Resource Quality, Mutualism, and Energy Partitioning in Food Chains

Abstract

Energy of net primary production (NPP) flows to heterotrophs in the form of living tissue, the grazing food chain, or as nonliving organic matter, the detritus food chain. When resource quality and the nature of control of transfer at the autotroph-heterotroph interface are considered, it is evident that both grazing and detritus flow routes involve pathways that are sufficiently different to justify recognition as distinct food chains. Accordingly, we propose a multichannel food chain model with six major pathways as follows (fig. 1): (1) direct grazing; (2) granivorous; (3) detrital POM (particulate organic matter); (4) detrital DOM (dissolved organic matter extruded); (5) active extraction (of photosynthate); and (6) nectarivorous. The active extraction pathway (5) can be subdivided into mutualistic and parasitic. Over the short term, paths 3 or 4 are donor controlled, and 1 and 5 are recipient controlled, while 2, 5A, and 6 involve both. Over evolutionary time, most, if not all, autotroph-heterotroph transfers appear to be mutualistic since numerous positive feedbacks have been documented that enhance the welfare of primary producers and contribute to homeostasis at the ecosystem level. A picture that seems to be emerging is that homeostasis at the ecosystem level can be mediated through autotroph-heterotroph mutualism that involves a few highly specialized, coevolved species (the "keystone mutualists" of Gilbert [1980]). Accordingly, the food web network may be an important part of the diffuse subsystem cybernetics that distinguishes ecosystems from organisms and man-made servomechanisms (as recently pointed out by Patten and Odum [1981]). The granivorous and nectarivorous food chains are absent in aquatic ecosystems that lack flowering plants, but in shallow water ecosystems (wetlands, estuaries, etc.) a secondary detritus-based carbon and energy recovery anaerobic route involving reduced inorganic compounds and chemolithotrophic bacteria may be important. Conventional methods of measuring NPP often fail to include, or grossly under-estimate, the high resource quality flows such as those involving DOM exudates from roots and algal cells, and active extraction of photosynthate by mutualistic microorganisms. Data from Biever's study of sporocarp production in pine plantations are presented to show that more than 15% of NPP may flow along this mycorrhizal path. Since feedback response is rapid, that is, nutrients flow back to the plant very soon after the outflow of photosynthate, mycorrhizal networks could very well be a major cybernetic control subsystem, especially in terrestrial communities on nutrient-poor substrate.