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The decomposition of plant litter of terrestrial origin is a key process for the functioning of many stream ecosystems with notable relevance for global biogeochemical cycles. The process has received much attention in the literature but, since the iconic paper of Kaushik and Hynes (1971), we lack a comprehensive review of its patterns of variation and drivers. This book provides an updated compendium of the ecology of plant litter decomposition in streams, through 22 chapters grouped in four sections that focus on (I) the analysis of the patterns and drivers of decomposition, the importance of (II) biodiversity and (III) multiple aspects of global change, and (IV) the methodological approaches used to study litter decomposition and its applications. By summarizing decades of fruitful research, we hope this will be a reference textbook for ecologists and students that will stimulate further research and promote collaboration among researchers in this field.

A key or keystone species is defined as a species with disproportionately large effects on the ecosystem relative to its abundance. In freshwater ecology it is often used with a bottom-up perspective, to refer to riparian plant species whose litter resources are of particular importance for invertebrate communities and ecosystem processes. This includes fast-decomposing species that represent an important litter supply in terms of nutrients (e.g., alder) and slow-decomposing species that last for long in the stream and are able to sustain communities in periods were preferred resources have disappeared (e.g., oak). This chapter will focus on the major role that litter of the genus Alnus (i.e., alder) plays in the decomposition process, a crucial component of stream ecosystem functioning. Alder litter often determines overall decomposition rates and how these are affected by factors such as plant diversity as well as rates of nutrient cycling or secondary production. We take advantage of the wide use of alder litter in multiple studies conducted at different spatial scales (from local to global) and with different approaches (from laboratory to field studies) to illustrate how the presence and abundance of a key riparian plant species can drive stream ecosystem functioning.

Understanding ecological patterns and processes at the global scale is becoming increasingly important in view of the rapid pace of environmental change and consequent impacts on ecosystems. This chapter reviews current knowledge about how plant litter decomposition—a key stream ecosystem process—and its major biotic and abiotic drivers vary globally along geographic gradients. The evidence available suggests that thermal regime is the main factor influencing microbial decomposition rate, which tends to increase with temperature. The presence or absence of litter-consuming detritivores is a major determinant of overall decomposition rate, the scarcity of these invertebrates being a common pattern in tropical streams. However, relating detritivore-mediated decomposition rates to environmental factors is complex, because of the interplay between detritivore abundance, body size distribution, diversity and community composition, as well as plant litter traits and diversity, all of which are influenced by climate, geology and biogeography. Meta-analyses and a growing number of coordinated large-scale studies have greatly enhanced our understanding of geographical variation of litter decomposition in streams, and have enabled first projections of how climate warming and a range of other aspects of global environmental change will affect the process.

Terrestrial litter that decomposes in streams is critical to carbon and nutrient fluxes and aquatic food web dynamics. Litter dynamics is influenced by biogeochemical, morphological, environmental and climatic factors, making it challenging to understand how these factors relate to each other and to litter decomposition across different spatial scales. Here, we present a hierarchical framework that accommodates the links among a wide variety of local and regional factors (e.g., litter quality, water chemistry, flow) in relation to climate, geology, biogeography and phylogeny. These factors ultimately influence the agents or processes (e.g., microbes, detritivores, physical fragmentation, retention capacity) that govern litter inputs, storage and decomposition in streams. This framework highlights the dependence of litter dynamics on spatial scale and cautions against extrapolations across scales without quantifying the influence of biophysical variables on the different agents and processes. The framework can be used as a basis for experimental and observational studies of those interactions to develop broader mechanistic understanding of litter dynamics.

Many stream invertebrates consume substantial amounts of plant litter at some time during their life cycle and thus play a key role in plant litter decomposition. These litter-consuming invertebrates include mainly shredders, a functional feeding group defined by their ability to shred litter, but also some scrapers, leaf miners, wood borers, and generalist consumers. This chapter describes methods to estimate invertebrate abundance and biomass in litter samples, and to identify litter-consuming invertebrates through gut content analysis. The latter involves gut dissection and mounting, and subsequent identification and quantification of different food items under a microscope. Additionally, the chapter provides an extensive list of litter-consuming invertebrate families recorded from different biogeographic regions with a particular focus on the tropics.