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  • Albrectsen, Benedicte R., 1960-, et al. (författare)
  • Nutrient addition extends flowering display, which gets tracked by seed predators, but not by their parasitoids
  • 2008
  • Ingår i: Oikos. - : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; 117, s. 473-480
  • Tidskriftsartikel (refereegranskat)abstract
    • Although phenological matching between two and three trophic interactions has received some attention, it has largely been disregarded in explaining the lack of strong cascade dynamics in terrestrial systems. We studied the response of the specialist seed predator, Paroxyna plantaginis (Tephritidae) and associated generalist parasitoids (Chalcidoidea) to controlled fertilisation of individuals of naturally growing Tripolium vulgare (Asteraceae) on four island populations (Skeppsvik Archipelago, Sweden). We consistently found evidence of nutrient limitation: fertilised plants increased their biomass, produced more capitula (the oviposition units for tephritid flies), were more at risk of attack by the tephritids, and puparia were heavier in fertilised plants. During some parts of the season tephritids became more heavily parasitized, supporting the presence of cascade dynamics, however net parasitism over season decreased in response to nutrient addition. We found no evidence that capitulum size complicated parasitoid access to the tephritids, however the extended bud production prolonged the flowering season. Thus, tephritids utilized the surplus production of capitula throughout the entire season, while parasitoids did not expand their oviposition time window accordingly. Implications for top down regulation and cascade dynamics in the system are discussed.
  • Brodersen, Jakob, et al. (författare)
  • Interplay between temperature, fish partial migration and trophic dynamics
  • 2011
  • Ingår i: Oikos. - : Wiley-Blackwell. - 1600-0706 .- 0030-1299. ; 120:12, s. 1838-1846
  • Tidskriftsartikel (refereegranskat)abstract
    • Whereas many studies have addressed the mechanisms driving partial migration, few have focused on the consequences of partial migration on trophic dynamics, and integrated studies combining the two approaches are virtually nonexistent. Here we show that temperature affects seasonal partial migration of cyprinid fish from lakes to predation refuges in streams during winter and that this migration in combination with temperature affects the characteristics and phenology of lower trophic levels in the lake ecosystem. Specifically, our six-year study showed that the proportion of fish migrating was positively related to lake temperature during the pre-migration growth period, i.e. during summer. Migration from the lake occurred later when autumn water temperatures were high, and timing of return migration to the lake occurred earlier at higher spring water temperatures. Moreover, the winter mean size of zooplankton in the lake increased with the proportion of fish being away from the lake, likely as a consequence of decreased predation pressure. Peak biomass of phytoplankton in spring occurred earlier at higher spring water temperatures and with less fish being away from the lake. Accordingly, peak zooplankton biomass occurred earlier at higher spring water temperature, but relatively later if less fish were away from the lake. Hence, the time between phyto- and zooplankton peaks depended only on the amount of fish being away from the lake, and not on temperature. The intensity of fish migration thereby had a major effect on plankton spring dynamics. These results significantly contribute to our understanding of the interplay between partial migration and trophic dynamics, and suggest that ongoing climate change may significantly affect such dynamics.
  • Clausen, P, et al. (författare)
  • Energy limitations for spring migration and breeding: the case of brent geese Branta bernicla tracked by satellite telemetry to Svalbard and Greenland
  • 2003
  • Ingår i: Oikos. - : Wiley-Blackwell. - 1600-0706 .- 0030-1299. ; 103:2, s. 426-445
  • Konferensbidrag (refereegranskat)abstract
    • Brent geese were tracked by satellite telemetry from spring staging areas in Denmark to Arctic breeding areas in Svalbard and Greenland in 1997 and 2001. From estimated departure masses and carcass analysis we used flight mechnical theory to estimate maximum flight ranges of both sexes, and remaining stores of fat and protein upon arrival in females. Model predictions suggested that all birds but one exceptionally thin male could easily reach Svalbard, but that approximately one third of the males and half of the females would have problems with flying to Greenland. Nevertheless, some birds even flew longer than the models predicted. In addition, females predicted to be capable of making the flight to Greenland, were predicted to arrive almost lean of fat. This contradicts our expectation that these birds are capital breeders - that they depend on endogenous stores of fat and protein when initiating and incubating their eggs. We discuss how the Greenland breeding sub-population during 1985-1998 has been able to grow at the same rate as the sub-population breeding in Svalbard, despite the added flight distance of 700-1000 km, and despite the birds predicted shortage of fat stores on arrival. We suggest four hypotheses that alone or in combination could explain the discrepancy between model predictions and observations. These are that most birds: (1) refuel on stop-overs in Spitsbergen en route to Greenland; (2) pick favourable tail-winds enabling them to reduce flight costs; (3) fly in formation and thereby save energy; and/or (4) undergo gut atrophy immediately prior to departure, and use the nutrients mobilised by catabolism of the digestive system to build larger pectoral muscles. The latter option would both reduce their airframe fraction, and increase their fat and flight-muscle fractions, enabling them to fly longer. We conclude that the latter option seems less likely to operate in brent geese.
  • Hagberg, Jacob, et al. (författare)
  • Uncertain biotic and abiotic interactions in benthic communities
  • 2003
  • Ingår i: Oikos. - : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; 100:2, s. 353-361
  • Tidskriftsartikel (refereegranskat)abstract
    • We analyze marine benthic communities at different sites in Skagerrak with the purpose of understanding the role of exogenous and endogenous factors in explaining the species' temporal dynamics. The previous finding that the dynamics of these species communities are mainly driven and synchronized by environmental (temperature) forcing was only weakly supported when analyzing single-species dynamics at five sites where four of the species were present every year. There was no consistent pattern in how the temperature affected the realized per capita growth rate, either across species at a given site, or among sites for a given species. Furthermore, there was no net-interaction from the community on a given species strong enough to give rise to second-order dynamics. However, when implementing a Multi Dimensional Scaling (MDS) analysis and incorporating all sampling sites and species -we found that the different communities clustered in relation to depth, hence, communities at the same depth were more "similar" than communities at different depth. Revealing the underlying interactions shaping these marine benthic communities is a challenge that calls for an array of various and complementary approaches.
  • Knape, Jonas, et al. (författare)
  • An analysis of hatching success in the great reed warbler Acrocephalus arundinaceus
  • 2008
  • Ingår i: Oikos. - : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; :117, s. 430-438
  • Tidskriftsartikel (refereegranskat)abstract
    • Hatching success is a potentially important fitness component for avian species. Previous studies of hatching success in natural populations have primarily focused on effects of inbreeding but a general understanding of variation in hatching success is lacking. We analyse data on hatching success in a population of great reed warblers Acrocephalus arundinaceus in Lake Kvismaren in south central Sweden. The effects of a range of covariates, including three measures of inbreeding as well as effects of classifications in the data (such as identities of individuals), on hatching success are analysed simultaneously. This is done by means of fitting Bayesian binomial mixed models using Markov chain Monte Carlo methods. Using random effects for each individual parent we check for unexplained variation in hatching success among male and female individuals and compare it to effects of covariates such as degree of inbreeding. Model selection showed that there was a significant amount of unexplained variation in hatching probability between females. This was manifested by a few females laying eggs with a substantially lower hatching success than the majority of the females. The deviations were of the same order of magnitude as the significant effect of parent relatedness on hatching success. Whereas the negative effect of parent relatedness on hatchability is an expression of inbreeding, the female individual effect is not due to inbreeding and could reflect maternal effects, that females differ in fertilisation and/or incubation ability, or an over representation of genetic components from the female acting on the early developing embryo.
  • Lankinen, Åsa, et al. (författare)
  • Allocation to pollen competitive ability versus seed production in Viola tricolor as an effect of plant size, soil nutrients and presence of a root competitor
  • 2013
  • Ingår i: Oikos. - : Wiley: 12 months / Nordic Ecological Society. - 0030-1299 .- 1600-0706. ; 122, s. 779-789
  • Tidskriftsartikel (refereegranskat)abstract
    • In hermaphroditic plants, the effect of a social environment on sex allocation has not been studied to our knowledge, while in hermaphroditic animals such effects are known to be common. In recent years, studies on root competition beyond the effects of nutrients have shown that plants can respond to their conspecific root competitors (social environment), making it interesting to ask if these effects could also influence sex allocation in addition to more commonly studied factors, such as plant size or resources. In this study on hermaphroditic Viola tricolor, we investigated how plant size, soil nutrients and presence of a root competitor influenced allocation to pollen competitive ability versus seed production, i.e. male and female reproductive functions. We allowed plants to grow in pairs with partly intermingled or separate roots in the same amount of soil. In additional treatments with intermingled roots (as part of the same experiment) one of the two competitors was given combinations of nitrogen (N), phosphorous (P) and micro nutrients. We found that pollen performance but not seed production increased when plants were in contact in the soil. Additionally, pollen performance was negatively correlated to plant size across fertilisation treatments. For seed production, the opposite relation to plant size was seen, indicating that large, fertilized plants invest relatively more in the female function. In conclusion, in violets, sex allocation appears to be size-dependent and influenced by both the presence of a root competitor and by nutrients. These results suggest that social environment can influence sex allocation in plants as well as in animals, indicating that such effects are important to consider in sex allocation studies across taxa.
  • Lindström, Åke, et al. (författare)
  • Density-dependent reproductive output in relation to a drastically varying food supply: getting the density measure right
  • 2005
  • Ingår i: Oikos. - : Wiley-Blackwell. - 1600-0706. ; 110:1, s. 155-163
  • Tidskriftsartikel (refereegranskat)abstract
    • When a limiting resource (e.g. food) varies drastically between years, and population density is measured in the conventional way as individuals per area, demographic processes such as productivity and survival may erroneously be considered density-independent. We tested the hypothesis that if the variation in a limiting resource is not taken into account in the density measure, this may lead to erroneous conclusions about the density-dependence of demographic variables. We studied the food-related variation in productivity of bramblings Fringilla montifringilla, an insectivorous passerine bird, using 19 years of standardised insect censusing, bird censusing and mist-netting of birds in subalpine birch forest in Swedish Lapland. The yearly variation in our measure of brambling per capita productivity (numbers of juveniles per adult trapped) was explained to 30-40% by the larvae abundance of the moth Epirrita autumnata. Taking larvae density into account, no other environmental variable (inferred predation pressure, breeding phenology, and summer temperature) was significantly related to variation in reproductive output. There was no effect of brambling population density on per capita productivity, that is, when density was measured the conventional way, productivity seemed density-independent. However, per capita productivity was significantly and negatively correlated to the food-related population density (population density divided by larval density), supporting the hypothesis that not including a limiting resource into the density measure may indeed lead to erroneous conclusions about the density-dependence of demographic variables.
  • Lönn, M, et al. (författare)
  • Gene diversity and demographic turnover in central and peripheral populations of the perennial herb Gypsophila fastigiata
  • 2002
  • Ingår i: Oikos. - : Wiley-Blackwell. - 1600-0706 .- 0030-1299. ; 99:3, s. 489-498
  • Tidskriftsartikel (refereegranskat)abstract
    • Within-population gene diversity (H-S) was estimated (using allozyme markers) for 16 populations of the perennial, outcrossing plant, Gypsophila fastigiata, on the Baltic island of Oland. The populations were characterized by data on extent, density, life-stages, and habitat diversity. Populations were classed as central or peripheral in relation to the distribution of ''alvar" (habitats with shallow, calcareous soils on limestone bedrock) on southern Oland. Three minimal adequate models were used to explain H-S and the proportions of juveniles and dead adults. In the first model, H-S was significantly lower in peripheral populations and there were no significant additional effects of other explanatory variables. The lower diversity in peripheral populations can be explained by a combination of genetic drift (in populations that vary in size in response to habitat fragmentation) and lower levels of interpopulation gene flow than in central populations. In the two life-stage models, peripheral populations had significantly larger proportions of both juveniles and dead adults indicating a greater demographic turnover than in the central populations. There were also significant effects of H-S and species diversity on the proportion of juveniles. The central or peripheral position of populations is the strop est predictor of both within-population gene diversity and life-stage dynamics in Oland G. fastigiata.
  • Marini, Lorenzo, et al. (författare)
  • Population dynamics of the spruce bark beetle: a long-term study
  • 2013
  • Ingår i: Oikos. - : Wiley-Blackwell. - 1600-0706 .- 0030-1299. ; 122:12, s. 1768-1776
  • Tidskriftsartikel (refereegranskat)abstract
    • Bark beetle population dynamics is thought to be primarily driven by bottom-up forces affecting insect performance and host tree resistance. Although there are theoretical predictions and empirical evidences that predation and parasitism may play an important role in driving bark beetle population fluctuations, long-term studies testing the role of both biotic and abiotic controls on population dynamics are still rare. The aim of the study was to quantify the relative importance of predation, negative density feedback and abiotic factors in driving Ips typographus population dynamics. We analyzed a unique time series of population density of I. typographus and its main predator Thanasimus formicarius over almost two decades in four regions across Sweden. We used a discrete population model and a multi-model inference approach to evaluate the importance of both bottom up and top down factors. We found that availability of breeding substrates in the form of storm-felled trees was the main outbreak trigger, while strong intra-specific competition for host trees was the main endogenous regulating factor. Although temperature-related metrics are known to have strong individual effect on I. typographus development and number of generations, they did not emerge as important drivers of population dynamics. A positive effect of low summer rainfall was evident only in the region located in the southernmost and warmest part of the spruce distribution range in Sweden. Predator density did not emerge as an important prey regulating factor. As the reported damage from storms seems to have increased across whole Europe, spruce forests are expected to be increasingly susceptible to large outbreaks of I. typographus with important economic and ecological consequences for boreal ecosystems. However, the observed negative density feedback seems to be a natural regulating mechanism that impedes a strong long-term propagation of the outbreaks.
  • Schröder, Arne, et al. (författare)
  • Direct experimental evidence for alternative stable states : a review
  • 2005
  • Ingår i: Oikos. - 0030-1299 .- 1600-0706. ; 110:1, s. 3-19
  • Tidskriftsartikel (refereegranskat)abstract
    • A large number of studies have presented empirical arguments for the existence of alternative stable states (ASS) in a wide range of ecological systems. However, most of these studies have used non-manipulative, indirect methods, which findings remain open for alternative explanations. Here, we review the direct evidence for ASS resulting from manipulation experiments. We distinguish four conclusive experimental approaches which test for predictions made by the hysteresis effect: (1) discontinuity in the response to an environmental driving parameter, (2) lack of recovery potential after a perturbation, (3) divergence due to different initial conditions and (4) random divergence. Based on an extensive literature search we found 35 corresponding experiments. We assessed the ecological stability of the reported contrasting states using the minimum turnover of individuals in terms of life span and classified the studies according to 4 categories: (1) experimental system, (2) habitat type, (3) involved organisms and (4) theoretical framework. 13 experiments have directly demonstrated the existence of alternative stable states while 8 showed the absence of ASS in other cases. 14 experiments did not fulfil the requirements of a conclusive test, mostly because they applied a too short time scale. We found a bias towards laboratory experiments compared to field experiments in demonstrating bistability. There was no clear pattern of the distribution of ASS over categories. The absence of ASS in 38% of the tested systems indicates that ASS are just one possibility of how ecological systems can behave. The relevance of the concept of ASS for natural systems is discussed, in particular under consideration of the observed laboratory bias, perturbation frequency and variable environments. It is argued, that even for a permanently transient system, alternative attractors may still be of relevance.
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