Tangled webs: reciprocal flows of invertebrate prey link streams and riparian zones

Abstract

1. Streams and their adjacent riparian zones are closely linked by reciprocal flows of invertebrate prey. We review characteristics of these prey subsidies and their strong direct and indirect effects on consumers and recipient food webs. 2. Fluxes of terrestrial invertebrates to streams can provide up to half the annual energy budget for drift-feeding fishes such as salmonids, despite the fact that input occurs principally in summer. Inputs appear highest from closed-canopy riparian zones with deciduous vegetation and vary markedly with invertebrate phenology and weather. Two field experiments that manipulated this prey subsidy showed that it affected both foraging and local abundance of stream fishes. 3. Emergence of adult insects from streams can constitute a substantial export of benthic production to riparian consumers such as birds, bats, lizards, and spiders, and contributes 25-100% of the energy or carbon to such species. Emergence typically peaks in early summer in the temperate zone, but also provides a low-level flux from autumn to spring in ice-free streams. This flux varies with in-stream productivity, and declines exponentially with distance from the stream edge. Some predators aggregate near streams and forage on these prey during periods of peak emergence, whereas others rely on the lower subsidy from autumn through spring when terrestrial prey are scarce. Several field experiments that manipulated this subsidy showed that it affected the short-term behaviour, growth, and abundance of terrestrial consumers. 4. Reciprocal prey subsidies also have important indirect effects on both stream and riparian food webs. Theory predicts that allochthonous prey should increase density of subsidised predators, thereby increasing predation on in situ prey and causing a negative indirect effect via apparent competition. However, short-term experiments have produced either positive or negative indirect effects. These contrasting results may be due to characteristics of the subsidies and individual consumers, but could also result from differences in experimental designs. 5. New study approaches are needed to better determine the direct and indirect effects of reciprocal prey subsidies. Experiments coupled with comparative research will be required to measure their effects on individual consumer fitness and population demographics. Future work should investigate whether reciprocal prey fluxes stabilise linked stream-riparian ecosystems, explore how landscape context affects the magnitude and importance of subsidies, and determine how impacts of human disturbance can propagate between streams and riparian zones via these trophic linkages. 201 reciprocal connections is helping to define a more holistic perspective of catchments, and has the potential to shape new directions for ecology in general.

Diet of invertebrates sampled in leaf-bags incubated in a tropical headwater stream

“…This study corroborates the recent literature (ROSI-MARSHALL & WALLACE 2002, BENSTEAD & PRINGLE 2004, MOORE et al 2004, MOTTA & UIEDA 2004, BAXTER et al 2005, CHESHIRE et al 2005, LANCASTER et al 2005, ALBARIÑO & VILLANUEVA 2006) that emphasizes the great importance of organic matter as food resource for the macroinvertebrates community. However, the high diversity of the macrofauna associated to litter compared to the low diversity of shredders suggests that the coarse particulate organic matter is used more as substrate than as direct food, becoming an important food source to macroinvertebrates after its decomposition until fine particles.…”

“…Shredders and collectors are considered the major primary consumers in forested streams, providing the main link between the organic inputs and the predatory invertebrates (CHESHIRE et al 2005). However, these primary consumers (leaf specialists) are often microendemic and particularly vulnerable to deforestation (CUMMINS & KLUG 1979, BENSTEAD & PRINGLE 2004, BAXTER et al 2005.…”

In a state of flux: The energetic pathways that move contaminants from aquatic to terrestrial environments

“…We highlight the ways in which a new understanding of the aquatic-terrestrial interface has prompted an integrated view of cross-boundary contaminant flows within complex ecological networks. We pay particular attention to aquatic insects, which as an important source of energy for riparian consumers such as arthropods, birds, mammals, and reptiles, are especially likely to move contaminants into terrestrial ecosystems [4][5][6] (Fig. 1).…”

“…The complex relationships among these dimensions have led, in part, to a perspective that recognizes riverine landscapes as both internally heterogeneous and intimately linked with their surroundings via boundary dynamics. Indeed, a growing body of literature provides evidence that reciprocal exchanges of energy, matter, and organisms among streams and their adjacent riparian zones are essential for functional ecosystems (as reviewed in [5]) (Fig. 2).…”

Zebra mussel (Dreissena polymorpha) affects the feeding ecology of early stage striped bass (Morone saxatilis) in the Hudson River estuary

“…Diet analyses also provide insight towards broader subjects such as population dynamics (Braga et al 2012), habitat use (Feitosa and Ferreira 2015), evolution (Collar et al 2009), energy flow between ecosystems (Baxter et al 2005), and conservation (Alcaraz et al 2015;Donadelli et al 2015). Characterizing the diet of early-stage fishes is particularly important because this is when fish grow most rapidly and undergo ontogenetic shifts that are critical to survival.…”