Trophic Ecology: Bottom-Up and Top-Down Interactions across Aquatic and Terrestrial Systems

Context The rates and pathways of nutrient cycling through ecosystems depend on interactions between both bottom-up forces, including the chemical characteristics of biomass that influence its decomposition and consumption by higher trophic levels, and top-down forces, such as the nutritional requirements andmetabolic efficiencies of consumers and decomposers that influence their feeding and excretion. At the base of the food web, nutrient cycling is influenced by whether NPP becomes detritus, entering the so-called “brown” food web, or is consumed by herbivores before death or senescence. Here, we use “nutrient” to refer to essential elements other than carbon (C), such as nitrogen (N), phosphorus (P), and calcium (Ca). The fraction of NPP in ecosystems that is consumed by decomposers (primarily bacteria and fungi) versus herbivores is hugely variable among ecosystems, with a larger fraction being consumed by herbivores in aquatic than in terrestrial ecosystems on average (Cyr and Pace, 1993; Cebrian and Lartigue, 2004). This pattern arises because primary producers in aquatic environments have higher nutritional quality than terrestrial primary producers (Cebrian and Lartigue, 2004) and are largely unicellular, whereas those in terrestrial ecosystems are multicellular and structurally complex – requiring compounds, such as lignin, that are difficult to digest (Lindeman, 1942; Shurin et al., 2006). In this chapter, we discuss bottom-up and top-down influences on nutrient cycling (Fig. 9.1), focusing first on decomposer foodwebs, and the characteristics of primary producers (bottom-up forces) and decomposers (top-down forces)