ways, since the subcomponents may not all scale with size in the same way
(Thompson,1975 ; Kislalioglu & Gibson,1976 ;Hewett,1980 ;Pastorok,1981 ;
Eggleston,1990 ; Tripet & Perrin,1994 ; Perssonet al., 1998 ;Aljetlawi,Sparrevick&
Leonardsson, 2004 ). The interaction of these costs and benefits will yield a size-
dependent profit function, which should, according to foraging theory, drive
diet choice (Stephens & Krebs,1986). Thus, size is embedded in the relationships
that determine the feeding adaptations selected for in individuals and this, in
turn, shapes the fundamental trophic niche: i.e. the range of prey types a species
is equipped to consume.
Evidence for size-dependent foraging comes mostly from systems that con-
tain single predator and prey species, where size can be varied independently of
other things. Such studies in aquatic systems support the idea that both attack
rate and handling time are related to predator–prey size differences (Thompson,
1975 ; Kislalioglu & Gibson,1976; Hewett, 1980 ; Scott & Murdoch,1983; Spitze,
1985 ; Eggleston, 1990 ; Woodward & Hildrew,2002c; Tripet & Perrin,1994;
Aljetlawiet al., 2004), although the exact nature of the relationships varies.
Handling time tends to decrease as predators become larger relative to their
prey (see references above and Fig.6.1). Attack rate has been suggested to follow
a unimodal relationship (Perssonet al., 1998; Persson & De Roos, this volume),
although experimental results have yielded increasing, decreasing and hump-
backed forms (see references above). Some of this variation is because individual
prey species encompass only limited size ranges, so experiments sample differ-
ent parts of the profitability curve, and some is probably due to fundamental(^0123)
50
100
150
Handling time (s)
r^2 = 0.77, p < 0.001
(^0123)
0.5
1
1.5
2
log 10 mg predator body-mass log 10 mg predator body-mass
log
10
mg prey biomass consumed 24 h
–1
r^2 = 0.85, p < 0.001
Figure 6.1(a) Handling time (i.e. from prey capture to complete ingestion) forCordulegaster boltonii
as a function of predator body mass. Prey¼oneNemurella pictetii(head-capsule width
¼1.0 mm). (b) Maximum daily consumption ofN.pictetiibyC.boltoniias a function of
predator body mass.
BODY SIZE AND PREDATORY INTERACTIONS IN FRESHWATERS 101