WLF 448: Fish & Wildlife Population Ecology

Fall 2004

Population Dynamics

IV.  Predation

Important questions for predator-prey systems

  1. What is the numerical response of the predator to varying levels of prey density?

  2. What is the functional response of the predator to varying levels of prey density?

  1. What is the numerical response of the prey to varying levels of predator density?

  2. Is predation limiting prey population?

  3. Is predation regulating prey populations

Density independent processes

Additive vs. Compensatory mortality


A.  Theoretical Models of Predator-Prey Systems

  1. Prey grows exponentially in absence of predator

  2. Predator declines exponentially in absence of prey

  3. Solutions of differential equations results in an unstable equilibrium point and neutral limit cycles dependent on starting conditions









  1. Logistic growth of prey stabilizes system and results in a stable equilibrium point

  2. Logistic growth of predators (Leslie 1948) results in a stable equilibrium point where predator equilibrium densities are dependent on prey abundance.








  1. Because predators can only handle a finite number of prey, the prey death rate should be a nonlinear function of prey density (e.g., Holling's Type I, II, and III functional responses)

  2. Incorporating predator functional response results in a hyperbolic prey isocline

  3. Vertical predator isocline implies that the rate of increase of the predator population is controlled completely by prey density

  4. Stability is dependent on which side of the hump the predator isocline intercepts the prey isocline











  1. Tanner (1975) looked at the stability of predator-prey models using a logistic predator model combined with a prey model incorporating Holling's type 2 predator response.
  2. Arditi and Ginzburg (1989) assumed predation rate (i.e., functional response) depends on the ratio of predators to prey resulting in a predator isocline that slopes upward instead of vertical as the Rosenzweig-MacArthur model
  3. Sinclair's (1989) predator pit







B.  Role of Predators in Limiting Prey Populations

Red kangaroo populations and dingos in Australia (from Krebs 2001)

Paul Errington (1963) studied muskrats and suggested that while mink were the primary cause of death in muskrats, mink were only removing surplus muskrats that were doomed to die of other causes (i.e., compensatory mortality).


The Effectiveness of Removing Predators to Protect Bird Populations

Isabelle M. Cote; William J. Sutherland

Conservation Biology, Vol. 11, No. 2. (Apr., 1997), pp. 395-405.


The control of predators for nature conservation purposes is becoming an increasingly important issue. The growing populations of predator species in some areas and the introduction of predators in other areas have led to concerns about their impact on vulnerable bird species and to the implementation of predator control in some cases. This is set against a background of increasingly fragmented semi-natural habitats and declining populations for many species. To assess the efficiency of predator removal as a conservation measure, the results of 20 published studies of predator removal programs were meta-analyzed. Removing predators had a large, positive effect on hatching success of the target bird species, with removal areas showing higher hatching success, on average, than 75% of the control areas. Similarly, predator removal increased significantly post-breeding population sizes (i.e. autumn densities) of the target bird species. The effect of predator removal on breeding population sizes was not significant, however, with studies differing widely in their reported effects. We conclude that predator removal often fulfills the goal of game management, which is to enhance harvestable post-breeding populations, but that it is much less consistent in achieving the usual aim of conservation managers, which is to maintain and, where appropriate, increase bird breeding population size. This may be due to inherent characteristics of avian population regulation, but also to ineffective predator removal and inadequate subsequent monitoring of the prey populations


Gadomski, Dena M. and Judy A. Hall-Griswold.  1992.  Predation by northern squawfish on live and dead juvenile chinook salmon.  Transactions of the American Fisheries Society. 121(5):680-685.
Abstract: Northern squawfish Ptychocheilus oregonensis is a major predator of juvenile salmonids Oncorhynchus spp. migrating downstream through the Columbia River. High predation rates occur just below dams. If northern squawfish selectively consume salmonids killed or injured during dam passage, previous estimates of predation mortality may be too high. We conducted laboratory experiments that indicate northern squawfish prefer dead juvenile chinook salmon O. tshawytscha over live individuals. When equal numbers of dead and live chinook salmon were offered to northern squawfish maintained on a natural photoperiod (15 h light:9 h darkness), significantly more (P < 0.05) dead than live fish were consumed, both in 1,400-L circular tanks and in an 11,300-L raceway (62% and 79% of prey consumed were dead, respectively). When dead and live juvenile chinook salmon were provided in proportions more similar to those below dams (20% dead, 80% live), northern squawfish still selected for dead prey (36% of fish consumed were dead). In additional experiments, northern squawfish were offered a proportion of 20% dead juvenile chinook salmon during 4-h periods of either light or darkness. The predators were much more selective for dead chinook salmon during bright light (88% of fish consumed were dead) than during darkness (31% were dead).

Beamesderfer, Raymond C., Bruce E. Rieman, Lewis J. Bledsoe, and Steven Vigg.  1990.  Management implications of a model of predation by a resident fish on juvenile salmonids migrating through a Columbia River reservoir.  North American Journal of Fisheries Management. 10(3):290-304.
Abstract: We constructed a model of predation by northern squawfish Ptychocheilus oregonensis on juvenile salmonids migrating through John Day Reservoir. The model predicts salmonid survival as a function of number and distribution of northern squawfish, number and timing of juvenile salmonids entering the reservoir, salmonid residence time, water temperature, and flow. The model predicted survival similar to independent estimates for 1983-1986 and also approximated differences among areas and months. Uncertainty analyses showed that the number of salmonids surviving predation may vary 5% with normal annual variation in predator number, temperature, and flow. Survival in 1983-1986 was near the average predicted from 30 years of historic environmental data. Sensitivity analyses implied that the best avenues of reducing predation are to reduce the number of northern squawfish, pass salmonids earlier in the year, and maintain sizes of runs of juvenile salmonids at or above present levels. Survival of salmonids, as simulated by the model, is weakly affected by changes in predator distribution, changes in predator consumption rate near the upstream dam, residence time, or flow.


Large mammals of Africa:  Sinclair and Arcese (1995) suggested large predators (i.e., lions, leopards, cheetahs, wild dogs, and spotted hyenas) seemed to have little effect on their large mammal prey abundance.  Likely due to migratory prey and resident predators

Wolves and Moose:  Boutin (1992) looked at the results of 5 experiments, 3 showed and increase in calf survival but only 1 showed an increase in the moose population.  The combined effect of grizzly bears, black bears, and wolves may be more important.

Wolves and Caribou:  Boutin (1992) suggested that improper experimental design has contributed to ongoing controversy about the effect of wolf predation on caribou population size.

Skogland, T.  1991.  What are the effects of predators on large ungulate populations?  Oikos 61:401-441.  Several cases of limitation of prey by predators were found.

C.  Role of Predators in Regulating Prey Populations

  1. Evolutionary changes in predators and prey may act to stabilize otherwise unstable systems

  2. Notable exceptions include snowshoe hare-lynx cycles, and lemming-weasel cycles


  1. Generalist predators tend to stabilize prey numbers

  2. Specialist predators tend to cause instability in prey numbers

  1. Predator effects - cyclic populations are an inherent result of many predator-prey models

  2. Physical effects - cycles reflect the response of birth and death rates to external physical factors (e.g., periodic climate)

  3. Pathogen effects - simple models of disease transmission can generate cycles in host (prey) and pathogen (predator) populations

  4. Plant effects - one hypothesis is that plants are the prey and herbivores are the predators resulting in cyclic populations following predator-prey models; another hypothesis involves nutrient cycling; yet another involves induced resistance of plants to herbivory

  5. Genetic effects - Chitty (1957, 1967) argued natural selection favors low fecundity and/or survival when populations are dense and the opposite when populations are sparse



Clepper, H. (ed.). 1979. Predator-prey systems in fisheries management. Sport Fishing Institute, Washington, D.C. 504pp.

Holling, C. S. 1959. The components of predation as revealed by a study of small mammal predation of the European pine sawfly. Can. Entom. 91:293-320.

Holling, C. S. 1959. Some characteristics of simple types of predation and parasitism. Can. Entom. 91:385-398.

Hornocker, M. G. 1970. An analysis of mountain lion predation upon mule deer and elk in the Idaho Primitive Area. Wildl. Manage. No. 21. 39pp.

Ivlev, V. S. 1961. Experimental ecology of the feeding of fishes. Yale Univ. Press, New Haven.

May, R. M. 1981. Models for two interacting populations. Pp. 78-104 In May, R. M. (ed.). Theoretical ecology. Second edition. Sinauer Assoc., Sunderland, Mass. 489pp.

Mech, L. D. 1966. The wolves of Isle Royal. Fauna Natl. Parks U.S. Fauna Series 7. 210pp.

Mech, L. D. 1970. The wolf. Natural History Press, Garden City, N.Y. 384pp.

Murdock, W. W., and A. Oaten. 1975. Predation and population stability. Advances in Ecol. Res. 9:1-131.

Pyke, G. H., H. R. Pulliam, and E. L. Charmov. 1977. Optimal foraging: a selective review of theory and tests. Quant. Rev. Biol. 52:137-154.

Royama, T. 1971. A comparative study of models for predation and parasitism. Res. Pop. Ecol. Kyoto Suppl. 1:1-91.

Tanner, J. T. 1975. The stability and the intrinsic growth rates of prey and predator populations. Ecology 56:855-867.

Tinbergen, L. 1960. The natural control of insects in pine woods. 1. Factors influencing the intensity of predation by songbirds. Arch. Neerl. Zool. 13:265-343.

Ware, D. M. 1972. Predation by rainbow trout (Salmo gairdneri): the influence of hunger, prey density, and prey size. J. Fish. Res. Board Can. 29:1193-1201.

Werner, E. E. 1974. The fish size, prey size, handling time relation in several sunfishes and some implications. J. Fish Res. Board Can. 31:1531-1536.


Revised: 30 November 2004