Long-term population dynamics of a coral reef gastropod and responses to disturbance

Abstract

The age-specific density of the red-lipped stromb Strombus luhuanus (Mollusca: Gastropoda) was monitored over 13 years (1981-1993) at four locations on the intertidal reef flat at Heron Island, Great Barrier Reef. Densities were highly variable, but there were persistent, location-specific differences in population density, age structure and adult body size, the latter indicating that the populations were not extensively linked by adult movement. There was relatively high recruitment at most locations in 1984, 1989 and 1993, each occurring approximately 2 years after El NiᯯSouthern Oscillation events, although recruit density during these years varied in both space and time. The studied strombs experienced three disturbance events: (i) experimental harvesting at two locations (1984-1985); (ii) siltation from a harbour dredging operation (1987-1988); and (iii) a severe cyclone (1992). Resilience to harvesting at a local scale (0.5-2 ha) was high: density had recovered within a year, due to immigration of adults and older juveniles. Strombus luhuanus responded much more strongly to broad-scale changes to its environment than to localized harvesting. After dredging, there was a progressive density decline coupled with low recruitment at two locations, and a later decline at a third location, followed by a recruitment-driven rebound after the cyclone. Generalized environmental effects of siltation and the cyclone were also reflected in substantial changes in algal cover. Long-term variations in environmental conditions probably cause high temporal variation over large spatial scales through effects on the survival of larvae or recruits. Localized short-term field monitoring of such species would give a misleading picture of key factors affecting population dynamics.