| 51. |
- Nilsson, Anders
(författare)
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Avoid your neighbours: size-determined spatial distribution patterns among northern pike individuals
- 2006
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 113:2, s. 251-258
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Tidskriftsartikel (refereegranskat)abstract
- The spatial distribution of individual animals may both cause and be caused by intra- and/or interspecific interactions. This work aims primarily on effects of intraspecific interactions. Agonistic and unequally strong interactions among conspecifics should make the within-population distribution of individuals to be characterised by spatial avoidance of potentially risky conspecifics, according to the individual risks perceived. This process should affect individual performance and involve individual tradeoffs, as failing to adequately avoid risky conspecifics could incur unnecessary costs, while, at the same time, successful conspecific avoidance may reduce access to patches favourable for e.g. foraging or sheltering. Intraspecific agonistic behaviours, such as cannibalism and competition, are likely to have prominent effects in size-structured populations. It is therefore reasonable to assume spatial avoidance of intraspecific risks according to individuals' size relationships in such populations. With this field investigation I show that individuals of northern pike spatially avoid larger conspecifics. This avoidance creates a size-influenced and spatially clumped distribution pattern among pike individuals. At low pike densities, however, distances between individuals increase, allowing for an even distribution pattern to appear. The spatial distribution patterns among piscivore individuals should affect both the individual performance of predators and the potential for spatial antipredatory responses of their prey, and hence be a factor in consumer-resource interactions.
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| 52. |
- Nilsson, Anders, et al.
(författare)
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Prey vulnerability to a gape-size limited predator: behavioural and morphological impacts on northern pike piscivory
- 2000
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 88:3, s. 539-546
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Tidskriftsartikel (refereegranskat)abstract
- The northern pike (Esox lucius) is a selective and important predator in lake ecosystems. Prey size in pike is limited by pike gape size, which is a linear function of pike body length. Here we show that the absolute gape-size limit in pike is greater than previously considered, and that maximum ingestible prey size is limited by prey body depth. Further, we experimentally show that pike prefer shallow-bodied roach before deeper-bodied common bream, and small prey sizes within each prey species. Handling lime in pike increases with prey body depth, and since common bream are deeper-bodied than roach, handling time is longer for bream than for roach of the same length, but equal considering body depth. Prey handling time is suggested to be a major cost to the pike, since it increases the risk of Losing the prey, as well as exposure to predation, kleptoparasitism and cannibalism. Consequently, prey vulnerability is determined by risk of predation and intraspecific interactions, and behavioural preferences in the pike, and not by pike gape-size limits. The consequences for natural populations is evaluated by analysing size structures of predator and prey fish populations in a eutrophic lake.
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| 53. |
- Nilsson, Anders, et al.
(författare)
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Temporally fluctuating prey and coexistence among unequal conspecific interferers
- 2003
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Ingår i: Oikos. - : Wiley. ; 101:2, s. 411-415
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Tidskriftsartikel (refereegranskat)abstract
- Coexistence among unequal conspecific interferers should be unlikely to persist if stronger interferers always experience a relative fitness increment from their higher foraging rates. In this study, we suggest that decreased relative costs to weaker interferers with increasing temporal fluctuations in prey availability may be a mechanism enhancing coexistence of unequal conspecific interferers. Previous work on fluctuation and coexistence has dealt with oscillations over a time-scale measured in generations of competitor species and their resources, while our work shows that fluctuations in prey availability facilitate coexistence of different phenotypic strategies within species and generations, and over short time-scales. With increasing amplitude of temporal fluctuation about an average prey density, cumulative intakes for differently strong interferers are affected differently. Because of the prey-dependent effect of interference, high amplitudes of fluctuation allow for relatively lower foraging-rate costs in weaker interferers, which decreases the difference in foraging success between strong and weaker interferers. This decreased difference in foraging success could thus significantly relax the conditions allowing for unequal interferer coexistence.
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| 54. |
- Nilsson, Elna, et al.
(författare)
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Death odour changes movement pattern of a Collembola
- 2004
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 104:3, s. 509-517
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Tidskriftsartikel (refereegranskat)abstract
- We used video-tracking of individuals of a Collembola, Protaphorura armata, on a clay surface in a petri dish to analyse their movement pattern in an environment with attractive and repellent cues. An area with dead conspecifics was repellent whilst live conspecifics made the area attractive. An area which had been occupied for 24 hours by the predatory mite, Hypoaspis aculeifer, was avoided only if the mite had preyed upon P. armata before it was placed in the area. P. armata lost their looping behaviour, moved faster and more straightened out (decreased turning rate) in the presence of attractive or repellent odours. The resulting net squared displacement was faster than in the control and best described as a correlated random walk. Our results emphasise the importance of considering varying movement pattern in response to environmental cues when predicting dispersal and spatial distribution of an animal.
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| 55. |
- Nilsson, Jan-Åke, et al.
(författare)
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Postnatal effects of incubation length in mallard and pheasant chicks
- 2004
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 105:3, s. 588-594
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Tidskriftsartikel (refereegranskat)abstract
- Eggs of mallard ducks (Anas platyrhynchos) and ring-necked pheasants (Phasianus colchicus) were incubated in clutches arranged to stimulate embryos to hatch earlier or later than normal. This manipulation of hatching time was achieved by combining eggs of different age in the same clutch. To ensure hatching synchrony, embryos communicate with each other during the last stage of incubation, resulting in either a delay or an acceleration of hatching. Embryos of both species that accelerated their hatching time suffered a higher mortality rate after hatching. Combining mortality with the proportion of hatchlings that suffered from leg deformities, impeding their movements, resulted in a cost also to pheasant chicks delaying their hatching. Chicks of both species accelerating hatching time had a lower minimum mass and a shorter tarsus length than control chicks, whereas chicks delaying hatching time either grew as well or slightly better than control chicks. Mallard chicks had better balance and mobility immediately after hatching the longer they stayed in the egg. This indicates that the period immediately before hatching, is an important period for muscular and organ maturity. Reducing this period results in costs affecting post-hatching survival. The strategy to assure synchronous hatching in mallards and pheasants probably reflect a trade-off between the negative effects of shifting the age at hatching away from normal and differences in predation risk during different stages of reproduction.
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| 56. |
- Nilsson, Jan-Åke
(författare)
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Time-dependent reproductive decisions in the blue tit
- 2000
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 88:2, s. 351-361
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Tidskriftsartikel (refereegranskat)abstract
- Many breeding attempts in birds do not result in any fledged young due to predation on eggs or young. Consequently, the influence of time constraints on reproductive decisions are integrated parts of the reproductive behaviour of birds breeding within short, seasonal climate zones. In this study. I mimicked nest predation by removing blue tit (Parus caeruleus) clutches shortly after completion. Around 75% of the removed clutches were followed by a repeat clutch. Females producing their first clutch early in the season and females with an early onset of incubation in the laying sequence (an indication of high parental or territory quality) were most likely to initiate a repeat clutch. A trade-off between the benefits of a repeat clutch and survival likely stopped late Females in bad condition from investing more in the current reproductive season. Females producing a repeat clutch laid fewer eggs, had an earlier onset of incubation in the laying sequence and produced larger eggs than they did when producing their original first clutch. Eggs produced after the onset of incubation were especially large in the repeat clutches. Since food availability was presumably higher when the female produced her repeat clutch compared with her first clutch, females made a strategical decision when reducing clutch size, whereas onset of incubation and egg size may have been energetically constrained when producing the first clutch. Females that produced a relatively large clutch, had a relatively early onset of incubation, and laid relatively large eggs in their first clutch also did so when producing a repeat clutch, indicating that some of the variation in breeding parameters are due to differences in parental or territory quality. Differences between years in the temperature-dependent development rate of caterpillars seem to affect the time constraints on breeding. A year with a predicted early seasonal decline in caterpillars resulted in short intervals between removal and relaying, small clutches and an early onset of incubation.
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| 57. |
- Nyström, Per, et al.
(författare)
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Effects of fish chemical cues on the interactions between tadpoles and crayfish
- 2000
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 88:1, s. 181-190
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Tidskriftsartikel (refereegranskat)abstract
- We studied the effects of predatory crayfish (Pacifastacus leniusculus), the non-lethal effects of fish chemical cues (Oncorhynchus mykiss), and the combined effects of crayfish and fish chemical cues on the performance of tadpoles of two co-existing anuran species, Rana temporaria and Bufo bufo, in experimental pools. We also examined grazing effects on periphyton, the main food source for the tadpoles. Crayfish significantly reduced tadpole survival, particularly by feeding on Bufo. Rana benefited from reduced numbers of competitors, resulting from crayfish predation, by increased growth rate, whereas the growth rate of Bufo was unaffected by crayfish. The proportion of Rana in refuges (in relation to the number of survivors at the end of the experiment) was unaffected by crayfish, whereas proportionally more Bufo stayed in refuges in the presence of crayfish, relative to controls. Fish cues had no effect on tadpole survival of either species. During the entire larval period, Rana responded to fish cues by increasing the use of refuges relative to controls, whereas Bufo, did not show any significant behavioural response to fish cues. In accordance with these observations, the proportion of Rana in refuges at the end of the experiment was high in the presence of fish cues, whereas the use of refuges by Bufo was not affected by fish cues. Predatory crayfish and fish chemical cues had effects on tadpole survival, growth and refuge use. Tadpoles in all treatments reduced periphyton biomass. Both crayfish and fish cues had positive indirect effects on periphyton biomass. The positive indirect effect of fish cues on periphyton was likely an effect of reduced grazing from Rana. Thus lethal, as well as non-lethal, predator effects on prey populations can influence lower trophic levels.
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| 58. |
- Olsson, Ola
(författare)
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Bayesian foraging with only two patch types
- 2006
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 112:2, s. 285-297
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Tidskriftsartikel (refereegranskat)abstract
- model the optimal Bayesian foraging strategy in environments with only two patch qualities. That is, all patches either belong to one rich type, or to one poor type. This has been a situation created in several foraging experiments. In contrast, previous theories of Bayesian foraging have dealt with prey distributions where patches may belong to one out of a large range of qualities (binomial, Poisson and negative binomial distributions). This study shows that two-patch systems have some unique properties. One qualitative difference is that in many cases it will be possible for a Bayesian forager to gain perfect information about patch quality. As soon as it has found more than the number of prey items that should be available in a poor patch, it "knows" that it is in a rich patch. The model generates at least three testable predictions. 1) The distribution of giving-up densities, GUDs, should be bimodal in rich patches, when rich patches are rare in the environment. This is because the optimal strategy is then devoted to using the poor patches correctly, at the expense of missing a large fraction of the few rich patches available. 2) There should be a negative relation between GUD and search time in poor patches, when rich patches are much more valuable than poor. This is because the forager gets good news about potential patch quality from finding some food. It therefore accepts a lower instantaneous intake rate, making it more resistant against runs of bad luck, decreasing the risk of discarding rich patches. 3) When the energy gains required to remain in the patch are high (such as under high predation risk), the overuse of poor patches and the underuse of rich increases. This is because less information about patch quality is gained if leaving at high intake rates (after short times). The predictions given by this model may provide a much needed and effective conceptual framework for testing (both in the lab and the field) whether animals are using Bayesian assessment.
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| 59. |
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| 60. |
- Olsson, Ola, et al.
(författare)
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The foraging benefits of information and the penalty of ignorance
- 2006
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Ingår i: Oikos. - : Wiley. - 1600-0706 .- 0030-1299. ; 112:2, s. 260-273
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Tidskriftsartikel (refereegranskat)abstract
- Patch use theory and the marginal value theorem predict that a foraging patch should be abandoned when the costs and benefits of foraging in the patch are equal. This has generally been interpreted as all patches being abandoned when their instantaneous intake rate equals the foraging costs. Bayesian foraging – patch departure is based on a prior estimate of patch qualities and sampling information from the current patch – predicts that instantaneous quitting harvest rates sometimes are not constant across patches but increase with search time in the patch. That is, correct Bayesian foraging theory has appeared incompatible with the widely accepted cost–benefit theories of foraging. In this paper we reconcile Bayesian foraging with cost–benefit theories. The general solution is that a patch should be left not when instantaneous quitting harvest rate reaches a constant level, but when potential quitting harvest rate does. That is, the forager should base its decision on the value now and in the future until the patch is left. We define the difference between potential and instantaneous quitting harvest rates as the foraging benefit of information, FBI. For clumped prey the FBI is positive, and by including this additional benefit of patch harvest the forager is able to reduce its penalty of ignorance.
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