Geoscience Reference
In-Depth Information
well-known food webs in general in three of its key properties: (i) there are
many more feeding links detected than previously reported for marine food
webs (e.g.
Dunne et al., 2004; Woodward et al., 2010b
); (ii) the basal species
of the food web are (relatively) highly resolved and not lumped as phyto-
plankton or primary producers (but see
Brown et al., 2011; Layer et al., 2010,
2011
) and (iii) detritus is one of the most important food sources as reflected
by its high trophic vulnerability (
Jacob, 2005
; see also
Layer et al., 2011,
Mulder et al., 2011
).
The high number of feeding links can be explained by the generalist feeding
nature of most of the species of the Weddell Sea shelf and their well-docu-
mented capacity of diet shifting in response to availability (e.g.
Brenner et al.,
2001; Jacob et al., 2003
). This confirms early suggestions by
Glasser (1983)
that if resource abundances are highly variable and frequently tend to be
scarce, as in the high Antarctic indicated by the pulsed phytoplankton
bloom, consumers will be more likely be adapted to use many alternative
resources, as reflected by the high trophic vulnerability of detritus which is
also true for Broadstone Stream (
Layer et al., 2011
) where all primary
consumers depend on detritus as the most important food source.
Here, we use body size as a trait and a number of functional classifica-
tions of predatory types to understand the trophic role of the Weddell Sea
consumer species. The 'trophic level' of a species is the vertical position
within a food web, as defined by all links to or from this species (
Gilljam
et al., 2011
), and as such is typically described by a continuous, rather than
an integer, scale. Research on trophic levels focuses on (i) patterns common
to all ecological networks (
Elton, 1927; Pimm et al., 1991; Riede et al.,
2011
; Yodzis, 1998); (ii) patterns that distinguish types of systems (
Riede
et al., 2011
) and (iii) patterns that distinguish an organism's role within
ecological networks (
Elton, 1927; Riede et al., 2011; Williams and
Martinez, 2004
). Usually, predators are between one and three orders of
magnitude larger than their prey (
Cohen et al., 2003; Jonsson et al., 2005;
Woodward and Hildrew, 2002
), and the trophic level is positively corre-
lated with body size, and although there are some notable exceptions (e.g.
host-parasite and some host-parasitoid systems; see
Henri and Van Veen,
2011
, pack hunters and baleen whales), this general biological phenomenon
illustrates the links between the trophic structure of whole communities and
body size (
Brown et al., 2004; Riede et al., 2011
). Across the whole food
web, trophic level and body size are often positively related (
Jennings et al.,
2002; Riede et al., 2011
). Although species with a similar maximum body
size can evolve to feed at different trophic levels, there are fewer small
species feeding at high trophic levels than at low trophic levels (Jennings
et al., 2002), and many of these 'unusual' patterns may be due to artefacts
arising from the common practice of using species-averaged data (
Gilljam
et al., 2011; Woodward et al., 2010b
).
