The relationship of macrobenthic prey avaliability to shorebird use of blue-green algal flats in the upper Laguna Madre

Abstract

Shorebirds were censused and macrobenthic invertebrates sampled biweekly on two blue-green algal flats in the upper Laguna Madre from October 1991-October 1993. Twenty-two species of shorebirds were observed. Abundances were greatest from November to March. Bonferroni's inequality was used to determine microhabitat guilds. They were: Open water--tactile-feeding shorebirds; Wet--visual+tactile foragers; and, Damp-Wet--Piping, Snowy, and Semipalmated plovers (visual foragers). Proportions of benthic density and biomass were compared to proportions of birds in each microhabitat. Intertidal microhabitats were used significantly $(P<0.05)$ less than expected, and wet and damp microhabitats were used significantly $(P<0.05)$ more than expected. Wet and damp microhabitats may have been at or near carrying capacity or prey depletion may have occurred. Over 50 taxa of macrobenthic invertebrates were identified. Polychaetes, tanaids, and dipteran larvae were most frequently encountered. Macrobenthic invertebrate abundance was variable but was lowest from July-October. Abundance was usually greatest in surface (0-5 cm) section, but biomass was often greater in deep (5-10 cm) sections. Densities of aquatic-adapted organisms (i.e. polychaetes) increased as elevation decreased, while dominance, if not abundance, of semi-terrestrial organisms (i.e. fly larvae) increased as elevation increased. Relationships between shorebird density and macroinvertebrate density and/or biomass were determined using linear regression on log$sb{10}$(x + 1) transformed data. The best predictive models were for species, by site and season. Overall interpretation of models suggested that shorebird density was determined by sufficient tidal flat area estuary-wide, with enough inundation on particular sites to maintain a well-developed macrobenthic community including both ephemeral, aquatic organisms and persistent, semi-terrestrial organisms. An exclosure experiment using two exclosure treatments was conducted to determine effects of predation on the macrobenthic invertebrate community. One exclosure excluded all predators (shorebirds, fish, and crabs), and one excluded only shorebirds. Prey depletion due to shorebird predation was detected in three instances. Most effects detected were due to changes in the inundation-exposure regime, which resulted in decreased invertebrates within exclosures when compared to areas open to predation. Overall productivity and suitability of tidal flat habitats for shorebirds was closely tied to the inundation regime of the flat. Predicting which habitats were critical for shorebirds has little to do with the presence or absence of birds. Tidal flats with well-developed macrobenthic communities were likely to be used by shorebirds.