Oral Session #95: Trophic Structure.
Presiding: D. Wise
Thursday, August 8. 1:00 PM to 3:45 PM. Palo Verde Room, Radisson.

On the structural organization of food webs.

MELIAN, CARLOS*,1, BASCOMPTE, JORDI1, 1 Estación Biológica de Doñana, CSIC, Sevilla, Spain.

ABSTRACT- Current studies in food webs show how the degree distribution, that is, the probability P(k) to find a species with k-interactions decays either as a power-law or exponential distribution. However, little is known about how this pattern of interactions relates to the subweb structure and its implication on invariant structural organization. In this presentation, we integrate connectance, food web structure, and compartmentalization by studying how links are distributed within and among subwebs. We define a subweb as a subset of species which are connected to at least k-species from the same subset. We call this subweb a k-subweb. We study the subweb k-frequency distribution in four highly resolved food webs. First, we find that the subweb k-frequency distribution is highly skewed. That is, the most dense group of interacting species is unique and contains the bulk of interactions (from 10% in UK-Grassland to 30% in Ythan Estuary), despite including a low number of species (from 9% in UK-Grassland and Silwood Park to 22% in Ythan Estuary). Yet a large of small subwebs have just a few interactions. Thus, food webs have more structure than randomly generated webs. Second, while connectance is a scale-variant property, the most dense subweb and its connectance to the rest of the web remain scale-invariant from a minimum threshold to the most resolved webs. This keystone subweb articulates the ecosystem structure. That is, cohesion is mantained around the most dense group of interacting species. This shared structured pattern among food webs may provide critical information to detect the spreading of disturbances, species invasion, and pathogens within food webs.

KEY WORDS: Compartmentalization, Subweb k-frequency distribution, Densest subweb, Cohesion