Shorebirds depend on intertidal food resources, and evaluating how strongly birds' survival is related to the harvestible prey levels has been a major research theme in Europe. But it may be difficult to extrapolate from these northern cold-winter studies to Southern Hemisphere mild-summer conditions, or to the tropics, where benthic diversity, size, and productivity may differ greatly from other regions. Few investigations in Australasia have looked at prey selection by migratory shorebirds, or how tightly linked birds' distributions are with that of the benthos. Recent benthic mapping exercises in Roebuck Bay and Eighty Mile Beach in Northwest Australia have shown the feasibility of intensive large-scale surveys (see references below), and we recently made a similar, somewhat less intensive, survey of the 10,000 hectare tidal flats of Farewell Spit.
Using volunteers from the Nelson-Marlborough Institute of Technology Conservation Ranger course and the Ornithological Society of New Zealand, we sampled around 200 sites over a 1 km x 0.5 km grid, at distances up to around 7 km offshore (Figure 1). At each site we took three benthos core samples, a sediment sample, and made an estimate of the eelgrass Zostera muelleri coverage. All invertebrates (12,839) were identified to an appropriate taxonomic level and measured.
Figure 1. Sampling grid superimposed over the Farewell Spit tidal flats.
For each species or taxonomic group we generated density maps, density and size statistic summaries, and plots of size and density distributions. An example, the cockle Austrovenus stutchburyi is shown in Figures 2 and 3 (note that only small cockles are shown on the map).
Figure 2. Size and density distribution of cockles on Farewell Spit.
Figure 3. Distribution of cockles 1–10 mm on the Farewell Spit tidal flats.
The data were also used to look for underlying environmental relationships. Zostera cover turned out to be an important influence on the benthic communities. Diversity increased with Zostera cover (Figure 4), and multivariate analyses showed that where taxa varied significantly with Zostera cover, in almost all cases they increased as Zostera cover became denser. Only a small number of taxa were associated with 'clean' sandy sediments (including the pipi Paphies australis, a good food for knots).
Figure 4. Diversity versus Zostera surface cover.
By selecting taxa and size-classes relevant as prey for different bird species, we also mapped the distribution of potential food across the flats. For knots (Figure 5) the highest densities of suitable food (in this case small pipi) are on the extreme spring low-water mark on the Southwest part of the central tidal flats. These resources will only be available on some tides, meaning that knots must usually forage higher on the flats, where Zostera beds present different prey harvesting issues.
Figure 5. Spatial distribution of potential prey for red knots. Points represent the sampling grid on the tidal flats, and the size of the points is proportional to the density of suitable prey items (numbers per m2)
Battley, P.F., D.S. Melville, R. Schuckard & P.F. Ballance. 2005. Quantitative Survey of the Intertidal Benthos of Farewell Spit, Golden Bay. Marine Biodiversity Biosecurity Report. No. 7. 119 pp. See executive summary. This report will become available for downloading from the Ministry of Fisheries website in due course.
Pepping, M., T. Piersma, G. Pearson, and M. Lavaleye. 1999. Intertidal sediments and benthic animals of Roebuck Bay, Western Australia. NIOZ-report 1999-3, Netherlands Institute for Sea Research, Texel.
de Goeij, P., M. Lavaleye, G. B. Pearson, and T. Piersma. 2003. Seasonal changes in the macro-zoobenthos of a tropical mudflat. NIOZ-report 2003-4, Netherlands Institute for Sea Research, Texel.
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