waders and their estuarine food supplies - Vlaams Instituut voor de ...

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PREY PROFITABILITY AND INTAKE RATE
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10 50 100 500 1000
prey weighl (mg)
Kin. 7. Handling time as a function of prey weighl (mg AFDW > in
earthworms tKns unpubl.). Nereis iBoale- isiaid I9S9.
Em el ul. 1996a, Hulscher unpubl.I. Tipula iHloinen & Zvvans unpubl..
Ens unpubl.I and Arenicola (liunskoeke 1988 & unpubl).
The handling limes of ihe four species differ sigmlic.mil> according
to a covariance analysis (R 2 =0.918 for prev weight and R s =0.025
lor ihe species i.
10 50 100 500 1000
prey weight (mg)
The large scalier in the profitability of armoured
prey could largely be explained by the species concerned
and the technique used to open them. An analysis
of covariance revealed lhat the 11 prey species
alone explained 39.1 '< of the variance, prey weight
15.1'.. whereas 24.11', could be allnhntcd K) both. lb
visualize the differences in profitability between
species, independently of prey weighl. we regressed
profitability againsi prey weight for the armoured and
soft-bodied prey, without distinguishing among the
species, and calculated for each species the average deviation
Irom ihe regression line. Fig. 9 is based upon
these calculations and shows ihe average profitability
per species, standardized lo a prey weighl of 200 mg.
Four types ol prey can be distinguished. First, soli
bodied prey taken from the surface had a profitability
of 30-35 mg s'. Second, if soft-bodied prey were extracted
from the substrate, their profitability was reduced
to about half, as indicated for Leathcr|,ickcts,
bul not shown as a separate category in Fig. 9. Third,
the average profitability of bivalves living buried in the
substrate was 10-15 mg s '. but was about half this
value when prey had to be taken from a great depth.
Fourth, hard-shelled prey found just beneath, or from
the surface, were the least profitable prey (4-8 mg s ').
Fig. 8. Profitability tmg s ' handling) as a t unction of prey weight (mg AFDW). given separate I v lot soft-bodied prev (same data as lig 7) and
armoured prey im addition to the dala given in I-igs. 2-1. also Anadara (Swennen 1990), Lillorina (Boaies St Gott-Custard 1992), I'alclla
iS.itnel 1967) and Ilea (Ens etal. 1993 & unpubl). The regression lines are shown separately for sofl-bodied and armoured prey. A multiple
iion analysis revealed that non-parallel regression lines did not explain more variance than parallel ones iR = 0.51s gad 0314. respective!
v i Hence a common exponent (0.421) may be used with different intercepts: 0.788 for armoured prev and 1.490 for soft-bodied prey.
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