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temperature (
Ives and Cardinale, 2004
). While the relationship between
functional traits and the susceptibility of extinction risk is complex, recent
research does suggest some clear and consistent patterns, namely, that smal-
ler species are apparently less vulnerable to extinction than larger species
(
Cardillo, 2003; McKinney, 1997
). In the light of this, it is perhaps heartening
that in our topological simulations it was the sequences of deletions based on
the removal of small, and not large, organisms that caused a cascade of
secondary extinctions. It appears that within the Weddell Sea network, larger
bodied species can be lost without causing a direct collapse of the network
topology, which is the opposite case in other marine systems, where the loss
of large apex predators has lead to multiple trophic cascades (i.e.
Myers et al.,
2007; Raffaelli, 2007
).
We should of course interpret these findings with a great deal of care.
A major caveat, which needs to be borne in mind with such topological
analysis, is that population dynamics are ignored and therefore no top-
down extinctions, or other indirect effects, can occur (
Montoya et al.,
2009
). Further, the strength of the bottom-up extinctions will be reduced in
the absence of population dynamics, as species need to lose all their prey
items before going extinct and not just part of their diet, this need not be the
case in real ecosystems (
Ebenman and Jonsson, 2005
). There are a range of
studies showing the strong effect of top-down control (e.g.
Berger et al., 2001;
Borrvall and Ebenman, 2006; Estes and Palmisano, 1974; Reisewitz et al.,
2006; Terborgh et al., 2006
) and the importance of considering interaction
strength when analyzing the response of perturbations in ecological commu-
nities (e.g.
McCann et al., 1998; Montoya et al., 2009; Novak et al., 2011
) and
hence the analysis of topological robustness should be considered only as a
best-case scenario.
Despite these considerations, there are several studies that have used
mass-balanced models of population dynamics to examine the role
of large whales and the consequences of their loss within the Antarctic
food web. These studies indicate weak top-down control in these systems
(
Bredesen, 2003; Trites et al., 2004
) and suggest that the removal of large
whales might have little measurable effect on lower trophic levels or on the
population dynamics of other species in the food web. It is also worth
noting that, in these models, once the populations of large whales were
reduced to small numbers, they take a long time to recover (
Bredesen, 2003;
Trites et al., 2004
).
Secondary extinction cascades can occur from a body mass-based
sequence of primary extinctions suggesting the importance of trophic
size-structure for this system. It is also intriguing that this is despite the
lack of any clear relationships between body mass and vulnerability, gener-
ality, or trophic level when examined across all species in the network. That is
to say, the robustness analysis still makes clear the importance of the smaller
