis salina, Rhodomonas sp.) and larger, better- defended cells (Chaetoceros decipens, Coscinodiscus sp., Melosira sp.). Each richness treatment consisted of three species combinations, each in triplicate, with the exception of the 6-species treatment that contained all species. All cultures were grown in 20 mL tubes maintained on a rotating mixer at 19°C and a 12:12 L:D cycle. Cultures were sampled repeatedly for cell number, total biovolume and O2 production.
Results/Conclusions
In general, polycultures were more productive (higher community biomass and O2 production); stable (lower temporal CV); and resilient after predator addition than monocultures. The deviation of productivity from expected productivity based on monoculture performance (DT = [OT – ET]/ET, sensu Loreau 1998) showed that polycultures consistently outperformed monocultures, although there were no differences in relative yield between levels of richness beyond one species. Most polycultures recovered after predator addition, whereas monocultures did not, apparently due to the density of preferred prey. In monocultures with high densities of Nannochloropsis or Dunaliella, rotifers exhibited high growth rates, reaching densities of 100s mL-1, and decimated algal populations. In contrast, rotifers grew slowly in polycultures. Although algal biomass biomass decreased after rotifer addition, biomass subsequently increased to pre-rotifer abundances after a few days. We used partitioning methods to separate the effects of selection and complementarily. Our results have important implications not only for understanding and managing ecosystems, but also for designing stable cultures for mass cultivation.
