| 1. |
- Albrectsen, Benedicte R., 1960-, et al.
(författare)
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Nutrient addition extends flowering display, which gets tracked by seed predators, but not by their parasitoids
- 2008
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Ingår i: Oikos. - : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; 117, s. 473-480
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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.
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| 2. |
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| 3. |
- Schröder, Arne, et al.
(författare)
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Direct experimental evidence for alternative stable states : a review
- 2005
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 110:1, s. 3-19
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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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| 4. |
- Sundqvist, Maja K., et al.
(författare)
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Interactive effects of vegetation type and elevation on aboveground and belowground properties in a subarctic tundra
- 2011
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Ingår i: Oikos. - Copenhagen : Munksgaard. - 0030-1299 .- 1600-0706. ; 120:1, s. 128-142
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Tidskriftsartikel (refereegranskat)abstract
- An improved knowledge of how contrasting types of plant communities and their associated soil biota differ in their responses to climatic variables is important for better understanding the future impacts of climate change on terrestrial ecosystems. Elevational gradients serve as powerful study systems for answering questions on how ecological processes can be affected by changes in temperature and associated climatic variables. In this study, we evaluated how plant and soil microbial communities, and abiotic soil properties, change with increasing elevation in subarctic tundra in northern Sweden, for each of two dominant but highly contrasting vegetation types, namely heath (dominated by woody dwarf shrubs) and meadow (dominated by herbaceous species). To achieve this, we measured plant community characteristics, microbial community properties and several soil abiotic properties for both vegetation types across an elevation gradient of 500 to 1000 m. We found that the two vegetation types differed not only in several above- and belowground properties, but also in how these properties responded to elevation, pointing to important interactive effects between vegetation type and elevation. Specifically, for the heath, available soil nitrogen and phosphorus decreased with elevation whereas fungal dominance increased, while for the meadow, idiosyncratic responses to elevation for these variables were found. These differences in belowground responses to elevation among vegetation types were linked to shifts in the species and functional group composition of the vegetation. Our results highlight that these two dominant vegetation types in subarctic tundra differ greatly not only in fundamental aboveground and belowground properties, but also in how these properties respond to elevation and are therefore likely to be influenced by temperature. As such they highlight that vegetation type, and the soil abiotic properties that determine this, may serve as powerful determinants of how both aboveground and belowground properties respond to strong environmental gradients.
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| 5. |
- O'Sullivan, Daniel, et al.
(författare)
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Inter-patch movement in an experimental system : the effects of life history and the environment
- 2014
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Ingår i: Oikos. - : John Wiley & Sons. - 0030-1299 .- 1600-0706. ; 123:5, s. 623-629
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Tidskriftsartikel (refereegranskat)abstract
- An important process for the persistence of populations subjected to habitat loss and fragmentation is the dispersal of individuals between habitat patches. Dispersal involves emigration from a habitat patch, movement between patches through the surrounding landscape, and immigration into a new suitable habitat patch. Both landscape and physical condition of the disperser are known to influence dispersal ability, although disentangling these effects can often be difficult in the wild. In one of the first studies of its kind, we used an invertebrate model system to investigate how dispersal success is affected by the interaction between the habitat condition, as determined by food availability, and life history characteristics (which are also influenced by food availability). Dispersal of juvenile and adult mites (male and female) from either high food or low food natal patches were tested separately in connected three patch systems where the intervening habitat patches were suitable (food supplied) or unsuitable (no food supplied). We found that dispersal success was reduced when low food habitat patches were coupled to colonising patches via unsuitable intervening patches. Larger body size was shown to be a good predictor of dispersal success, particularly when the intervening landscape is unsuitable. Our results suggest that there is an interaction between habitat fragmentation and habitat suitability in determining dispersal success: if patches degrade in suitability and this affects the ability to disperse successfully then the effective connectance across landscapes may be lowered. Understanding these consequences will be important in informing our understanding of how species, and the communities in which they are embedded, may potentially respond to habitat fragmentation.
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| 6. |
- Albrectsen, Benedicte Riber, 1960-, et al.
(författare)
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Slugs, willow seedlings and nutrient fertilization: intrinsic vigor inversely affects palatability
- 2004
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Ingår i: Oikos. - : John Wiley & Sons. - 0030-1299 .- 1600-0706. ; 105, s. 268-278
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Tidskriftsartikel (refereegranskat)abstract
- This study evaluates how preference by a generalist slug herbivore Arion subfuscus changes inversely with seedling size across three levels of fertilization for three full‐sib families of willow seedlings. We analyzed seedlings for condensed tannin and protein concentration, and related these data to changes in palatability. In preference tests over time, leaf discs from more fertilized seedlings experienced an extended window of vulnerability compared to discs from less fertilized seedlings, which were also more tannin‐rich. In a whole seedling selection study, slugs readily attacked smaller seedlings (<5 cm) but rarely attacked taller seedlings (>10 cm). However, a general difference in risk of damage close to 50% existed when comparing shorter and taller individuals within each family and level of fertilizer. The decrease in palatability with height of the seedlings was positively correlated with an increase in condensed tannin concentration. We found no effect of seedling size on protein concentration. Akaiki index criterion model comparisons suggested that only main effects were important for explaining seedling choice by slugs as well as the ratio between proteins and condensed tannins. Seedling size, had the largest effect, followed by fertilizer level and family. Surprisingly, seedling size and fertilizer treatment had opposite effects on palatability to slugs. Size decreased probability of damage, whereas fertilization extended the window of susceptibility. Because the seedlings were even‐aged, differences in size are interpreted as differences in growth rate or vigor. The positive phenotypic correlation found between size and tannin production in the less preferred willow seedlings confirms that several plant defense traits may be selected for simultaneously, because fast growth may allow an early development of plant defenses. We discuss these results in the light of plant‐defense theories that predict a negative correlation between the allocation to growth and the production of secondary defense compounds.
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| 7. |
- Bourdeau, Paul E., et al.
(författare)
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Predator-induced morphological defences as by-products of prey behaviour : a review and prospectus
- 2012
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 121:8, s. 1175-1190
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Forskningsöversikt (refereegranskat)abstract
- Predator-induced morphological defences (PIMDs) are ubiquitous. Many PIMDs may be mediated by prey behaviour rather than directly cued by predators. A survey of 92 studies indicated 40 that quantified prey behaviour, all of which document positive associations between defence production and activity reduction. Thus, PIMDs are associated with changes in prey activity, which could have caused the morphological change. We propose two possible mechanisms: 1) decreased activity reduces feeding rate, resulting in lower growth and morphological change; and 2) activity reduction conserves energy, which is reallocated for growth, subsequently changing morphology. Resource availability also causes similar morphological change to predator presence, suggesting confounding effects of resources and predators with current methodology. Future studies should estimate food ingestion, assimilation efficiency, and growth rate in the presence and absence of predators, crossing predator presence with resource levels. Not all PIMDs will be behaviourally-mediated, but consideration of causal linkages between prey behaviour and PIMDs is warranted.
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| 8. |
- Carlsson-Granér, Ulla, et al.
(författare)
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The spatial distribution of plant populations, disease dynamics and evolution of resistance
- 2002
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Ingår i: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 97:1, s. 97-110
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Tidskriftsartikel (refereegranskat)abstract
- Empirical studies of the interaction between the anther smut fungus Microbotryum violaceum and its host plant Lychnis alpina were combined with modelling approaches to investigate how variation in the spatial distribution of host populations influences disease dynamics and variation in resistance. Patterns of disease incidence and prevalence were surveyed in three contrasting systems of natural L. alpina populations where there is substantial variation in spatial structure, ranging from large continuous populations through to small isolated patches. Disease incidence (fraction of populations where disease was present) was highest in the continuous situation, and lowest in the most isolated populations. The reverse was true for prevalence (fraction of individuals diseased). To better understand the long-term ecological and evolutionary consequences of differences in among population spatial structure, we developed a two-dimensional spatially explicit simulation model in which host-population spacing was modelled by varying the percentage of sites suitable for the host. The general patterns of disease incidence and prevalence generated in the simulations corresponded well with the patterns observed in natural populations of L. alpina and M. violaceum; i.e. the fraction of sites with disease increased while the average disease prevalence in diseased populations decreased when host populations became more connected. One likely explanation for the differences in disease incidence and prevalence seen in natural populations is that the evolution of host resistance varies as a function of the degree of fragmentation. This is supported by simulation results that were qualitatively similar to the survey data when resistance was allowed to vary, but not when hosts were assumed to be uniformly susceptible. In the former, the frequency of resistance increased markedly as host populations became more connected.
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| 9. |
- Cherif, Mehdi, et al.
(författare)
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Towards a more biologically realistic use of Droop's equations to model growth under multiple nutrient limitation
- 2010
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Ingår i: Oikos. - : Wiley-Blackwell. - 0030-1299 .- 1600-0706. ; 119:6, s. 897-907
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Tidskriftsartikel (refereegranskat)abstract
- Droop's model was originally designed to describe the growth of unicellular phytoplankton species in chemostats but it is now commonly used for a variety of organisms in models of trophic interactions, ecosystem functioning, and evolution. Despite its ubiquitous use, Droop's model is still limited by several simplifying assumptions. For example, the assumption of equal theoretical maximum growth rates for all nutrients is commonly used to describe growth limited by multiple nutrients. This assumption, however, is both biologically unrealistic and potentially misleading. We propose the alternative hypothesis of equal realized maximum growth rates for all nutrients. We support our hypothesis with empirical and theoretical arguments and discuss how it may improve our understanding of the biology of growth, while avoiding some of the pitfalls of the previous assumption.
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| 10. |
- Doi, Hideyuki, et al.
(författare)
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Integrating elements and energy through the metabolic dependencies of gross growth efficiency and the threshold elemental ratio
- 2010
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Ingår i: Oikos. - : WILEY-BLACKWELL. - 0030-1299 .- 1600-0706. ; 119:5, s. 752-765
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Tidskriftsartikel (refereegranskat)abstract
- Metabolic theory proposes that individual growth is governed through the mass- and temperature-dependence of metabolism, and ecological stoichiometry posits that growth is maximized at consumer-specific optima of resource elemental composition. A given consumer's optimum, the threshold elemental ratio (TER), is proportional to the ratio of its maximum elemental gross growth efficiencies (GGEs). GGE is defined by the ratio of metabolism-dependent processes such that GGEs should be independent of body mass and temperature. Understanding the metabolic-dependencies of GGEs and TERs may open the path towards a theoretical framework integrating the flow of energy and chemical elements through ecosystems. However, the mass and temperature scaling of GGEs and TERs have not been broadly evaluated. Here, we use data from 95 published studies to evaluate these metabolic-dependencies for C, N and P from unicells to vertebrates. We show that maximum GGEs commonly decline as power functions of asymptotic body mass and exponential functions of temperature. The rates of change in maximum GGEs with mass and temperature are relatively slow, however, suggesting that metabolism may not causally influence maximum GGEs. We additionally derived the theoretical expectation that the TER for C:P should not vary with body mass and this was supported empirically. A strong linear relationship between carbon and nitrogen GGEs further suggests that variation in the TER for C:N should be due to variation in consumer C:N. In general we show that GGEs may scale with metabolic rate, but it is unclear if there is a causal link between metabolism and GGEs. Further integrating stoichiometry and metabolism will provide better understanding of the processes governing the flow of energy and elements from organisms to ecosystems.
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