The concept of carrying capacity and shorebirds

Goss-Custard, J. D.; West, A. D.. 1997 The concept of carrying capacity and shorebirds. In: Goss-Custard, J. D.; Rufino, R.; Luis, A., (eds.) Effect of habitat loss and change on waterbirds. London, The Stationery Office, 52-62, 144pp. (ITE Symposium, 30).

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This paper discusses the term 'carrying capacity, defined in terms of the 'one-in, one-out rule'. On this definition, capacity is reached when, for every additional bird that arrives, another one either dies or emigrates, so that numbers in a locality cannot increase and the habitat is fully occupied. Simulations with an individuals-based and physiologically structured model showed that this definition is appropriate for wintering oystercatchers eating mussels, a system in which interference between foraging birds occurs; as the numbers of recruits settling on the mussel beds in September increased to very high levels, the number remaining by spring reached a plateau. Whether birds were assumed to remain on the estuary until they died at some point during the winter or whether they emigrated before they starved made no difference to the predicted capacity of the mussel beds. Carrying capacity in this system could also be defined in terms of the total bird-days per winter because this quantity likewise tended to plateau as the numbers of recruits settling in September increased. However, the plateau was less clearly defined and it took a very much higher number of September recruits for capacity defined this way to be reached. Further simulations showed that, without interference, the numbers of autumn recruits could be increased to the point at which no birds remained on the mussel beds by spring because food density had been depleted to below the levels required for even the most efficient birds to balance their energy budget. The relationship between numbers remaining by March and the numbers settling the previous September was thus dome-shaped. In systems with little or no interference, therefore, carrying capacity may only be measured as the total number of bird-days depends critically on the numbers of birds that arrive in September. Defining carrying capacity in terms of bird-days per winter rather than in terms of the numbers remaining in March does not solve the difficulties of defining and measuring capacity in systems without interference. In such systems, measuring capacity in bird-days per winter — as is so often done — provides a poor prediction because the total number of bird-days depends on the numbers that arrive in autumn. Whether carrying capacity is actually reached depends on there being a sufficiently large number of potential recruits to the estuary in autumn; in the oystercatcher— mussel system, it required 8000 recruits in September for the carrying capacity of ca 320 to be achieved. In many species and localities there may not normally enough recruits fo e capacity of a locality to be reached. However this should not be taken to mean that a reduction in food supplies through habitat loss or change would not affect local numbers If competition for food is sufficiently intense or the rates of emigration and/or mortality to be density-dependentyouo increased , a reduction in the food suppl ld lead t ' rates of emigration and/or mortality, and so to a reduction in local bird numbers. This was confirmed by simulations which showed that winter habitat loss used local mortality or emigration to increase in oystercatchers long before carrying capacity was reached.

Item Type: Publication - Book Section
Programmes: CEH Programmes pre-2009 publications > Other
CEH Sections: _ Pre-2000 sections
NORA Subject Terms: Zoology
Ecology and Environment
Date made live: 19 Sep 2008 14:26

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