Ideas for Moving Beyond Structure to Dynamics of Ecological Networks - Biological Networks - page 311

(A)
Three species
food−web motifs
(B)
Fig. 14.2. Food-web motifs and the scale of food-web stability. (A): The set of 13 unique food-web
motifs composed of three species [Milo et al. (2002); Stouer et al. (2007)]. Each vertex represents
a species and arrows represent predator-prey interactions pointing from prey to predator. (B): A
signicant unanswered question in the study of food webs is how the dynamic properties and
stability of food-web motifs relate to those of the entire food web.
14.2.2. The scale of food-web stability
The complexity of empirical food webs has lead many ecologists to concentrate on
dynamic ecosystem studies in terms of small sub-webs | \community modules"
[Holt (1997); Holt and Hochberg (2001)] | which bridge the gap between \the
baroque complexity of entire communities and the bare bones of single and pair-
wise population dynamics" [Holt (1997)]. Community modules are comprised of
three to ve species and a set of interactions likely to have ecological relevance,
such as a three-species food chain or intraguild predation.
The dynamic stability of community modules has been investigated previously;
particular focus has been placed upon the implications of varying species interaction
strengths and the role of weak interactions [McCann et al. (1998); Fussmann and
Heber (2002); Emmerson and Yearsley (2004); Bascompte et al. (2005); Nakazawa
and Yamamura (2006); Rooney et al. (2006)]. More recent research has also uncov-
ered the important role of predator-prey body size ratios in stabilization of specic
community modules such as a tri-trophic food chain [Otto et al. (2007)]. These
bottom-up studies provide a theoretical foundation for the stability of individual
modules, but leave open the question of which modules empirical food webs are
actually composed of. To address this question, scientists have investigated \food-
web motifs", the structural counter-part to community modules [Milo et al. (2002);
Bascompte and Melian (2005); Stouer et al. (2007)]. Food-web motifs consist of
the complete set of unique connected subgraphs containing n-species [Milo et al.
(2002)] (Fig. 14.2). These top-down studies provide an indication of the subgraphs