Following on from my last post on how sea-urchins alter how much they eat in response to nutrients and CO2, here Owen Burnell describes his latest paper (in as many months!) that shows why Amphibolis antarctica and other morphologically similar species of seagrass may be so susceptible to grazing.
Sea urchins are important marine invertebrates, which in many parts of the world can shape sub-tidal habitats via their grazing. In South Australian seagrass meadows the short-spined sea urchin Amblypneustes pallidus generally occurs in low densities, however, population increases of the species have recently been documented by researchers at The University of Adelaide.
It was observed that the grazing activity of these urchins was impacting seagrass meadows, in particular the species Amphibolis antarctica, when compared with adjacent Posidonia spp. By manipulating urchin density to measure seagrass loss and then simulating urchin grazing to study seagrass recovery, we found that while urchins grazed equally upon both seagrass species, Posidonia sinuosa recovered much faster from simulated grazing than Amphibolis antarctica. It appears the different morphology of these two seagrass species, in particular the meristem location (or centre of growth) of Amphibolis spp., which is elevated within the canopy and thus exposed to grazers, is likely to be the cause of these asymmetric grazing impacts. In essence, if the urchins eat the meristem that seagrasses grow from they don’t recover as quickly!
While sea-urchins are by no means a rampant force destroying local seagrass meadows, population expansions such as these are important to document, particularly if they have deep seated connections with changing trophic interactions or urchin fecundity. In many marine systems worldwide population expansion of macro-grazers such as urchins can be linked to over-exploitation of their predators (e.g. fish or crustaceans, or before their protection, sea otters!) or changes to temperature that affect their reproduction and metabolism.
The persistence of seagrass meadows reflects variation in factors that influence their productivity and consumption. Sea urchins (Amblypneustes pallidus) can over-graze seagrass (Amphibolis antarctica) to create sparse meadows in South Australia, but this effect is not observed in adjacent Posidonia sinuosa meadows despite greater densities of inhabiting urchins. To test the effect of urchin grazing on seagrass biomass, we elevated the density of urchins in meadows of A. antarctica and P. sinuosa and quantified seagrass decline. Urchins removed similar amounts of biomass from both seagrass species, but the loss of leaf meristems was 11-times greater in A. antarctica than P. sinuosa. In a second experiment to assess the recovery of seagrass, we simulated urchin grazing by clipping seagrass to mimic impacts measured in the first experiment, as well as completely removing all above ground biomass in one treatment. Following simulated grazing, P. sinuosa showed a rapid trajectory toward recovery, while A. antarctica meadows continued to decline relative to control treatments. While both A. antarctica and P. sinuosa were susceptible to heavy grazing loss, consumption of the exposed meristems of A. antarctica appears to reduce its capacity to recover, which may increase its vulnerability to long-term habitat phase-shifts and associated cascading ecosystem changes.