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Food hoarding in animals
TREE vol. 5, no. 12, December
22 Mazia, D. (1984) Exp. Cell Res. 153, 1-15 23 Birky, C.W. (1978) Annu. Rev. Genet. 12,471-512 24 Felsenstein, J. (1974) Genetics 78, 737-756 25 Williams, G.C. (1975) Sex and Evolution. Princeton Universitv Press 26 Maynard Smith, J. (1978) ihe Evolution of Sex, Cambridge University
Press 27 Hamilton, W.D. (1982) in Population Biology of Infectious Diseases (Dahlem Konferenzen) (Anderson, R.M. and May, R.M., eds), pp. 269-296, Springer-Verlag 28 Bell, G. (1982) The Masterpiece of Nature: The Evolution and Genetics of Sexuality, University of California Press 29 Hickey, D.A. and Rose, MR. (1988) in
7990
The Evolution of Sex (Michod, R.E. and Levin, B.R., eds), pp. 161-175, Sinauer Associates 30 Bernstein, H., Byerly, H.C., Hopf, F.A. and Michod, R.E. (1985) Science 229, 1277-1281 31 Marin, G. and Argenton, F. (1990) fxp.
Cell Res. 187, l-3 32 Johnson, K.A. and Rosenbaum, J.L. (1990) Gel/62,615-619
Lettersto the Editor A New Role for Predation The role of predation in the population dynamics of vertebrates has been a very controversial subject. In the 195Os’, many game biologists denied that it had any importance, while an implicit idea of recent environmentalists has been that predation is responsible for ‘the balance of nature’. Mathematical ecologists have instead looked at population cycles in terms of Lotka-Volterra equations with predation as one possible driving variable*. Newsome’s recent article3 fits well into the pattern of more recent thinking, in which predation is considered to interact with other environmental variables. However, the author proposes that control by predators only operates within ‘a limited range of population densities’; this seems difficult to reconcile with the role he assumes for predation in cyclic populations. The emphasis should probably be changed to predator-prey ratios. In faintly fluctuating high-density prey, with diverse and seasonally fairly constant food, a reduction in prey food or prey numbers (e.g. by drought) will increase predator-prey ratios and lead to control by pred-
predator-prey equilibrium3. There appear, thus, to be good reasons to examine more closely the role of predation in many overcompensating ecological systems. Recent findings in such systems3 appear to unravel certain persistent enigmas in vertebrate dynamics.
ators as long as alternative prey types are available3. In cyclic populations, prey relying on a seasonally strongly varying food supply will face food shortage at winter peak numbers4f5. Specialist predator populations will by then have increased considerably due to a strong numerical response6, and the nutritionally or socially6 depressed prey growth rate will lead to another type of control by predators, lasting for a short time until the very few alternative prey types are also devoured. The latter scenario will apply to folivorous prey, such as hares4 and microtines5, as well as to granivorous rodents, for which there occurs an over-compensating mortality one and a half years after irregular mast seeding7. A high diversity of alternative prey may lead to a more or less continuous control (or rather’regulation’) bygeneralist predators8. Such an equilibrium will, however, easily be broken by any disturbance affecting the predator-prey ratios. Deep snow will prevent predation by generalist microtine predators3 and pave the way for cyclic fluctuations5. Persecution of predators will release prey from a possibly temporary
304-307 2 May, R.M. (1981) in Theoretical Ecology: Principles and Applications (May. R.M., ed.), pp. 78-87, Blackwell 3 Newsome. A. (1990) Trends Ecol. Evol. 5, 187-191 4 Keith, L.B., Cary, J.B., Rongstad, O.J. and Brittingham, M. (1984) Wild/. Monogr. 90. l-43 5 Hansson, L. (1987) Oikos 50,308318 6 Henttonen, H., Oksanen, T., Jortikka, A. and Haukisalmi, V. (1987) Oikos 50, 353-365 7 Jensen, T.S. (1982) Oecologia 54, 184-l 92 8 Erlinge, S. (1987) Oikos 50,347-352 9 Halpin, M.A. and Bissonette, J.A. (1988) Can. J. Zoo/. 66,587-592
lations increase and is irrelevant at high numbers. Nor has any effect been measured on populations collapsing due to drought and failing food supplies. Once prey numbers are low, however, predation can become regulatory once more4. The Australian experience fits the concept of a ‘predator-pit’ well, and recent studies have attempted to measure its extent for densities of rabbits’ and house mice (ARE. Sinclair, P. Olsen and T.D. Redhead, unpublished). Lennart’s other major point is predator-prey ratios. Of course they
matter. Their measurement in cyclical systems, where predator numbers build up over a few years in response to increasing prey, may well predict phases in the cycle. In unpredictably irruptive systems, however, food supplies and weather play vital roles. Predator-prey ratios will tell us the obvious only at low prey numbers, post-drought. This is the essence of the concept of ‘environmentally modulated predation’4. Sweden presents the fascinating picture, now unfolding, of cyclicity at high latitudes but low populations of rodents at slightly lower latitudes5-8.
Lennart Hansson Dept of WildlifeEcology,SwedishUniversityof AgriculturalSciences, S-750 07 Uppsala, Sweden References
1 Errington, P.L (1956) Science 124,
Reply fromAlan Newsome Lennart Hansson’s points are well taken. The generality that I was seeking’ was that carnivores can effectively suppress populations of prey once their numbers are low - for whatever reason. I especially had in mind cyclic and irruptive species. In cyclical species, predation may be involved at all phases in the population dynamics; but, as recent experiments confirm, there can be links to other factors, notably food2s3. A major difference in unpredictably irruptive species (rodents, rabbits in Australia) is that predation diminishes in effectiveness as prey popu422
22 Mazia, D. (1984) Exp. Cell Res. 153, 1-15 23 Birky, C.W. (1978) Annu. Rev. Genet. 12,471-512 24 Felsenstein, J. (1974) Genetics 78, 737-756 25 Williams, G.C. (1975) Sex and Evolution. Princeton Universitv Press 26 Maynard Smith, J. (1978) ihe Evolution of Sex, Cambridge University
Press 27 Hamilton, W.D. (1982) in Population Biology of Infectious Diseases (Dahlem Konferenzen) (Anderson, R.M. and May, R.M., eds), pp. 269-296, Springer-Verlag 28 Bell, G. (1982) The Masterpiece of Nature: The Evolution and Genetics of Sexuality, University of California Press 29 Hickey, D.A. and Rose, MR. (1988) in
7990
The Evolution of Sex (Michod, R.E. and Levin, B.R., eds), pp. 161-175, Sinauer Associates 30 Bernstein, H., Byerly, H.C., Hopf, F.A. and Michod, R.E. (1985) Science 229, 1277-1281 31 Marin, G. and Argenton, F. (1990) fxp.
Cell Res. 187, l-3 32 Johnson, K.A. and Rosenbaum, J.L. (1990) Gel/62,615-619
Lettersto the Editor A New Role for Predation The role of predation in the population dynamics of vertebrates has been a very controversial subject. In the 195Os’, many game biologists denied that it had any importance, while an implicit idea of recent environmentalists has been that predation is responsible for ‘the balance of nature’. Mathematical ecologists have instead looked at population cycles in terms of Lotka-Volterra equations with predation as one possible driving variable*. Newsome’s recent article3 fits well into the pattern of more recent thinking, in which predation is considered to interact with other environmental variables. However, the author proposes that control by predators only operates within ‘a limited range of population densities’; this seems difficult to reconcile with the role he assumes for predation in cyclic populations. The emphasis should probably be changed to predator-prey ratios. In faintly fluctuating high-density prey, with diverse and seasonally fairly constant food, a reduction in prey food or prey numbers (e.g. by drought) will increase predator-prey ratios and lead to control by pred-
predator-prey equilibrium3. There appear, thus, to be good reasons to examine more closely the role of predation in many overcompensating ecological systems. Recent findings in such systems3 appear to unravel certain persistent enigmas in vertebrate dynamics.
ators as long as alternative prey types are available3. In cyclic populations, prey relying on a seasonally strongly varying food supply will face food shortage at winter peak numbers4f5. Specialist predator populations will by then have increased considerably due to a strong numerical response6, and the nutritionally or socially6 depressed prey growth rate will lead to another type of control by predators, lasting for a short time until the very few alternative prey types are also devoured. The latter scenario will apply to folivorous prey, such as hares4 and microtines5, as well as to granivorous rodents, for which there occurs an over-compensating mortality one and a half years after irregular mast seeding7. A high diversity of alternative prey may lead to a more or less continuous control (or rather’regulation’) bygeneralist predators8. Such an equilibrium will, however, easily be broken by any disturbance affecting the predator-prey ratios. Deep snow will prevent predation by generalist microtine predators3 and pave the way for cyclic fluctuations5. Persecution of predators will release prey from a possibly temporary
304-307 2 May, R.M. (1981) in Theoretical Ecology: Principles and Applications (May. R.M., ed.), pp. 78-87, Blackwell 3 Newsome. A. (1990) Trends Ecol. Evol. 5, 187-191 4 Keith, L.B., Cary, J.B., Rongstad, O.J. and Brittingham, M. (1984) Wild/. Monogr. 90. l-43 5 Hansson, L. (1987) Oikos 50,308318 6 Henttonen, H., Oksanen, T., Jortikka, A. and Haukisalmi, V. (1987) Oikos 50, 353-365 7 Jensen, T.S. (1982) Oecologia 54, 184-l 92 8 Erlinge, S. (1987) Oikos 50,347-352 9 Halpin, M.A. and Bissonette, J.A. (1988) Can. J. Zoo/. 66,587-592
lations increase and is irrelevant at high numbers. Nor has any effect been measured on populations collapsing due to drought and failing food supplies. Once prey numbers are low, however, predation can become regulatory once more4. The Australian experience fits the concept of a ‘predator-pit’ well, and recent studies have attempted to measure its extent for densities of rabbits’ and house mice (ARE. Sinclair, P. Olsen and T.D. Redhead, unpublished). Lennart’s other major point is predator-prey ratios. Of course they
matter. Their measurement in cyclical systems, where predator numbers build up over a few years in response to increasing prey, may well predict phases in the cycle. In unpredictably irruptive systems, however, food supplies and weather play vital roles. Predator-prey ratios will tell us the obvious only at low prey numbers, post-drought. This is the essence of the concept of ‘environmentally modulated predation’4. Sweden presents the fascinating picture, now unfolding, of cyclicity at high latitudes but low populations of rodents at slightly lower latitudes5-8.
Lennart Hansson Dept of WildlifeEcology,SwedishUniversityof AgriculturalSciences, S-750 07 Uppsala, Sweden References
1 Errington, P.L (1956) Science 124,
Reply fromAlan Newsome Lennart Hansson’s points are well taken. The generality that I was seeking’ was that carnivores can effectively suppress populations of prey once their numbers are low - for whatever reason. I especially had in mind cyclic and irruptive species. In cyclical species, predation may be involved at all phases in the population dynamics; but, as recent experiments confirm, there can be links to other factors, notably food2s3. A major difference in unpredictably irruptive species (rodents, rabbits in Australia) is that predation diminishes in effectiveness as prey popu422