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# Great Horned Owls and Snowshoe Hares: What Causes the Time Lag in the Numerical Response of Predators to Cyclic Prey?

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## Abstract

Predator populations often decline with a time lag after the peak of prey cycles. Theoretical models of predator-prey interactions predict that this delay is caused by a higher rate of population growth in prey, which leaves predators with super-abundant food after the peak and buffers their decline. This situation is met when predator populations have a lower innate capacity for increase than their prey or when the increase is inhibited because of territorial behaviour. Here, I refer to this hypothesis as 'single prey hypothesis' (SPH) in contrast to the 'multiple prey hypothesis' (MPH), which predicts that the delayed decline is caused by high availability of other prey species. Results on population growth rates of great horned owls showed that the predictions of SPH were met, although the predicted difference was small when floaters were taken into account or social exclusion from breeding was removed in a population model. In their diet, great horned owls relied to a large degree on the main cyclic prey (snowshoe hares), and thus the results were not in agreement with the MPH. Inverse density-dependent growth rates in the territorial population, density-dependent accumulation of floaters, and replacements of territorial vacancies were consistent with the hypothesis that social behaviour limited the number of owl territories. Reproduction of resident owls was immediately affected by the prey decline, indicating that there was no buffering effect of super-abundant food. Therefore, neither MPH nor SPH were satisfactory explanations, and I propose a mechanism based on individual behaviour to explain delayed numerical responses: territorial predators monopolize a disproportionately large amount of resources for reproduction during the increase and peak of the cycle, and are then buffered against prey declines by adjusting their breeding activities. Non-territorial floaters have lower access to resources and their numbers are affected more immediately by declining prey.

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... A time-lag, the necessity for coupled predator Á/prey oscillations, in numerical response may be caused by a lower reproduction rate of the predator than its prey (Tanner 1975, Hanski et al. 1991, Valkama et al. 2005, which may not necessarily be due to lower intrinsic reproductive rate of the predator, but also due to some limiting factors, like territoriality (Rohner 1995). A time lag in the numerical response has most often been found in resident mammalian predators , Krebs 1996, Korpimäki and Krebs 1996, O'Donoghue et al. 1997, but in few cases it has been demonstrated also for raptor Á/prey interaction: great-horned owls Bubo virginianus and goshawks Accipiter gentilis preying on snowshoe hares Lepus americanus in North America (Keith et al. 1977, Doyle and Smith 1994, 2001, Rohner 1995, Rohner et al. 2001, and gyrfalcons Falco rusticolus preying on ptarmigans Lagopus mutus in Iceland (Nielsen 1999). ...
... A time-lag, the necessity for coupled predator Á/prey oscillations, in numerical response may be caused by a lower reproduction rate of the predator than its prey (Tanner 1975, Hanski et al. 1991, Valkama et al. 2005, which may not necessarily be due to lower intrinsic reproductive rate of the predator, but also due to some limiting factors, like territoriality (Rohner 1995). A time lag in the numerical response has most often been found in resident mammalian predators , Krebs 1996, Korpimäki and Krebs 1996, O'Donoghue et al. 1997, but in few cases it has been demonstrated also for raptor Á/prey interaction: great-horned owls Bubo virginianus and goshawks Accipiter gentilis preying on snowshoe hares Lepus americanus in North America (Keith et al. 1977, Doyle and Smith 1994, 2001, Rohner 1995, Rohner et al. 2001, and gyrfalcons Falco rusticolus preying on ptarmigans Lagopus mutus in Iceland (Nielsen 1999). In addition, the brood size of goshawks living in boreal forests of northern Finland seems to lag one year after grouse (Huhtala and Sulkava 1981, Sulkava et al. 1994, Tornberg 2001. ...
... First, the alternative prey hypothesis (hereafter APH) states that grouse cycles are driven by generalist predators that switch from main prey (voles) to alternative prey during or after a crash of the main prey (Hagen 1952, Lack 1954, Angelstam et al. 1984. In North America, a similar mechanism is apparent with the difference that switching takes place from snowshoe hares to grouse (Keith et al. 1977, Keith and Rusch 1989, Rohner 1995. This pattern, however, does not work in areas where grouse do not follow 3 Á/4 year vole cycle, as is the case in Finland (Lindén 1988, Ranta et al. 1995. ...
... As the number of established owl pairs increased and territories were packed more densely in the study area, not only the addition of further territories was reduced but also the floater pool increased strongly ( fig. 5b, y = 3.89x -44.84, r 2 = 0.96, p < 0.05; details in Rohner 1995). ...
... The density of non-territorial floaters was estimated based on a population model including productivity, survival, and emigration (details in Rohner 1996). At peak hare densities, reproductive success and juvenile survival (Rohner 1995). A: Growth rates of the territorial population decline as numbers of owl territories increase in the area (inverse density-dependent growth rate). ...
... Fatal fighting can evolve when a major part of a contestant's lifetime reproductive success is at stake (Enquist and Leimar 1990). This, for example, (Rohner 1995 (8) may occur in saturated populations of Golden Eagles (Aquila chrysaetos) (Haller 1996). Many diurnal raptors have conspicuous immature plumages (Newton 1979) and display this bright coloration to approaching territory owners (Jenny 1992, pers. ...
Article
The ecology and behavior of non-territorial owls are basi-cally unknown. I studied the integration of young Great Horned Owls (Bubo virginianus) into the territorial breeding population from 1988-1993 in the southwestern Yukon, Canada, during a peak and decline of the population cycle of snowshoe hares (Lepus americanus). Fifty-five fledglings were equipped with radio-transmitters that allowed weekly monitoring of individuals for 2-3 years. After a synchronized dispersal phase in each September, 29-45 percent remained within 35 km of their natal territories. Although 15 percent settled in a territory and were capable of reproducing before the end of their first year of life, most of these owls became non-territorial floaters. Sev-eral lines of evidence indicated that this behavior was caused by territorial exclusion of breeding pairs. Floaters were secretive and mostly resident within home ranges that were about five times the size of average territories. Movement patterns suggested that floaters were not involved in extra-pair matings, and that floating is not an alternative reproductive strategy. Survival of floaters was very high during peak densities of prey, leading to a proportion of 40-50 per-cent of non-territorial owls in the population. When numbers of snowshoe hares declined, emigration and mortality rates increased in floaters before territory owners were affected. The results of this study show how a large proportion of secretive floaters can delay the detection of population declines in traditional censuses of territorial birds, and can lead to serious underestimates of the impacts of predation. Non-territorial 'floaters', which live a secretive life and form a 'shadow population', are well known for some bird species and assumed for many others (Brown 1964, Newton 1992, Smith 1978, Watson and Moss 1970). Sometimes, such 'surplus' birds live in areas separate from breeding territories, and they may become directly observable when they form social groups (Birkhead et al. 1986, Charles 1972) or they may be detectable in open habitat (Haller 1996, Hannon and Martin 1996, Jenny 1992, Watson 1985). Most of the knowledge about floaters, however, is indirect and is derived from experimental removals of territory holders (review in Newton 1992). The majority of owl species are territorial, and ecological field studies are usually based on territorial birds. Very little is known about floaters in territorial owl populations.
... A time-lag, the necessity for coupled predator Á/prey oscillations, in numerical response may be caused by a lower reproduction rate of the predator than its prey (Tanner 1975, Hanski et al. 1991, Valkama et al. 2005, which may not necessarily be due to lower intrinsic reproductive rate of the predator, but also due to some limiting factors, like territoriality (Rohner 1995). A time lag in the numerical response has most often been found in resident mammalian predators , Krebs 1996, Korpimäki and Krebs 1996, O'Donoghue et al. 1997, but in few cases it has been demonstrated also for raptor Á/prey interaction: great-horned owls Bubo virginianus and goshawks Accipiter gentilis preying on snowshoe hares Lepus americanus in North America (Keith et al. 1977, Doyle and Smith 1994, 2001, Rohner 1995, Rohner et al. 2001, and gyrfalcons Falco rusticolus preying on ptarmigans Lagopus mutus in Iceland (Nielsen 1999). ...
... A time-lag, the necessity for coupled predator Á/prey oscillations, in numerical response may be caused by a lower reproduction rate of the predator than its prey (Tanner 1975, Hanski et al. 1991, Valkama et al. 2005, which may not necessarily be due to lower intrinsic reproductive rate of the predator, but also due to some limiting factors, like territoriality (Rohner 1995). A time lag in the numerical response has most often been found in resident mammalian predators , Krebs 1996, Korpimäki and Krebs 1996, O'Donoghue et al. 1997, but in few cases it has been demonstrated also for raptor Á/prey interaction: great-horned owls Bubo virginianus and goshawks Accipiter gentilis preying on snowshoe hares Lepus americanus in North America (Keith et al. 1977, Doyle and Smith 1994, 2001, Rohner 1995, Rohner et al. 2001, and gyrfalcons Falco rusticolus preying on ptarmigans Lagopus mutus in Iceland (Nielsen 1999). In addition, the brood size of goshawks living in boreal forests of northern Finland seems to lag one year after grouse (Huhtala and Sulkava 1981, Sulkava et al. 1994, Tornberg 2001. ...
... First, the alternative prey hypothesis (hereafter APH) states that grouse cycles are driven by generalist predators that switch from main prey (voles) to alternative prey during or after a crash of the main prey (Hagen 1952, Lack 1954, Angelstam et al. 1984. In North America, a similar mechanism is apparent with the difference that switching takes place from snowshoe hares to grouse (Keith et al. 1977, Keith and Rusch 1989, Rohner 1995. This pattern, however, does not work in areas where grouse do not follow 3 Á/4 year vole cycle, as is the case in Finland (Lindén 1988, Ranta et al. 1995. ...
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V. 2005. Delayed numerical response of goshawks to population fluctuations of forest grouse. Á/ Oikos 111: 408 Á/415. Delayed density-dependent mortality induced by delayed numerical response of predators can drive prey populations to fluctuate in high-amplitude cycles. We studied numerical response of goshawks Accipiter gentilis to varying densities of their main prey (forest grouse) in western Finland during 1979 Á/1996. Occupancy rate of goshawk territories tracked grouse numbers with a two year lag. Occupancy rate of goshawk territories and pooled number of adult and young goshawks correlated negatively with a 1 Á/2 year lag to the chick production of grouse. Goshawk to grouse ratio was negatively related to grouse density. This suggests that goshawk predation on grouse is inversely dependent on grouse density. We conclude that in northern Europe with few alternative preys, goshawk predation might contribute to the generation of multiannual cycles of forest grouse. This should be tested experimentally. Predation is one possible mechanism maintaining popu-lation regulation provided that the predator acts in a density-dependent way (Hanski et al. 1991, Murdoch 1994, Sinclair and Pech 1996, Turchin 1999). Outcome of predation in prey population is largely dependent on timing and amplitude effect of predator's response (Andersson and Erlinge A time-lag, the necessity for coupled predator Á/prey oscillations, in numerical response may be caused by a lower reproduction rate of the predator than its prey (Tanner 1975, Hanski et al. 1991, Valkama et al. 2005), which may not necessarily be due to lower intrinsic reproductive rate of the predator, but also due to some limiting factors, like territoriality (Rohner 1995). A time lag in the numerical response has most often been found in resident mammalian predators (Korpimäki et al. 1991, Krebs 1996, Korpimäki and Krebs 1996, O'Donoghue et al. 1997), but in few cases it has been demonstrated
... The prey population grows faster than the predator population and overshoots an equilibrium density. Predators are not immediately affected when the prey begin to decline, and as a consequence they overshoot the carrying capacity of the prey and then decrease with a time lag (Keith, 1963;Keith & Windberg, 1978 ; see also Rohner, 1995). This process leads to an extended decline in the prey population, and only when predator densities have declined low enough can a new cycle begin. ...
... This process leads to an extended decline in the prey population, and only when predator densities have declined low enough can a new cycle begin. However, in reality predator-prey dynamics exceed simple two-species interactions (Rohner, 1995). Second, specialist predators are more likely to cause population cycles than generalist predators because they are less able to switch to alternative prey when prey populations decline. ...
... Here we present a review of the key findings from past or ongoing studies on the relationship between birds of prey and gamebirds across Europe. Diet and prey choice of various raptor species have been examined in many European countries and in North America (summarised by Marti, Korpimäki & Jaksic, 1993;Korpimäki & Marti, 1995), but here we aim to synthesise results from those European studies in which the impacts of raptor predation on their prey populations have been estimated (for comparison with North America, see Section VI, and also Rohner, 1995Rohner, , 1996Rohner, Doyle & Smith, 2001). ...
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Whether predators can limit their prey has been a topic of scientific debate for decades. Traditionally it was believed that predators take only wounded, sick, old or otherwise low-quality individuals, and thus have little impact on prey populations. However, there is increasing evidence that, at least under certain circumstances, vertebrate predators may indeed limit prey numbers. This potential role of predators as limiting factors of prey populations has created conflicts between predators and human hunters, because the hunters may see predators as competitors for the same resources. A particularly acute conflict has emerged over the past few decades between gamebird hunters and birds of prey in Europe. As a part of a European-wide research project, we reviewed literature on the relationships between birds of prey and gamebirds. We start by analysing available data on the diets of 52 European raptor and owl species. There are some 32 species, mostly specialist predators feeding on small mammals, small passerine birds or insects, which never or very rarely include game animals (e.g. hares, rabbits, gamebirds) in their diet. A second group (20 species) consists of medium-sized and large raptors which prey on game, but for which the proportion in the diet varies temporally and spatially. Only three raptor species can have rather large proportions of gamebirds in their diet, and another seven species may utilise gamebirds locally to a great extent. We point out that the percentage of a given prey species in the diet of an avian predator does not necessarily reflect the impact of that predator on densities of prey populations. Next, we summarise available data on the numerical responses of avian predators to changing gamebird numbers. In half of these studies, no numerical response was found, while in the remainder a response was detected such that either raptor density or breeding success increased with density of gamebirds. Data on the functional responses of raptors were scarce. Most studies of the interaction between raptors and gamebird populations give some estimate of the predation rate (per cent of prey population taken by predator), but less often do they evaluate the subsequent reduction in the pre-harvest population or the potential limiting effect on breeding numbers. The few existing studies indicate that, under certain conditions, raptor predation may limit gamebird populations and reduce gamebird harvests. However, the number and extent of such studies are too modest to draw firm conclusions. Furthermore, their geographical bias to northern Europe, where predator-prey communities are typically simpler than in the south, precludes extrapolation to more diverse southern European ecosystems. There is an urgent need to develop further studies, particularly in southern Europe, to determine the functional and numerical responses of raptors to gamebird populations in species and environments other than those already evaluated in existing studies. Furthermore, additional field experiments are needed in which raptor and possibly also mammalian predator numbers are manipulated on a sufficiently large spatial and temporal scale. Other aspects that have been little studied are the role of predation by the non-breeding part of the raptor population, or floaters, on the breeding success and survival of gamebirds, as well as the effect of intra-guild predation. Finally there is a need for further research on practical methods to reduce raptor predation on gamebirds and thus reduce conflict between raptor conservation and gamebird management.
... At the same time that we were studying hares, we obtained detailed data on the major predators in this ecosystem: lynx (Lynx canadensis), coyotes (Canis latrans), Great Horned Owls (Bubo virginianus), and other smaller predators Rohner, 1995;O'Donoghue et al., 1997). All of these predators fluctuate with hare numbers, but respond with a time lag because of their lower reproductive rates (Fig. 9). ...
... The Great Horned Owl is the major bird predator of hares, and like the lynx, it fluctuates with hare numbers, but with a time lag because of its low reproductive rate (Rohner, 1995(Rohner, , 1997. Figure 10 shows the density of nesting Great Horned Owls from 1987 to 2011. These owl data mirror the lynx and coyote data in showing a delayed, density-dependent response to hare density. ...
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The trophic dynamics of the Yukon boreal forest have been under investigation at the Kluane Lake Research Station since 1973. We monitored and conducted experiments on the major species in this ecosystem, except the large mammals (for logistic reasons). The central problem has been to determine the causes of the 9-10 year cycle of snowshoe hares, and to achieve this we carried out several large-scale experiments manipulating food supplies, predator pressure, and soil nutrient availability to test hypotheses that food, predation, or habitat quality regulate populations. The hare cycle is driven top-down by predators, and most hares die because they are killed by predators. Predators also cause stress in female hares, and the stress response seems to be responsible for the loss of reproductive potential in the decline and low phases of the hare cycle. Many of the specialist predators and some herbivores in this ecosystem fuctuate with the hare cycle. Arctic ground squirrels do, but red squirrels do not, being linked closely to white spruce seed masting years. Small rodents fuctuate in numbers in two patterns. Red-backed voles and four species of Microtus voles have a 3-4 year cycle that seems to be driven by food supplies and social behaviour. Deer mice, in contrast, have fuctuated dramatically in the 38 years we have monitored them, but not cyclically. White spruce seed production varies with temperature and rainfall, but was not affected by adding nutrients in fertilizer. Global warming and reduced hare browsing in the last 20 years have helped to increase the abundance of shrubs in these forests. It will be challenging to predict how this system will change as climatic warming proceeds, because even closely related species in the same trophic level respond differently to perturbations. We recommend continued monitoring of the major species in these boreal forests.
... Of 11 aged birds seen trespassing on occupied nesting territories during this same period, ®ve individuals (45%) were yearlings (Nielsen & Cade 1990b). The role of¯oaters in the population dynamics of raptors has been much discussed in the literature (Newton 1979;Hunt 1988Hunt , 1998Rohner 1995), and this aspect of gyrfalcon ecology calls for a study using modern telemetry technology. ...
... Changes in density of resident specialist microtine and lagomorph predators lag behind changes in their cyclic prey (Keith 1963;Keith et al. 1977;Henttonen et al. 1987;KorpimaÈ ki, Norrdahl & Rinta-Jaskari 1991;Rohner 1995). My data show that the same applies to the gyrfalcon, as the total numerical response (adults +¯edglings) tracked ptarmigan population with a 2-year lag, in agreement with prediction one of the Introduction, which states that the predator should show a delayed density-dependent response to changes in prey numbers. ...
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1. Gyrfalcon predation on ptarmigan during the breeding season was studied in north-east Iceland 1981–97. The ptarmigan population went through a complete 10-year cycle of numbers with a 4·3-fold difference in density between high and low years. The yearly number of occupied gyrfalcon territories was correlated with ptarmigan density with a 3-year time-lag. Total falcon numbers in late summer (territorial adults + fledglings) showed a 2-year lag with ptarmigan numbers. Variability in falcon density was significantly less than that of ptarmigan. It is suggested that the factors contributing to the time-lag between the two populations are the year-round residency of falcons on nesting territories, and late maturity (2- to 4-year-old). Mean brood size and the proportion of the territorial falcon population breeding successfully showed no relation to ptarmigan numbers.
... This process leads to an extended decline in the prey population, and only when predator densities have declined low enough can a new cycle begin. However, it must be kept in mind that in reality predator-prey dynamics exceed simple two-species interactions (Rohner 1995). Second, specialist predators are more likely to cause cyclicity than generalist predators because they are unable to switch to alternative prey when prey populations decline. ...
... Another aspect that would be important to consider when evaluating the impact of raptors on gamebirds is that the influence of floaters, or the non-breeding part of the raptor population, on breeding success and survival of grouse is relatively poorly known (Rohner 1995Rohner ,1996, Korpimäki and Krebs 1996). However, most studies that have evaluated the predation rate on gamebirds have not separated between breeders and non-breeders, only levels of predation have been assessed. ...
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55 pages.-- REGHAB Project: Report on Workpackage 3 – Deliverable no 5. The traditional Erringtonian view suggested that predators are generally not harmful to prey populations as they only take a doomed surplus of the prey population (Errington 1956). It was also frequently assumed that the predated individuals were ill, injured or otherwise of low quality and that predators acted as health officers in nature. This view has, however, been questioned in many studies conducted during 1980s and 1990s, and recent studies indicate that predation may, at least under certain environmental conditions, have profound effects on vertebrate prey populations (Marcström et al. 1988, Newton 1993, Krebs et al. 1995, Korpimäki and Krebs 1996, Tapper et al. 1996, Hubbs and Boonstra 1997, Korpimäki and Norrdahl 1998, Byrom et al. 2000, Thirgood et al. 2000b, Korpimäki et al. 2002). From a theoretical point of view, predators can either stabilise or destabilise prey populations, depending on the type of responses of the predator and carrying capacity of the prey (e.g. Hanski et al. 1991, Sinclair and Pech 1996). Increasing rate of generalist predation decreases the length and amplitude of the prey cycle which is driven by specialist predators, and with high enough density of generalist predators the prey cycle turns to a stable equilibrium point (Hanski et al. 1991). REGHAB Project is funded by European Union Contract No. EVK2-CT-2000-200004 within the 5th Research Framework Program. Peer reviewed
... We studied the response of Great Horned Owls (Bubo virginianus) to the 10-year population cycle of snowshoe hares Lepus americanus in the boreal forest from 1989-1992 , Rohner 1996. Great Horned Owls are large and long-lived predators feeding mainly on lagomorphs, they defend long-term territories, and are widely distributed in North and South America (Voous 1988, Donazar et al. 1989, Rohner 1995. Several demographic parameters in Great Horned Owls were strongly affected by the population cycle of snowshoe hares (Rohner 1996). ...
Article
We document a shift in roosting behavior of Great Horned Owls (Bubo virginianus) from winter and late spring to summer. During summer, Great Horned Owls roosted near the ground or exposed on open ground, whereas they chose concealed perches at mid-canopy level for the rest of the year as typical for forest owls. This shift of roosting behavior coincided with the emergence of ornithophilic black flies, which transmit avian malaria (Leucocytozoon spp.). The shift in roosting behavior was consistent with measurements of parasite exposure at different habitat positions. Black fly activity was highest at mid-canopy level, and almost no black flies were active on open ground. Ground-roosting was not caused by poorly developed flying capability of juveniles, because solitarily-roosting adult owls showed the same behavioral shift in a second year of study. Black flies and avian malaria are widely distributed, and the effect of the vertical distribution of these parasites in forests on roosting, nesting, and foraging of sylvatic birds deserves further study.
... Delayed numerical response of resident generalist predators to increase in prey abundance is characterized by a time lag through higher natality and lower mortality (Goszczynski 1977). Delayed numerical response has been documented in resident mammalian predators , Korpimäki & Krebs 1996, Krebs 1996, O'Donoghue et al. 1997, and in birds of prey as well (Keith et al. 1977, Doyle & Smith 1994, Rohner 1995, Nielsen 1999, Rohner et al. 2001. Three types of functional responses -linear, convex and sigmoid -are distinguished according to the nature of relationship between predation rate and prey density (Holling 1959). ...
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Összefoglalás A kisemlősök denzitás változásainak az egerészölyv reprodukciójára gyakorolt hatását (un. numerikus választ) vizsgáltam a Hortobágyon, 2000-2001-ben. A fészekellenőrzések során regisztráltam a lerakott tojások, valamint a kikelt és a kirepült fiókák számát. Az egyik legfontosabb zsákmánycsoport, a kisemlősök állomány változásait élvefogó csapdázással monitoroztam. Vizsgáltam az időjárásnak az áttelelő kisemlősök túlélésére gyakorolt hatását is. Az ölyvek átlagos fészekalja a két évben szignifikánsan különbözött: 2000-ben 2,3, míg 2001-ben 3,1 tojás volt. Ez a kisemlősök - 2000-ről 2001-re bekövetkezett - jelentős mértékű állománynövekedésével magyarázható. 2000-ben nagyon alacsony volt a kisemlősök egyedszáma (9 példány/ha), míg 2001- ben ennek nyolcszorosát regisztráltam (76 példány/ha). A kisemlősök egyedszámában tapasztalt óriási eltéréseket a vizsgált két év tél végi-tavasz eleji időjárási különbségei okozhatták. 2000 február-márciusában, a napi minimum hőmérsékleteket tekintetve, 4 rövid, enyhe periódus váltakozott 4 fagypont alattival, ugyanakkor az enyhe időszakokban jelentős mennyiségű (6-8 mm) eső is hullott. Ekkor az áttelelő kisemlősök járatai ismételten beáztak, az állatok megfáztak, kihűltek, szinte kipusztultak a területről, ezért létszámuk a költési időszakban is rendkívül alacsony volt. 2001-ben a tél vége sokkal enyhébb volt, 3 héttel korábban emelkedtek fagypont fölé a napi minimum hőmérsékletek, mint 2000-ben, nem alakultak ki váltakozó hideg-meleg időszakok sem. Ez kedvezően hatott az áttelelő kisemlősökre, létszámuk gyorsan emelkedett és nyár elejére mezei pocok gradáció alakult ki.
... Over the cycle, the behavior of both predators and prey changed, sometimes in unpredictable directions. Territorial systems broke down and raptor species began preying on others in their own trophic level (Doyle and Smith 1994, Rohner 1995. A breakdown in territorial behavior among great horned owls (Bubo virginianus) and several other raptors required researchers to modify their census and capture techniques during the study. ...
... Because the nesting territories are usually defended with the regularly spaced nest sites as centre, I argue that a considerable increase in the amount of food available is necessary to allow surplus Goshawks to establish nesting territories between the existing ones . Similarly, a time lag in the population increase of the Great Horned Owl (Bubo virginianus) relative to the marked increase in cyclic populations of the Snowshoe Hare (Lepus americanus) has been suggested to be caused by territorial behaviour (Rohner 1995). ...
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Despite many years of protection, nesting Goshawks (Accipiter gentilis) are still killed by man in Norway to save small game, especially grouse, from predation. In a 650-km2 area in southern Norway, the re-establishment of Goshawk nesting territories was studied during four, 4-year periods 1972-75, 1980-83, 1984-87 and 1988-91. After a reduction in the Red Fox (Vulpes vulpes) population because of an infestation of sarcoptic mange, the grouse population increased during 1984-87, and then remained high for the rest of the study period. The number of Goshawk nesting territories per 100 km2 was three/year in 1972-75 and in 1980-83, and four/year in 1988-91. During these periods, the nesting territories were regularly spaced, and their number and distribution were unaffected by the removal of breeding birds. During 1984-87, five nesting territories, in which one or both of the breeding hawks had been removed by man, were replaced by eleven new ones. The study results indicate that removal of breeding pairs of goshawks may lead to an increase in the breeding density during periods of increasing food availability.
... We encourage future studies with intensive monitoring of predator and prey populations to provide more understanding of the numerical response (see e.g. Hone et al., 2007;Rohner, 1995Rohner, , 1996. Anyway, in this study the use of a regional approach provided a wide range of abundance estimates and valuable large-scale observations of dynamics of natural species in their home-range conditions. ...
... Indeed, annual movements of greathorned owls Bubo virginianus , a strigidae of similar size than the snowy owl inhabiting the boreal forest, are very limited even if its main prey, snowshoe hares Lepus americanus , vary considerably in abundance from one year to another (Rohner 1996). Its capacity to turn to alternative prey (Rohner 1995) and the benefi ts of remaining on territory likely outweigh the potential gains of moving. ...
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Mobility and irruptive movements have been proposed as mechanisms that could allow some diet specialists to inhabit and breed in environments with highly unpredictable resources, like the arctic tundra. The snowy owl, one of the main avian predators of the tundra, is known to specialize on lemmings during the breeding season. These small mammals are also well known for their tremendous spatial and temporal variations in abundance. We examined the spring (pre-breeding, from March to June) movements of snowy owls by tracking 9 breeding females in the Canadian Arctic for up to 3 yr with satellite transmitters. We used state-space modeling to assess searching behavior and measure breeding dispersal distances. We also ascertain lemming abundance at some of the sites used by the marked owls. Tracked owls displayed searching movements for extended periods (up to 108 d) and traveled over large distances (up to 4093 km) each spring. The distance between furthest apart searching areas in a given year averaged 828 km (range 220 to 2433 km). Settlement date, distance between searching areas, traveled distance and the duration of prospecting movements were longer in the year where density of lemmings recorded in the eastern High-Arctic (Bylot Island) was lowest. Nonetheless, snowy owls settled in areas where local lemming abundance was relatively high. Individual breeding dispersal distance between consecutive years averaged 725 km (range 18 to 2224). Overall, the high mobility of female snowy owls allowed these diet specialists to behave as irruptive migrants and to sustain their reproductive activities during consecutive years even under highly fluctuating resources.
... Ground squirrel activity is centred around an underground burrow system; the farther a squirrel is from its burrow system, the greater its risk of predation (Bonenfant and Kramer 1996). Since ground squirrels are an important food source for many predator species (e.g., Elliott and Guetig 1990;Rohner 1995), they should be risk-sensitive and adjust their movements accordingly. However, antipredator behaviours must be balanced against the conflicting demands of foraging. ...
... En los vertebrados, los flotantes no territoriales son típicos de las poblaciones saturadas y tienen un éxito reproductor y una supervivencia mucho menores que los individuos territoriales (Smith y Arcese 1989). Los flotantes forman una especie de colchón amortiguador de las poblaciones, parecen tener menor acceso a los recursos, son los primeros en morir cuando la población está en declive y son prácticamente indetectables en los censos que no utilizan radiomarcaje, como Rohner (1995Rohner ( , 1996Rohner ( y 1997) ha demostrado con el búho de Virginia (Bubo virginianus). ...
... In other words, it is possible that Lepus n. nigricollis was the prime basic food of Bubo bengalensis in this region before human pressures had an impact on prey density. Closely related species Bubo bubo and Bubo virginianus are specialised hare and rabbit hunters, and their lifestyles are in sync with the highs and lows of their prey cycles (Adamcik et al., 1978; Bayle et al., 1987; Blondel & Badan, 1976; Delibes & Hiraldo, 1979; Herrera & Hiraldo, 1976; Houston & Francis, 1995; Martinez, 2003; Penteriani et al., 2002; Rohner, 1995 Rohner, , 1996 Rohner & Hunter, 1996; Rohner & Smith, 1996). That Bubo bengalensis could switch to alternate primary prey resources (rodents) shows the adaptibility of the species. ...
Article
A total of 2,467 prey items of the Indian Eagle Owl Bubo bengalensis were identified accounting for an estimated biomass (dry weight) of 1,35,575.37g in 35 months at four study sites. Mammals accounted for an estimated biomass of 86.93% of which rodents occupied pride of place with 64.91%. Tatera indica (24.96%), Rattus rattus (20.43%), Bandicota bengalensis (12.28%), Mus spp. (4.67%), Bandicota indica (2.34%) and Funambulus palmarum (0.15%) featured prominently among rodent food, but Millardia meltada (0.06%) was conspicuous by its near absence. Increased predation was noticeable only when young were present, but other than this no distinct statistical variations were discernable. Trapping exercises showed only marginal seasonal fluctuations in rodent populations (the greatest monthly variation being 4.63% for Rattus rattus), and this was linked to the year round availability of plant food in the region. Another basic food was Lepus nigricollis (20.06%), though, at one study site free from poaching pressure, it accounted for 30.16%, leading to the postulation that historically, before poaching became rampant, it could have been the primary prey resource. Suncus murinus (1.36%) and Chiroptera (0.58%) were the other mammal prey. Birds (8.28%) were the most important non-mammal food, followed by batrachians (2.75%), both of which showed distinct seasonal fluctuations. Varanus bengalensis (1.64%) and a single Amphiesma stolata were the reptiles consumed. Arthropods accounted for 0.34%, of which Coleoptera dominated with 0.24%. The venomous Heterometrus swammerdami and Scolopendra morsitans also formed part of the prey spectrum, albeit in negligible quantities (a combined biomass of 0.022%).
... Ground squirrel activity is centred around an underground burrow system; the farther a squirrel is from its burrow system, the greater its risk of predation (Bonenfant and Kramer 1996). Since ground squirrels are an important food source for many predator species (e.g., Elliott and Guetig 1990;Rohner 1995), they should be risk-sensitive and adjust their movements accordingly. However, antipredator behaviours must be balanced against the conflicting demands of foraging. ...
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We used radiotelemetry to study the effects of food addition and predator reduction on the home-range sizes of adult Arctic ground squirrels (Spermophilus parryii) on large-scale experimental grids in the boreal forest of the southwestern Yukon Territory. Home ranges were 2-7 times smaller on food-supplemented grids than on nonsupplemented grids, regardless of whether large mammalian predators were present. Similarly, core areas (where 50% of activities occur) were 8-11 times smaller on food-supplemented grids. Food availability rather than predator presence primarily determined the sizes of home ranges and core areas of Arctic ground squirrels.
... The fact that both species take similar prey types from the same areas suggests that they could compete for food resources. Prey availability is assumed to limit reproduction and/or survival of both spotted and great horned owls in some years (Adamcik and others 1978;Rohner 1995;Rohner and Hunter 1996;Verner and others 1992;Ward 2001), and this appears to be true both for many other owls (Korpimäki and Norrdahl 1991;Lundberg 1981;Southern 1970) and for raptorial birds in general (Newton 1979). Further, prey numbers in this area may be lower than historical levels due to changes in forest structure and especially reductions in herbaceous vegetation (Block and others 2005;Covington and Moore 1994;Ganey and others 1997;Reynolds and others 1996). ...
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We estimated diet composition of sympatric Mexican spotted (Strix occidentalis lucida, n = 7 pairs of owls) and great horned owls (Bubo virginianus, n = 4 pairs) in ponderosa pine (Pinus ponderosa)-Gambel oak (Quercus gambelii) forest, northern Arizona. Both species preyed on mammals, birds, and insects; great horned owls also ate lizards. Mammals dominated the diet of both species. Mammals comprised 63 and 62% of all prey items identified in diets of spotted and great horned owls, respectively, and 94 and 95% of prey biomass. Both species primarily preyed on a few groups of small mammals. Observed overlap in diet composition between species (0.95) was greater than expected based on null models of diet overlap, and the size range of prey taken overlapped entirely. Mean prey mass was similar for both species (great horned owl, 47.0 ± 7.4 g [SE], n = 94 items; spotted owl, 40.1 ± 1.8 g, n = 1,125 items). Great horned owls consumed larger proportions of diurnally active prey than spotted owls, which primarily consumed nocturnally active mammals. Our results, coupled with a previous analysis showing that these owls foraged in the same general areas (Ganey and others 1997), suggests that they could compete for food resources, which are assumed to be limiting in at least some years. They may minimize the potential for resource competition, however, by concentrating foraging activities in different habitats (Ganey and others 1997) and by foraging at different times, when different suites of prey species are active.
... The risk of a hare being killed by a coyote is dominated by changes in the numerical and functional responses of coyotes (O'Donoghue et al. 1997(O'Donoghue et al. , 1998b. Hare predation patterns in general are driven by numeric and functional changes of their main predators (coyotes and lynx (O'Donoghue et al. 1997(O'Donoghue et al. , 1998b; great horned owls (Bubo virginianus Gmelin) (Rohner 1995(Rohner , 1996; goshawks (Accipiter gentilis L.) (Doyle and Smith 1994)). The results presented here show that coyotes have distinct seasonal impacts as well and may prey disproportionately upon whichever category of hares is most susceptible at a given time. ...
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Differential predation on particular sex or age classes of a population can arise as a result of predator preferences or prey attributes. I examined the impacts of age, size, and body mass of snowshoe hares, Lepus americanus, on their susceptibility to predation by coyotes, Canis latrans. I observed coyote predation on naïve radio-collared hares during a fortuitous natural experiment: a coyote entered a predator exclosure fence in which hares of all ages had no previous experience with terrestrial predators, thus separating age from experience with this predator. I contrasted this manipulation with populations in which hares grew up in the presence of coyotes. Prey naiveté per se did not influence coyote predation, but older hares appeared to be more susceptible to coyote predation than younger ones. There were no obvious effects of body mass or size on coyote predation during the winter.
... Strong territoriality was one of the main factors that limited breeding densities and thus time-lagged the numerical responses of great horned owls to 10-year population fluctuations of snow-shoe hares (Rohner 1995(Rohner , 1996. A considerable time lag of 1-3 years has also been documented for other avian predators that remain resident in their territories when densities of main prey populations decline. ...
Book
Preface Acknowledgements 1. Introduction 2. Boreal (or Tengmalm's) Owls: briefly 3. Study areas and research methods 4. Habitat use, roosts and nest sites 5. Interactions with prey animals 6. Life-history 7. Mating and parental care 8. Reproduction 9. Dispersal and autumn movements 10. Survival and mortality under temporally varying food conditions 11. Old forests increase survival and lifetime reproductive success 12. Family planning under fluctuating food conditions 13. Population dynamics 14. Population regulation 15. Conservation of Boreal Owl populations References Index.
... I hypothesize that in seasonally reproducing organisms such as mammals, predation on neonates tends to create directional selection against late birthing and that it represents a seasonal analog to timelagged predator-prey cycles (Hanski and Korpimaki 1995;May 1981;Rohner 1995) or predator pits (Gasaway et al. 1992;Messier 1994). By this analogy, newborn young represent an irruption of vulnerable prey to which predators can respond by moving to areas or habitats of highest density of newborns and by adapting their search and capture efforts to capture them more efficiently. ...
Article
I hypothesize that predation on newborn young represents a seasonal analog to time-lagged predator–prey cycles and can cause directional selection against late birthing. Newborn offspring represent an irruption of vulnerable prey to which predators can respond by adapting their search efforts to find and capture these prey. Those born early would be the 1st to achieve the size or mobility necessary to escape predators. Simple models of predation on moose calves were used to demonstrate how changes in predator efficiency as newborns appear, combined with a short period of offspring vulnerability, would produce selection against late birthing. Of the 9 published studies of mammals, 6 showed evidence of selection against late-born young that could be driven by predation on neonates. If true, this hypothesis has consequences for the way we interpret evidence for “predator swamping” and optimal birthing periods in mammals and other taxa with synchronous reproduction.
... In a countrywide investigation using longer time series, breeding goshawks seem to lag two years behind the black grouse population (Väisänen et al. 1998 ). Two-year time lags have been reported for gyrfalcons Falco rusticolus in relation to ptarmigans in Iceland (Nielsen 1999) and for great horned owl Bubo virginianus in relation to snow-shoe hares in North America (Keith et al. 1977, Rohner 1995 ). Theoretically , time lags of two years would be enough to generate grouse cycles in northern boreal forests because a predator driven cyclicity of a prey population requires a time lag of 1 /4 of the cycle length (May 1981). ...
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I studied predator-prey relationships between goshawk Accipiter gentilis and four species of forest grouse (Tetraonidae) in northern Finland during 1988-1998. The main purpose of my study was to evaluate the impact of goshawk predation and its possible effect on multiannual cycling patterns in grouse numbers. Theoretically specialist predators should tend to cause stable-limit cycles in prey populations if there is a time-lag in the predator's response to prey density and the prey species should be most affected at low densities. Four grouse species, willow grouse Lagopus lagopus, black grouse Tetrao tetrix, capercaillie Tetrao urogallus and hazel grouse Bonasa bonasia, form the main food of the goshawk in boreal forests in northern Finland. Grouse constituted >40% of the goshawk's diet during the breeding season. The impact of predation by breeding goshawks on grouse varied depending on grouse species within 7-32% during the breeding season. Losses were highest for willow grouse and lowest for capercaillie. On average, goshawks took 6% of grouse chicks. On an annual basis breeding goshawks took 2-31% of the August grouse population. The goshawk's share of the total mortality in grouse was also species related. The most reliable estimates were found for black grouse of which 35% were removed and for hazel grouse of which 40% were removed. Goshawks are relatively specialised on forest grouse in northern boreal forests as was demonstrated by a weak functional response of the hawks to changes in grouse density. Breeding goshawks showed no numerical response to changes in grouse density but the production of young tended to lag one year behind black grouse density. The predation rate of goshawks was inversely density dependent on changes in grouse density, which may have had a destabilising effect on the grouse populations. A positive relationship existed between summer predation on willow grouse and changes in the population the previous year.
... Shortterm studies not running long enough to encompass low survival events may thus underestimate the temporal variance in fitness components, as was suggested for barn owl (Altwegg et al. 2003). We expected these fluctuations to be correlated with the dynamics of voles as observed for other owl species (Hakkarainen et al. 2002;Hone and Sibly 2002;Klok and de Roos 2007;Rohner 1995). However, models including measures of vole dynamics did not fit the data well. ...
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... The GHO diet varies in both composition and integration of trophic levels. The GHO diet has been well-characterized [26,35,4445464748495051 , and while it consistently comprises small mammals , birds, and invertebrates, the diet has been noted for its variability among habitats. A marked difference in dietary composition between agricultural and rangeland regions was observed in a correlation between habitat and dietary composition in Idaho, USA [45]. ...
Article
Soils and sediments in the floodplain of the Tittabawassee River downstream of Midland, Michigan, USA contain elevated concentrations of polychlorinated dibenzofurans (PCDF) and polychlorinated dibenzo-p-dioxins (PCDD). As a long-lived, resident top predator, the great horned owl (Bubo virginianus; GHO) has the potential to be exposed to bioaccumulative compounds such as PCDD/DF. Site-specific components of the GHO diet were collected along 115 km of the Tittabawassee, Pine, Chippewa, and Saginaw Rivers during 2005 and 2006. The site-specific GHO biomass-based diet was dominated by cottontail rabbits (Sylvilagus floridanus) and muskrats (Ondatra zibethicus). Incidental soil ingestion and cottontail rabbits were the primary contributors of PCDD/DF to the GHO diet. The great horned owl daily dietary exposure estimates were greater in the study area (SA) (3.3 to 5.0 ng 2,3,7,8-TCDD equivalents (TEQ(WHO-avian))/kg body wt/d) than the reference area (RA) (0.07 ng TEQ(WHO-Avian)/kg body wt/d). Hazard quotients (HQs) based on central tendency estimates of the average daily dose and no-observable-adverse effect level (NOAEL) for the screech owl and uncertainty factors were <1.0 for both the RA and the SA. Hazard quotients based on upper end estimates of the average daily dose and NOAEL were <1.0 in the RA and up to 3.4 in the SA.
Article
This book presents an up-to-date, detailed and thorough review of the most fascinating ecological findings of bird migration. It deals with all aspects of this absorbing subject, including the problems of navigation and vagrancy, the timing and physiological control of migration, the factors that limit their populations, and more. Author, Ian Newton, reveals the extraordinary adaptability of birds to the variable and changing conditions across the globe, including current climate change. This adventurous book places emphasis on ecological aspects, which have received only scant attention in previous publications. Overall, the book provides the most thorough and in-depth appraisal of current information available, with abundant tables, maps and diagrams, and many new insights. Written in a clear and readable style, this book appeals not only to migration researchers in the field and Ornithologists, but to anyone with an interest in this fascinating subject. * Hot ecological aspects include: various types of bird movements, including dispersal and nomadism, and how they relate to food supplies and other external conditions * Contains numerous tables, maps and diagrams, a glossary, and a bibliography of more than 2,700 references * Written by an active researcher with a distinguished career in avian ecology, including migration research.
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Article
Predators that aggregate in "hot spots" of high prey density have been hypothesized to synchronize population cycles of small mammals. During a peak and decline in a snowshoe hare (Lepus americanus) cycle, we created artificial hot spots of increased hare abundance by adding food and excluding mammalian predators on three $1\text{-}{\rm km}^{2}$ blocks and then recorded the response of radio-marked Great Horned Owls (Bubo virginianus) to these food additions. Territorial owls showed a decrease in home range size and patchiness of spatial use as hare densities peaked and declined, although this was better explained by smaller territory sizes due to a growing owl population rather than a direct behavioral response to changing food density. Experimental owls on food-enriched territories did not show a difference in conventional measurements of home-range size and patchiness of spatial use compared with controls, but the distances of owl locations to treatment blocks revealed concentrations of spatial use on experimental hot spots. At a larger scale, neither territorial owls nor nonterritorial floaters showed a tendency to leave poorer patches and move toward hot spots, and the territorial system of Great Horned Owls was largely resistant to extreme variations in prey density. The effect of social interference between predators has been assumed for several models of predator-prey interactions, but empirical evidence has rarely been demonstrated. Our results suggest that territorial behavior, in addition to limiting the growth of a predator population, also prevents large aggregations of predators at an intermediate spatial scale.
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I have studied the population ecology of the Gyrfalcon (Falco rusticolus) since 1981 on a 5,300-km 2 study area in northeast Iceland harbouring 83 traditional Gyrfalcon territories. The main questions addressed relate to the predator–prey relationship of the Gyrfalcon and its main prey, the Rock Ptarmigan (Lagopus muta). Specifically, how does the Gyrfalcon respond func-tionally and numerically to changes in ptarmigan numbers? Field work involved an annual census to determine occupancy of Gyrfalcon territories, breeding success, and food. Also, Rock Ptarmi-gan were censused annually on six census plots within the study area. The Rock Ptarmigan pop-ulation showed multi-annual cycles, with peaks in 1986, 1998 and 2005. Cycle period was 11 or 12 years based on the 1981−2003 data. The Gyrfalcon in Iceland is a resident specialist predator and the Rock Ptarmigan is the main food in all years. The functional response curve was just slightly concave but ptarmigan densities never reached levels low enough to reveal the lower end of the trajectory. Occupancy rate of Gyrfalcon territories followed Rock Ptarmigan numbers with a 3−4 year time-lag. Gyrfalcons reproduced in all years, and all measures of Gyrfalcon breeding success—laying rate, success rate, mean brood size, and population productivity—were signifi-cantly related to March and April weather and to Rock Ptarmigan density. The Gyrfalcon data show characteristics suggesting that the falcon could be one of the forces driving the Rock Ptarmi-gan cycle (resident specialist predator, time-lag). The Iceland Gyrfalcon and Rock Ptarmigan data are the longest time series on the Gyrfalcon–Rock Ptarmigan relationship, and the only series that indicate a coupled predator–prey cycle for the falcon and its ptarmigan prey.
Article
During 1995–97, reproductive requirements and success were quantified for raptor assemblages of up to 10 species on (usually dry) creeks in the south-west of the Northern Territory. Most breeders chose to nest in River Red Gums (E. camaldulensis) that were taller, of greater girth and more foliated than other trees generally available on the local drainages, but few interspecific differences in nest tree or nest site requirements were identified. Breeding densities and success fluctuated greatly during the study. Total assemblage productivity in 1996, a drought year, was 6–10 times lower than in 1995 and 8–15 times lower than in 1997. Declines in territory occupancy, breeding density, breeding success and the number of young fledged per active nest were characteristic responses to the drought for most species.
Article
The ultimate and proximate causes of natal dispersal have been extensively investigated, but the behaviour of dispersers in relation to social interactions has been largely neglected. Here, we investigated the social organisation of floating individuals during their dispersal by analysing the behaviour of 40 radio-tagged eagle owls Bubo bubo during the wandering and stop phases of dispersal. Unexpectedly, eagle owl floaters mixed with conspecifics independently of their sex, age, phase of dispersal, birthplace, health status and habitat features, showing an ‘underworld’ of interactions characterised by the absence of obvious social organisation or short-term strategies. Non-breeding owls were not transient floaters that occurred at numerous sites for short periods of time but rather had fairly stable home ranges: they attempted to settle as soon as possible within well-defined home ranges. The spatial distribution pattern of floaters and high rates of home range overlap support the prediction that floating individuals are not spatially segregated, challenging the expectation that dominance by size, age and/or health status may determine the exclusive use of some portions of the dispersal area. Finally, (1) the short distances among conspecifics and the extensive home range overlaps allowed us to discard the possibility that neighbouring floaters represent a real cost during dispersal and (2) floater interactions showed a lack of clear mechanisms for avoidance of kin competition among offspring or inbreeding.
Article
Most bird species have low survival rates in their first year of life, and the highest losses occur when juveniles become independent and disperse. Young great horned owls (Bubo virginianus), monitored by telemetry in the southwestern Yukon, Canada, survived well during the peak of the population cycle of snowshoe hares (Lepus americanus). Subsequently, juvenile survival collapsed parallel to the decline in hare densities. The proportion of starving owls did not increase, but there was a significant increase in mortalities involving parasitism and predation, probably as an interaction with food shortage. The mortality rates of juvenile great horned owls peaked before, not during, dispersal. We propose that extended parental care makes the postfledging stage safe during optimal conditions, but that the relatively slow development during this stage incurs the cost of increased susceptibility to disease and other mortality factors under environmental stress.
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We document a shift in roosting behavior of Great Horned Owls (Bubo virginianus) from winter and late spring to summer. During summer, Great Horned Owls roosted near the ground or exposed on open ground, whereas they chose concealed perches at mid-canopy level for the rest of the year as typical for forest owls. This shift of roosting behavior coincided with the emergence of ornithophilic black flies, which transmit avian malaria (Leucocytozoon spp.). The shift in roosting behavior was consistent with measurements of parasite exposure at different habitat positions. Black fly activity was highest at mid-canopy level, and almost no black flies were active on open ground. Ground-roosting was not caused by poorly developed flying capability of juveniles, because solitarily-roosting adult owls showed the same behavioral shift in a second year of study. Black flies and avian malaria are widely distributed, and the effect of the vertical distribution of these parasites in forests on roosting, nesting, and foraging of sylvatic birds deserves further study.
Article
Four hypotheses regarding the role of predation in the population dynamics of eruptive small mammal communities were tested using the small mammal assemblage found in mixed forests in New Zealand. Large-scale (750 ha) predator removal was conducted, targeting stoats (Mustela erminea). House mouse (Mus musculus) and ship rat (Rattus rattus) Population dynamics during an eruption were compared in areas with and without predator reduction. The success of predator reduction Was measured by comparing live-capture rates of predators on treatment and non-treatment areas, and by recruitment rates of the threatened northern brown kiwi (Apteryx australis mantelli). Overall, predator reduction was successful, although there was a continual low rate of reinvasion. The predictions and results were that 1) Predators can slow but not prevent a Population eruption. Supported: Populations of mice and rats erupted to high densities in areas with and Without predator reduction, following synchronous southern beech (Nothofagus spp.) seeding. 2) Predators cannot truncate peak prey population size. Supported: Peak densities of mice and rats were not significantly different between treatment and non-treatment areas. 3) Predators call hasten the rate of decline in prey populations during the crash phase. Not supported: There was evidence Of Populations of mice and rats declining slower in areas with predators removed, but none of the trends were significant. 4) Predators can limit low-phase prey populations. Equivocal: Populations of rats in beech forest, and population of mice and rats in coastline habitats were significantly higher in areas with predators removed, but were not significantly different in tawa-podocarp forest. Therefore, the role of food in driving the early stages Of the Mouse and rat eruption was demonstrated, but the role of predation in the decline and low phases is unclear.
Article
The reproduction of raptors strongly depends on food resources. It is unclear whether predators experience superabundant food during cyclic peaks of prey populations. In order to test this hypothesis, four pairs of Great Horned Owls Bubo virginianus with two young were subjected to brood size manipulations during high densities of cyclic Snowshoe Hare Lepus americanus populations in southwestern Yukon, Canada. Broods older than 35 days were temporarily enlarged by one, and then by two, young. No effects were observed when one owlet was added, but the addition of two young resulted in significant weight losses in manipulated broods. Females with enlarged broods moved farther from their nest sites at night, presumably reflecting increased hunting effort, and also spent less time near the nest during the day. Food additions to enlarged broods returned the parental behaviour to normal. We conclude that these large predators did not experience superabundant food at this stage of the breeding season during a peak in cyclic prey.
Article
We show evidence of differential predation on snowshoe hares (Lepus americanus) by great horned owls (Bubo virginianus) and ask whether predation mortality is related to antipredator behaviour in prey. We predicted higher predation on (1) young and inexperienced hares, (2) hares in open habitats lacking cover to protect from owl predation, and (3) hares in above average condition assuming that rich food patches are under highest risk of predation. Information on killed hares was obtained at nest sites of owls and by monitoring hares using radio-telemetry. The availability of age classes within the hare population was established from live-trapping and field data on reproduction and survival. Great horned owls preferred juvenile over adult hares. Juveniles were more vulnerable to owl predation before rather than after dispersal, suggesting that displacement or increased mobility were not causes for this increased mortality. Owls killed ratio-collared hares more often in open than in closed forest types, and they avoided or had less hunting success in habitats with dense shrub cover. Also, owls took hares in above average condition, although it is unclear whether samples from early spring are representative for other seasons. In conclusion, these results are consistent with the hypothesis that variation in antipredator behaviours of snowshoe hares leads to differential predation by great horned owls.
Article
The ecology and behaviour of non-territorial owls are little known. During a population peak of snowshoe hares,Lepus americanus, the main prey of great horned owls,Bubo virginianus, in the boreal forest, fledglings were equipped with radiotransmitters, and 30 successful dispersers were monitored in 1988–1991. Of those, nine became resident floaters in the study area. Transient floaters were not recorded, although floaters shifted the centre of their home ranges more than territorial owls. Floater home ranges were about five times larger than defended territories, but the space use did not differ significantly. Floaters intruded regularly into territories and their locations overlapped broadly with those of territory owners and other floaters, but were concentrated on the periphery of defended territories. This is consistent with other evidence that territorial behaviour limits the breeding density of great horned owls even at extreme peaks of prey availability. None of the monitored floaters bred as secondary females, and the intrusion rates and movement patterns of floaters did not change during the fertile period of females, as predicted if male floaters were seeking extra-pair copulations.
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We studied hawk-owls in the southwestern Yukon, Canada, from 1987-1993. Most information on hawk-owls originates from studies in Europe, and very little is known about the subspecies Surnia ulula caparoch in North America. The boreal forest communities in the two continents differ remarkably in the composition of cyclic herbivore populations. Fennoscandia is dominated by 3-4 year microtine cycles, whereas northern Canada and Alaska experience a 10-year cycle in snowshoe hare numbers, with voles fluctuating at lower levels. We studied the diets of nine nesting pairs by pellet analysis, and we observed prey deliveries at five nests. The proportion of voles in the diets was lower than reported from Fennoscandia, and snowshoe hares made up 40-50% during the peak of the hare cycle. Estimates of prey densities by live-trapping revealed that hawk-owls strongly prefer voles over snowshoe hares and squirrels. Among voles, Microtus were preferred and Clethrionomys were avoided. Hawk-owls showed, however, a functional response not only to voles but also to juvenile hares, and they may be critically dependent on larger prey during certain nesting stages when vole abundance is moderate or low. Breeding densities and winter observations changed concurrently over years of different prey abundance. Prey selection translated into population consequences: hawk-owls did not respond numerically to Clethrionomys outbreaks, but to the combined densities of Microtus and snowshoe hares. We conclude that the Northern Hawk-Owl is less of a vole specialist and more affected by the prey composition in specific systems than commonly assumed, and we discuss this pattern from an evolutionary perspective.
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Synchronized fluctuations in vole and grouse populations in boreal Fennoscandia have been explained as the result of shifting predation pressure: the alternative prey hypothesis. We tested this hypothesis by supplying additional food to predators in only one of two 615 ha areas during the crash in vole density in 1985. As predicted from the hypothesis the proportion of young grouse in autumn was higher in the experimental area than in the control area. Moreover, the proportion decreased in the control area in comparison with the year before, but did not do so in the experimental area. Thus, with respect to synchronized short-term population fluctuations of voles and grouse, we find the hypothesis that predators take only a doomed surplus less probable. However, we cannot discard the hypothesis that grouse decline during years of strong vole decline for example because a poor quality or low quantity of food makes them more vulnerable to predators. Hence, predation is a necessary but perhaps not a sufficient factor in mediating short-term fluctuations from voles to grouse in boreal Fennoscandia.
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The most common age class of females was 1-yr old and that of males was 2-yr old. A larger proportion of females than males bred as yearlings, possibly because the breeding success of a pair is largely dependent on the male's ability to provide food. First-year breeders were most frequent in peak years, when many immigrant young owls entered the breeding population. Yearling-yearling pairs performed less well in their breeding attempts than did pairs of 2-yr old mates, which in turn did less well than did older pairs. Nests of 1st-yr males had smaller prey stores than those of 2nd-yr males, which in turn had smaller stores than nests of older males. A lack of experience and shortage of food seemed to cause poor performance of yearlings. -from Author
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The alternative prey hypothesis (APH) states that synchronous population fluctuations of small game are caused by varying predation impact. Voles that show 3-4 yr population cycles in C and N Fennoscandia, are the staple food of several predators. APH predicts that these predators partly switch their diet from voles to small game as the staple prey decrease. The authors collected food samples of breeding eagle and Ural owls (Bubo bubo, Strix uralensis) during 1973-87 in S Ostrobothnia (SO) and during 1965-80 in C Ostrobothnia (CO), Finland, and tested the following predictions of APH: 1) The yearly abundances of voles in the field should correlate positively with the proportions of voles in the diet. Data from eagle and Ural owls in SO were consistent with this. 2) The proportion of small game in the diet should be negatively related to the abundance of voles in the field. This was true for the Ural owl in SO. 3) The owls should take more small game in poor vole years than in good ones, independently of the proportion of voles in the diet. This was the case for the 2 owls in CO. 4) The proportion of small game in the diet is nearly independent of its abundance in the field. This held true for the eagle owl in CO and the Ural owl in the 2 areas, while most data supported the 4 predictions of APH, additional data on their predation impact on small game are needed to better assess how much these two owls are responsible for crashes of hare and grouse populations. -from Authors
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We studied numerical and functional responses of breeding European Kestrels (EK) (Falco tinnunculus), Short-eared Owls (SO) (Asio flammeus), and Long-eared Owls (LO) (Asio otus) during 1977-1987 in 47 km2 of farmland in western Finland. The pooled mean yearly breeding density varied from 0.1 to 2.4 pairs/km2. The number of nesting EKs (range 2-46 pairs), SOs (0-49), and LOs (0-19) fluctuated in close accordance with the spring density of Microtus (M. agrestis and M. epiroticus) voles. The mean yearly number of fledglings produced per pair ranged from 0.4 to 3.8 and, for each species, was positively correlated with spring density of Microtus voles. Due to their high degree of mobility, EKs, SOs, and LOs were able to track the population fluctuations of their microtine prey without time lags. An increase in microtine densities caused a rapid immigration into the study area and a decrease caused a rapid emigration from the area. Microtus voles were the most important prey group by mass in the diet of each species. Water voles, bank voles, shrews, and small birds were the most frequent alternate prey. The spring density of Microtus spp. was positively correlated with the percentage of these voles in the diet of EK, SO, and LO. The pooled functional response curve of these three raptor species to the fluctuating densities of Microtus spp. was close to linear, indicating that consumption rates are independent of vole densities. Breeding EKs, SOs, and LOs seemed to take a larger proportion of voles available in peak years than in low ones.
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Food habits and trophic convergence between Palearctic Eagle Owl and Nearctic Great Horned Owl were studied on the basis of the taxonomic composition of their diet, trophic diversity and mean weight of prey. Within each faunal region six biomes were distinguished: Mediterranean shrubland, temperate deciduous forest, boreal conifer forest, grassland, temperate desert, and mountain. Three prey assemblages dominated the diet of the two owls but maximum frequencies were not always attained in similar Palearctic and Nearctic biomes. Thus, lagomorphs dominated in the Mediterranean shrubland (Eagle Owl) and temperate deciduous forest (Great Horned Owl), respectively; voles (Cricetidae) dominated the diet of both owls in the boreal conifer forest; desert rodents (gerbils, jerboas and kangaroo rats) and invertebrates dominated their diet in deserts. The diet of the two owl species reached maximum trophic diversity in temperate deciduous forest and deserts and minimum mean weight of prey in the deserts. Great Horned Owls tended to have a more diverse diet than Eagle Owls. Limited trophic convergence between the two species of owl could be due to differences between similar Palearctic and Nearctic biomes in the composition of prey assemblages and abundance of the preferred prey (lagomorphs), and to intra- and interspecific differences in the body weight of the owls.
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A mortality factor should be density-dependent to regulate population density. In western Finland, vole (Microtus spp.) populations fluctuated in 3-4-yr cycles in a farmland area. During 1977-87, the mortality rate of voles by breeding avian predators was positively correlated with vole density in spring. Accordingly, vole mortality by avian predators directly depended on prey density, and avian predators have thus a potential to reduce the amplitude of the multiannual vole cycle. Vole kill rates of least weasels during six winters were significantly positively related to the prey densities in the preceding spring (3/4-yr lag). The same relationship for the stoat was also positive but not significant. In seasonal environments, 3-4-yr vole cycles might be driven by a density-dependent mechanism affecting the growth rate of vole populations with a 9-month lag (May 1981). My results suggest that one mechanism producing this critical lag could be mustelid, especially least weasel, predation.
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A generalized descriptive equation for the effect of prey density on the instantaneous rate of attack was proposed; A(N0 ) = \fracTt th + 1/[ a¢N0 exp(cN0 ) ]A(N_0 ) = \frac{{T_t }}{{t_h + 1/\left[ {a'N_0 \exp (cN_0 )} \right]}} whereA(N o ) is the number of attacks per predator during timeT t ,N o is the prey density,T t is the total time the prey was exposed,T h is the handling time per prey,a′ is the rate of successful search, andc is the facilitation coefficient. The proposed equation can describe all types of the functional response curves; withc=a′ t h (pseudo Type I), withc=0 (Type II), withc>a′ t h (Type III), and withc<0 (Type IV). The applicability of the equation was tested against the results of a simulation model and available laboratory and field data on predation. The equation described these data very well and in many cases yielded biologically interesting insights, although the equation was proposed primarily for descriptive purposes.
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(1) Predators were removed in an experiment to study the impact of mammal predation on woodland tetraonid populations during vole cycles. Foxes and martens were killed from 1976 to 1980 on one of two similar islands in the northern Baltic. The treatment was then reversed until 1984. (2) When predators were killed, tetraonid brood sizes averaged 5.52 in August, and 77% of hens had chicks. When predator were not killed, broods averaged 3.29 chicks and 59% of hens had chicks. (3) Counts of adult capercaillie and black grouse during July and August increased by 56-80% after 2 years of predator removal. Counts at leks increased by 166-174%. (4) Removing foxes and martens had no significant effect on vole abundance during two 4-year cycles. (5) When predators were not removed, tetraonid brood sizes and the proportion of females that bred successfully were each positively correlated with vole abundance in summer. There were most chicks per adult hen when vole numbers were high and increasing slowly from summer to autumn. When foxes and martens were killed, neither brood size nor subsequent adult numbers were significantly correlated with vole abundance in summer, although losses of whole broods increased slightly when vole numbers grew most rapidly from summer to autumn. We conclude that large vole populations resulted in large autumn grouse populations mainly because they reduced predation on breeding grouse. (6) The vole numbers and increase rates that were asociated with high grouse breeding success in one summer were also associated with low counts of adult grouse the next year, and thus with an increase in grouse losses from one summer to the next.
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Male Aegolius funereus collect nearly all the food for their females and young in the breeding season. Brood sizes of males between one year and the following year were not significantly correlated. Males rearing large broods survived only marginally better the next year than those rearing small broods. In 1985 and 1986, 1 newly hatched young was in some cases transferred between 2 nests. In control nests, one young was exchanged for one young from another brood. The proportion of males surviving and breeding in the next year was similar in the 3 groups that reared reduced, control or enlarged broods in the preceding year. Enlarged broods did not affect breeding performance in the next year. In 1985 (when the vole supply was increasing), the owls could rear enlarged broods, whereas in 1986 (vole supply decreasing), they tended to be unable to do so. In 1986 but not in 1985, fledglings in enlarged broods were slightly lighter than those in control or reduced broods. Neither correlative nor experimental data from the males supported the hypothesis of reproductive costs. The probable reason was that males given enlarged broods did not increase their reproductive effort. In the increase phase of the vole cycle, the number of coles available to the egg-laying female probably limits clutch size, whereas in the decrease phase food available during the nestling and fledging periods is even more limited. -from Author
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1. The relationship between the yearly densities of avian predators and their microtine prey in western Finland was studied. Predator densities were determined by checking nest-boxes in forests [Tengmalm's owl (Aegolius funereus), hawk owl (Surnia ulula) and Ural owl (Strix uralensis)] and by searching for nests in farmland [short-eared owl (Asio flammeus), long-eared owl (Asio otus) and kestrel (Falco tinnunculus)]. Tengmalm's, Ural and hawk owls overwinter in Fennoscandia, long-eared owls are partially migratory, and short-eared owls and kestrels are migratory. 2. Prey densities were estimated by snap-trapping in spring (early May) and autumn (mid-September), and by snow-tracking in late February to early March (early spring) and late November to early December (late autumn). 3. The breeding densities of Tengmalm's owls in two study areas were significantly correlated with trap indices of voles in the prevailing spring and in the preceding autumn (6-month lag), but not in the preceding spring (1-year lag). Tengmalm's owl breeding densities in one study area covaried with track indices of voles in the early spring (i.e. the settling period of owls), but not with those in the late autumn. 4. The yearly breeding densities of Ural owls and hawk owls, and the wintering densities of hawk owls were positively related to spring trap indices of voles, but not to indices in the previous autumn and spring. 5. The breeding densities of long-eared owls and short-eared owls were dependent on vole abundances both in the current spring and the preceding autumn, but not in the preceding spring. Kestrel breeding densities fluctuated in accordance with spring vole abundances. 6. The densities of most avian predators tracked rapidly, without obvious time lags, vole abundances at the time the birds of prey settled on their territories. This rapid tracking is mostly based on vole-supply dependent immigration and emigration. Densities of most avian predators did not lag 9 months behind prey densities which, in theory, may drive 3-4-year vole cycles (May 1981).
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Reviews 138 cases in which terrestrial vertebrates received supplemental food under field conditions. These cases are strongly biased toward small-bodied herbivores in north temperate environments. Most studies address population level questions and have supplied food over a short term (<1 yr) and on a small spatial scale (to <50 individuals). Individuals receiving supplemental food usually had smaller home ranges, higher body weights and advanced breeding relative to those on control areas. The typical population response to food supplementation was 2- to 3-fold increase in density, but no change in the pattern of population dynamics. In particular, food addition did not prevent major declines in fluctuating populations. Researchers have failed to examine behaviour of individuals under conditions of supplemental food when addressing questions of population regulation. -from Author
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Yearling male song sparrows Melospiza melodia on Mandarte Island, British Columbia, were either territorial or nonterritorial. Some territorial yearlings failed to gain mates. Mated yearlings with territories and floaters were much more common than unmated yearlings with territories. Floaters made up a higher proportion of the population in years of high population density. Unmated territorial birds accounted for a higher proportion of all yearling males at low population densities. Most floaters and unmated territorial yearlings remained unmated during their first breeding season. Floating males were more likely to disappear from the island after the 1st year of life. Nearly all birds that survived to 2 yr of age defended territories and bred. Birds that were floaters as yearlings and later gained a territory did not reproduce better in later life than birds that acquired territories as yearlings. Males that obtained territories and mates as yearlings thus raised more than twice as many breeding offspring during their lifetimes. Yearling territorial males without mates survived well but did not breed more successfully than floaters over their life spans. -from Authors
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Populations of certain mammal species and their predators show cyclic fluctuations in northern latitudes, and the amplitude of cycles in some cases increases towards the north. The evidence reviewed suggests that (1) abiotic factors and intrinsic mechanisms are unable to explain cycles; (2) quantity and quality of food resources of the herbivore population have important effects on population dynamics, although plant-herbivore interaction cannot explain the cycles by itself; (3) predation is another important factor for these populations, and probably essential for cyclicity of herbivore populations. A number of mathematical models have shown that prey-predator models can produce limit cycles, but they have not demonstrated that the cyclic fluctuations observed in natural populations can be explained by the mechanisms they incorporate and the parameters they define. In this study a mathematical model is developed to predict specific patterns of prey-predator cycles observed in nature with independently estimated parameters. This prey-predator model is based on the concept of "ratio dependence": the trophic functions (functional and numerical responses) are modeled as functions of prey-to-predator ratio rather than as functions of prey density only, as in traditional prey-predator models. This approach incorporates the concept of interference in a simple way by describing trophic interactions as functions of per capital resources. The parameters of the model are estimated from studies on the biology of cyclic lynx and hare populations, rather than by fitting time-series data to the model. Parameters of the model give rise to limit cycles when they are changed in the way they are expected to change from south to north, which is consistent with the observations on the latitudinal patterns in cyclicity. The major quantitative prediction of the model is the cycle period. The period is predicted to be around 10 yr, which is the observed period of hare-lynx fluctuations.
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1. Many species of forest Lepidoptera have cyclic population dynamics. Although there are numerous potential causes, including interactions with predators, parasitoids, pathogens, and food-plant quality, strongly density-dependent interactions are often difficult to demonstrate. Both autocorrelation analysis and attractor-reconstruction methods have recently been applied to a number of species' time series. Results suggest that complex dynamics, i.e. cycles or deterministic choas, may be more prevalent than once thought, and that higher-dimensioned models are necessary. 2. We develop a two-dimensional difference equation model that relates the average quality of individuals to patterns of abundance. The delayed density dependence is caused by transmission of quality through generations via maternal effects. We show that the maternal effect model can produce patterns of population fluctuations similar to those displayed by one class of host-parasitoid models. 3. We review empirical evidence for maternal and quality effects in dynamics of forest Lepidoptera. We fit the maternal effect and delayed logistic models to six species of forest moths for which delayed density dependence and maternal or quality effects have been found. The maternal effect model was a good predictor of the period of the oscillations for the species that we examined. We discuss why models of this type give better fits to moth cycles than do first order models with added delays.
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In a peak vole year in W Finland, owl pairs that were given supplementary food in the pre-laying and laying periods laid more eggs earlier than control pairs. Supplementary feeding was stopped after clutch completion. The fed pairs subsequently had larger prey stores in their nest-boxes, showed better hatching success and produced more fledgings than did the unfed pairs. Thus, food availablility restricted both start of laying and clutch size of Tengmalm's owl, even in a peak vole year when natural food conditions were very good. Results are consistent with the "food limitation hypothesis' of Perrins (1970). -from Author
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We tested the hypothesis that synchronous fluctuations in small game species in boreal Fennoscandia are caused by varying predation pressure. The main prey of predators are the cyclically superabundant voles. Small game species (alternative prey) are rare compared to voles. The following 4 predictions were checked: (1) Predators should shift their diet from main prey to alternative prey as main prey decline. — This was confirmed using data on red fox (Vulpes vulpes L.) diet.; (2) The mortality rate of alternative prey should be inversely correlated to the abundance of main prey. — This was true for mountain hare (Lepus timidus L.) mortality rates and the rate of nest predation on black grouse (Tetrao tetrix L.).; (3) The total consumption of prey by all the predators should at least equal the critical losses in alternative prey during a decline year. — A tentative estimate of predator consumption amounted to 10 times the losses in grouse and hare.; and (4) The absence of synchrony between the species in the boreonemoral region should be associated with a more diverse diet of predators. — This was the case for red fox diets throughout Sweden. Although all 4 predictions were confirmed, we could not necessarily exclude other hypotheses involving changes in quality or quantity of plant food.
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Populations are regulated intrinsically (self-regulated) when the animals lower their rate of increase behaviorally or physiologically as a reaction to rising density. They are regulated extrinsically if the equilibrium is a mechanical consequence of interaction between the population and the organisms providing its food. We suggest that, at least for mammalian herbivores, self-regulation is unlikely to evolve unless the population's intrinsic rate of increase exceeds about 0.45 on a yearly basis. That value corresponds to a body weight of about 30 kg, the intrinsic rate being related inversely to body weight by r m=1.5 W-0.36 with W in kg.The two dynamic strategies, self-regulation and extrinsic regulation, should enforce a bimodality of the frequency distribution of observed intrinsic rates of increase. This in turn might be reflected in a bimodality of body sizes, the smaller herbivores constituting the lower mode generally showing intrinsic regulation and the larger herbivores of the upper mode generally being regulated by extrinsic mechanisms. There is some empirical support for these predictions but it is by no means clearcut.Mechanisms of self-regulation can evolve either by individual or group selection. Individual selection may act in two ways. By inhibiting their neighbours with some form of interference, individuals may increase their relative fitness without increasing their reproductive rate. Alternatively, individual selection may raise the absolute fitness of individuals and thereby raise the populations's intrinsic rate of increase. The population is destabilized if that process continues beyond a certain threshold and the population is then at significant risk of extinction at the troughs of the consequent oscillations. Selection between such populations will favour those carrying the beginnings of a self-regulating mechanism, and with that mechanism strengthened and fixed by continuing group selection, individual selection is again freed of the dynamic restraints on raising further the intrinsic rate of increase.
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We relate causes of mortality of snowshoe hares to density of hares over an 8-year period that included a peak in numbers. We then use simulation modeling to examine whether these density-dependent relationships could produce changes in hare density similar to those observed in our study are in Yukon, Canada. Predation during winter was the largest source of mortality for snowshoe hares at Kluane, Yukon during 1978–84. There was a one-year lag in the response of winter predation mortality rate to hare density. There was a two-year lag in the response of winter mortality not caused by predators to hare density. A simple simulation model with density-dependent predation produced 8–11 year cycles only within a narrow range of parameters that are inconsistent with data from the Kluane region. However, a simulation model that predicted winter mortality rates using a delayed density-dependent numerical response and a Type II functional response by predators, produced 8–11 year cycles within the range of parameter values measured in our study. Yet another simulation model that predicted both summer and winter mortality rates using a delayed density-dependent numerical response and a Type II functional response by predators, did not produce 8–11 year cycles within the range of parameter values measured in our study. Lack of data on juvenile mortality may be one reason for this result.