Geoscience Reference
In-Depth Information
In general, body size is positively correlated to trophic position
and generality of consumers (
Cohen et al., 2003; Peters, 1983; Riede et al.,
2010, 2011
). However, this relationship is poorly developed in theWeddell Sea
system. Very large animals can feed on very small prey (whales
!
myctophid
fish
phytoplankton); small omnivorous species (e.g. amphipods,
nemertines and gastropods) feed up and down the food chain, seemingly
irrespective of their size (Nyssen et al., 2002;
Jacob, 2005
), and large benthic
and pelagic suspension and filter feeders feed on small POM. These findings
support a recent study which showed that predators on intermediate
trophic levels do not necessarily feed on smaller or prey similar in size but
depending on their foraging strategy have a wider prey size range available
(
Riede et al., 2011
).
It needs to be borne in mind that size-based analyses based on species
average body size can obscure the size-structure of ecological commu-
nities.
Gilljam et al. (2011)
found that prey mass as a function of predator
mass was consistently underestimated when species mass averages were
used instead of the individual size data. For the Weddell Sea data,
ontogenetic stages would certainly shift some of the observed patterns in
the sense that in true carnivores the relationship between trophic level and
body mass would become even more apparent, whereas in benthic omniv-
orous predators and scavengers, the pattern observed should not change
substantially as the prey size range available for a scavenger remains
the same.
Species of intermediate size, such as the ophiuroid Ophiosparte gigas, have
the highest generality, whereas the most important food source with the
highest vulnerability was phytodetritus. There was no strong correlation of
generality and body size, intermediate-sized based species have the highest
generality whereas larger predators have more specialized diet. This reflects
the high degree of complexity of the Weddell Sea shelf ecosystem, which
results from the opportunistic feeding behaviour observed (
Brenner et al.,
2001
; Jacob et al., 2003) and from different predatory types of most of the
benthic invertebrate species.
When dealing with natural communities at large spatial scales, logistical
constraints prevent measuring traits across all species in a perfectly consis-
tent and comparable manner and, consequently, several important caveats
concerning the reliability and interpretation of the resulting data come
readily to mind. The usefulness of functional feeding categories has not
been clearly demonstrated before in marine systems, although it is well
established in freshwaters where it has been used for several decades in the
context of trophic ecology (e.g.
Cummins and Klug, 1979; Ledger et al.,
2011
). Difficulties with the correct assignment to functional feeding groups
have contributed to the inability to consider these metrics previously in
marine studies. However, gaining the distribution of predatory types will
!
krill
!
