PREDATION 1 Overview a General sense organisms that harm another living organism by consuming part or all of it A positive negative ecological interaction Positive is to the predator and the negative is to the prey b Curious Observation predator prey cycles e g wasp bean weevil lab Wasp is predator bean weevil is the prey and they cycle They prey increases and peaks then predator increases and peaks etc e g lynx hare natural populations Lynx is predator snowshoe hair is prey Rabbit peaks then lynx peaks Then rabbit decreases then lynx decreases and etc Why this behavior 2 Functional classi cation a True predators carnivores Typically kill and ingest prey Prey are animals b grazers c Parasites d Parasitoids eat modules usually not killing prey White tailed deer are herbivores It is predation pos neg Live on or in host gain nutrition from the host usually doesn t kill host Parasite bene ts and host doesn t Plus minus interaction includes pathogens like HIV Insects that lay eggs inside of other insects Developing parasitoid eats host from inside out Like in the movie Alien 3 Expected Effects of interaction of Predation a On prey population i reduction removal of individuals lowers N ii compensation For within species competition If a predator reduces N the observed r increases Yet lower N results in reduced intraspeci c competition Hence increased robs compensates in part for removal b On predator population i Increase Because competitor converts prey into offspring Predator population increases ii Consumption threshold for reproduction Certain amount of food predator must ingest in order to reproduce Predator increase may not occur if consumption level does not permit reproduction The predator needs to consume a given minimum amount of food before it even begins to reproduce If it doesn t meet that threshold it cannot reproduce iii time lag Example within context of predation white tailed deer grazer Deer munch on snow drop owers the consumption does not immediately mean that they will reproduce Their is an inherent lag between consumption and reproduction iv mutual interference Imagine group of conspeci c predators that increase in pop size On average prey will see them more frequently The prey could change their habits and behaviors to avoid the predators e g eat less frequently hide more As the predator increases prey may become harder to catch 4 Simple predator prey models by Lotka and Volterra N prey population size C predator population size Note only measures predation not competition dN dt rN a CN Prey pop if predator is absent where a proportion of encounters that yields capture between 0 1 dC dt qC f a CN Predator pop if prey is abundant where f number of predator offspring produced per ingested prey q instantaneous per capita mortality rate Next establish state space zero isocline cross them use vector analysis to see what model predicts SIMPLE MODEL SIMPLE MODEL ABOVE dN dt is when prey pop is neither increasing nor decreasing dC dt is when predator pop is neither increasing nor decreasing Predicts oscillatory behavior Note r a is on left because we set dN dt 0 and the same for other Point 1 prey at its intermediate predator at its maximum Model predicts coupled oscillatory behavior But how are they coupled The prey peaks 1 4 cycle before the predator They are offset by 1 4 cycle The amplitude is constant for each but not identical The period is also constant It looks like predator and prey reach equal highest abundance But in nature the predator oscillates at a much lower density of the prey They are still coupled though Can predator convert 100 of the prey it ingests to offspring no it has to convert some energy to maintain its own livelihood Predators never except in very rare examples reach a density higher than the prey Note assume in this model that the predator only ingests 1 type of prey If the system starts at the center of the circle model there will be no oscillation Because dN dt dC dt The amplitude of oscillation is set by the initial population sizes 5 Population Dynamics Rosenzweig and MacArthur model This is same MacArthur for theory of island biography and warbler model SIMPLE MODEL For the third graph imagine really low prey densities an increase in density caused robs to increase positive density dependence the Allee effect Simple Model Imagine the predators are competing amongst themselves The predator can t persist at huge densities if they are competing against each other for nest sites for example So the predator will reach a plateau at that point If heritable phenotype arrises where a predator will make 2 offspring per unit k then k will increase Over evolutionary time we expect what form of natural selection for species that never reach k r selection If they are at k for a long period of time it is k selection It causes carrying capacity to increase The logic for transition to 4th graph is that a point to the left of the isocline may not be shifted to the right of the isocline after the shift Therefore ef ciency of using prey is increased The point is equilibrium and will want to reach it This case is damped oscillations The system will oscillate and eventually the oscillations will stop like spiral Oscillation is like this When it stops oscillating it is at equilibrium If it is thrown off of equilib it will damp oscillation again until it reaches equilib again They intersect at 90 deg Just like simple model You get stable oscillations just like that model Get constant oscillations Imagine further increase in ef ciency of use of prey by predator Intersection will be to left of prey s peak Because predator is really ef cient it drives prey to be lower And lower prey means less predator too THIS IS NOT 90 DEGREES Note that oscillation is bigger This is Stable Limit Cycle Imagine even greater ef ciency amplitude will increase even further The circle overlaps that y axis predator goes extinct if they don t switch prey THEREFORE predator prey relationships are unstable because the predator gets too ef cient But extinction only occurs if they don t switch prey This model makes a prediction that is unrealistic that they won t switch prey Damped Oscillations At point of intersection predator isn t close to its k but prey is close to its k Predator and prey start to oscillate but it stabilizes since prey is close enough to k that it does not rebound as far and eventually the oscillations stop 6 Lab and eld studies a Gause s lab test Paramecium and