Coexistence of Sea Urchins in Seagreas Meadows: An Experimental Analysis of Competition and Predation

An experimental examination at two sites of the effects of competition and predation on Tripneustes ventricosus and Lytechinus variegatus, the common sea urchins in meadows of the Caribbean seagrass, Thalassia testuinum, suggests that intraspecific competition and predation on recruits influence their population dynamics. Grazing by Tripneustes strongly influenced the biomass of Thalassia. Density—dependent mortality caused the number of caged Tripneustes to converge on naturally occurring densities (about 1 urchin/m²), but Tripneustes mortality was not significantly affected by Lytechinus. Recruitment of Tripneustes was sparse at one site and did not increase when predators were excluded. However, recruitment at a more exposed site was an order of magnitude greater in predator exclusion cages than in an unmanipulated control plot. Tripneustes recruitment was inversely related to the density of adults at this site, but was not significantly affected by Lytechinus. Lytechinus has a more generalized diet than Tripneustes, and had a moderate effect on the amount of Thalassia. Substantial mortality of Lytechinus occurred only at a density 16 times greater than its naturally occurring abundance. Lytechinus mortality was not significantly affected by Tripneustes. Recruitment of Lytechinus at a protected site decreased as the density of Tripneustes increased, but did not change significantly when predators were excluded. However, at a more exposed site predation reduced the density of recruits by an order of magnitude. Tripneustes had a higher growth rate than Lytechinus, and both urchins grew more rapidly at the exposed site. These differences were consistent with the size of the urchins; Tripneustes was larger than Lytechinus and both species were larger at the more exposed site. The growth and reproductive condition of Lytechinus were density dependent and differed between sexes; Lytechinus males grew to a larger size than females, but females had larger gonads. Naturally occurring densities of Tripneustes, which were closed to carrying capacity, were somewhat less variable than those of Lytechinus, although the differences were not significant. Coexistence of these urchins appears to stem from differences in their diets; there was no evidence of significant interspecific competitive effects on mortality, growth, and fecundity. Although predation substantially reduced urchin recruitment at one site, this effect appeared to be superimposed on, rather than necessary for, coexistence.