Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 399

Carnivorous plants are notable sit-and-wait predators on insects. Most
species tolerate or require saturated soils and are wetland species (Juniper
et al., 1989). Passive trapping strategies are used, which include the use of
adhesive traps such as in sun dew (Drosera), chambers that can be entered
but not left as in the pitcher plants (e.g., Sarracenia ), snap traps such as
the Venus flytrap (Dionaea), and triggered chambers as in the bladderworts
(Utriculara). These plants also need to attract insects, and adaptations in-
clude visual stimuli such as UV patterns visible to insects. Olfactory sub-
stances can be produced, including those with nectar scent or the smell of
putrefaction, and nectar rewards may be used to lure insects. Tactile stim-
uli are also important in some cases, such as that of Utricularia, which has
filamentous extensions that mimic filamentous algae and attract epiphyte
feeders; when the invertebrate contacts the extensions, the chamber fills
rapidly with water and sucks in the prey (Juniper et al., 1989). Carnivo-
rous plants are generally found in low-nutrient environments and are pho-
tosynthetic, so they use their prey as a source of nitrogen and phosphorus.
Carnivorous plants are an excellent example of convergent evolution lead-
ing to solution of a problem (nutrient limitation) from divergent plant lin-
eages using a wide variety of capture and attraction mechanisms.
Sensing prey can be accomplished by a variety of adaptations, de-
pending on the organisms and their prey. Invertebrates use visual (in a few
organisms with well-developed eyes), mechanical, tactile, and chemical
cues (Peckarsky, 1982). In an ingenious demonstration of the importance
of the use of mechanical cues, Peckarsky and Wilcox (1989) recorded hy-
drodynamic pressure wave patterns associated with escaping Baetis nymphs.
Predatory stonefly nymphs (Kogotus modestus) attacked Baetis models in
greater frequency when the wave patterns were played back than when
they were not. Fish may sense prey visually, chemically, electrically, or hy-
drodynamically. Electrical sensory systems in fish are highly developed; the
paddlefish (Polyodon spathula) can sense the electrical activity of a swarm
of Daphnia 5 cm away (Russell et al., 1999).
The idea that evolution through selection leads to maximization of net
energy gained per unit time feeding led to optimal foraging ecology (Pyke
et al., 1977; Schoener, 1987). This simple concept can be applied to all
phases of predation (encounter, detection, attack, capture, and ingestion).
I have already discussed the relative merits of sitting and waiting for prey
as opposed to active searching. Food quality, quantity, and spatial distrib-
ution are often additional considerations for optimal foraging. Optimal
foraging has been well established for several fish species (Mittelbach and
Osenberg, 1994). If a food item is high quality but very rare relative to a
lower quality food source, it may not be preferred. All items may be taken
when food is limiting, but only the most profitable may be taken when
more is available. For example, bluegill will take all sizes of zooplankton
in equal amounts when they are at low density but will take large Daph-
nia preferentially at higher zooplankton concentrations (Fig. 19.6).
Predictions can also be made regarding how long a predator will re-
main in a patch of food. If a patch is of low quality, then moving to an-
other patch may be more beneficial. However, if the cost of moving to an-
other patch is high, it can be beneficial to extract more food from the
current patch. Obviously, temporal and spatial scale are important consid-
erations when making predictions using optimal foraging models.