Predation • Competition (intra- and inter-) involve interactions between same trophic level • Predation involves interactions between trophic levels • One species negatively effects the other • Very important for management goals • Conservation • Control • Harvesting
Types of predation • Herbivory • Animals prey on green plants • Death of plant not usual • Parasitism • Usually don’t kill host • Carnivory • Predator kills and eats prey • Cannibalism • Predator and prey same species
Predator behavior and prey survival • Herbivores gain nutrition from plants • Have to deal with plant defenses • chemical and physical • Must also avoid predators • Protective shapes and coloration • Active defense • Alertness and swiftness • Protective cover • morphology • Predators must catch prey • Speed, agility, claws, teeth, smell, vision, etc
Predation • Many different techniques • Normally involves large investment of time and effort per unit food • Usually target physically weakened or in a vulnerable place • Typically 4.5 to 10.8 % capture success • e.g. 124 moose, 7% success rate by wolves
Predation • Often select prey by oddity • Color • Behavior • Location
Predation • Conspicuousness and crypsis
Predation • Conspicuousness and crypsis • Prey change behavior in absence of predators
Predation • Conspicuousness and crypsis • Prey change behavior in absence of predators • Marginal habitat
Effect of predators on prey density • Predator numbers also correlated with prey numbers • Based on correlations • Are predators keeping prey below food supply, or killing only those that are malnourished? • Need to understand the behavior of predators
The behavior of predators How do predators respond to: • Changes in prey density? • Changes in predator density? • Differences in the degree of clumping of prey?
Response to changes in prey density Response of predators to prey depends on: • Feeding behavior of individual predators • Functional response • Response of the predator population through reproduction, immigration and emigration • Numerical response
Functional response • Searches randomly for its prey • Has an unlimited appetite • Spends a constant amount of time searching • Then the number of prey found will increase in direct proportion to prey density • Type I response Percent of Prey Population Eaten Number of Prey Eaten Per Predator Prey Density Prey Density
Functional Response • Assumptions are usually unrealistic • Some predators show an approximation of a type 1 function response • Reindeer feeding on lichens • No animal has an unlimited appetite. • Constant search time unlikely • Handling time – more prey eaten per unit time, more time is taken up with handling and less for searching.
Number of Prey Eaten Per Predator Prey Density Functional response Na = (aTtN) / (1 + ahN) • Type II response • Number eaten per time increases to an asymptote as prey density increases Percent of Prey Population Eaten Prey Density
Percent of Prey Population Eaten Prey Density Functional response • Type III response • Prey switching Number of Prey Eaten Per Predator Prey Density
Functional response • Success at catching depends on prey density • Predators react to individuals of own species by dispersing • Territoriality • Eviction • Interference • Interference reduces the searching efficiency of the predator as predator density increases • Interference stabilizes predator numbers
Functional response • Prey live in small patches of high density and low density in between • Clumped distribution • Predators concentrate on areas of high density • Predators thereby have a regulating effect on prey numbers
Numerical response • Trend of predator numbers against prey numbers • Predators increase as prey density increases • Due to • Increased rate of predator reproduction when prey are abundant • Numerical response • Attraction of predators to prey aggregations • Aggregational response
Numerical response • Reproduction and mortality rate of predators depends on predation rate • More prey, more energy • Predator numbers increase to an asymptote determined by interference
Total response • Total number eaten = number eaten by one predator multiplied by number of predators • Can be plotted as Total response curves • Need to also incorporate recruitment rate of prey • Different types of curves indicate different types of relationships • Predators regulate prey population • Prey regulated by intraspecific competition for food • Multiple stable states
Behavior of prey • How does the behavior of the prey influence predation? • Migration • Herding and spacing • Birth synchrony
Migration • If a prey species can migrate beyond the range of its predators the population can escape predator regulation • Predators have slow growing young and are restricted to a small area to breed • Ungulates have precocial young that can move within a few hours • Thus predators follow food resource, predators cannot.
Herding and spacing • Animals reduce risk of predation by forming groups • Group size predicted to increase with increasing predator densities • Can also leave group when most vulnerable • Leave group when give birth as predators are concentrated around herds
Birth synchrony • Synchronize births to reduce predation rate • Predator swamping • Also influenced by seasonal availability of resources • Unlikely predation is cause of synchrony, but likely intensifies it.
Conservation and management • Predator and prey populations usually coexist • Prey at low density by regulation • Prey at high density by intraspecific competition for food with predation depensatory • Both systems can operate in one area • Type III functional response or density-dependent numerical response • Disturbance moves from one state to other • Explains outbreaks of pest species, decline of hunted species
Conservation and management • Prey population can become extinct • Type II response with no prey refuge, no alternative prey • Important in management where there are habitat changes • Which situation occurs depends on • Ability of predator to catch prey • Ability of prey to escape predation • Reproduction