Factors constraining the structure of food webs can be investigated by comparing classes of ecosystems. We find that pelagic ecosystems, those based on one-celled primary producers, have longer food chains than terrestrial ecosystems. Yet pelagic ecosystems have lower primary productivity, contrary to the hypothesis that greater energy flows permit higher trophic levels. We hypothesize that longer food chain length in pelagic ecosystems, compared with terrestrial ecosystems, is associated with smaller pelagic animal body size permitting more rapid trophic energy transfer. Assuming negative allometric dependence of biomass production rate on body mass at each trophic level, the lowest three pelagic animal trophic levels are estimated to add biomass more rapidly than their terrestrial counterparts by factors of 12, 4.8, and 2.6. Pelagic animals consequently transport primary production to a fifth trophic level 50–190 times more rapidly than animals in terrestrial webs. This difference overcomes the approximately fivefold slower pelagic basal productivity, energetically explaining longer pelagic food chains. In addition, ectotherms, dominant at lower pelagic animal trophic levels, have high metabolic efficiency, also favoring higher rates of trophic energy transfer in pelagic ecosystems. These two animal trophic flow mechanisms imply longer pelagic food chains, reestablishing an important role for energetics in food web structure.
SupplementaryTables: Data on trophic levels and body lengths of trophic species in 9 pelagic and 9 terrestrial food webs
Adult body lengths of all trophic species in these 18 webs were taken from reported values in the literature and on the Internet. The trophic level of each trophic species was determined from food web graphs, designating primary producers as trophic level 1, herbivores as trophic level 2, and so on. Six of 9 pelagic and 2 of 9 terrestrial webs explicitly indicated principal energy flow pathways in the food web graphs. Where food web diagrams indicated the relative strength of multiple trophic pathways (such as a predator consuming a range of prey species from various trophic levels, e.g. by the width of arrows), the strongest pathways were used to determine food chain length and trophic levels. Otherwise, directed trophic links were used as given. The 18 webs are selected from the 113 food webs analyzed by Briand and Cohen (1987, Science 238:956-960). These data were used to estimate geometric mean body length by trophic level for animals in pelagic and terrestrial food webs. Table S1 gives summary information and a description for all 18 webs. Table S2 gives the lengths and trophic levels for all trophic species in the 9 pelagic webs. Table S3 gives the lengths and trophic levels for all trophic species in the 9 terrestrial webs. The 9 pelagic webs chosen were the 9 of 113 Briand and Cohen webs with longest maximum food chain length that also had only one-celled primary producers. The 9 terrestrial webs were individually chosen as those having similar numbers of trophic species as the 9 pelagic webs (to minimize differences between pelagic and terrestrial food webs arising from more detail having been included in the published food web graphs).
SupplementaryTables.docx
