Forest canopy cover determines invertebrate diversity and ecosystem process rates in depositional zones of headwater streams

Abstract

Summary Previous studies of the ecological linkages between forest and headwater streams have focussed primarily on patterns and processes in erosional habitats, typically riffles. Depositional zones trap large amounts of sediments and particulate organic matter, suggesting that they may be important for forest–stream linkages. We studied the invertebrate benthos and two key ecological processes, surface sediment reworking and leaf litter breakdown, in the depositional zones of streams bordered by contrasting riparian vegetation. We compared three stream reaches, draining open canopy forest regenerating after recent clearcut harvesting, with reaches on three different streams bordered by older forests with closed canopies. We also assessed whether, and to what extent, forest canopy cover determined abiotic factors at the reach scale (physicochemistry of stream water) and patch scale (sediment properties). Depositional zones in both types of stream harboured a taxonomically and functionally diverse invertebrate community, including efficient sediment reworkers and specialised shredders. Higher diversity was found in open canopy than in closed canopy streams, despite similarities in habitat morphology and sediment properties. Water temperature and sediment reworking rate were higher in open canopy forest than in closed canopy forest. As rates of sediment reworking, adjusted for temperature, did not differ between forest types, temperature was probably a key factor linking the forest canopy to stream depositional zones. The rates of leaf litter breakdown sometimes varied substantially between streams, but no consistent forest effect was detected for this process. Temperature‐adjusted rates of surface sediment reworking and litter breakdown were positively correlated with the density of invertebrates that rework sediments and shredders, respectively. A relationship between these two ecological processes was found across depositional zones in closed canopy forest, but not in open canopy forest. This study on depositional zones provides new evidence of the strong linkage between forest and headwater streams. By moderating stream summer temperature, riparian canopy cover has the potential to affect invertebrate metabolic rates and, indirectly, the intensity of surface sediment reworking. However, other factors, such as the quality and diversity of basal trophic resources, may also account for invertebrate diversity pattern across streams and the positive relationship between litter breakdown and sediment reworking in closed canopy forest.

Fish introductions and light modulate food web fluxes in tropical streams: a whole‐ecosystem experimental approach

“…, De Nadai‐Monoury et al. ). Many of these studies also demonstrate significant effects on functional properties such as consumer growth, primary production and decomposition, yet relatively little is known about how top‐down and bottom‐up drivers influence fluxes of energy and materials in food webs, especially at the natural ecosystem scale.…”

“…There is substantial evidence that both pathways are important in structuring ecosystems (Leroux and Loreau 2015). Many seminal studies have been conducted in freshwater systems (reviewed by Taylor et al 2015), where investigators have documented significant effects of predatory or planktivorous fishes (Carpenter and Kitchell 1988, Power 1990, Flecker and Townsend 1994, nutrient enrichment (Schindler 1977, Hambright et al 2007, Davis et al 2010, and light availability (Hill et al 1995, Ask et al 2009, De FISH AND LIGHT EFFECTS ON FOOD WEBS Nadai-Monoury et al 2014). Many of these studies also demonstrate significant effects on functional properties such as consumer growth, primary production and decomposition, yet relatively little is known about how top-down and bottom-up drivers influence fluxes of energy and materials in food webs, especially at the natural ecosystem scale.…”

Habitat specialization and sensitivity to change in a threatened crayfish occupying upland streams

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Effects of flatworm predators on sediment communities and ecosystem functions: a microcosm approach

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