Lauri Oksanen

University of Turku | UTU · Department of Biology

Long‐term studies of cyclic rodent populations have contributed fundamentally to the development of population ecology. Pioneering rodent studies have shown macroecological patterns of population dynamics in relation to latitude and have inspired similar studies in several other taxa. Nevertheless, such studies have not been able to disentangle the...

Reports of fading vole and lemming population cycles and persisting low populations in some parts of the Arctic have raised concerns about the spread of these fundamental changes to tundra food web dynamics. By compiling 24 unique time series of lemming population fluctuations across the circumpolar region, we show that virtually all populations di...

Long-term studies of cyclic rodent populations have contributed fundamentally to the development of population ecology. Previous research has shown macroecological patterns of population dynamics in relation to latitude, but without disentangling the role of underlying ecological and climate drivers. We collected 26 rodent time-series from the tund...

The searching efficiency of predators depends on the balance between the adaptations of the predator and the counter‐adaptations of the prey. In this evolutionary race, the prey should normally have the upper hand, as it can perform tradeoffs between efficiency in resource use and ability to avoid predators. In terrestrial predator–herbivore system...

Overexploitation of natural resources is often viewed as a problem characteristic of only the human species. However, any species could evolve a capacity to overexploit its essential resources through natural selection and competition, even to the point of resource collapse. Here, we describe the processes that potentially lead to overexploitation...

Many terrestrial endotherm food webs constitute three trophic level cascades. Others have two trophic level dynamics (food limited herbivores; plants adapted to tackle intense herbivory) or one trophic level dynamic (herbivorous endotherms absent, thus plants compete for the few places where they can survive and grow). According to the Exploitation...

Lemmings are a key component of tundra food webs and changes in their dynamics can affect the whole ecosystem. We present a comprehensive overview of lemming monitoring and research activities, and assess recent trends in lemming abundance across the circumpolar Arctic. Since 2000, lemmings have been monitored at 49 sites of which 38 are still acti...

In the original published article, some of the symbols in figure 1A were modified incorrectly during the typesetting and publication process. The correct version of the figure is provided in this correction.

Mammalian herbivores shape the structure and function of many nutrient-limited or low-productive terrestrial ecosystems through modification of plant communities and plant–soil feedbacks. In the tundra biome, mammalian herbivores may both accelerate and decelerate plant biomass growth, microbial activity and nutrient cycling, that is, ecosystem pro...

Herbivory is a process where animals obtain energy and nutrients from vegetative plant parts (leaves, stems, etc.). Herbivory is called grazing, browsing or folivory, depending on the size of the herbivore and the type of plant tissue consumed. The consumption is actually performed by microbes in the digestive system. The role of the animal is to p...

Questions We investigated the importance of climate and herbivory on native and alien conifer colonisation of the birch‐dominated Fennoscandian treeline by addressing the following questions: (1) Are treeline and tundra habitats similarly suitable for conifer seedling recruitment? (2) Do ungulate and rodent herbivores differentially impact seedling...

Understanding the determinants of spatial and temporal differences in the relative strength of consumer–resource interactions is an important endeavour in ecology. Here, we explore the necessary conditions for temporal shifts in the relative strength of rodent–plant interactions in an area characterised by profound spatial differences in trophic co...

Large herbivores can control plant community composition and, under certain conditions, even induce vegetation shifts to alternative ecosystem states. As different plant assemblages maintain contrasting carbon (C) cycling patterns, herbivores have the potential to alter C sequestration at regional scales. Their influence is of particular interest i...

Many primary livelihoods in Arctic and sub-Arctic regions experience accelerating effects of environmental change. The often close connection between indigenous peoples and their respective territories allows them to make detailed observations of how these changes transform the landscapes where they practice their daily activities. Here, we report...

In the forest-tundra ecotone of the North Fennoscandian inland, summer and winter temperatures have increased by two to three centigrades since 1965, which is expected to result in major vegetation changes. To document the expected expansion of woodlands and scrublands and its impact on the arctic vegetation, we repeated a vegetation transect study...

According to some treatises, arctic and alpine sub-biomes are ecologically similar, whereas others find them highly dissimilar. Most peculiarly, large areas of northern tundra highlands fall outside of the two recent subdivisions of the tundra biome. We seek an ecologically natural resolution to this long-standing and far-reaching problem. We studi...

Appendix S1 Weather stations used for the analysis of winter climate patterns. Appendix S2 Sources and descriptions of vegetation data material and analysis methods. Appendix S3 The elevation ranges of the twelve 25 × 25 km tundra sites of Fennoscandia.

The Arctic region is forecasted to warm approximately twice as much as the Earth on average. This will cause dramatic transformations in the northern ecosystems. Further pressure to the environment is imposed by changes in land-use. Natural resources extraction dissects the tundra with pipelines, open-pit mines and road networks. Simultaneously, ou...

Spatial variation in the strength of trophic cascades in arctic tundra has been related to flows of subsidies across ecosystem boundaries. Here, we ask whether the input of marine subsidies in tundra systems would cause spatial variation in the strength of rodent-plant interactions between coastal areas, where predators have access to marine-derive...

Our transdisciplinary research tackles the interaction between the ecological phenomenon of top-down impacts in food webs and climate–vegetation interactions in a changing climate and integrates this perspective with the reindeer husbandry and the Sámi culture dependent on it 1. The Arctic region will warm more rapidly than the global mean, and mea...

The observed substantial warming of the northern polar regions since the mid-20th century is expected to encourage the transformation of the current arctic-alpine tundra to forest or dense scrubland. An abundant forest-scrub vegetation penetrating through the snowpack in spring-time will decrease the albedo and hence, trigger positive warming feedb...

Both the theory and the observations suggest that, there are strong links between herbivores and plants in terrestrial ecosystems; although, the effect of herbivores on plant community biomass is often attributed to variations in plant palatability. The existence of a strong link is commonly tested by constructing exclosures that exclude herbivores...

The relative importance of top-down and bottom-up mechanisms in shaping community structure is still a highly controversial topic in ecology. Predatory top-down control of herbivores is thought to relax herbivore impact on the vegetation through trophic cascades. However, trophic cascades may be weak in terrestrial systems as the complexity of food...

Understanding the population dynamics of arctic small rodents is paramount given their importance in arctic and subarctic ecosystems as food resource for multiple avian and mammalian predators, their multiannual fluctuations and recent climate change. Most studies on population dynamics focus on average patterns over large areas and/or a limited se...

Questions: Assuming that arctic lemming oscillations are generated by interactions between lemmings and depletable plants, how should these oscillations change in response to varying densities of marine-subsidized predators and differences in the production of herbaceous forage? Are the patterns thus generated consistent with existing data? Feature...

Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind’s most pervasive influence on nature. Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, t...

Background: According to the current predominant view, intraguild predation leads to the replacement of intermediate predators from highly productive habitats, whereas top predators and intermediate predators can co-exist in habitats with intermediate primary productivity. These predictions are contradicted by the observed abundance of intermediate...

Background: Evergreen ericaceous dwarf shrubs form a dominating component of low arctic and low alpine vegetation. They typically produce high contents of secondary chemicals such as phenolics. The primary function of these chemicals may be to defend the shrubs by making them less palatable to herbivores. Question: Does the production of secondary...

Background: According to the Green World Hypothesis of Hairston, Smith, and Slobodkin, all plants are edible for some herbivores. Hence, the copious abundance of plant biomass, typical for terrestrial ecosystems, depends on the collective regulatory action of predators on the herbivore guild. According to the counterarguments of Polis and Strong, t...

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures w...

Herbivory is a process where animals obtain energy and nutrients from vegetative plant parts (leaves, stems, etc.). Herbivory is called grazing, browsing or folivory, depending on the size of the herbivore and the type of plant tissue consumed. The consumption is actually performed by microbes in the digestive system. The role of the animal is to p...

Recent Pan-Arctic shrub expansion has been interpreted as a response to a warmer climate. However, herbivores can also influence the abundance of shrubs in arctic ecosystems. We addressed these alternative explanations by following the changes in plant community composition during the last 10 years in permanent plots inside and outside exclosures w...

We have monitored population fluctuations of microtine rodents since 1977 in two habitat complexes in Finnmark, northernmost Norway – a low arctic plains landscape, with patches of willow scrubland embedded in lichen-dwarf birch tundra, and in adjacent highlands, occupied by scrub-free heaths, snow-beds and bogs. In the plains landscape, voles were...

Question: What do the evolution and the fluctuation patterns of arctic lemmings – Lemmus spp. and Dicrostonyx spp. – tell us about their population dynamics and the influence of lemmings on the ecology and evolution of arctic plants? Methods: We reviewed the literature concerning the evolution of arctic lemmings and analysed their current fluctuati...

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base,...

Vole–vegetation interactions in a predation-free taiga environment of northern Fennoscandia were studied by transferring vegetation from natural Microtus habitats into a greenhouse, where three habitat islands of about 30 m² were created. The ‘islands’ were subjected to simulated summer conditions and a paired female field vole, Microtusagrestis, w...

We compared the abundance, population structure and palatability of bilberry ramets on vole-free islands, islands with voles but no predators (predator-free islands) and mainland sites with both voles and predators. As expected, bilberry biomass was strongly correlated with the herbivory pressure exerted by the voles, since it was significantly low...

Question: How is tundra vegetation related to climatic, soil chemical, geological variables and grazing across a very large section of the Eurasian arctic area? We were particularly interested in broad-scale vegetation-environment relationships and how well do the patterns conform to climate-vegetation schemes. Material and Methods: We sampled vege...

Question What is the relationship between species richness of vascular plants, bryophytes and macrolichens, and two important gradients in the alpine environment, altitude and local topography? Location Northernmost Fennoscandia, 250–1525 m a.s.l. corresponding to the range between timberline and mountain top. Methods The vegetation was sampled i...

Herbivores influence the structure of plant communities in arctic-alpine ecosystems. However, little is known of the effect of herbivores on plant colonisation following disturbance, and on its variability depending on the identity of herbivores and the characteristics of the habitats. To quantify the role of large and small vertebrate herbivores,...

During 1991–95, mammalian predators (weasel, Mustela nivalis, stoat, M. erminea, mink, M. vison, and red fox, Vulpes vulpes) were excluded in late summers from a 2 ha piece of a north Norwegian mountain slope. The exclosure extended from an outpost of luxuriant sub-arctic birch forest to a typical arctic–alpine habitat complex, including productive...

Both theoretical arguments and empirical evidence suggests that herbivory in general and mammalian winter herbivory in particular is important in arctic–alpine ecosystems. Although knowledge of the effect of herbivores on specific plants and communities is quite extensive, little is known about the relative impact of large and small vertebrate herb...

The role of predators in controlling herbivores and indirectly affecting plant abundance is controversial, and some have argued that such trophic cascades are rare in terrestrial habitats. To examine the potential of trophic cascades in a shrubby tundra ecosystem, vole densities, plant damage and plant cover were examined in areas with and without...

Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands have replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transpla...

The effects of long-term (11 yr) exclusion of vertebrate herbivores on competition intensity and plant community structure were studied using manipulative field experiments in two arctic-alpine plant communities with contrasting productivity: an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed, the exclusion...

In studies on dynamics of northern predator-prey systems, two assumptions are often made. First, the bifurcation from stable to cyclic dynamics is seen as a consequence of changing generalist-specialist ratio, ultimately due to reduced prey diversity at high latitudes and the negative impact of snow on the efficiency of generalists as predators of...

The hypothesis that the regular multiannual population oscillations of boreal and arctic small rodents (voles and lemmings) are driven by predation is as old as the scientific study of rodent cycles itself. Subsequently, for several decades, the predation hypothesis fell into disrepute, possibly because the views about predation and rodent dy-namic...

In this study, we investigated the effect of reindeer grazing on tundra heath vegetation in northern Norway. Fences, erected 30 yr ago, allowed us to compare winter grazed, lightly summer grazed and heavily summer grazed vegetation at four different sites. At two sites, graminoids dominated the heavily grazed zone completely, while ericoid dwarf sh...

Theoretical analyses and data suggest that the interaction between bank voles and their predators generates a locally stable equilibrium, regardless to the composition of the predator guild. The suggested primary proximate source for stability is female territoriality. Predation_per se_appears to be destabilizing. Whether or not predation regulates...

Positive interactions between plants typically occur where the presence of a species ameliorates the abiotic environment for another. However, there is also a potential for resource competition to act at the same time, which creates a situation where the net outcome is a balance between positive and negative interactions. We present data from a nin...

Vegetation patterns of oceanic heath vegetation in northern Norway are described on the basis of systematically sampled vegetation data from two coastal mountain areas, Vannøya and northern Varanger Peninsula. The data are classified into community types by TWINSPAN clustering. On Vannøya, heaths of the Calluna-Bryophyta type prevail at the lowest...

During 1987-1994 we monitored changes in vegetation in exclosures and permanent open plots established in two contrasting habitats: a productive hemiarctic tall herb meadow and a less productive alpine snow-bed. In addition, we studied the survival and growth of transplanted tall herbs, woody plants and arctic-alpine plants together with their seed...

Shoot survival of a toxic herb, Actaea spicata, and a herb with a digestibility-reducing defense, Geranium sylvaticum, was studied in an experimental set-up where microtine rodents were allowed to graze freely. It was hypothesized that the defense system of Actaea would work in a risk-reducing way, while the defense system of Geranium would have co...

Current discussion on structure and dynamics of food webs includes two seemingly incompatible views on the role of energy supply for food chain length. Authors of food web reviews (Pimm, 1979, 1982, 1991; Briand and Cohen, 1987; Cohen et al., 1990) conclude that the energy constraint is trivial and that chain length is determined by other factors (...

In landscapes in which habitat patches are larger than the home ranges of consumers, ideal free habitat selection is incompatible with pseudointerference Source-sink dynamics and spillover consumption can be generated only by nonequilibrium situations. At equilibrium, patterns in consumer and resource standing crops and resource mortality are ident...

The timberline woodlands of N Fennoscandia are shaped by the interaction between climate, and grazing. Oceanic woodlands are characterized by tall, monocormic or oligocormic birches Betula, with at least a discontinuous willow Salix scrub layer. In continental woodlands, the birches are lower and generally more polycormic and willows only occur in...

In N Fennoscandia, timberland forests are birch-dominated Betula both in oceanic areas and in areas with relatively continental climate. In E. Fennoscandia and N Russia, weakly and moderately continental areas are characterized by spruce-dominated timberline forests. A similar situation prevails in parts of S and C Scandinavia, too. Possible causes...

In lichen heath of Finnmark, Norway, four woody species showed the strongest increases as a response to the grazer exclosure. In meadow, a tall grass showed the strongest increase in the exclosures, while a creeping woody plant increased in snowbed exclosures. The species that increased in the exclosures were consistently those which were capable o...

In many productive terrestrial biomes there seems to be an abundance of forage which is not exploited by herbivores. This observation has generated several hypotheses. One set of hypotheses suggests that the herbivores are either regulated by predators, social interactions or pathogens, and that the abundant plant biomass is an effect of relatively...

The impact of a lemming population on a snowbed in northern Fennoscandia was examined during a population peak. Twelve exclosures and 12 open plots were established on a moderately early snowbed. The plots were photographed in autumn 1988, spring 1989, autumn 1989, and autumn 1990, and the cover of graminoids, woody plants, lichens, litter, lemming...

Standard exploiter-victim models assume an instantaneous connection between foraging and population dynamics. However, in nature this connection is inevitably time-delayed: it takes time to convert food into offspring. R. Arditi and L. R. Ginzburg proposed that this time delay implies ratio-dependent exploitation. Models with ratio-dependent exploi...

The model of exploitation ecosystems was re-analysed, assuming that habitat patches are so small that they form only parts of the home range of an individual predator. For habitat complexes where productive patches abound, the results suggested that predation will strongly spill over from productive patches, which set the tune for population dynami...

The habitat use of small mustelids in a tundra area in Norwegian Lapland was studied chiefly by means of snow-tracking 1986-89 Stoats showed strong peference to a habitat complex immediately beneath the thrust line of the Scandes, with exceptional abundance of luxuriant habitats, whereas weasel activity was more evenly spread over the lowland tundr...

Densities of microtine rodents in two habitat complexes in the tundra of Finnmarks-vidda, Norwegian Lapland, were studied during 1977-89 by means of snap trapping (Small Quadrat Method) Predator populations were studied by mapping breeding raptors and by snow-tracking small mustelids During 1977-85, snow-trackmg was conducted only during peak and d...

The model of exploitation ecosystems by Oksanen et al. contains a tacit assumption of constancy of the physical environment. Seasonality is a source of time delay in density dependent processes and thus potentially destabilizing. An analysis of grazing chain dynamics in seasonal environments where primary production is concentrated to a short and i...

Discusses the rarity of undisturbed vegetation in barren areas and grazer plant dynamics and coevolution in these areas, including a summary of theoretical viewpoints. Gore Range, Colorado is used as a case study on a gradient with increasing vegetational fragmentation. The pattern of vegetational change from forests to arid steppes and arctic or a...

The Tihomirov hypothesis, according to which microtines have a strong impact upon the reproductive performance of northern plant populations, and the hypothesis of Kalela and Tast, stating that northern environments are characterized by strong, synchronous pulses of generative and vegetative reproduction, which create microtine cycles, were tested...