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  • 1.
    Elmberg, Johan
    et al.
    Kristianstad University, School of Education and Environment, Avdelningen för Naturvetenskap.
    Gunnarsson, Gunnar
    Kristianstad University, School of Education and Environment, Avdelningen för Naturvetenskap.
    Pöysä, Hannu
    Finland.
    Sjöberg, Kjell
    SLU, Umeå.
    Nummi, Petri
    Finland.
    Within-season sequential density dependence regulates breeding success in mallards Anas platyrhynchos2005In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 108, p. 582-590Article in journal (Refereed)
    Abstract [en]

    Density dependence in vital rates is a key issue in population ecology but remains largely unexplored experimentally. We studied breeding success, lake use, and prey availability in wild mallards Anas platyrhynchos on small nemoral lakes in a replicated, two-year cross-over experiment in which pair density was increased. The number of wild mallards that settled on lakes prior to introductions of extra pairs did not differ between control and introduction years. Introductions led to a lake-level reduction in the number of broods observed. However, the number of stage 2/ (almost fledged) ducklings did not differ between treatments, nor did lake utilization by nonbreeding adults, broods and ducklings. Prey resource availability differed greatly among lakes, but it did not correlate with breeding success. Partialling out the possible effect of food competition from wild adult nonbreeding mallards did not change this conclusion. Our study demonstrates sequential density dependence in breeding success; introductions caused a decrease in brood number, but despite fewer broods a similar number of nearly fledged ducklings were produced. We suggest that predation and/or lake change of broods soon after hatching created these patterns. We conclude that using a single and late measure of breeding success such as fledged birds can mask regulatory processes. Implications of density dependence and its relation to individual reproductive success are understood better if breeding success is decomposed into nest success, duckling survival and fledgling survival.

  • 2.
    Elmberg, Johan
    et al.
    Kristianstad University, Department of Mathematics and Science.
    Gunnarsson, Gunnar
    Kristianstad University, Department of Mathematics and Science.
    Pöysä, Hannu
    Finnish Game and Fisheries Research Institute, Joensuu Game and Fisheries Research.
    Sjöberg, Kjell
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Nummi, Petri
    Department of Applied Biology, University of Helsinki.
    Within-season sequential density dependence regulates breeding success in mallards (Anas platyrhynchos)2005In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 108, no 3, p. 582-590Article in journal (Refereed)
    Abstract [en]

    Density dependence in vital rates is a key issue in population ecology but remains largely unexplored experimentally. We studied breeding success, lake use, and prey availability in wild mallards Anas platyrhynchos on small nemoral lakes in a replicated, two-year cross-over experiment in which pair density was increased. The number of wild mallards that settled on lakes prior to introductions of extra pairs did not differ between control and introduction years. Introductions led to a lake-level reduction in the number of broods observed. However, the number of stage 2+ (almost fledged) ducklings did not differ between treatments, nor did lake utilization by nonbreeding adults, broods and ducklings. Prey resource availability differed greatly among lakes, but it did not correlate with breeding success. Partialling out the possible effect of food competition from wild adult nonbreeding mallards did not change this conclusion. Our study demonstrates sequential density dependence in breeding success; introductions caused a decrease in brood number, but despite fewer broods a similar number of nearly fledged ducklings were produced. We suggest that predation and/or lake change of broods soon after hatching created these patterns. We conclude that using a single and late measure of breeding success such as fledged birds can mask regulatory processes. Implications of density dependence and its relation to individual reproductive success are understood better if breeding success is decomposed into nest success, duckling survival and fledgling survival.

  • 3.
    Elmberg, Johan
    et al.
    Kristianstad University, Department of Mathematics and Science.
    Nummi, Petri
    Department of Applied Biology, University of Helsinki.
    Pöysä, Hannu
    Finnish Game and Fisheries Res. Inst., Joensuu Game and Fisheries Research.
    Sjöberg, Kjell
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Breeding success of sympatric dabbling ducks in relation to population density and food resources2003In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 100, no 2, p. 333-341Article in journal (Refereed)
    Abstract [en]

    Breeding success in sympatric mallard Anas platyrhynchos, teal A. crecca and wigeon A. penelope in a boreal watershed in Finland was studied for 12 years. Benthic and surface-emerging prey animals were trapped to obtain annual indices of food abundance. Mallard and teal were equally abundant over the years, being roughly twice as numerous as wigeon. Pair density, brood:pair ratio and duckling:pair ratio were used to test the hypothesis that per capita breeding success decreases in a density-dependent fashion as either pair density or the number of nesting pairs per available food unit increases. In mallard we found no density-dependent patterns at all. In teal per capita brood production decreased as prey animals became relatively scarcer, but this interpretation may not be robust. In wigeon, however, there were two independent significant patterns of direct density-dependence in a temporal succession, i.e. between pair density and per capita brood production in the early part of the breeding season, and then between per capita abundance of surface-emerging insect prey and the number of ducklings per pair. Despite wide dietary overlap and frequent co-occurrence on single lakes among species in the guild, we found no evidence for interspecific density-dependent effects. We hypothesize that there is no or infrequent food limitation for breeding dabblers in this system, and that behavior may be the process behind the pattern of density-dependence in wigeon.

  • 4.
    Nudds, Thomas D.
    et al.
    Deptartment of Zoology, University of Guelph.
    Elmberg, Johan
    Deptartment of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Sjöberg, Kjell
    Deptartment of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Pöysä, Hannu
    Game and Fisheries Research Institute, Evo Game Research Station.
    Nummi, Petri
    Deptartment of Applied Zoology, University of Helsinki.
    Ecomorphology in breeding Holarctic dabbling ducks: the importance of lamellar density and body length varies with habitat type2000In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 91, no 3, p. 583-588Article in journal (Refereed)
  • 5.
    Pöysä, Hannu
    et al.
    Finnish Game and Fisheries Research Institute, Joensuu Game and Fisheries Research.
    Elmberg, Johan
    Kristianstad University, Department of Mathematics and Science.
    Gunnarsson, Gunnar
    Kristianstad University, Department of Mathematics and Science.
    Nummi, Petri
    Department of Applied Biology, University of Helsinki.
    Sjöberg, Kjell
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Ecological basis of sustainable harvesting: is the prevailing paradigm of compensatory mortality still valid?2004In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 104, no 3, p. 612-615Article in journal (Refereed)
  • 6.
    Pöysä, Hannu
    et al.
    Finnish Game and Fisheries Research Institute, Evo Game Research Station.
    Elmberg, Johan
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Nummi, Petri
    Department of Applied Zoology, University of Helsinki.
    Sjöberg, Kjell
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Are ecomorphological associations among dabbling ducks consistent at different spatial scales?1996In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 76, no 3, p. 608-612Article in journal (Refereed)
  • 7.
    Ranta, Esa
    et al.
    Integrative Ecology Unit, Department of Zoology, University of Helsinki.
    Laurila, Anssi
    Integrative Ecology Unit, Department of Zoology, University of Helsinki.
    Elmberg, Johan
    Department of Animal Ecology, Swedish University of Agricultural Sciences, Umeå.
    Reinventing the wheel: analysis of sexual dimorphism in body size1994In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 70, no 3, p. 313-321Article in journal (Refereed)
    Abstract [en]

    Sexual dimorphism in body size has received increasing scientific interest during the past decade. In most of the papers on the subject, the magnitude of sexual size dimorphism is expressed as some conversion of body size ratio. There are several possible flaws in using such ratios, however, the most serious one concers the scaling of body size and size differences between sexes. Using ratios may easily lead to wrong conclusions. In this paper we emphasize some fallacies related to ratios and present a less biased method, based on residuals of linear regression. This method is suggested to be used in comparative analyses of sexual size dimorphism.

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