Population Dynamics of Benthic Species on Tidal Flats: the Possible Roles of Shorebird Predation
In late summer millions of shorebirds leave their breeding grounds in the Arctic and move to tidal mudflats in the temperate and tropical zones. These mudflats can either be used as stopover sites during migration, or as overwintering areas. Shorebirds ne
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15.1 Introduction In late summer millions of shorebirds leave their breeding grounds in the Arctic and move to tidal mudflats in the temperate and tropical zones. These mudflats can either be used as stopover sites during migration, or as overwintering areas. Shorebirds necessarily have high feeding rates and the densities of these salient predators can be high. Not surprisingly, many researchers have asked the question what the impact is of shorebirds on their intertidal invertebrate prey, and whether this impact may differ, for example, between temperate and tropical regions (Piersma and Beukema 1993). Generally, two approaches have been adopted to answer these questions. First, estimates of annual consumption by shorebirds were compared with annual invertebrate production estimates. Second, experiments were carried out using exclosure cages. We argue that neither of the two methods is very relevant in answering the questions posed. We suggest a more appropriate, but time-consuming, approach based on long-term observations.
15.2 Production-Consumption Comparisons A widely applied approach in assessing the impact of migrating or overwintering shorebirds on intertidal invertebrates is the estimation of the amount of annual invertebrate production that is on average consumed by shorebirds. The published accounts that we were able to find (Table 15.1) suggest that consumption/production ratios vary around a value of 0.3 (range 0.12-0.52), a value that does not seem to consistently differ between temperate and tropical regions {Fig. 15.1}. Yet, two things should be kept in mind. First, it may be seriously questioned whether the reliability of these published data allows any conclusions at all. Second, even if such conclusions as Ecological Studies, Vol. 151 K. Reise (ed.) Ecological Comparisons of Sedimentary Shores © Springer-Verlag Berlin Heidelberg 200 1
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made above are warranted, one may wonder how relevant the results actually are. Preferably, production is directly estimated by, for example, the mortalitysummation method (Crisp 1984). This method, however, requires knowledge of the age-structure of the population and the changes therein. As this knowledge is rarely available, in most cases published species-specific production/biomass ratios (P /B ratio) were used instead, in combination with species biomass estimates. Only in one study (Baird and Milne 1981) were estimates based on direct methods and then only for one or two species (Table 15.1). Predation causes mortality of the prey and may thus enhance production and turnover rate. Thus, predation itself may increase the P/B ratio, and a certain amount of circularity occurs when consumption/production ratios are used to study the impact of predation assuming constant P/B ratios.
Table 15.1. Annual consumption by shorebirds C (g AFDM m- 2 ), macrozoobenthos biomass B (g AFDM m-2 ) and annual zoobenthic production P (g AFDM m- 2 ) for various intertidal areas Area
Region a
C
B
P
Methodb
Source(s)
Ythan Tees Wadden Sea
W
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