In 113 community food webs from natural communities, the average and maximal lengths of food chains are independent of primary productivity, contrary to the hypothesis that longer food chains should arise when more energy is available at their base. Environmental variability alone also does not appear to constrain average or maximal chain length. Environments that are three dimensional or solid, however, such as a forest canopy or the water column of the open ocean, have distinctly longer food chains than environments that are two dimensional or flat, such as a grassland or lake bottom.

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The structure of 40 real food webs, representing aquatic and terrestrial communities fro~ all latitudes, is_ found to be markedly affected by the degree of variability of the physical environment. In particular, food webs in_fluctuating ecosystems are characterized by a significantly lower connecta?~e than webs representative of more constant systems. This is interpreted within the context of stability theory as a means to optimize feeding in the face of increasing disturbance. It is ~h~wn f'!rther that_the nature of the habitat itself imposes additional constraints on food web structure m mtert!dal, pelagic, estuarine, and forest ecosystems.

This report describes and explains regularities in the numbers and kinds of trophic links in community food webs. To a first approximation, the mean number of trophic links in a community food web is proportional to the total number of trophic species. The mean number of trophic links between any two categories of trophic species (basal, intermediate, and top) is proportional to the geometric mean number of species in the categories joined. These linear relationships, and scale-invariance in the proportions of basal, intermediate, and top species, make it possible to predict with remarkable precision the proportions of each kind of trophic link among all community food webs. The differences between food webs in constant and in fluctuating environments reflect apparently greater constraints on the trophic organization of food webs in fluctuating environments. PROBLEM AND HYPOTHESESHow does the total number L of links in a web vary as the number S of species increases? At least three hypotheses are plausible. First, the number of potential links increases as S2 because the maximal number of edges in a directed graph on S nodes is S(S-1). If there were a constant probability that any potential link were a real link, the mean E(L) of L would be proportional to S2. Second, if each species could eat or serve as food for only a finite number of other species, regardless of how many species were present in the community, the mean E(L) would be proportional to S. Third, both of the preceding hypotheses might apply over different ranges of values of S. When the number of species in a community is small, L may be constrained only by the availability of potential links and hence vary as S2. When the number of species in a community is large, L may be limited by the potential for interaction of each species and hence vary as S. The same relation between L and S might also arise because field ecologists might be more thorough in recording links when the total number of species in the community is small, but proportionally more prone to omission when the number of species is large.According to these three hypotheses, plots against S, on the abscissa, of (a) the square root of L, (b) L, or (c) some power of L between 1/2 and 1, on the ordinate, should be approximately linear.

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