Dynamics of coastal meta-ecosystems: The intermittent upwelling hypothesis and a test in rocky intertidal regions

Background/Question/Methods

Meta-ecosystems are defined as ecosystems that are linked by flows of propagules and materials, and offer a conceptual context for global-scale models of ecosystem dynamics. The intermittent upwelling hypothesis (IUH) predicts that the strength of ecological subsidies, organismal growth responses, and species interactions will vary unimodally along a gradient of upwelling from persistent downwelling to persistent upwelling, with maximal levels at an intermediate or “intermittent” state of upwelling.  To test this model, I employed the comparative-experimental method to investigate these processes at 24 wave-exposed rocky intertidal sites in Oregon, California, and New Zealand spread across a downwelling-upwelling gradient.  

Results/Conclusions

As predicted by the IUH, abundance of phytoplankton, and rates of recruitment and colonization of prey (ecological subsidies) were unimodally related to upwelling, as were rates of competition, predation, the barnacle colonization rate, and effects of predation during the colonization phase and at the end of field experiments.  Facilitation rate also showed a unimodal relationship to upwelling, but only marginally so.  The unimodal relationships explained ~20 to 45% of the variance in the various processes.  Regressions among the ecological subsidies and species interactions were used to determine potential linkages that underpinned the unimodal patterns.  Abundance of phytoplankton was associated with increases in rates of barnacle recruitment, intensity of competition, facilitation, predation, and with predation effects, and rates of barnacle and mussel recruitment were associated with increases in colonization (barnacles only) and species interactions.  Strong positive effects on interactions were also seen for rates of colonization, competition, facilitation, and predation.  Many responses were non-linear, suggestive of threshold or saturation effects.  These results suggest the IUH has geographic generality, and are also consistent with earlier arguments that bottom-up effects and propagule subsidies are strongly linked to the dynamics of higher trophic levels, or top-down effects, as well as to non-trophic interactions.  The ~55% to 80% of the variance not explained by upwelling is likely due to more regional to local influences on the processes examined, and future efforts should focus on incorporating such effects into the IUH.