Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 401

Type I
Type II
Type III
Density of prey population
FIGURE 19.7 Holling's three types of functional response curves and the proportion of prey
consumed assuming constant predator numbers.
Several strategies are used to consume prey. Many predators need to
consume prey whole. These are referred to as gape limited predators, and
the size of prey they can consume is limited by the size of their mouth or
gape (Zaret, 1980). The dominant vertebrate predators in freshwaters are
mostly of this type. Other strategies include piercing or sucking prey and
removing bites or chunks of prey. Such predators include copepods, preda-
tory cladocera, some midge larvae, and other insect larvae. Parasites are a
special class of predators that have a wide variety of ways to attack prey.
Generally, gape limited predators are highly dependent on the size of their
prey, whereas others may be dependent to various degrees.
NONLETHAL EFFECTS OF PREDATION
The previous discussion assumes that the predator will kill its prey;
however, other effects can occur, including injury, restriction in habitat use
or foraging behavior, and changes in life history (Allan, 1995). Potential
examples have been described elsewhere in the topic and include herbi-
vores that consume part of a macrophyte, the scale-eating Cichlids of
Africa, restricted foraging time in the presence of a predator, and most
cases of parasitism. Also, earlier in this chapter I described how the pres-
ence of crayfish could lead to delayed reproduction of snails so they more
quickly attain a size too large to be consumed. I also described a variety
of behavioral responses that alter behavior of prey. Most cases of predator
avoidance will be classified as nonlethal effects of predation because of the
associated energetic cost.
An example of sublethal effects and how they can alter a lake food
web was documented by Hill and Lodge (1995). Omnivorous crayfish (Or-
conectes) were exposed to largemouth bass (Micropterus salmoides) that
were too small to eat them. These bass were apparently perceived as a pre-
dation risk because crayfish survival and their feeding on macrophytes and