Unique structural properties of pelagic food webs.

Two large food web data sets, each characterized by comprehensive sampling and high taxonomic resolution, display major differences in their structural properties. In 60 food webs compiled by Sugihara et al., representing such diverse habitats as springs, creeks, rotting logs, treeholes, and forest canopies, there is no significant increase in web complexity (SC) with species number (S), and predator/prey (PP)ratios average 2.2. The fractions of top (TF), intermediate (IF) andbasal (BF) species average 0.47, 0.35, and 0.18, respectively. Alonga gradient of increasing S, IF increases and BF declines. Maximal food chain length (MAX) also increases with S in the Sugihara webs, andaverages 2.7. In 50 food webs compiled by Havens, representing the pelagic communities of small lakes, SC increases rapidly with S, and PP averages only 0.6. Species fractions average 0.06 (TF), 0.44 (IF), and 0.50 (BF), and only IF is significantly correlated with S. MAX increases more rapidly with S than in the Sugihara webs, and it averages 4.7. Distinct properties of pelagic food webs, including high SC that increases rapidly with S, low PP, and high BF, are evidenced by the results of other studies, and may be due to (1) the predominance offilter-feeding predators that consume numerous prey taxa; and (2) tothe very high diversity of small particulate autotrophic prey species found in pelagic communities. [ABSTRACT FROM AUTHOR]