The Role of Body Size in Complex Food Webs: A Cold Case - Advances in Ecological Research - page 184

ecosystemwithin the marine Antarctic ( Arntz et al., 1994; 1997 ). In contrast to
the previous pioneering trophic studies of the Weddell Sea ecosystem, which
focused on a simple pelagic food chain ( Tranter, 1982 ), the data set analyzed
here reveals an extraordinarily complex food web that includes the benthos (see
Brose et al., 2006a,b; Jacob, 2005 ). This complexity reflects the high species
numbers ( Brey et al., 1994; Gutt et al., 2004 ), the great variety of foraging
strategies (e.g. Brenner et al., 2001; Dahm, 1996; Nyssen et al., 2002 ), the
enormous range in body mass of species and the large proportion of omnivo-
rous species in the system ( Jacob et al., 2003; 2005 ).
Because of the highly resolved nature of the data, we were able to classify the
key functional roles of many species. We did this by focussing on consumers in
this food web and developing a new classification scheme (i.e. sorting consu-
mers into 11 different categories) which takes different consumer traits into
account and includes feeding strategy (predator, grazer, etc.), prey type (herbi-
vore, etc.), motility and habitat. Based on the recent theoretical advances
described above, we assume that these consumer characteristics and their
body size will determine food web properties and that we can therefore analyse
the contribution of different species categories to community robustness.
Food web robustness estimates the impact of species loss on one aspect of
food web stability: that is, its potential to experience secondary extinctions
based on its topology (sensu Dunne et al., 2002 ). To investigate how robust the
Weddell Sea food web is to the loss of species, we carried out a topological
extinction analysis ( Dunne et al., 2002, 2004; Staniczenko et al., 2010 ). In this
approach, computer simulations are used to investigate how susceptible a food
web is to sequential collapse as a result of secondary extinction cascades.
To summarize, we focus on what traits characterise the consumers in a
large pristine food web and how the robustness of this food web depends on
the sequence in which these species are assumed to go extinct. We aimed to
explore (i) how body size is correlated with network structure (i.e. the trophic
level of a species or its generality/vulnerability) over all species in the Weddell
Sea and across a variety of consumer feeding types and (ii) the role of species
characteristics such as body size regarding network robustness to provide
first steps towards the understanding on how body size of a species con-
straints the likelihood of extinctions.

II. METHODS

A. The Weddell Sea Data Set

The Southern Ocean ( Figure 1 ) ecosystem exhibits a number of unique
features, including 25 million years of bio-geographic isolation ( Barnes,

2005; Clarke, 1985 ; Hempel, 1985 ) and, in the form of the annual formation