Ecological Stoichiometry as an Integrative Framework in Stream Fish Ecology

—Ecological stoichiometry refers to the relative availability of elements in ecosystems as both an influence upon and result of ecological interactions. Nutrient ratios have long been analyzed in primary producers, but their application to animals is more recent. Here, we summarize the ecological stoichiometry framework and highlight three key contexts in stream fish ecology: body stoichiometry, dietary stoichiometry, and roles in ecosystem nutrient cycling. Elemental demands for growth depend directly upon the stoichiometry of carbon and nutrients in body tissues. Body stoichiometry varies widely among the dozens of stream fish species for which data are available and exhibits some phylogenetic and size-based patterns. Due to the variety of foods consumed by stream fishes, the stoichiometry of their diets also varies widely. Consuming foods with high carbon:nutrient ratios can produce phosphorus-limited growth in algivores and potentially in insectivores as well. These expectations contrast with the prevailing belief that energy intake is the key nutritional control on growth of most fishes. Ingested nutrients that are not incorporated into body tissues must be defecated or excreted. These waste products can be a critical component of ecosystem nutrient cycles and offer the opportunity for species identity to affect ecosystem functioning. We argue that ecological stoichiometry provides an integrative framework for merging perspectives across individual, population, community, and ecosystem levels. Broader application of this approach to stream fishes will offer particular insight into consumer–resource interactions and ecosystem dynamics. * Corresponding author: [email protected] Introduction Ecological stoichiometry is one of the few ecological concepts that can be applied quantitatively across many levels of biological organization, thereby highlighting mechanistic linkages from individual cells all the way up to the biosphere (Reiners 1986; Sterner and Elser 2002). As a consequence, theoretical and empirical work on ecological stoichiometry continues to yield insights that bridge traditional disciplinary boundaries between environmental chemistry, biochemistry, ecology, and evolutionary biology. Such integration is made possible by using chemical elements as natural currencies 540 mcintyre and flecker whose pools and fluxes can be assessed at any spatial or ecological scale. The key insight of ecological stoichiometry is that the relative availability of elements determines which one most constrains ecological dynamics; all others are present in excess of potential demand and thus should not be limiting. To foster this relativistic perspective, researchers generally use elemental ratios expressed in atomic or molar units rather than by weight. Though the concept of ecological stoichiometry has deep roots (e.g., Lotka 1925; Redfield 1958), its exploration was largely limited to plant and phytoplankton ecology until two decades ago. In the mid-1980s, a seminal essay by Reiners (1986) laid out the framework for using elemental analyses to understand patterns and processes at scales from organisms to the biosphere. In the decade that followed, zooplankton ecologists discovered that low phytoplankton nutrient content could be a constraint on herbivore growth and fitness. Specifically, they noted that differences in nutrient requirements among zooplankton species could produce differential responses to algal nutrient content (Sterner and Hessen 1994), with potential for feedbacks on ecosystem nutrient availability through nutrient recycling (e.g., Elser et al. 1988). Subsequent theoretical, experimental, and observational research has explored these questions in detail for zooplankton communities and for some lake-dwelling fishes and stream invertebrates. However, the potential for ecological stoichiometry to elucidate aspects of the ecology of stream fishes remains largely unrealized. In this chapter, we outline many ways in which ecological stoichiometry can serve as an integrative framework in stream fish ecology. The rationale for incorporating the ecological stoichiometry perspective into research on stream fish ecology is simple—fish are the most nutrient-rich organisms in streams. When they are abundant, fish may constitute the dominant pool of phosphorus and nitrogen in a stream ecosystem, as observed in many lakes (Griffiths 2006). The availability of these nutrients often constrains rates of primary production and organic decomposition, therefore sequestering nutrients at upper trophic levels enables fish to play a regulatory role in ecosystems by enhancing the total pool of nutrients and reducing the rate of nutrient turnover. Even when fish biomass is low, the high concentration of nutrients in their tissues relative to the rest of the stream environment has profound consequences for fish physiology, growth, and predator–prey interactions. To explore these implications, we will begin by outlining two principles at the core of ecological stoichiometry: mass-balance constraints and homeostatic regulation. With these ground rules as background, we will then discuss three key aspects of ecological stoichiometry in stream fish ecology: dietary stoichiometry, body stoichiometry, and roles in ecosystem nutrient cycling. Ground Rules of Ecological