Vidar Selås

Cyclic population fluctuations of herbivores as an effect of cyclic seed cropping of plants : The mast depression hypothesis

Three sets of time series on plant seed production and herbivore number were analysed in order to test the proposed mast depression hypothesis, which states that cyclic population fluctuations of herbivores is a result of cyclic seed cropping of plants, because high seed crops, termed masts, are produced at the expense of chemical defence against herbivores. From 1880 to 1916, the annual export of small game, mainly willow grouse Lagopus lagopus, from Oslo was positively correlated with the quantity of berries, mainly bilberry Vaccinium myrtillus and cowberry V. ritis-Idaea, exported in the previous year. In vegarshei, South Norway, the intervals between years of high access of bilberry and cow-berry varied from 3 to 5 yr during 1952 95. All mast years were followed by a year of high microtine population. In South Norway, mast years of Norway spruce Picea abies, which usually shows high flowering intensity in the same years as mountain birch Betula pubescens, occurred with a mean interval of 9.8 yr (range 7 13 yr) during 1930-85. Almost all attacks of autumnal moth Epirrita autumnata on mountain birch were reported after a mast year of forest trees. The synchronised flowering cycles of plants indicate that high seed crops are induced by some climatic conditions. Allocating a large proportion of available resources, including chemical defence compounds, to seed production in some years of advantageous conditions may improve a plants total life-time reproduction, despite the subsequent risk of serious damage by herbivores.

The effect of a natural reduction of red fox Vulpes vulpes on small game hunting bags in Norway

An epizootic of sarcoptic mange among red foxes Vulpes vulpes reached central Norway in 1976, and by 1986 it had spread to the whole country, resulting in a severe decline in the red fox population. We analyse the change in the hunting bags of the predator species red fox and pine marten Martes martes, and the prey species capercaillie Tetrao urogallus, black grouse Tetrao tetrix, mountain hare Lepus timidus and willow grouse Lagopus I. lagopus from the period prior to and during the mange epizootic. The data are gathered from publications by Statistics Norway and are based on answers from more than 5,000 hunters yearly. On the national level, the hunting bags of capercaillie, black grouse, mountain hare and pine marten were significantly negatively correlated with that of the red fox. When the time series at the national level were detrended, there were positive correlations between the hunting bags of all species without time lag, except that of pine marten, which lagged one year behind the other species. At the local level there were negative correlations between the hunting bag of red fox and those of the small game species except for willow grouse. The study confirms that the red fox is a keystone predator in Scandinavia.

Effects of seed masting of bilberry, oak and spruce on sympatric populations of bank vole (Clethrionomys glareolus) and wood mouse (Apodemus sylvaticus) in southern Norway

In southern Norway, population fluctuations of the herbivorous bank vole Clethrionomys glareolus and the granivorous wood mouse Apodemus sylvaticus were analysed in relation to mast seeding of bilberry Vaccinium myrtillus , sessile oak Quercus petraea and Norway spruce Picea abies . Bilberries ripen in August, acorns are dispersed in September, whereas spruce seeds are usually dispersed during winter or spring. The bilberry index of both the current and previous year contributed significantly to explain the population growth indices of bank vole. The spruce seed index significantly influenced only the bank vole, whereas the acorn index significantly influenced only the wood mouse, apparently through increased winter survival. Because of the lack of significant responses to bilberry masting by granivorous wood mice, we conclude that the most likely explanation for the observed peak populations of bank voles in post-mast years of bilberry is that high seed crops have a positive influence on the chemical composition of bilberry plants, which are important as food for bank voles during winter.

Lichen Compounds Restrain Lichen Feeding by Bank Voles (Myodes glareolus)

Some lichen compounds are known to deter feeding by invertebrate herbivores. We attempted to quantify the deterring efficiency of lichen compounds against a generalist vertebrate, the bank vole (Myodes glareolus). In two separate experiments, caged bank voles had the choice to feed on lichens with natural or reduced concentrations of secondary compounds. We rinsed air-dry intact lichens in 100% acetone to remove extracellular compounds non-destructively. In the first experiment, pairs of control and rinsed lichen thalli were hydrated and offered to the bank voles. Because the lichens desiccated fast, we ran a second experiment with pairs of ground control and compound-deficient thalli, each mixed with water to porridge. Eight and six lichen species were tested in the first and second experiment, respectively. In the first, bank voles preferred compound-deficient thalli of Cladonia stellaris and Lobaria pulmonaria, but did not discriminate between the other thallus pairs. This was likely a result of deterring levels of usnic and stictic acid in the control thalli. When lichens were served as porridge, significant preference was found for acetone-rinsed pieces of Cladonia arbuscula, C. rangiferina, Platismatia glauca, and Evernia prunastri. The increased preference was caused mainly by lower consumption of control thalli. Grinding and mixing of thallus structures prevented bank voles from selecting thallus parts with lower concentration of secondary compounds and/or strengthened their deterring capacity. We conclude that some lichen secondary compounds deter feeding by bank voles.

Autumn population size of capercaillie Tetrao urogallus in relation to bilberry Vaccinium myrtillus production and weather: an analysis of Norwegian game reports

From a study area in Aust-Agder, southern Norway, game reports from 1920–1978, supplemented with autumn counts carried out during 1968–1984, were used to determine whether the autumn population size of capercaillie Tetrao urogallus showed no increase, a slight increase or a strong increase compared to the population size the previous year. Based on the mast depression hypothesis, it was predicted that adverse weather conditions should have less influence on the reproduction and thus also on the autumn population size in postmast years of bilberry Vaccinium myrtillus than in other years, because of a higher food quality and therefore also a higher body condition of the birds. In a logistic regression model, only the bilberry index and the June–September temperature of the previous year contributed significantly to explain the status of the capercaillie population. A negative effect of high summer temperatures in the previous year was highly significant when analysing post-mast years separately, possibly because bilberry plants were less depressed after a high seed crop if summer temperatures and thus primary production were high. Only when years with high bilberry production were analysed separately, did I find effects of weather conditions which could be assumed to have direct impacts on breeding success, such as snow conditions in spring and precipitation in early summer.

Explaining bank vole cycles in southern Norway 1980–2004 from bilberry reports 1932–1977 and climate

Correlations between mast fruiting of bilberry Vaccinium myrtillus and peak levels of Clethrionomys-voles have been reported from both Norway and Finland, but there has been a discussion whether this is a bottom-up or a top-down relationship. In a multiple regression model, 65% of the variation in a bilberry production index calculated from game reports from southern Norway 1932–1977 could be explained by the berry index of the two preceding years and climate factors acting during key stages of the flowering cycle. High vole populations in previous years did not contribute to explain the fluctuation in berry production. I used the selected model and climate data to predict bilberry production for the period 1978–2004. Predicted berry indices of the current and previous year explained 38% and the total amount of precipitation in May–June explained 16% of the variation in a log-transformed snap-trapping index of bank vole Clethrionomys glareolus 1980–2004. The vole index was not related to any of the climate variables used to predict berry production. This pattern supports the hypothesis that vole cycles are generated by changes in plant chemistry due to climate-synchronized mast fruiting.

Large brood sizes of pied flycatcher, sparrowhawk and goshawk in peak microtine years: support for the mast depression hypothesis

Abstract The mast depression hypothesis (MDH) proposes that cyclic population fluctuations of microtines and other herbivores are an effect of cyclic seed cropping of plants. This is because high seed crops, termed masts, are produced at the expense of chemical defence against herbivores. It has generally been assumed that bird-hunting raptors produce high numbers of offspring when microtine prey are abundant because of reduced competition from generalist predators. However, this may also be caused by higher production of herbivorous insects, and thus insectivorous bird prey, because of lower contents of chemical defence compounds in some plant species, such as bilberry Vaccinium myrtillus and cowberry V. vitis-idaea. In Aust-Agder county, southern Norway, the mean brood size of pied flycatcher Ficedula hypoleuca, sparrowhawk Accipiter nisus and goshawk A. gentilis was higher in peak vole years than in other years. The effect was not due to variation in nest predation, as only successful nesting attempts were included in the analyses. For the pied flycatcher, the annual proportion of large broods (>6 fledglings) was positively correlated with the vole trapping index. No correlation was found between the offspring production of goshawks and the proportion of voles in their diet. During a 3-year light-trapping study of nocturnal moths prior to our study, four moth species whose larvae ate Vaccinium were commonest in the vole peak year. All these results are consistent with the MDH.

Prey Delivery Rates as Estimates of Prey Consumption by Eurasian Kestrel Falco tinnunculus Nestlings

Steen R., Løw L.M., Sonerud G.A., Selås V. & Slagsvold T. 2011. Prey delivery rates as estimates of prey consumption by Eurasian Kestrel Falco tinnunculus nestlings. Ardea 99: 1–8. In altricial birds the type of prey selected by parents for their nestlings may affect the allocation of time and energy spent on hunting, preparing prey and feeding the nestlings, which in turn may affect the rate of provisioning. Raptors take relatively large prey items, which facilitates the quantification of rates of prey items and prey mass delivered to nestlings. Estimates of rates of prey delivery in raptors are nevertheless few and have been based on direct observations from a hide in combination with analyses of prey remnants and regurgitated pellets. To obtain better estimates we video monitored prey deliveries at 55 nests of Eurasian Kestrels Falco tinnunculus. Of the 2282 prey items recorded, voles were most abundant by number, followed by birds, shrews and lizards, while insects and frogs were rare. An average brood of 4.3 nestlings was estimated to consume 18.3 g/h, hence a nestling consumed on average 4.2 g/h. This is equivalent to 67.8 g/d, given an average daily activity period of 16.1 h. The estimated delivery rate of prey items required to feed an average brood in our study was 91 per h if the kestrels had provided only insects, and 3.4, 1.9, 0.83 and 0.52 if they had provided only lizards, shrews, voles or birds, - respectively. This corresponds to one prey delivery per 40 s if feeding solely on insects and one per 18, 32, 75 and 120 min if feeding solely on lizards, shrews, voles or birds, respectively. We argue that kestrels in the boreal forest would be unable to raise an average brood solely on insects or lizards, unlikely to do so solely on shrews, but able to do so solely on voles in a vole peak year.

The arctic fox Alopex lagopus in Fennoscandia: a victim of human-induced changes in interspecific competition and predation?

After a marked decline at the beginning of the 1900s, the arctic fox Alopex lagopus population in Fennoscandia has remained at a very low level. We suggest that the main cause for the population crash was winter starvation caused by (1) over-hunting of reindeer Rangifer tarandus populations, and thus reduced carcass availability in the mountains, and (2) increased interspecific competition for these carcasses because of increased invasion of red foxes Vulpes vulpes from lower altitudes. The failure of arctic fox populations to recover, despite increasing reindeer populations in the mid 1900s, can be explained by a concurrent strong increase in red fox numbers. Analyses of countywide hunting statistics from Norway 1891–1920 suggest that there actually was an increase in red fox numbers in the period of arctic fox decline, and that the increase in reindeer populations from the 1920s to the 1950s was accompanied by a new increase in red fox numbers. We conclude that restoring arctic fox populations most likely will require a substantial and lasting reduction of red fox populations.

Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors.

In birds with bi-parental care, the provisioning link between prey capture and delivery to dependent offspring is regarded as often symmetric between the mates. However, in raptors, the larger female usually broods and feeds the nestlings, while the smaller male provides food for the family, assisted by the female in the latter part of the nestling period, if at all. Prey items are relatively large and often impossible for nestlings to handle without extended maternal assistance. We video-recorded prey delivery and handling in nests of a raptor with a wide diet, the Eurasian kestrel Falco tinnunculus, and simultaneously observed prey transfer from male to female outside the nest. The male selectively allocated larger items, in particular birds and larger mammals, to the female for further processing and feeding of nestlings, and smaller items, in particular lizards and smaller mammals, directly to the nestlings for unassisted feeding. Hence, from the video, the female appeared to have captured larger prey than the male, while in reality no difference existed. The female’s size-biased interception of the male’s prey provisioning line would maximize the male’s foraging time, and maximize the female’s control of the allocation of food between her own need and that of the offspring. The male would maximize his control of food allocation by capturing smaller prey. This conflict would select for larger dominant females and smaller energy-efficient males, and induce stronger selection the longer the female depends on the male for self-feeding, as a proportion of the offspring dependence period.