Food webs are more than the sum of their tritrophic parts

Abstract

Many studies have aimed to understand food webs by investigating components such as trophic links (one consumer taxon eats one resource taxon), tritrophic interactions (one consumer eats an intermediate taxon, which eats a resource), or longer chains of links. We show here that none of these components (links, tritrophic interactions, and longer chains), individually or as an ensemble, accounts fully for the properties of the next higher level of organization. As a cell is more than its molecules, as an organ is more than its cells, and as an organism is more than its organs, in a food web, new structure emerges at every organizational level up to and including the whole web. We demonstrate the emergence of properties at progressively higher levels of structure by using all of the directly observed, appropriately organized, publicly available food web datasets with relatively complete trophic link data and with average body mass and population density data for each taxon. There are only three such webs, those of Tuesday Lake, Michigan, in 1984 and 1986, and Ythan Estuary, Scotland. We make the data freely available online with this report. Differences in web patterns between Tuesday Lake and Ythan Estuary, and similarities of Tuesday Lake in 1984 and 1986 despite 50% turnover of species, suggest that the patterns we describe respond to major differences between ecosystem types.

Fig. 1.

Properties of food webs in Tuesday Lake 1984 (A and D), Tuesday Lake 1986 (B and E), and Ythan Estuary (C and F). (A–C) Log population density N declines approximately linearly as a function of increasing log average body mass M of taxa. Trophic links were omitted for clarity, but food generally flowed from upper left toward lower right, because consumers were generally less abundant and had larger average body mass than the resource taxa they consumed. In Tuesday Lake (A and B), basal, intermediate, and top taxa were distinct with high, intermediate, and low population density. In Ythan Estuary (C), basal and top taxa spread to the middle of the body mass distribution. Numbers, means, and standard deviations of log(M) and log(N) of taxa in each category are in Table S1. Linear regression coefficients are in Table S2. (D–F) Upper angle A_upper was a horseshoe-shaped function of lower angle A_lower of 2-chains. Vertical and horizontal lines represent median lower and upper angles for all 2-chains (solid line), median angle of all links (dashed line), and allometric angle (dash-dotted line). Counts of data points in each quadrant refer to quadrants formed by medians of lower and upper angle [#dots(l.u.)], median angle of all links [#dots(med)], and allometric angle [#dots(allom)].

Fig. 2.

Definitions of angles. (A) Link angle was negative (the typical case) when the link from R to C resulted from a clockwise turn with respect to a horizontal line (dashed line here) starting from R and pointing right parallel to the positive log M axis. (B) When the upper link (I, C) turned counterclockwise from the lower link (R, I), between-angle was positive. (C) When the upper link (I, C) turned clockwise from the lower link (R, I), between-angle was negative. R, resource taxon; I, intermediate taxon; C, consumer taxon.

Fig. 3.

Typical Tuesday Lake (A) and Ythan Estuary (B) chains inferred from average between-angles. Tuesday Lake upper links tended to turn counterclockwise from lower links, the opposite of Ythan Estuary.

Fig. 4.

Upper and lower angle correlated negatively. A lower angle less than the median angle, which was approximately −40° in Tuesday Lake and approximately −52° in Ythan Estuary (for a very rough overall central tendency around −45°), was often followed by an upper angle greater than the median angle (A), and vice versa (B).