BIEB 102 Final Exam

Consumption

• predation: consumption of one organism by another

• predators: kill prey immediately & consume prey item (consume many prey over lifetime, some predators also scavenge)

• scavenging predators: combine predation + scavenging

Food webs and trophic position

• trophic position: number of links btw species & primary producers

• food web: map of feeding links

• trophic levels/guilds: groups of species at same trophic position (consume similar resources)

How can predators impart non-lethal effects on prey?

• defense can be expensive to develop & maintain

• avoiding predation can result in lost opportunities for growth & reproduction

• ex: moose hanging near roads when there’s lots of bears

Predator Regulation (Top-Down Control)

• predators regulate pops of prey

• pop growth rate comparable to that of prey (ex: whales can’t respond to rapid change of krill bc large differences in generation time)

• high dispersal ability (prey easy to find bc they can’t hide)

• ability to switch resources based on availability (dietary specialists may crash once prey depleted)

• ex: predatory mites control herbivorous mites (pesticides remove predatory mites

Donor Control (Bottom-Up Control)

• some predators don’t regulate populations of prey → instead, prey abundance affects predators

• predators only have access to small part of prey population (prey has refuge to hide)

• predators pops controlled by something other than prey availability (ex: nests, intra-specific aggression)

Cyclical population behavior

caused by time delay between predator’s numerical response + changes in prey pop size

Empirical examples of predator-prey cycles

• Hudson Bay Company observing trends in pelts between good and bad years (not due to variation in trapping)

• Lynx and hares in Arctic

• CB Huffaker explored herbivory & predatory mites (prey survival could be prolonged by dispersing oranges randomly + increase of spatial complexity of environment; barriers impeded predator movement + wooden pegs erected so prey could move to new oranges)

Gause experiment

using Paramecium & Didinium cultures to illustrate importance of prey refuges & cycles (no refuge, prey refuge, restocking of predator-prey cycles)

When do predator-prey cycles occur?

• predators have strong regulating impact on prey

• prey allows predator pop to grow fast (predator pop responds quickly by reproduction to increasing numbers of prey)

When do predator-prey cycles not occur?

• prey have high-quality refuge from predators

• prey can employ defenses that lower predator growth (ex: algae aggregate into colonies to avoid predation)

Isocline

• condition in which the pop size of predator/prey doesn’t change in size for given number of prey/predators

• dashed, horizontal line reps number of predators needed to keep prey pop from changing

• dashed, vertical line reps number of prey needed to keep predator pop from changing

Joint equilibrium

point where two isoclines cross (superimposes predator & prey isoclines to follow elliptical counterclockwise path)

What influences changes to theoretical models?

• initial numbers of predators & prey changes amplitude of cycles

• the higher population turnover, faster the system oscillates

• if initial number of predators & prey at joint equilibrium, pops stay there

• if initial conditions too extreme, something crashes

What stabilizes predator-prey cycles?

• reduced time delays in predator’s response to changes in prey abundance (reduces prey crash bc predators don’t allow prey pop to get so high to begin with)

• prey phenotypic plasticity (as predator pop increases, prey becomes harder to catch + less nutritious; reduces growth of predator pop)

Functional Response curves

• Type I: straight line unrealistic (no predator satiation, no time lost handling prey, no consideration of animal behavior)

• Type II: initially like Type I w/ decelerating predation rate at hig prey density (predator satiation at high prey density)

• Type III: accelerating predation at low prey density, declerating at high density (prey switching at low density, predator satiation at high density, prey has limited number of refuges)

What do predators do for ecosystems?

• no mountain lions → lots of deer → windy rivers

• mountain lions → few deer → limited erosion → straight rivers

• no predators → lots of large cottonwood trees but no young trees to replace them

What does predation rate depend on?

• predator density

• prey density (challenging to find if few prey)

• attack rate of predator (predators not always hungry + successful)

Prey population growth equation

• exponential growth - # lost to predator (rN - aNP)

• aN = functional response (rate of prey capture by individual predator as function of prey abundance → losses to predators proport to NP; predators find prey at random)

Predator population growth equation

• aeNP - dP

• e increase w/ value of individual prey items (nutritious prey converted into new predators more efficiently)

• aeN = numerical response (predators produced for every prey available)

Predator equilibrium

• P_equil = r/a

• when growth rate of prey pop high (r), more predators needed to keep prey pop from growing

• when predators feed at high rates (a),  fewer of them needed to keep prey pop from growing

Prey equilibrium

• N_equil  = d/ae

• w/ increasing predator death rate (d), more prey needed to keep predator pop from declining

• w/ greate conversion efficiency (e) or predator attack rate (a), fewer prey needed to keep predator pop at equil

Numerical response

• add. predators added to pop for every prey available

• individual predators increase consumption of prey only up to satiation

• continued response to increasing prey density can be achieved only thru increased size of predator pop

• immigration: mobile predators can track prey over lage areas (ex: Bay-breasted Warblers follow outbreaks of spruce budworm)

• reproduction: for less mobile species, numerical response results from local pop growth

• numerical response of predator lags behind pop growth (cycling around joint equilibrium; prey increase = predators scarce while prey decrease = predators plentiful)