Spoonbills as indicator of the Wadden Sea condition
a study with stable δ13C & δ15N isotope analysis PhD-student: Jeltje Jouta 1,2 Promotor: Theunis Piersma 1,2 MSc-student: Suzanne Bakker1 1 Royal Netherlands Institute for Sea Research. Dept. Marine Ecology. PO Box 59, 1790 AB Den Burg, Texel, The Netherlands 2 University of Groningen. Dept. Animal Ecology. PO Box 11103, 9700 CC Groningen, The Netherlands INTRODUCTION Ecologically the Wadden Sea impoverished for the last decades. It is thought that a West- East gradient exists along the Dutch Wadden Sea, where the Western part is in an ecologically poorer state then the Eastern part.
Spoonbills are an ideal species to study the ecological states along this West-East gradient. Unlike most Wadden Sea species, the Spoonbill populations still seemed to grow the last decades but *. Spoonbills are opportunistic and tactile foragers. They breed on all Dutch Wadden Sea islands, where they feed their nestlings with prey from nearby areas. The diet of Spoonbills nestlings on different Wadden Sea islands, will therefore reflect differences in prey species abundance and occurrence along the West-East gradient. Diet analysis of Spoonbills is done by stable δ15N and δ13C isotope analysis and regurgitate analysis. Terschelling Vlieland * Tamar Lok (NIOZ) recently found that the yearly Schiermonnikoog survival of Spoonbills decreases. Wieringen
RESULTS Spoonbills show heterogeneousness in trophic level (δ15N) along the West-East SHRIMP IN SPOONBILL DIET gradient of the Dutch Wadden Sea. These differences are caused by two factors:
GRAPH 3, shows the total length of 1. The prey of Spoonbills also show a heterogeneous pattern in trophic position. Brown Shrimp found in regurgitates of Spoonbill nestlings on Wadden Sea islands 19 p =0,893 GRAPH 1: in 2011. Spoonbills on Terschelling and *** *** Spoonbills show a Schiermonnikoog eat significantly bigger 18 *** p =0,055 heterogeneous Brown Shrimp, compared to those on 17 pattern along the Vlieland. 16 West-East gradient. *** The three main prey 60 *** n.s. ) 15 species of Spoonbills N
15 also vary along this 50
δ 14 gradient. ( 13 Although the variance 40 in trophic position of N= 7 N= 135 12
level level the three main prey 30 11 species may partly N= 358 explain the Spoonbill 20 10 heterogeneousness in total length pf shrimp (mm)
Trophic trophic position, it 9 Vlieland Terschelling Schiermonnikoog cannot be the only 8 Spoonbill causing factor. Spoonbill diet 7 (= minus one trophic level) PREY: Flatfish 6 PREY: Brown shrimp PREY: Stickleback WEST EAST 5
Wieringen Vlieland Terschelling Schiermonnikoog
2. The diet ratio of the Spoonbills fluctuates along the West-East gradient.
Proportions by group: 1
GRAPH 2a/b:
1 1
1.0 Terschelling Schiermonnikoog With help of “Stable Isotope
Analysis in R” (SIAR) a best fit of
the diet ratios of the three main
0.8
0,8
0,8
prey species of Spoonbill nestlings
is depicted for two islands.
Spoonbill nestlings on Schiermon-
0.6 0,6
0,6 nikoog have a much higher
proportion of Shrimp in their diet,
proportion
proportion
Proportion
0.4 compared to the Spoonbill
nestlings on Terschelling. Diet Diet It should be noted that this model is 0.2 runned by using Spoonbills from 2011 and prey from 2012. Currently, Spoonbill
isotopes of 2012 are measured.
0 0 0,2 0,4
0 0 0,2 0,4 0.0
Brown 1Shrimp Flatfish2 Stickleback3 Brown Shrimp Flatfish Stickleback Source MAIN CONCLUSION Spoonbills on Dutch Wadden Sea islands differ in trophic position along the West-East gradient, because the trophic position of their prey differ and also because Spoonbills have a different diet composition along the West-East gradient of the Dutch Waden Sea. CONTINUING RESEARCH To get a precise view of (the reason of) the differences in the diet of Spoonbills; we analyse δ15N & δ13C of Spoonbills on all islands for multiple years, we analyse the Spoonbill diet by regurgitate analysis and we complement the ‘prey’ stable isotope database.
NIOZ is an institute of the Netherlands Organisation for Scientific Research (NWO)" [email protected]