SEASONAL-VARIATIONS IN INTERTIDAL DENSITY OF INVERTEBRATE PREY IN A TROPICAL LAGOON AND EFFECTS OF SHOREBIRD PREDATION

We analyzed the relationship between the abundance of shorebirds and that of their intertidal invertebrate prey, and attempted to determine if shorebird predation significantly affects prey density. The study was conducted at three sites in the Chacopata Lagoon complex in northeastern Venezuela between January 1985 and September 1986, using shorebird census data, monthly sampling of intertidal invertebrates, and exclosure experiments. The invertebrates collected were divided into three groups: (i) polychaetes (less than or equal to 50 mm in length), (ii) small bivalves (2-3 mm in length), and (iii) other species. The overall density of shorebirds foraging in the Chacopata Lagoon complex was very high, exceeding the densities reported for most other staging and overwintering areas by a wide margin. The abundance of shorebirds in the study area was related primarily to the density of polychaetes, their main prey. At two of the study sites, the arrival of fall migrants followed a significant increase in polychaete numbers. The increase in shorebird numbers in autumn was negligible at the third site, where poly chaete density was lower than at the other sites. The impact of shorebird predation on their invertebrate prey varied seasonally, as did the extent of the foraging area available to them. The exclosure experiment revealed significant differences in the density of polychaetes inside and outside the exclosures only during, or shortly after, the fall migration period. The variability in the conclusions drawn from other studies examining prey depletion by shorebirds in tropical environments may be explained by differences in the length of sampling periods and seasonal variations in the numbers of foraging shorebirds, the energy demands of moult, fat deposition, and the intertidal surface area available for feeding. The impact of seasonal variables such as these should be taken into consideration when designing studies to measure prey depletion by shorebirds; such objectives may necessitate long-term studies.