Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 396

same form is used for carbon. A mass spectrometer is usually used to de-
termine the isotopic ratios.
Typically, 15 N is the first choice for determining food habits. There is
generally a 3-5‰ increase in

15 N for each trophic transfer, allowing res-
olution of the feeding levels (Fig. 19.3). In addition, different food sources
(e.g., terrestrial vegetation versus algal material) often differ significantly in
their signature natural abundance.
The use of 13 C ratios is more difficult. There is not consistent fractiona-
tion of 13 C across food webs and the fractionation is not as great (Fig. 19.3).
However, 13 C ratios can often be used to resolve food web differences where
there is overlap in the 15 N data. Use of natural abundance of both isotopes
simultaneously can provide data that laboratory experiments cannot.
There are numerous examples of the utility of isotope analyses; a few
are noted here. Littoral fishes and crayfish in Canadian Shield lakes are de-
pendent on terrestrial vegetation (France, 1996, 1997b). Habitat usage
varies and food preferences at different life history stages changed in fishes
in a subtropical lake (Fry et al., 1999). It was demonstrated that ocean-
derived nitrogen is transported into streams and lakes by spawning salmon
(Kline et al., 1990; Finney et al., 2000). Nitrogen fluxes in stream food
webs have been quantified (Hall et al., 1998; Peterson et al., 1997). Also,
it has been established that bacteria are an important carbon source in
stream food webs (Hall and Meyer, 1998).

vision to find prey. At daybreak they swim down to darker parts of the
lake to avoid the visual feeders. In some cases, larger Daphnia that would
be more susceptible to predation move deeper than smaller individuals (De
Meester et al., 1995; Hutchinson, 1967). Chemical cues excreted by preda-
tors or released by predators during feeding may be required to trigger the
migration response (Folt and Burns, 1999; Tollrian and Dodson, 1999).
A chemically mediated behavioral defense of Daphnia to predation by
fish involves predator-avoidance behavior triggered by alarm chemicals re-
leased when other Daphnia are eaten. In this case, Daphnia swim down-
ward, form aggregate swarms, or increase avoidance behavior after expo-
sure to water with crushed Daphnia (Pijanowska, 1997). Studies have
demonstrated that these behaviors lower predation loss to bream (Abramis
brama). Similar defenses have been documented for the related cladoceran
Ceriodaphnia reticulata in response to chemicals produced by green sun-
fish, Lepomis cyanellus (Seely and Lutnesky, 1998). Chemicals released by
fish can also increase sensitivity of Daphnia to mechanical signals arising
from movement of predators (Brewer et al., 1999).
Hydromechanical cues can be an important component of predator
avoidance (Peckarsky and Penton, 1989). These pressure waves are trans-
mitted rapidly at the small spatial scales at which insect larvae and smaller
organisms operate. Although use of hydromechanical cues may be wide-
spread, much less is known about these than chemical cues (Dodson et al.,
1994).