Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 395

METHOD 19.1.
Natural Abundance of Stable Isotopes of Carbon and Nitrogen in Food Web
and Nutrition Studies

Stable carbon and nitrogen isotopes occur naturally and are useful in the
determination of food sources in food webs (Peterson and Fry, 1987; Pe-
terson, 1999). Nitrogen and carbon isotopes 15 N and 13 C are not ra-
dioactive but are heavier than their more abundant counterparts ( 14 N and
12 C) in the natural environment. These isotopes are useful because they are
fractionated (selected for or against) by physical and biological processes
to some degree, causing slight but often consistent variations in natural
abundance. Natural abundance is the ratio of the trace isotope to the more
abundant isotope. Because these ratios are often very small, natural abun-
dance is commonly expressed as a value relative to some known stan-

dard. The equation used for 15 N is

⁄

14 N sample
15 N standard

15 N sample

15 N

1 1000

⁄

14 N standard

which is simply the 15/14 N ratio in a sample to that of a standard (at-
mospheric N 2 gas). Units are parts per thousand (‰). An equation of the

have been sought, they have been well documented; there are numerous ex-
amples of toxin production in Coleoptera and Hemiptera (Scrimshaw and
Kerfoot, 1987). An example of poor food quality leading to decreased suc-
cess of a predator is the relationship between highly unsaturated fatty acid
content in phytoplankton and zooplankton success. These fatty acids are
almost exclusively synthesized by photosynthetic organisms but are re-
quired for animal growth. Low-phosphorus algae that are high in these
fatty acids are a better food source for Daphnia than high-phosphorus,
low-fatty-acid phytoplankton (Brett and Müller-Navarra, 1997). There-
fore, simple measures of food quality, such as nitrogen and phosphorus
content, may not adequately represent nutritional quality.
Animals exhibit numerous behavioral responses to predation. Behav-
ioral defenses may be quite complex and require sensing of predators. In-
vertebrates may use visual, hydromechanical, or chemical cues to avoid
predators (Dodson et al., 1994). Chemical cues to predators include
kairomones, compounds produced by predators that affect the behavior,
morphology, or life history characteristics of prey species, and alarm chem-
icals, chemicals that are produced by damaged or stressed organisms. Al-
teration of behavior by alarm chemicals is common in fish.
Probably one of the most studied behavioral responses to predation in
aquatic systems is the diel vertical migration of Daphnia and other zoo-
plankton in lakes (Fig. 19.5). These zooplankton enter surface waters at
night when the darkness lowers predation by zooplanktivores that rely on