Sea urchin herbivory: evidence for long-lasting effects in subtropical seagrass meadows

Abstract

A 4-month enclosure experiment, using high (40/m 2) sea urchin densities ( Lytechinus variegatus) (Lamarck), examined the effects of urchin grazing during winter-spring in St. Joseph Bay, Florida. This was done to test a prediction generated in earlier grazing experiments that repeated sea urchin cropping during fall and winter would likely produce permanently unvegetated plots. Three treatments were replicated in this experiment: (1) continuous grazing; (2) intermittent grazing of one week/month; and (3) no grazing (control). In addition, predation potential on various sized urchins was assessed using tethering experiments, and urchin residence times were estimated by direct observation. Sea urchin grazing dramatically affected seagrass habitat structure for long periods of time (> 3.5 yr). Intermittent grazing produced significant reductions in aboveground plant biomass compared to controls, while continuous grazing produced apparently permanent loss of sea-grasses. The winter start of the experiments was important in producing the large grazing effects observed because low productivity during winter increases the probability that plants can exhaust stored rhizome reserves. Published productivity values of seagrasses and urchin feeding rates, combined with our field densities of resident urchins, suggest that most aboveground production at our study site is consumed by urchins. Tethering experiments showed that small urchins (< 30 mm test diameter) were consumed in significantly greater numbers than large urchins (31–60 mm test diameter) and that significantly more urchins were consumed on sand than in grass. We hypothesize that urchins and seagrasses can coexist because newly recruited urchins are consumed at high rates when seagrass density is lowered by large amounts of urchin grazing, which subsequently leads to lower urchin densities. This eventually allows seagrasses to recover to high densities, which results in increased urchin survival. This, in turn, leads to high rates of grazing and, once again, reduced seagrass density.