Factors Affecting Rate of Food Consumption

Factors Affecting Rate of Food Consumption

Factors Affecting Rate of Food Consumption Prey density (availabilty) Attack rate Handling time Learning, switching, microhabitat selection

Functional Response: a framework for understanding how these factors affect rate of food consumption

Type I Functional Response Number of prey consumed depends just on and x (prey density) Predator s perspective: # of prey eaten is proportional to # of prey in the environment. Prey s perspective: per capita mortality of prey (m = prob of death/min) is simply m= P, where P = # of predators. m does not depend on x

What sorts of factors might affect, attack rate? Search speed (how quickly the predator can cover an area) Time spent searching Prey detectability Predator sensory radius Patterns of active preference/avoidance by predators

Mobile predators searching for prey: the importance of the sensory field Larger sensory radius will find more prey

Examples of Type I Functional Filter feeder Response Stoneroller feeding on algae Type I functional responses only work when there is minimal handling time.

Type II Functional Response Number of prey consumed depends on prey density, predator attack rate, and handling time Predator s perspective: feeding rate increases with increased prey density, but the increase in less than proportional Prey perspective: probability of mortality declines with increased prey density. M is proportional to the slope of the line connecting the origin with f(x) [m= p/(1+ hx)]

What is Handling Time? Capture time: time from initiation of attack to completion of the feeding process and resumption of search behavior

Gut clearance time: time to vacate gut and make room for more food.

Example of a Type II Functional Response Weasels eating rodents Brown trout drift feeding on inverts

Type III Functional Response Number of prey consumed depends on prey density, predator attack rate, and handling time. The effect of prey density is non-linear (x e )

Predator s perspective: feeding rate increases with increased prey density. At low density, the increase is more than proportional (predator s become more efficient), but at higher prey density the increase is less than proportional (predators become less efficient)

Prey perspective: At low prey density, the probability of mortality increases with increasing prey density. At higher prey density, the probability of mortality declines with increasing prey density.

Why might predators become better at feeding on prey as prey density increases? More opportunities for learning Search images Increased capture efficiency as predators learn prey escape strategies

Limited refuges for prey As prey become more abundant, they overflow the refuges available and become more vulnerable to predators Switching behavior Predators avoid foraging in habitats with very few prey They feed on prey that is most available

Giant Water Bugs Feeding on Mayfli 100 90 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 120 Percent of Mayf lies in Environment

Factors affecting the sorts of things predators eat: Understanding diet preference and selectivity

1. Visibility Reaction distance, and radius of search area, depend on prey size, prey transparency, and cryptic coloring Ex of small body size as a defense: Small bodied zooplankton. Large zoops are more vulnerable to fish predators than small zoops simply because they can be seen from further away. Ex of transparency: silverside minnow

Ex of cryptic coloration: countershading, colorshifts, camouflage

2. Toxins and palatability (chemical defense) Many inverts sequester or make secondary compounds which make them toxic or unpalatable to predators Chemical defenses are most effective if they re advertised (bright coloring, aposematic coloration) Ex: aposmatic, or warning coloration (water mites) Chem. Defenses: whirly gig beetles and bullfrog tadpoles.

3. Refuge use Prey detect predators and move to refugia, where they are less detectable Ex. Juvenile bluegills forage in open water w/o bass. Forage in thick cover w/ bass Shiners avoid pools with trout Snails on tops of rocks w/o fish, under rocks w/ fish. Top of rocks w/ crayfish.

4. Evasive Behaviors: Some prey are good at avoiding capture once detected Ex. Deer, schooled fish leaping from water

5. Active selectivity: Sometimes predators choose not to pursue prey (active prey selection, as opposed to passive patterns outlined above).

6. Morphological defenses Spines on advanced teleosts fish Deep bodies Spines on porcupines