Roebuck Bay near Broome (NW Australia) is with itsextensive tidal flats one of the foremost internationallyimportant sites for shorebirds in the Asia-Pacificflyway system. It is home to 150,000 shorebirds (or‘waders’) in the nonbreeding season, which suggeststhat the intertidal flats of the bay have abundantinvertebrate food to offer. To answer the question whyand how so many birds are able to make a living in themud of Roebuck Bay, about a quarter of the intertidalarea was quantitatively sampled for benthic animals inJune 1997 by a team of about 30 volunteers andprofessionals. For a series of 12 successive days atalmost every low tide 2-4 three-person teams walkedparts of the study area. In addition there was a twoperson Hovercraft team working the outlying sites. Tothe best of our knowledge, this is the first detailedmapping of benthic biodiversity on tropical intertidalmudflats. Covering the entire northem shore ofRoebuck Bay, benthos and sediment cores were takenat more than 500 stations laid out in a gridlike fashionwith intersections every 200 m. Each sampling stationyielded a list of invertebrate species, along with theirnumber and sizes, and a value for median gram size ofthe sediment. Most molluscs and many crustaceanswere sorted to species (even though many of thesespecies are presently unknown to science and needformal description). All worms (including thepolychaetes, but also groups such as phoronids,nemertines and sipunculids) were sorted and countedto family level (polychaetes) or at least to the phylumlevel (e.g. phoronids and nemertines). A total of 161taxa were identified from the quantitative samples;another 30 taxa were discovered opportunistically. Theactual number of species within the different habitatsof Roebuck Bay will be much higher, because first ofall, most of the animals were identified to highertaxonomic levels, and secondly, no samples weretaken from rocky habitats and within the mangals. Thetotal density of macrobenthic animals retained on a 1-mm sieve was 1,287 ind./m 2.The polychaete familiesChaetopteridae and Oweniidae dominated the fauna interms of abundance and biomass. The 20 mostabundant taxa made up more than 90% of the totalnumbers. Many rare taxa occurring at less than 2% ofthe stations were found. Polychaetes were most abundant with 70% of the individuals, followed by bivalves(12.5%), crustaceans (8%), brittle stars (4.2%), andgastropods (2.5%). All the remaining taxa amounted toonly 2%. In terms of biomass polychaetes contributeda lower proportion (45.4%). Thefew large sipunculidsmade up 23%, followed by bivalves (16.3%),crustaceans (10.5%), and gastropods (3%). Brittlestars, though high in number, made up only 0.4 % ofthe biomass. Thus, the macrobenthic invertebratefauna is very diverse, a feature quite typical for biotain the Indo-Pacific region. However, only 10% of thetaxa could be confidently assigned a species namewithin the three months of volunteer specialist’s workallotted to this aspect. This large proportion impliesthat we have only scratched the surface of describingtrue biodiversity in Roebuck Bay. Clear verticalzonation pattems of faunal assemblages were notobvious and were probably veiled by other factors.Gastropods and bivalves were characteristic of theupper intertidal areas independent of gram size. Themid- and lower intertidal areas were numericallydominated by polychaetes in the sandier regions, butbivalves prevailed in muddy substrates. At the earliestpossible occasion, in October 1997, the shorebirdswere mapped over the surveyed area. For shorebirdspecies following the tideline, such as the two knotspecies (Red Knot Calidris canutus and Great Knot C.tenuirostris) and the two godwit species (Bar-tailedGodwit Limosa lapponica and Black-tailed Godwit L.limosa), it was rather difficult to find associationsbetween their occurrence and the densities of preferredprey. Presumably these prey behave in ways that makethem more easily detectable near the sea-edge thanelsewhere. Studies on the burying behaviour of threebivalve species demonstrate that behavioural featureslinked to seawater movements may indeed beimportant in determining their availability asshorebird-food. The concentration of speciesspecialized in feeding on large crabs and mudskippers(Eastern Curlew Numenius madagascariensis,Whimbrel Numenius phaeopus, and to a lesser extentGreenshank Tringa nebularia) on the soft and muddysediments in the northeast comer of Roebuck Baycoincided nicely with the distribution of theirpresumed prey. |
