GLM applied on presence-absence polychaeta data

Abstract

Benthic monitoring programs can be used to investigate the relationship between shore birds and their intertidal food supply, particularly in areas where rapid habitat fragmentation and deterioration take place due to human disturbance. Samborombon Bay (Buenos Aires, province, Argentina) is an area of importance for migratory shorebirds due to the vast extension of saltmarshes inhabited by infaunal species that play an essential role in the food web of the coastal ecosystem (Ieno 2000; Ieno and Bastida 1998; Martin 2002). Main shorebird preying species in the area during the annual stop over migrations are the Red Knot (Calidris canutus rufa), the White-rumped Sandpiper (Calidris fuscicollis), the Hudsonian Godwit (Limosa haemastica), the American Golden Plover (Pluvialis dominica), among others (Blanco 1998; Botto et al. 1998; Ieno 2000; Myers and Myers 1979). Through March and April, when shorebirds migrate north along the shores of Argentina, they prey mainly on the polychate species Laeoneries acuta, Neanthes succinea, and to a less extend, the abundant capitelid Heteromastus similis. This species tends to aggregate in deeper muddy and medium sand substrate with moderate organic material levels. Preliminary observations showed that remnants of this polychaete species could also be found in the bird?s droppings. Knowing the deep preferences of H. similis, an interesting question is if this species can be reached within the bill?s length of birds and whether this depth pattern is always the same among areas within the bay that exhibit different man-made disturbance. To answer this question, 73 core samples were taken in 3 locations in Samborombon Bay. The presence and absence of H. similis was recorded in the upper (<10 cm) and lower (>10 cm) part of the substrate. We model the presence and absence of H. similis as a function of medium sand content (MedSand), the 3 locations (Location) and the depth preference (Level). Interactions between all 3 covariates may be expected.