Is the shape of the density–growth relationship for stream salmonids evidence for exploitative rather than interference competition?

Authors

  • D. M. WARD,

    1. Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA; USDA Forest Service Northeastern Research Station, 201 Holdsworth NRC, 160 Holdsworth Way, Amherst, MA 01003, USA;
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  • K. H. NISLOW,

    1. Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA; USDA Forest Service Northeastern Research Station, 201 Holdsworth NRC, 160 Holdsworth Way, Amherst, MA 01003, USA;
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  • J. D. ARMSTRONG,

    1. Fisheries Research Services Freshwater Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, Scotland, UK;
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  • S. EINUM,

    1. Norwegian Institute for Nature Research, Tungasletta 2, NO-7485 Trondheim, Norway
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  • C. L. FOLT

    1. Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA; USDA Forest Service Northeastern Research Station, 201 Holdsworth NRC, 160 Holdsworth Way, Amherst, MA 01003, USA;
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D. M. Ward, Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA. E-mail: darren.ward@dartmouth.edu

Summary

  • 1Empirical studies show that average growth of stream-dwelling salmon and trout often declines with increasing density in a characteristic concave relationship. However, the mechanisms that generate negative density–growth relationships in populations in natural streams are not certain.
  • 2In a recent study, Imre, Grant & Cunjak (2005; Journal of Animal Ecology, 74, 508–516) argue that density-dependent growth due to exploitative competition for prey causes the negative density–growth relationships for stream salmonids. They argue that the concave shape of empirical density–growth relationships is consistent with a simple model of exploitative competition and not consistent with interference competition for space.
  • 3We use a simple model to show that competition for space can yield concave density–growth relationships consistent with the empirical pattern when individuals compete for foraging sites that vary spatially in quality and lower-quality sites predominate. Thus, the predictions of the exploitative competition and spatial competition models overlap.
  • 4The shape of the density–growth relationship does not differentiate between candidate mechanisms underlying density-dependent growth for stream salmonids. Our results highlight the general problem with determining the mechanism driving an ecological process from patterns in observational data within the context of linking population demographics to habitat structure and animal behaviour.

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