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

Abstract

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 10 sites ranging from persistent upwelling to persistent downwelling regimes ran for 26–29 months in Oregon and California, and New Zealand (NZ) 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 NZ. 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 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.

Testing effects of bottom‐up factors, grazing, and competition on New Zealand rocky intertidal algal communities

“…For example, grazing appeared to have the strongest effect at Raramai on the east coast, particularly on sheet algae, with the greatest effect occurring in cleared treatments. This result is consistent with the study by Menge et al (2023) in which macrophyte-herbivore interactions weakened with increasing nutrient inputs. In low-nutrient environments, strong herbivory and low macrophyte productivity occurred and vice versa in high-nutrient environments.…”

“…We believe that future studies allowing longer succession time to further delineate interactions among organisms are warranted. However, based on trends observed in the experiments in the 5th and 6th months and long-term (~20 years) observations of high limpet density and large limpet size (Menge et al, 2023), we suggest that in uncleared plots at Raramai, grazers would have kept macroalgae sparse in +herbivore treatments, while sheet algae would likely persist in herbivore treatments, although winter storms likely would reduce their cover (Figure 6). Final trends in cleared Raramai plots suggested similar long-term grazing outcomes.…”

“…A strong effect of light, or in other words desiccation stress, would likely have been suppressing the abundance of crustose algae in both cleared and uncleared plots (Figure 6). Thus, at Raramai under reference conditions, grazers (mostly large limpets) would likely maintain a system with much bare space and little macroalgae for some intermediate successional period until slowly colonizing barnacles and mussels gradually became relatively abundant (Menge et al, 2003(Menge et al, , 2023.…”

“…High algal productivity is often associated with higher grazing activity (Menge et al., 1999) or greater average biomass of grazers (Bustamante, Branch, & Eekhout, 1995; Bustamante, Branch, Eekhout, Robertson, et al., 1995). Grazing activity has been documented to positively affect producer community structure in highly productive (e.g., high nutrient) environments (Hillebrand et al., 2007; Menge et al., 2023; Sellers et al., 2020). In such environments where rare species can now use new resources, herbivores can attenuate competitive effects by reducing competitively dominant species and promoting producer coexistence.…”

“…In such environments where rare species can now use new resources, herbivores can attenuate competitive effects by reducing competitively dominant species and promoting producer coexistence. However, grazing mostly negatively affects producers in unproductive (e.g., nutrient poor) environments (Hillebrand et al., 2007, Menge et al., 2023, Sellers et al., 2020). In low productivity environments, herbivores can negatively impact diversity by unselective removal of species or targeted removal of rare species.…”