Abstract Relationships between predators and prey may affect population dynamics of both parties. Predators may also serve as a link between populations of different prey, e.g., small game and small mammals. I used available data on the diet and reproduction of birds of prey (mainly common buzzards Buteo buteo and goshawks Accipiter gentilis) and video surveillance of their nests, as well as multiannual data on numbers of grouse and small mammals for studying food habits and population dynamics of raptors and their links with population fluctuations of voles and grouse (capercaillie Tetrao urogallus, black grouse Tetrao tetrix and hazel grouse Bonasa bonasia) in western Finland during 1980–1990s when grouse and vole numbers fluctuated in regular cycles. Microtus voles were the main prey of the buzzards which partly switched their diet to small game (juvenile grouse and hares) in years when vole numbers declined. The nesting rate of buzzards also correlated with vole abundance, but the productivity rate and brood size tended to lag behind the vole cycle. This mismatch between the buzzards' functional and numerical responses resulted in a fairly small impact of buzzards on juvenile grouse, which did not correlate with vole density. The productivity of goshawks followed the fluctuations of grouse density closely whereas the occupancy rate of goshawk territories did so with a two-year lag. The annual numerical ratio of goshawk to grouse was inversely related to grouse density, suggesting that this predator may be a destabilising factor for grouse population dynamics. However, the goshawks' kill rate of grouse showed no clear relations to grouse density. In June–July, these birds of prey (including hen harriers Circus cyaneus) usually killed a relatively small number of grouse chicks. Losses to raptors constituted up to one quarter of grouse juvenile mortality during the two months. We did not find a strong effect of avian predators on grouse juvenile mortality. In boreal forests, predators and other factors of grouse mortality do not operate as one, and there is probably no single factor responsible for the reproductive success of grouse.

Rodents often colonise agricultural lands and become pests that cause economic loss and crop damage. Eradicating or regulating pest numbers has economic and environmental costs and has always been of great interest to farmers and nature conservationists. To reduce rodent numbers poison baits are used more often than biological control. However, poisoning has adverse environmental impacts and ignores the underlying biological factors that influence pest abundance. This study examined the interactions of the common rat species of oil palm plantations, namely Rattus rattus diardii, R. argentiventer and R. tiomanicus, with their food resource, habitat structure and introduced predators in an attempt to provide a better understanding and approach for their control. To investigate the interrelationships among rodents, owls and oil palms, rodent trapping was conducted simultaneously with pellet collection and assessment of the breeding of owls over six study plots and seven trapping intervals in one oil palm plantation. Trapping records suggested that relative abundances of rodent species differed in plots with palms of differing ages. There were more R. r. diardii as a proportion of the total captures in older palms (seven years old) while R. argentiventer was the most common rat captured in younger palms (three years old). The abundance of R. tiomanicus remained low throughout trapping sessions for palms of all ages. A numerical response of rats to fruit availability in older palms was demonstrated by a positive and significant correlation between the numbers of fruit bunches present and the total number of rats captured. This relationship was observed in both older and younger palms only for R. argentiventer but not R. r. diardii or R. tiomanicus in either age of palms. This suggests that the competitiveness of R. argentiventer may be higher than that of the other two species providing that there were no factors other than food availability that limit the numbers of other two species. However, the levels of damage to palm fruit were significantly correlated with the relative abundance of R. argentiventer only in younger palms, not in older ones where R. r. diardii were more abundant. The more terrestrial nature of R. argentiventer may have restricted their acquisition of food when the palms became taller. In addition to differences in feeding niche, habitat heterogeneity may also be important in determining the rat species compositions at different ages of palms. Vegetation cover may act as a refuge for rats; the overall occurrence of rats, and especially R. argentiventer, was positively correlated with vegetation cover and height, and even certain vegetation associations. This study supported the continued use of the barn owl Tyto alba javanica for biological control of rodent pests in the palm oil plantations. The numbers of different rat species consumed by owls, as measured by pellet numbers, were proportional to prey captures with R. r. diardii dominating the prey items. Weights and sexes of rats, based upon sizes of bones recovered from owl pellets, indicated that the birds did not preferentially prey on any size or sex classes of rats. Although there was no differential predation by the owls, a functional, and possibly a numerical, response of the birds to changes in rat numbers was demonstrated. A functional response of barn owls to prey abundance was evidenced by a significant positive relationship between the relative abundance of rats captured and numbers of pellets collected. Some form of numerical response of barn owls was suggested by higher breeding records when rat abundances were significantly higher. Since both functional and numerical responses are important determinants of whether predators are likely to be able to regulate prey numbers, the role of barn owls as a practical biological control agent in oil palm plantations was supported. This study suggested that the regulation of rodent pests in oil palm plantations should not be limited to chemical measures but can be complemented by other biological factors including interspecific interactions, manipulation of the availability and density of food and habitat structure, in addition to predation by barn owls. The findings suggested that an integrated approach to rodent control, considering all biological factors that influence rat numbers, should be properly applied if an environmentally friendly and possibly cost effective approach is to be applied for the palm oil industry.

Bank voles, grey-sided voles, and field voles had synchronous 3-4 year density cycles with variable amplitudes which averaged about 200-fold in each species. Cycles of vole predators (red fox and Tengmalm's owl), and their (foxes') alternative prey (mountain hare and forest grouse) lagged behind the vole cycles. The nomadic Tengmalm's owl responded with a very rapid and strong numerical increase to the initial cyclic summer increase of voles (the owl’s staple food). Owl breeding densities in the springs were highly correlated with vole supply in the previous autumns. This suggested that the number of breeding owls was largely determined in the autumn at the time of the owl's nomadic migrations, and that immigration was crucial for the rapid rise in owl numbers. The owl's numerical response was reinforced by the laying of earlier and larger clutches when food was plentiful. In addition, the owl has an early maturation at one year of age. The transition between subsequent vole cycles was characterized by a distinct shift in rate of change in numbers from low to high or markedly higher values in both summer and winter. Regulation increased progressively throughout the cycle since the rate of change decreased continuously in the summers. Moreover, there was a similar decrease of the rate of change in winter. Rate of change was delayed density-dependent. The delayed density-dependence had an 8 month time-lag in the summers and a 4 month time-lag in the winters relative to the density in previous autumns and springs, respectively. These findings suggest that vole cycles are likely to be generated by a time-lag mechanism. On theoretical grounds, it has been found that a delayed density- dependence of population growth rate with a 9 month time-lag caused stable limit cycles with a period between 3 and 4 years. Some mechanisms for the delayed density-dependence are suggested and discussed. The mechanisms are assumed to be related to remaining effects of vole populations past interactions with predators, food supplies, and/or diseases. Unlike the other voles, the bank vole had regular and distinct seasonal declines in density over winter. These declines are proposed to be due to predation, mainly by Tengmalm's owl. Supranivean foraging for epiphytic tree lichens and conifer seeds most likely explains why this species was frequently taken by the owl under snow-rich conditions. The alternative prey hypothesis predicts that a reduction of predator numbers should increase the number of alternative prey. Alternative prey should be less effectively synchronized to the vole cycle by predation at declining and low vole (main prey) densities; they may also lose their 3-4 year cyclicity. The appearance of sarcoptic mange among foxes in northern Sweden in the mid 1970s provided an opportunity to "test" these ideas, and these were found to be supported. In areas with highest mange infection rates, foxes declined markedly from the late 1970s to mid 1980s, whereas hare numbers rose rapidly and appeared non-cyclic. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1991, härtill 7 uppsatser</p> / digitalisering@umu