PARENT SESSION
Oral Session # 71: Trophic Structure I: Theory; Aquatic Systems.
Presiding: D Chalcraft
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 202.

Intuitive compartments: The other half of trophic structure in food webs.

Krause, Ann^*,1, Frank, Kenneth^{1, 2}, Mason, Doran³, Ulanowicz, Robert⁴, Taylor, William¹, ¹ Michigan State University, East Lansing, MI² Michigan State University, East Lansing, MI³ NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI⁴ University of Maryland, Solomons, MD

ABSTRACT- Food webs are representations of the feeding interactions among organisms in a community. When examining the structure or organization of these interactions, most food web studies focus on trophic levels, where measures quantify the number of energy transfers, and organisms are grouped based on the similarity of their interactions. In contrast, compartments are underutilized and are defined as groups of organisms where interactions within the group are stronger than interactions between groups. In this study, we applied a method from the social networking science to identify compartments within an established food web of Chesapeake Bay, USA. This method has distinct advantages over previous methods in that organisms are assigned to only one compartment, the number of compartments is not identified a priori, and the results are tested for significance. Using this method on different weightings of interactions of the same Chesapeake Bay food web, we found that the results of some of the weightings were greater than expected by chance ( = 0.05). The connectance, or realized density of interactions relative to the potential density of interactions within compartments was higher than the overall connectance, and the connectance between compartments was lower than either within or overall. Depending on the weighting of the interactions, the analysis identified two to six compartments within the web. Compartments contained organisms across several different trophic levels and represented groups of organisms found in similar habitats, such as the pelagia or benthos. Using a multidimensional scaling technique based on the results, we arranged compartments and their associated organisms in relation to each other. This graphical representation gave us an intuitive understanding of the compartmental structure of the food web. Identifying and understanding compartmentalization in food webs is necessary to ecological theories such as the relationship between stability and diversity in real food webs.

Key words: compartments, food web, trophic structure, trophic levels