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Lecture 28: Predation
Reading: None.
Volterra’s Principle Reduction of predators and prey by the same proportion results in faster recovery by prey than predators. Let: C = 100 and N = 100, so CN = 10,000 Given that the growth rates of prey and predators are:
If there were a 50% reduction in both predator and prey numbers, then C = 50 and N = 50, so CN = 2500. Prey birth rate were reduced by 50% and death rates were reduced by 75% while predator birth rates were reduced by 75% and death rate were reduced by 50%, so prey recover faster than predators. Predator limitation of prey does occur in nature. Volterra’s Principle and the importance of predators in limiting prey population growth is shown in the densities of an insect pest on lemon trees, California red scale (Aonidiella aurantii). Monthly application of an indiscriminate pesticide, DDT (dichloro-diphenyl-trichloroethane) eliminated the natural predators of the red scale which permitted rapid recovery by the red scale prey (and the evolution of DDT resistant red scale populations) compared to populations on lemon trees that were not sprayed. The density of red scale insects that causes economic injury in the production of lemons is shown by the dashed line. Note that the trees that were not sprayed maintained red scale populations that were kept in check by natural predators and parasites, such as the parasitoid wasp Aphytis melinus (after Krebs, 1994, p 381, Fig. 18.2). Volterra’s Principle is also shown in the densities of an arthropod herbivore, cyclamen mites (Tarsonemus pallidus), on strawberry plants. Application of another indiscriminate in pesticide, parathion, eliminated the natural predators of cyclamen mites which permitted rapid recovery by this herbivore. In a separate experiment, no pesticide was used, and this permitted predatory Typhlodromus mites to keep the populations of cyclamen mites in check (Ricklefs, 1996, p 449, Fig. 20.2). Importance of Predators Herbivores can have a very significant limiting influence on prey plant biomass. In grasslands, insect and mammalian herbivores may consume 30% - 60% of the total plant biomass production. An exclosure experiment in a California grassland was conducted during a two year period in which voles were excluded from some areas with fencing. Other areas (control plots) were evaluated where voles had free access. Relative biomass was evaluated as the summed height of plants in a 100 cm2 area. Large differences in relative biomass were observed among the annual grasses that are the principle food plants of voles in exclosure and control plots, with much smaller biomass measured in the control plots which were subject to herbivory. Among perennial grasses and herbs that are not food plants for voles, relative biomass was actually greater in the control plots than in the exclosures (Ricklefs, 1996, p 452, Fig. 20.4).
Dingo predation on red kangaroos. Fencing programs to exclude dingo (Canis familiaris dingo, wild dogs) from sheep pasture areas has resulted in 166x increase among red kangaroo (Macropus rufus) in the absence of dingo (after Krebs, 1994, p 277, Fig. 14.11).
Sea lamprey predation on lake trout. Sea lamprey (Petromyzon marinus) is marine fish species that parasitizes other fishes and normally migrates to freshwater streams to spawn. Prior to the construction of the Welland Canal in 1829, sea lamprey were excluded from the North American Great Lakes by Niagara Falls. The first sea lamprey were reported in Lake Superior in 1945 and their predation on lake trout (Salvelinus namaycush) decreased lake trout production to near zero within 20 years (after Krebs, 1994, p 279, Fig. 14.13).
The importance of predation can also be observed in multi-level predator-prey interactions. Interactions between white oak, Quercus alba, insect herbivores on oak, and insectivorous birds were evaluated in a two year study using insecticides and bird exclosure mesh (which permitted insect passage) on oak saplings. Control saplings were neither sprayed with insecticide nor covered with bird exclosure mesh. Insect densities on oak saplings and the percentage of leaf damage increased from the sprayed saplings to the controls, and was greatest on the caged (bird exclosure mesh) saplings. Total plant biomass, leaf biomass, and twig biomass were greatest in the sprayed and smallest in the caged saplings. Birds are therefore very important in controlling herbivorous insect and insects herbivory has an important negative effect on oak sapling biomass (Ricklefs, 1996, p 459, Fig. 20.9). The Lotka-Volterra model makes the assumption that the rate of prey consumption changes linearly depending on the number of prey available (-a’CN). This simple model does not permit predator satiation which does occur in nature. A non-linear response to prey density would be more realistic. The functional response of a predator is the relationship between prey consumption rate and prey density. Three types of predator functional responses. Type I: Linear response as in the Lotka-Volterra model where a constant proportion of prey is consumed independent of absolute prey density. Type II: Predation rate decreases with increased prey density until satiation. Type III: Predation rate lags behind initial prey density increases, response is linear for a small range of prey densities, then predation rate decreases to satiation. Functional responses are shown as numbers of prey consumed and proportion of prey consumed as a function of prey density (Ricklefs, 1996, p 463, Fig. 20.12).
A functional response by predators to prey density is seen in laboratory experiments with damsel fly larvae (Ischnura elegans) feeding on the water flea (Daphnia magna) (after Krebs, 1994, p 280, Fig. 14.14).
Numerical responses by predators to prey are changes in predator population density that result from migration and/or reproduction. Numerical responses to changes in brown lemming (prey) at Barrow, Alaska are seen in the predatory bird populations of pomarine jaeger (Stercorarius pomarinus), snowy owl (Nyctea scandiaca), and short-eared owl (Asio flammeus) (Ricklefs, 1996, p 466, Table 20.1).
Both functional and numerical responses are seen in bay-breasted warblers (Dendroica castanea) in response to spruce budworm (Choristoneura fumiferana) prey in eastern Canada (Krebs, 1994, p 284, Fig. 14.18).
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