Predation on meiofaunal and macrofaunal invertebrates by western sandpipers (Calidris mauri): evidence for dual foraging modes

Western sandpiper (Calidris mauri) predation was examined by concurrent experiments and direct observations of foraging behaviour on high intertidal mudflats of the Fraser River estuary, British Columbia. Western sandpipers foraged by either “pecking” on the surface (64% of observational time) or probing into sediment (29%). The first experiment (probe-mark method) consisted of collecting small-volume cores (21.2 cm³) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min feeding period. The second experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collection of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal and meiofaunal size fractions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control sediments revealed significant differences, attributed to shorebird predation, for both experiments. The probe-mark experiment detected the removal of large infaunal polychaetes (∼ 20 mm), while the exclosure experiment showed depletion of epifaunal harpacticoid copepods (0.063–0.5 mm). Predation on macrofaunal cumaceans was detected in both experiments. Invertebrates selected by western sandpipers neither fell within traditional infaunal size classifications (macro- vs. meiofauna; 500 μm delineation) nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western sandpipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal size ranges and (2) selective probing for larger infauna, such as polychaetes. These findings were facilitated by the combination of methodologies employed. Received: 29 December 1999 / Accepted: 11 September 2000     

Experimental evaluation of aggregation behavior in the sea urchin Strongylocentrotus droebachiensis

Defensive aggregation by Strongylocentrotus droebachiensis has been invoked as the fulcrum for the transformation of macroalgal beds into coralline barrens in the northwest Atlantic. We critically examined some of the mechanisms contributing to aggregation behavior by experimentally manipulating sea urchins, purported predators and food during autumn 1983 and spring 1984 both in the laboratory and field. We utilized several approaches to examine a range of sea urchin responses to the presence of food, tethered predators, caged predators, crushed con-specifics in the field and predators in laboratory tanks. Some of the field cages had the property of allowing free passage to sea urchins while retaining lobsters; this allowed distinctions to be made between artifacts caused by cage walls or topographic barriers and unrestricted behavioral responses of sea urchins. The results falsified the hypothesis that aggregations of S. droebachiensis are elicited by predators. Except in the presence of algae, sea urchins always avoided decapod predators (but not sea stars) and fled from the vicinity of injured conspecifics. However, avoidance behavior was subordinate to feeding responses, demonstrated by mass migrations of sea urchins into cages (with lobsters) to feed on algae. We reaffirmed by observation and manipulation previous studies which showed that sea urchins aggregate only in the presence of food. Two types of sea urchin groupings were delineated: (1) surficial two dimensional associations, often caused by topographic or other features which inhibited dispersal and (2) cohesive three-dimensional aggregations induced by food.     

Biofilm grazing in a higher vertebrate: the western sandpiper, Calidris mauri

We show that a higher vertebrate can graze surficial intertidal biofilm, previously only considered a food source for rasping invertebrates and a few specialized fish. Using evidence from video recordings, stomach contents, and stable isotopes, we describe for the first time the grazing behavior of Western Sandpipers (Calidris mauri) and estimate that biofilm accounts for 45-59% of their total diet or 50% of their daily energy budget. Our finding of shorebirds as herbivores extends the trophic range of shorebirds to primary consumers and potential competitors with grazing invertebrates. Also, given individual grazing rates estimated at seven times body mass per day and flock sizes into the tens of thousands, biofilm-feeding shorebirds could have major impacts on sediment dynamics. We stress the importance of the physical and biological processes maintaining biofilm to shorebird and intertidal conservation.     

Latitudinal clines in food distribution correlate with differential migration in the Western Sandpiper

We report that a latitudinal cline in intertidal food distribution is associated with the nonbreeding distribution of the Western Sandpiper (Calidris mauri). This novel result is the first to demonstrate a clear relationship between patterns of differential nonbreeding distribution and food availability for any shorebird species. Within each age class and sex, longer-billed Western Sandpipers winter further south. Moreover, females, the longer-billed sex, tend to winter south of males. Thus, both inter- and intra-sexual clines in bill morphology result in an overall gradient of increasing bill length from north to south. Longer-billed birds are able to extract prey that are buried more deeply in the sediment; therefore, we predicted shifts in the vertical distribution of food resources to coincide with the clines in bill morphology across the nonbreeding range. We tested our prediction by measuring biofilm density and the vertical distribution of macrofaunal invertebrates at six nonbreeding sites. Although no latitudinal trend was observed for biofilm, the vertical distribution of invertebrates was consistent with our prediction and revealed that the greatest relative abundance of surface prey occurred at northern nonbreeding sites and declined with decreasing latitude. We discuss the potential implications of these findings in the context of competing evolutionary hypotheses of differential migration and bill dimorphism in shorebirds.     

Fate of sprayed formulated aminocarb in freshwater

Following simulated aerial spraying of Matacil^® in field and tank experiments, the concentration of aminocarb increased in the subsurface water for some hours: 22 h in the case of the field and 72 h for the tank. Thereafter, the concentration of Matacil^® declined exponentially in subsurface waters. The equation for this, A = 4.834 + 0.165 g ? 0.155T, can be used to predict the concentration of aminocarb in the lentic environment (A, μgL^?1), providing the application rate (g ha^?1) and time after application (T in h) are known.