Diapauses driven by crowding promotes plankton coexistence: An experimental assessment

Background/Question/Methods

Many zooplankton species produce diapauses stages to overcome harsh environmental conditions. Resting stages production is proposed as promoter mechanism of coexistence between competitors under variable environmental conditions. Nevertheless, in zooplankton, crowding cues also inform about future resource limitation. Diapauses production occurs after switching from parthenogenesis to mixis. The usual partial mixis is a bet-hedging strategy that balances parthenogenetic growth and sexual reproduction, to optimize number of resting eggs produced. Questions formulated are: Does induction to sexual reproduction and resting eggs production promote coexistence between zooplankton species? If the investment in switching to sexual reproduction and diapauses production is costly, would exist a trade-off between latencies and parthenogenetic growth of active subpopulations? The aims are: assess if diapauses production change the competitive output between two herbivores competitors, rotifer Brachionus calyciflorus and cladoceran Moina micrura, and assess the existence of trade-off between diapauses and parthenogenetic growth.

Life tables and density trajectories were driven under conspecific densities gradient and resource level as factors, to estimate specific density threshold for induction to mixis. Density and diapauses of isolated and coexistence population, with and without crowding cues, and at resource levels, were daily counted to assess dominance, exclusion rates, number and time for diapauses.

Results/Conclusions

Crowding thresholds inducing sexual reproduction were 55 ind/mL for B. calyciflorus and 3 ind/mL for M. micrura.

Inverse correlation between latencies production and amictic growth rates revealed cost of resting eggs production for B. calyciflorus, while for M. micrura, growth rate was uncorrelated with diapauses production.

There was evidence of competition effect on density of B. calyciflorus at all resource levels, with or without induction cue. For M. micrura competition effect was observed at medium and high resource level.

B. calyciflorus produced earlier and more diapauses when competed for low resource, and progressively delayed and reduced diapauses when competition was at higher resource levels. For M. micrura, competition at different resource levels did not have effect on latencies production neither on time to produce them.

Finally, exclusion rates of competitors were negligible when production of latencies was induced, in other case, M. micrura exclude to B. calyciflorus at low and medium resource level.

In conclusion, B. calyciflorus was more affected by competition than M. micrura. A higher and earlier production of resting eggs, when competition was harder, permits B. calyciflorus to persist despite their costs.

Results show that sexual reproduction and resting eggs production promotes coexistence between zooplankton species.