Effects of predation by sea ducks on clam abundance in soft-bottom intertidal habitats

Abstract

Recent studies have documented strong, top-down predation effects of sea ducks on mussel populations in rocky intertidal communities. However, the impact of these gregarious predators in soft-bottom communities has been largely unexplored. We evaluated effects of predation by wintering surf scoters Melanitta perspicillata and white-winged scoters M. fusca on clam populations in soft-bottom intertidal habitats of the Strait of Georgia, British Columbia. Specifically, we documented spatial and temporal variation in clam density (clams m -2 ), scoter diet composition, and the consequences of scoter predation on clam abundance. Of the 3 most numerous clams, Manila clams Venerupis philippinarum and varnish clams Nuttallia obscurata were the primary prey items of both scoter species, while clams of the genus Macoma were rarely consumed by scoters. Between scoter arrival in the fall and departure in the spring, Manila clams decreased in density at most sample sites, while varnish clam densities did not change or declined slightly. Our estimates of numbers of clams consumed by scoters accounted for most of the observed declines in combined abundance of Manila and varnish clams, despite the presence of numerous other vertebrate and invertebrate species known to consume clams. For Macoma spp., we detected an over-winter increase in density, presumably due to growth of clams too small to be retained by our sieve (< 5 mm) during fall sampling, in addition to the lack of predation pressure by scoters. These results illustrate the strong predation potential of scoters in soft-bottom intertidal habitats, as well as their potentially important role in shaping community structure.

Morphology and density of mussels on natural and aquaculture structure habitats: implications for sea duck predators

“…If variation in prey quality affects habitat use patterns of predators, this may lead to variable prey depletion by habitat. There is a growing appreciation that sea ducks can have significant, top-down depletion effects on bivalves over winter (Guillemette et al 1996, Larsen & Guillemette 2000, Lewis et al 2007), which in turn might lead to temporal changes in availability of prey in preferred foraging habitats.…”

Shifts in foraging behavior of wintering Hooded Cranes (Grus monacha) in three different habitats at Shengjin Lake, China

“…During the three over-wintering periods, a total of 381 samples from these quadrats were collected, of which 43, 62 and 30 in meadows, 25, 29 and 55 in paddy fields and 47, 60 and 30 in the mudflats from the early, middle to late wintering stages, respectively (Gillespie and Kronlund 1999). The locations of the fall quadrats were not resampled in the spring (Lewis et al 2007). …”

Gray whales may increase feeding opportunities for avian benthivores

“…These impacts can alter prey availability for several months (Oliver & Slattery 1985), and thus we suggest that longer-term effects on many bottom-feeding animals may be important, even if direct feeding associations with gray whales are rare. Moreover, feeding by gray whales during their northward migration coincides with increasing energy needs of marine birds as they prepare for migration and reproduction, at the same time that typical winter foods may have declined (Lewis et al 2007). If numbers of Pacific gray whales were indeed far greater in the past as suggested by recent genetic analyses (Alter et al 2007), interactions such as the one reported here may have historically been major factors in the foraging strategies and trophic relations of a range of bottom-feeding vertebrates.…”