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Author

Tarja Oksanen

Other affiliations: University of Tromsø
Bio: Tarja Oksanen is an academic researcher from University of Turku. The author has contributed to research in topics: Tundra & Population. The author has an hindex of 17, co-authored 28 publications receiving 4036 citations. Previous affiliations of Tarja Oksanen include University of Tromsø.
Topics: Tundra, Population, Arctic, Vole, Trophic level

Papers
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Journal ArticleDOI
15 Jul 2011-Science
TL;DR: This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles.
Abstract: 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, terrestrial, and freshwater ecosystems worldwide. This empirical work supports long-standing theory about the role of top-down forcing in ecosystems but also highlights the unanticipated impacts of trophic cascades on processes as diverse as the dynamics of disease, wildfire, carbon sequestration, invasive species, and biogeochemical cycles. These findings emphasize the urgent need for interdisciplinary research to forecast the effects of trophic downgrading on process, function, and resilience in global ecosystems.

3,130 citations

Journal ArticleDOI
TL;DR: EEH is corroborated by biomass patterns, by patterns in the structure and dynamics of carnivore, folivore, and plant communities, and by experimental results, but critical empirical evidence for or against the capacity of folivorous insects to regulate plant biomass has not yet been obtained.
Abstract: Hypotheses on trophic dynamics in terrestrial ecosystems fall into two major categories: those in which plants are assumed to be invulnerable to their consumers and those in which the build‐up of plant biomass is assumed to require top‐down control of folivores. The hypothesis of exploitation ecosystems (EEH) belongs to the latter category and focuses particularly on the consequences of the high energetic costs of maintenance of endotherms. Carnivorous endotherms require relatively high prey densities in order to break even. Moreover, they are dependent on folivorous prey during the limiting season, at least at high latitudes. The endotherm branch of the grazing web is thus predicted to collapse from three‐link trophic dynamics (carnivores → folivores → plants → inorganic resources) to two‐link dynamics (folivores → plants → inorganic resources) along gradients of decreasing primary productivity. Consequently, the vegetation of cold and unproductive areas is predicted to be under intense winter ...

345 citations

Journal ArticleDOI
TL;DR: The numerical predator-to-prey ratio advocated by Pearson (1966, 1971) might not reflect the intensity of predation, and estimates ofpredation based on identifiable prey remains found from predator scats could be far too low.
Abstract: Studies on the impact of vertebrate predators on a prey population are often based on estimates of the numbers of predators and prey (e.g., Pearson 1966, 1971; Goszczynski 1977; Phelan and Robertson 1978; King 1980; Erlinge et al. 1983). This approach includes assumptions about the functional response of predators, that is, about the changes in hunting effort for different prey species and about the factors that determine the handling time (Holling 1965). Changes in the preference for different prey species, often called switching, have an important effect on the functional response curve, especially at relatively low prey densities (Greenwood and Elton 1979). The lower part of the functional response curve also will probably be influenced by transit time (Murdoch 1977; Oaten 1977) and by the costs of activity (Abrams 1982). The upper part of the functional response curve is largely determined by handling time, that is, the time between the capture and the resumption of searching behavior (Holling 1965). The extent of switching can be estimated by using scat analysis (e.g., Day 1968; Phelan and Robertson 1978; Tapper 1979; Erlinge 1981), and weight loss of predators can be used for estimating inadequate feeding rates. Handling time could be estimated readily if all predators were characterized by rigid hunting behavior in which a prey is attacked when seen within a fixed attack radius and consumed before the predator starts to search for new prey items. If, however, predatory behavior is more flexible, handling time becomes more difficult to estimate. Such flexibility may be created, for example, by selective feeding and consequently decreased consumption per prey with increased prey density (Mysterud 1980; Stenseth 1981; Abrams 1982). It is also possible for predators to kill prey without immediately consuming them (Nyholm 1961; Kruuk 1972; Curio 1976; Mysterud 1980; Elgmork 1982). Such behavior substantially shortens the handling sequence by eliminating the time used for feeding activities. This can make the functional response curve nearly linear or only slightly convex at even high prey densities. Consequently, the numerical predator-to-prey ratio advocated by Pearson (1966, 1971) might not reflect the intensity of predation, and estimates of predation based on identifiable prey remains found from predator scats could be far too low.

78 citations

Journal ArticleDOI
01 Sep 1999-Oikos
TL;DR: The observed scenario is consistent with the hypothesis that vole cycles represent a mustelid-microtine limit cycle, because cycles created by this mechanism should disappear when the productive habitats, capable of supporting resident predators, become fragmented and embedded in a vast unsuitable area.
Abstract: During 1991-1996. we studied population fluctuations of microtine rodents (primarily Clethrionomys rufocanus). of their winter food plants, and of their predators in a low arctic habitat complex, dominated by unproductive lichen dwarf-birch tundra. More productive habitats occurred patchwise throughout the landscape. On a south-facing slope, productive scrubland habitats prevailed, and luxuriant habitats were locally abundant. Our main method was live-trapping on 14 grids, representing typical lowland tundra (5 replicates), the productive slope (4 replicates) and barren high-altitude tundra (5 replicates). Within the slope vole populations were cyclic. In the lowland tundra. vole fluctuations were primarily seasonal, but the vole erash on the productive slope coincided with a phase of relatively low vole densities in the lowland. The highland was characterised by low vole densities. During the phase of rapid population growth, long-range dispersal occurred within the slope and from the slope to surrounding areas. Moreover, small mustelids which had initially been present only on the slope, started to move elsewhere, along natural dispersal corridors. Shoot mortalities of the main winter food plant. Vaccinium myrtillus. remained low. The observed scenario is consistent with the hypothesis that vole cycles represent a mustelid-microtine limit cycle, because cycles created by this mechanism should disappear when the productive habitats, capable of supporting resident predators, become fragmented and embedded in a vast unsuitable area.

78 citations


Cited by
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28 Jul 2005
TL;DR: PfPMP1)与感染红细胞、树突状组胞以及胎盘的单个或多个受体作用,在黏附及免疫逃避中起关键的作�ly.
Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1(PfPMP1)与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用,在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员,通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

18,940 citations

Journal ArticleDOI
13 Feb 2015-Science
TL;DR: An updated and extended analysis of the planetary boundary (PB) framework and identifies levels of anthropogenic perturbations below which the risk of destabilization of the Earth system (ES) is likely to remain low—a “safe operating space” for global societal development.
Abstract: The planetary boundaries framework defines a safe operating space for humanity based on the intrinsic biophysical processes that regulate the stability of the Earth system. Here, we revise and update the planetary boundary framework, with a focus on the underpinning biophysical science, based on targeted input from expert research communities and on more general scientific advances over the past 5 years. Several of the boundaries now have a two-tier approach, reflecting the importance of cross-scale interactions and the regional-level heterogeneity of the processes that underpin the boundaries. Two core boundaries—climate change and biosphere integrity—have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed.

7,169 citations

Journal ArticleDOI
07 Jun 2012-Nature
TL;DR: It is argued that human actions are dismantling the Earth’s ecosystems, eliminating genes, species and biological traits at an alarming rate, and the question of how such loss of biological diversity will alter the functioning of ecosystems and their ability to provide society with the goods and services needed to prosper is asked.
Abstract: The most unique feature of Earth is the existence of life, and the most extraordinary feature of life is its diversity. Approximately 9 million types of plants, animals, protists and fungi inhabit the Earth. So, too, do 7 billion people. Two decades ago, at the first Earth Summit, the vast majority of the world's nations declared that human actions were dismantling the Earth's ecosystems, eliminating genes, species and biological traits at an alarming rate. This observation led to the question of how such loss of biological diversity will alter the functioning of ecosystems and their ability to provide society with the goods and services needed to prosper.

5,244 citations

Journal ArticleDOI
25 Jul 2014-Science
TL;DR: Defaunation is both a pervasive component of the planet’s sixth mass extinction and also a major driver of global ecological change.
Abstract: We live amid a global wave of anthropogenically driven biodiversity loss: species and population extirpations and, critically, declines in local species abundance. Particularly, human impacts on animal biodiversity are an under-recognized form of global environmental change. Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25% average decline in abundance. Invertebrate patterns are equally dire: 67% of monitored populations show 45% mean abundance decline. Such animal declines will cascade onto ecosystem functioning and human well-being. Much remains unknown about this “Anthropocene defaunation”; these knowledge gaps hinder our capacity to predict and limit defaunation impacts. Clearly, however, defaunation is both a pervasive component of the planet’s sixth mass extinction and also a major driver of global ecological change.

2,697 citations

Journal ArticleDOI
10 Jan 2014-Science
TL;DR: The status, threats, and ecological importance of the 31 largest mammalian carnivores globally are reviewed and a Global Large Carnivore Initiative is proposed to coordinate local, national, and international research, conservation, and policy.
Abstract: Large carnivores face serious threats and are experiencing massive declines in their populations and geographic ranges around the world. We highlight how these threats have affected the conservation status and ecological functioning of the 31 largest mammalian carnivores on Earth. Consistent with theory, empirical studies increasingly show that large carnivores have substantial effects on the structure and function of diverse ecosystems. Significant cascading trophic interactions, mediated by their prey or sympatric mesopredators, arise when some of these carnivores are extirpated from or repatriated to ecosystems. Unexpected effects of trophic cascades on various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology; and crop damage. Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth's largest carnivores and all that depends upon them, including humans.

2,441 citations