A unified meta-ecosystem dynamics model: Integrating herbivore-plant subwebs with the intermittent upwelling hypothesis

Determining the relative influence of biotic and abiotic processes in
structuring communities at local to large spatial scales is best
understood using a biogeographic comparative-experimental approach. Using
this approach, previous work suggests that intertidal community dynamics
(top-down and bottom-up effects) vary unimodally along an upwelling-based
productivity gradient, termed the Intermittent Upwelling Hypothesis (IUH).
Evidence consistent with the IUH comes from the sessile
invertebrate/predator (SIP) subweb in certain rocky intertidal
communities, but whether this pattern extends to macrophyte/herbivore (MH)
subwebs is unknown. Here we ask: Are MH subwebs also structured as
predicted by the IUH? What is the relative importance of herbivory and
predation in structuring these communities? Under what conditions do
ecological subsidies like nutrients or propagule production drive
community dynamics? And are omnivorous interactions important? We
hypothesize that MH subwebs are driven by a new construct, the
Grazing-Weakening Hypothesis (GWH), which states that MH interactions
weaken monotonically with increasing nutrients, with strong (weak)
herbivory and low (high) macrophyte productivity at low (high) nutrients.
We explored local-to-large spatial scale dynamics of both subwebs using a
biogeographic comparative-experimental factorial field experiment testing
joint and separate effects of herbivores and predators between two
continents. Experiments at ten sites ranging across from persistent
upwelling to persistent downwelling regimes ran for 26-29 months in Oregon
and California, and New Zealand South Island. For the MH subweb, results
were consistent with the GWH: herbivory declined and macrophytes increased
with increasing nutrients. As expected, results for the SIP subweb were
consistent with the IUH: predator effect size was unimodally related to
upwelling. Overall, herbivory explained more variation in community
structure than did predation, especially in New Zealand. Omnivory was
weak, sessile invertebrates outcompeted macrophytes, and ocean-driven
subsidies provided the basic template driving ecosystem dynamics. We
propose a unified Meta-Ecosystem Dynamics Model (MEcoDynaMo) combining MH
and SIP results: with increased upwelling, sessile invertebrates and
underlying dynamics vary unimodally (as in the IUH), while herbivory
decreases and macrophytes generally increase. While this model was based
on research in temperate ecosystems varying in upwelling regime, its wider
applicability remains to be tested.