Interactive effects of plants, decomposers, herbivores, and predators on nutrient cycling

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 and metabolic 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 food webs, and the characteristics of primary producers (bottom-up forces) and decomposers (top-down forces) that affect nutrient cycling. Next, we discuss herbivore-based food webs and highlight mechanisms by which herbivores can have top-down effects that either enhance or depress rates of nutrient cycling. Then, we extend that discussion to the influence of higher trophic levels on nutrient cycling. Finally, we discuss mechanisms by which trophic interactions mediate the transfer of nutrients among ecosystems. Bottom-up and top-down influences on nutrient cycling in decomposer food webs. Bottom-up forces in decomposer food webs. Decomposers, primarily bacteria and fungi, but also detritivorous invertebrates and vertebrates, consume dead organic matter to obtain energy, and in the process of decomposition, break down macromolecules into smaller ones and excrete nutrients as waste products.