Global Patterns in Seagrass Herbivory: Why, Despite Existing Evidence, There Are Solid Arguments in Favor of Latitudinal Gradients in Seagrass Herbivory

The ecological paradigm that biological interactions are more intense in the tropics than in temperate or polar regions has existed since the mid-twentieth century, but several recent meta-analyses have provided scant evidence for latitudinal gradients in the intensity of herbivory. This contradictory evidence led us to carefully review the data and results of several of those papers that failed to find latitudinal gradients in rates of seagrass herbivory. To re-evaluate the arguments around the presence or absence of latitudinal gradients in herbivory in seagrass, we began by expanding the selection criteria to include more studies to compare the published latitudinal range of seagrass occurrences with the latitudes in which seagrass herbivory has been studied. We also compared the latitudinal range of known seagrass herbivores with the distribution of studies on seagrass herbivory. Finally, we investigated studies that provided seasonal data on net primary production and standing stock of seagrasses, which allowed an assessment of the relative amounts of production that could enter the seagrass grazing food web among latitudes and climatic regimes. Consistent with recent meta-analyses, we found little latitudinal effect on grazing rates. However, we argue that the following factors are likely to confound these findings and potentially mask latitudinal trends in seagrass herbivory: (1) the paucity of data available to test latitudinal trends in grazing rates at high latitudes; (2) the mismatch between the geographic distribution of important grazers and studies on seagrass herbivory; (3) the paucity of experimental studies from areas with little or no herbivory because few researchers would initiate a study on something not observed to be occurring; (4) the high level of seasonality in seagrass production in high latitudes, where seagrass production is very low or nonexistent in winter months; (5) the fact that temperate areas with Mediterranean climates behave very differently than temperate areas at similar latitudes with much greater seasonality, thereby making latitude a much less informative independent variable than annual range in temperature; and (6) anthropogenic disturbances, including the overharvesting to functional extinction of large seagrass herbivores in both temperate and tropical regions. Thus, while we currently cannot discount the lack of a latitudinal gradient in grazing intensity, we argue that the intensity of grazing is likely to be greater in the tropics than high-latitude regions where the carrying capacity of seagrass meadows is far less stable. Either way, there are clear gaps in our knowledge and ability to evaluate the role of grazing in seagrass ecosystems and inform future efforts to conserve and restore these extraordinarily valuable ecosystems.

Automated summary

Recent studies have found little latitudinal effect on grazing rates in seagrass ecosystems, but several factors may confound these findings and potentially mask latitudinal trends in seagrass herbivory.