Lauri Kalervo Oksanen
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The backbone of my research is the Exploitation Ecosystem’s Hypothesis (EEH, Oksanen et al. 1981, Am. Nat. 118: 240-261) which states that, everything else being equal, the strong interaction will inevitably shift from the predator-herbivore interface to the herbivore-plant interface, as there must be a productivity threshold where the herbivore density required for zero population growth by predators exceeds the density that the slowly growing vegetation can sustain. During the past decade, my research team has shown that this “everything else being equal” clause holds for the productivity gradients of northernmost Norway, where the critical threshold corresponds to the transition from low arctic shrublands to the tundra proper (see publications in Cristin). The success of EEH in organizing our ideas on food web dynamics has also been confirmed by its inclusion in the new edition of “Foundations of Ecology”.
In spite of its successes, EEH also has serious shortcomings, which I am now inspecting and remedying in collaboration with my colleague-wife Tarja Oksanen. In the 1981 paper, the “everything else being equal” clause was presented as if it were a fact, which is not true. In our recent paper (Oksanen, T., Oksanen, L., Vuorinen, K.E.M., Ripple, W.J., Wolf, C. Virtanen, R. Mäkynen, A., Olofsson, J., and Utsi, T. Aa. 2020. The impact of thermal seasonality on terrestrial endotherm food web dynamics: a revision of the Exploitation Ecosystem Hypothesis. – Ecography 43: 1859-1877) we explored the impact of thermal seasonality, which turned out to have a profound impact on the location of the critical productivity threshold. Without megaherbivores, the existence of wide areas where the strong interaction is between herbivores and plants requires genuine winters, when much of the energy fixed by plants is locked in the frozen soil and thus unavailable for herbivores.
With megaherbivores present, the situation changes radically, even if they were not absolutely invulnerable (there were megapredators, too). High size diversity of herbivores implies that only a small fraction of the total herbivore stock is within the size interval that a given predator is adapted to exploit. The community level predator-herbivore equilibrium is therefore reached at essentially higher over-all herbivore density than in ecosystems with lower size diversity among herbivores. We are currently working on this issue.
Moreover, EEH is no panacea for solving all ecological problems. When working with Norwegian lemmings and when discussing with our North American colleagues, we have become convinced that all lemmings are not born equal. There are essential dynamical and behavioral differences between the Norwegian lemming of Fennoscandia and its congeners. Behind the unique traits and dynamics of the Norwegian lemming lies a unique evolutionary history: its survival in a tiny set of refugia in the Andøya-Vesterålen region during the last glacial maximum (Lagerholm, V. K. et al. 2014. On the origin of the Norwegian lemming. – Molecular Ecology 23, 2060–2071). Moreover, modeling studies show that population cycles driven by the herbivore-plant interaction have two alternative stable states: one where herbivore peaks are extremely sharp and short-lived, being timed to the end of the growing season, and one where high densities are obtained in spring, making the area attractive for avian predators. We argue that the Norwegian lemming of Fennoscandia is locked in dynamics of the former type, while the latter type prevails elsewhere on the circumpolar tundra. The MS dealing with this issue is almost ready for submission.
Terrestrial food web dynamics have also been studied by applying the ECOPATH method, based on energetic calculus and originally designed for studies on pelagic food webs. By comparing the results of ECOPATH to experimental evidence, we show that its applicability to terrestrial food web dynamics is limited. Strong herbivore-plant interactions can be accompanied by modest (less than 0.1) consumption to primary production ratios. A MS on this topic will get ready within a few months.
At my age, an important task of a researcher is to ensure that the empirical studies conducted or initiated in the past will be useful for younger generations. This is especially important in the arctic where climate is changing rapidly. In 1976, I conducted a nested vegetation transect study on four different mountains, ranging from the continental inland of Finnish Lapland to the coastal mountains of Lyngen, Norway. One of these transect studies was repeated by two of my students, Katariina Vuorinen and Anne Pyykönen, in collaboration with some senior team members. The results were quite unexpected and extremely interesting (Vuorinen, K.E.M., et al. 2017. Open tundra persists, but arctic features decline: vegetation changes in the warming Fennoscandian tundra. – Global Change Biol. 23: 3794-3807). The other transect sites have now been marked in terrain and with GPS. Visual inspection indicated that the results will be equally exciting. (For instance, on the mountains at Kilpisjärvi, the formerly abundant arctic dwarf shrub Cassiope tetragona has been largely replaced by the wood rush Juncus trifidus, typical for South Scandinavian mountains.) These studies will be repeated as soon as possible.
Another gift for the next generation is the EU-financed study “DART” (Dynamic response of the forest-tundra ecotone to environmental change 1998-2000), where the core idea was to seed and to plant treeline forming Eurasian trees – mountain birches, Norway spruces, Siberian larches and Scots pines – in sites at the current treeline, with and without pivotal herbivores. In the seeding study, conducted at Joatka tundra lodge, small and large herbivores were excluded using fences with different mesh sizes. In the planting study on the shores and islands of Iešjávri, a 68 km2 lake just at the treeline, water barriers were used for the same effect. The fences were repaired by me and Katariina Vuorinen last summer. The planted trees were individually marked and the GPS bearings of the planting lines were taken 2014. Survival and geowth of planted trees has been studied recurrwntly. Many planted trees and some of the sown ones are now getting tall enough to have their leader shoots rising above the snow surface (or trying to do so and getting killed). The study is now ready for inerested young ecologists to harvest the results. An unexpeted result was that the in the conditions of interior Finnmark, the currently domianting mountain birch performed worst: it germinated poorly, it has grown poorly even when planted, it has survivived poorly and it has been sensitive to herbivory. Pines initially grew and survived well (see Bognounou F. et al. 2018, J. Veget. Sci. 29: 573-584), but many have died in recent years. Acccording to my visual impressions, larches and spruces, which do not naturally occur in the area, have performed best, especially at Iešjávri.