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Understanding the role that extrinsic processes play in shaping animal population dynamics and demography is a central tenet of population ecology and an issue of vital importance for conservation and wildlife management. The top-down impact of predation and bottom-up influence of food availability are thought to be two of the most important extrinsic processes affecting population dynamics and demography of species occupying middle trophic levels. However, many studies only focus on quantifying the impact of one of these processes in isolation and it is not clear whether the impact of one extrinsic factor on population dynamics and demographic rates is augmented or lessened by changes in other extrinsic factors. In this thesis I examine the extent to which both top-down and bottom-up processes shape population dynamics (population size, recruitment and immigration) and demography (survival, reproduction, life-history trade-offs and reproductive strategies) in a long-lived species, the tawny owl, by taking advantage of a natural increase in predation risk (goshawk abundance) and a decline in food availability (field vole densities). Despite the increase in predation and the decline in food availability, the owl population remained stable, which we posit is due to goshawk predation being selective on individuals with a low reproductive value (juveniles and old individuals) and an increasing number of immigrants entering the population. Selective predation on older owls had a negative impact on the survival of this age class and appeared to be shaping the pattern of actuarial senescence and influencing the strength of the intrinsic trade-off between survival and reproduction. As food availability declined and predation risk increased owls appeared to be switching from an 'eggs in one basket strategy' of saving resources to invest more in fewer breeding attempts in the future, to a 'bet-hedging' strategy of reproducing more often, but investing less per breeding attempt.

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Anthropogenic activities surrounding horticulture, agriculture and recreation have increased dispersal of invasive earthworms. The introduction of earthworms initiates many physical and chemical alterations in forest soils previously unoccupied by earthworms. Three trials were performed to assess the effects of earthworms on soil-water dynamics, C and N and defensive/storage compound production by a native plant. The first trial was a greenhouse experiment, performed to assess the impact of two ecologically contrasting invasive earthworm species on percolate and evaporative soil water loss. Mesocosms were constructed to simulate a typical forest Entisol commonly penetrated by the species of interest, Lumbricus terrestris and Amynthas agrestis. Earthworms were added in pair and combination to replicate an average population density observed in Vermont. Percolate water was collected and evaporative water loss balance was recorded over a period of 140 days. C and N were quantified in A-horizon bulk and aggregate soil as well as the subsoil. Residual surface leaf litter was also quantified. Results indicated significantly increased evaporative water loss where either worm was present. Conversely, percolate water loss was significantly reduced in presence L. terrestris alone. C and N analysis revealed that only L. terrestris had a significant effect on aggregate soil C. While the abundance of many herbaceous species is reduced at invasive earthworm sites, Arisaema triphyllum anecdotally have greater densities where earthworms are present. It has been hypothesized that the greater density is caused by a trait that allows this plant to store Ca, often observed at increased concentrations in earthworm invaded soils as Ca-oxalate Here, we tested the hypothesis that oxalate increases in A. triphyllum when earthworms are present. As such, we conducted a two-way factorial greenhouse trial to test whether the changes to soil properties made by two invasive earthworm species (Amynthas agrestis and Lumbricus rubellus) or their physical presence (and bioturbation) had an effect on the plant production of oxalate. Upon quantification of variable soluble oxalate in corms after senescence, we found that earthworm presence increased water soluble and total oxalate significantly as well as marginally significantly in the case of HCl soluble oxalate. No significant changes in oxalate concentrations were observed under soil treatments alone. Carbon and nitrogen are found extensively in both terrestrial and atmospheric cycles. A shift in the equilibrium of these elements can suggest a strong interaction between an introduced variable (invasive earthworms in this case) and the abiotic environment. To better understand changes in soil properties with earthworm invasion, a 112-day mesocosm study was undertaken to examine C and N dynamics. Two epi-endogeic invasive earthworm species Lumbricus rubellus and Amynthas agrestis were selected for study. Greenhouse gas production by total mesocosm and soil were monitored. Gas flux measurements on 11 dates indicate both worm species increase CO2 and N2O emitted from mesocosm system as well as soil. Mesocosm total C and N (mass balance) indicate significantly more N but no change in C where earthworms are present. This indicates a disruption of denitrification by earthworm invasion that results in increased N2O emissions. This research is the first to examine these variables in concert and confirms holistic view is essential when examining natural systems.

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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