Tino Schott

The importance of marine vs. human-induced subsidies in the maintenance of an expanding mesocarnivore in the arctic tundra

1. Most studies addressing the causes of the recent increases and expansions of mesopredators in many ecosystems have focused on the top-down, releasing effect of extinctions of large apex predators. However, in the case of the northward expansion of the red fox into the arctic tundra, a bottom-up effect of increased resource availability has been proposed, an effect that can counteract prey shortage in the low phase of the multi-annual rodent cycle. Resource subsidies both with marine and with terrestrial origins could potentially be involved. 2. During different phases of a multi-annual rodent cycle, we investigated the seasonal dynamics and spatial pattern of resource use by red foxes across a coast to inland low arctic tundra gradient, Varanger Peninsula, Norway. We employed two complementary methods of diet analyses: stomach contents and stable isotope analysis. 3. We found that inland red foxes primarily subsisted on reindeer carrions during the low phase of a small rodent population cycle. Lemmings became the most important food item towards the peak phase of the rodent cycle, despite being less abundant than sympatric voles. Isotopic signatures of tissue from both predator and prey also revealed that red foxes near the coast used marine-derived subsidies in the winter, but these allochthonous resources did not spillover to adult foxes living beyond 20-25 km from the coast. 4. Although more needs to be learned about the link between increasing primary productivity due to climatic warming and trophic dynamics in tundra ecosystems, we suggest that changes in reindeer management through a bottom-up effect, at least regionally, may have paved the way towards the establishment of a new mesopredator in the tundra biome.

Ecosystem Impacts of a Range Expanding Forest Defoliator at the Forest-Tundra Ecotone

Insect outbreaks in northern-boreal forests are expected to intensify owing to climate warming, but our understanding of direct and cascading impacts of insect outbreaks on forest ecosystem functioning is deficient. The duration and severity of outbreaks by geometrid moths in northern Fennoscandian mountain birch forests have been shown to be accentuated by a recent climate-mediated range expansion, in particular of winter moth (Operophtera brumata). Here, we assess the effect of moth outbreak severity, quantified from satellite-based defoliation maps, on the state of understory vegetation and the abundance of key vertebrate herbivores in mountain birch forest in northern Norway. We show that the most recent moth outbreak caused a regional-scale state change to the understory vegetation, mainly due to a shift in dominance from the allelopathic and unpalatable dwarf-shrub Empetrum nigrum to the productive and palatable grass Avenella flexuosa. Both these central understory plant species responded significantly and nonlinearly to increasing outbreak severity. We further provide evidence that the effects of the outbreak on understory vegetation cascaded to cause strong but opposite impacts on the abundance of the two most common herbivore groups. Rodents increased with defoliation, largely mirroring the increase in A. flexuosa, whereas ungulate abundance instead showed a decreasing trend. Our analyses also suggest that the response of understory vegetation to defoliation may depend on the initial state of the forest, with poorer forest types potentially allowing stronger responses to defoliation.

Are population outbreaks in sub-arctic geometrids terminated by larval parasitoids?

1. Larval parasitoids (i.e. parasitoids attacking host larvae) constitute a major source of mortality in many ecologically and economically important forest insects, but how this mortality affects spatio-temporal population dynamics is often not clear. 2. In sub-arctic Fennoscandian birch forest, the two geometrids Epirrita autumnata and Operophtera brumata exhibit pronounced outbreak cycles with significant ecosystem impacts. As mortality owing to larval parasitoids often is very high, the hypothesis that parasitism terminates outbreaks has been advocated, but without decisive empirical evidence. 3. We analysed the altitude- and species-specific timing of population outbreaks typically seen in the coastal section of the sub-arctic birch forest ecosystem to evaluate the critical premise that parasitoid-inflicted larval mortality ought to predict geometrid population growth. 4. However, despite temporally high rates of parasitism, this did not influence the strongly species- and altitude-patterned geometrid outbreaks. We therefore conclude that termination of cyclic outbreaks in these geometrids is caused by other regulatory mechanisms than larval parasitoids. 5. Regardless of their lack of effect on the altitude-specific outbreak dynamics, larval parasitoids accounted for some of the local spatial variance in the temporal dynamics. This implies that results from spatially localized observations and experiments, which dominate research on parasitoid-host interaction, may be misinterpreted with respect to their relevance for large-scale and long-term population dynamics.

Spatially mismatched trophic dynamics: cyclically outbreaking geometrids and their larval parasitoids

For trophic interactions to generate population cycles and complex spatio-temporal patterns, like travelling waves, the spatial dynamics must be matched across trophic levels. Here, we propose a spatial methodological approach for detecting such spatial match–mismatch and apply it to geometrid moths and their larval parasitoids in northern Norway, where outbreak cycles and travelling waves occur. We found clear evidence of spatial mismatch, suggesting that the spatially patterned moth cycles in this system are probably ruled by trophic interactions involving other agents than larval parasitoids.

Phenological diversity in the interactions between winter moth (Operophtera brumata) larvae and parasitoid wasps in sub-arctic mountain birch forest.

Population cycles of the winter moth (Operophtera brumata) in sub-arctic coastal birch forests show high spatiotemporal variation in amplitude. Peak larval densities range from levels causing little foliage damage to outbreaks causing spatially extensive defoliation. Moreover, outbreaks typically occur at or near the altitudinal treeline. It has been hypothesized that spatiotemporal variation in O. brumata cycle amplitude results from climate-induced variation in the degree of phenological matching between trophic levels, possibly between moth larvae and parasitoids. The likelihood of mismatching phenologies between larvae and parasitoids is expected to depend on how specialized parasitoids are, both as individual species and as a guild, to attacking specific larval developmental stages (i.e. instars). To investigate the larval instar-specificity of parasitoids, we studied the timing of parasitoid attacks relative to larval phenology. We employed an observational study design, with sequential sampling over the larval period, along an altitudinal gradient harbouring a pronounced treeline outbreak of O. brumata. Within the larval parasitoid guild, containing seven species groups, the timing of attack by different groups followed a successional sequence throughout the moths larval period and each group attacked 1-2 instars. Such phenological diversity within parasitoid guilds may lower the likelihood of climate-induced trophic mismatches between victim populations and many/all of their enemies. Parasitism rates declined with increasing altitude for most parasitoid groups and for the parasitoid guild as a whole. However, the observed spatiotemporal parasitism patterns provided no clear evidence for or against altitudinal mismatch between larval and parasitoid phenology.

Sources of variation in larval parasitism of two sympatrically outbreaking birch forest defoliators

1. Studies of insect communities rarely support the parasitoid–host regulation hypothesis. Spatio‐temporal variation in parasitoid prevalence due to complex food web interactions or abiotic factors may prevent parasitoids from regulating hosts.

Spatially patterned guild structure in larval parasitoids of cyclically outbreaking winter moth populations

1. Wide temporal fluctuations in host abundance are a potential source of instability and stochasticity in the spatiotemporal population dynamics of associated parasitoid species. Within parasitoid guilds (i.e. parasitoids with similar modes of host utilisation), a conceivable outcome is guild organisation according to a lottery model, in which guild members attain local dominance by colonising previously emptied habitats during increasing host density, before other guild members. In the spatial dimension, an expected manifestation of such dynamics is variable guild structure even across homogeneous habitats.