Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 394

herbivores, and detritivores. Others, such as predatory fish, can be zoo-
planktivores as fry and piscivores as adults. It can be difficult to determine
the trophic position of organisms given omnivory and technical difficulties
associated with determination of ingestion and assimilation.
Gut contents are often used to determine what organisms eat, but this
method can be misleading because it does not necessarily describe what or-
ganisms assimilate. For example, trout in some streams can have consid-
erable amounts of filamentous algae (e.g., Cladophora ) in their guts. They
are not able to digest this material. They simply ingest it when feeding on
invertebrates that live in the algae; thus, they ingest but do not assimilate
the algae. Feeding and growth experiments in the laboratory are one way
to assess the use of various food categories. Analysis of natural abundance
of stable isotopes, primarily the stable isotopes (not radioactive) of carbon
and nitrogen, is being used increasingly to determine trophic position of
animals and what food sources primarily supply food webs (Method 19.1).
ADAPTATION TO PREDATION PRESSURE
Defenses against predation include mechanical, chemical, life history,
and behavioral protections. Chemical protection includes both toxic or un-
palatable chemicals and low nutritional quality. Mechanical protection in-
cludes size (either too large or too small to be eaten) and protective spines
or projections. Behavioral responses to predation include avoidance and es-
cape behaviors.
One mechanical defense against predation is for the prey to be too
large for the predator to effectively consume. In Chapter 18, I discussed
how large colonial algae dominate the plankton of lakes with significant
zooplankton populations. In a benthic example involving both size and al-
teration of life history, a snail, Physella virgata, increases its growth rate
and delays reproduction in the presence of chemical cues from the preda-
tory crayfish Orconectes virilis . This allows the snail to reach a size at
which the crayfish can no longer effectively prey upon it. In the absence of
this predator, the snail reproduces at a smaller size (Crowl and Covich,
1990). Thus, evolution favors growth and deferred reproduction in the
presence of the predator. Fish also increase body size to limit predation, as
demonstrated by the crucian carp (Carassius carassius) in response to the
presence of predators (Brönmark et al., 1999).
Spiny fin rays of fish, hard shells of mollusks, gelatinous sheaths of al-
gae, and cases of caddis fly larvae and other aquatic insects are examples
of mechanical defense. Growth of spines in Daphnia can serve as a me-
chanical defense, increasing their effective size. Several invasive species of
zooplankton in the United States, such as Bythotrephes cederstroemi (the
spiny water flea) and Daphnia lumholtzi, have very long spines (East et al.,
1999; Fig. 19.4). This may allow them to resist predation and could be
partially responsible for their successful invasion of lakes that do not con-
tain zooplankton species with such long spines.
Chemical protection from predation includes toxic chemicals and poor
nutritional quality. Chemical defenses are probably common in freshwater
organisms, particularly those that are prey to fish. Where such defenses