Predator-Prey Relationships BIOL September 2015

Evidence Predators Can Regulate Prey Abundance  Achieved via controlled prey-transplant or predator-removal experiments  Also strongly suggested by introduction of new, exotic predators

Fig. 5.9 p. 73  Small mussels eliminated by crabs and starfish in Lough Ine, but waves and salinity limit predators on open coast  Large mussels disappeared in SE Lough, where they do not occur due to large crabs

Fig p. 74

Fig p. 200

Modelling Predator-Prey Interactions

Elton’s Oscillations (1924, 1942)  Apparent effect of prey density on predator density in pelt data Ups and downs in lynx seemed to come just after ups and downs of their primary prey, snowshoe hares, on a 9-10 year cycle Ups and downs in lynx seemed to come just after ups and downs of their primary prey, snowshoe hares, on a 9-10 year cycle Ups and downs in prey base of hares are probably also a part of this cycle Ups and downs in prey base of hares are probably also a part of this cycle

Fig p. 203

HANDOUT—Lynx and Hare Cycles

Fig p. 191  Assumptions of the model: Single predator species/single prey species Single predator species/single prey species Simple relationship of prey density to predation rate (i.e., predator density) Simple relationship of prey density to predation rate (i.e., predator density) Predator reproductive rate is proportional to prey density Predator reproductive rate is proportional to prey density

Figs a & p. 201

Laboratory Attempts to Generate Predator-Prey Oscillations

Fig. 11.7a p. 195 Gause 1934

Fig. 11.7b p. 195 Gause 1934

Fig. 11.7c p. 195 Gause 1934

Huffaker’s Mites and Oranges Experiments  Eotetranychus, a mite that feeds on oranges  Typhlodromus, a mite that feeds on Eotetranychus  Former disperses with threads of silk, latter only disperses overland

Predator and Prey on Single Orange  Extinction of prey  Starvation and extinction of predator

Fig p. 195 Huffaker 1958

Multiple Oranges Adjacent to One Another  Prey populations grew to per orange  Prey extinct in days  Starvation and extinction of predator

Multiple Oranges, Widely Dispersed  Prey populations grew to per orange  Prey extinct in 36 days  Starvation and extinction of predator

Vaseline Barriers, Oranges Dispersed  Four oscillations generated over 14 months

Fig p. 196

Why it is Generally Not That Simple in Nature  It's a food web, not a food chain  Prey may have refugia, and be less prone to predation at low densities  Predators may have search images that switch as prey become more abundant or less abundant  Other environmental factors may influence prey or predator density (e.g., salinity and starfish/crabs)  Predator and prey constantly are selected by one another in a co-evolutionary “arms race”

HANDOUT—Stenseth et al. 1997

Predator Responses to Prey Density

Fig p. 202

Numerical Response  Refers to both… …increases in predator N via reproduction …increases in predator N via reproduction …aggregation of predators in prey-rich areas …aggregation of predators in prey-rich areas

HANDOUT—Bowman et al. 2006

Functional Response  Change in per-capita rate of prey consumption Type I—constant increase in per-capita rate of consumption as prey density increases Type I—constant increase in per-capita rate of consumption as prey density increases Type II—predator satiation at high prey densities plus the effect of handling time Type II—predator satiation at high prey densities plus the effect of handling time Type III—satiation/handling time effect at high prey densities, and, at low prey densities, refugium saturation plus prey-switching behavior Type III—satiation/handling time effect at high prey densities, and, at low prey densities, refugium saturation plus prey-switching behavior

Fig p. 200

Fig p. 201

HANDOUT—Brown et al. 2010

Predator-Prey Model Incorporating a Functional Response

Panel a—Prey regulated near K prey

Panel b—Prey regulated near K prey or at very low density (B is unstable point)

Panel c—Prey regulated well below K prey

Panel d—Prey is driven to extinction

Indirect Effects and Predation

 An effect expressed upon a species, A, via an interaction between species B and C  B, by preying on C, may benefit A Exs: Keystone predators that limit strong competitors Exs: Keystone predators that limit strong competitors

Fig p. 392 Paine 1974

Fig p. 413

Fig p. 189  Left: Competition between two predators  Right: Apparent competition If H 1 increases, P 1 increases, H 2 decreases, and P 2 decreases Last change not necessarily due to competition between predators

Schmitt (1987)  Experiments with snails, clams, and their major predators A lobster, an octopus, and a whelkA lobster, an octopus, and a whelk  Adding either prey caused aggregative numerical response of predators, leading to reduced density of other prey  “Apparent competition” between snails and clams