Consumption of leaf detritus by a stream shredder: Influence of tree species and nutrient status

Four species of riparian vegetation (alder, birch, willow and poplar) were fertilized with nitrogen, phosphorus, nitrogen + phosphorus, or no fertilizer (control). The resulting leaf detritus (leached but not microbially colonized) was offered to a stream shredder, Hydatophylax variabilis (Trichoptera: Limnephilidae). In one experiment, shredder consumption of leaf detritus from different nutrient treatments (within tree species) was compared, and in a second experiment, consumption of different tree species (within nutrient treatments) was compared. Larvae preferred leaf detritus from nitrogen + phosphorus treatments (except in poplar where nitrogen treatment was preferred). Alder was preferred over other tree species for all treatments. Chemical and physical analyses of leaf litter showed differences between tree species and nutrient treatments in nutrient content, tannins and leaf toughness. Leaf consumption by larvae was positively associated with nitrogen content and negatively associated with condensed tannin content. Species composition and nutrient status of riparian vegetation may strongly influence detrital food webs in streams.     

THE TROPHIC ECOLOGY OF FRESHWATER GAMMARUS SPP. (CRUSTACEA:AMPHIPODA): PROBLEMS AND PERSPECTIVES CONCERNING THE FUNCTIONAL FEEDING GROUP CONCEPT

Gammarus spp. are widespread throughout a diverse range of freshwater habitats and can be the dominant part of many benthic macroinvertebrate assemblages, in terms of both numbers and/or biomass. Although the vast majority of studies have emphasized the herbivorous nature of Gammarus spp. and their ‘shredder’ functional feeding group (FFG) classification, we show that a far wider food base is exploited than has been previously acknowledged. This ‘plasticity’ as herbivore/predator is linked to the success of Gammarus spp. in persisting in and colonizing/invading disturbance-prone ecosystems. Intraguild predation and cannibalism are more common than previously realized. This behaviour appears to be a causal mechanism in many amphipod species replacements. Additionally, Gammarus spp. are major predators of other members of the macroinvertebrate community. Furthermore, while many studies have emphasized fish predation on Gammarus spp., we illustrate how this fish: amphipod, predator: prey interaction may be a two-way process, with Gammarus spp. themselves preying upon juvenile and wounded/trapped fish. We urge that a new realism be adopted towards the trophic ecology of Gammarus spp. and their role as predators and prey and that previously established FFG assumptions of both the food and the feeder be questioned critically.     

The effects of willow and eucalypt leaves on feeding preference and growth of some Australian aquatic macroinvertebrates

The effect of leaf species (willow, Salix fragilis L., and white gum, Eucalyptus viminalis Labill.) and leaf state (senescent or green) on the feeding selectivity and growth rates of three species of macroinvertebrate Notalina sp. Mosely (Trichoptera: Leptoceridae), Koorrnonga sp. Campbell and Suter (Ephemeroptera: Leptophlebiidae) and Physastra gibbosa (Gould) (Mollusca: Planorbidae) were tested in the laboratory. All three species of macroinvertebrate selected green willow most strongly over the other leaf types (senescent willow, green eucalypt and senescent eucalypt). Growth rates of P. gibbosa and Notalina sp. were significantly greater on green willow than on the other leaf types. We were unable to measure the growth of Koorrnonga sp. Invertebrates had access to softer internal tissues of leaf material during preference trials, therefore we do not think that leaf structure was the main influence on selection between these materials. Green willow material may have been a better food source because of the noticeably thicker biofilm that it supported, and this material may also retain higher levels of nutrients than abscissed leaves. We speculate that willow leaves may provide a preferred source of food but will be available for less time than native eucalypt detritus.     

The loss of aquatic and riparian plant communities: Implications for their consumers in a riverine food web

Abstract Human induced alterations to rivers and steams have resulted in significant changes to the structure and diversity of riparian and aquatic plant communities. These changes will impact on the dynamics of riverine carbon cycles and food web structure and function. Here we investigate the principal sources of organic carbon supporting local shredder communities across a gradient in different levels of anthropogenic development along riverine reaches, in South Australia. In forested/wooded reaches with minimum to limited development, semi‐emergent macrophytes were the principal sources of organic carbon supporting the local shredder communities. However, in developed reaches, course particulate organic matter and filamentous algae were the principal food sources. The C:N ratios of the food sources in developed reaches were higher than those of their consumers indicating a stoichiometric mismatch. This imbalanced consumer‐resource nutrient ratio in those developed reaches is likely to impose constraints on the growth and reproduction of their aquatic shredder communities with probable knock‐on effects to higher trophic levels.     

Feeding patterns of four macroinvertebrate taxa in the headwaters of the Buffalo River,eastern Cape

Gut content analysis, field and laboratory observations, and food choice experiments were used to assign four abundant macroinvertebrate taxa in the headwaters of the Buffalo River, eastern Cape, to functional feeding groups. The mayfly Adenophlebia auriculata (Leptophlebiidae) was classified as a collector: brusher; while the caddisflies, Dyschimus ensifer (Pisuliidae) and Goerodes caffrariae (Lepidostomatidae), and stoneflies Afronemoura spp. (Notonemouridae) were classified as shredders. The effects of organism size, season and biotope on dietary composition were tested, with size accounting for most of the dietary variability within each taxon. Larger individuals consumed more material, larger items, and, in the case of A. auriculata, a wider variety of food-types. There was little variation in the feeding of the taxa in different seasons or biotopes. Shredders ingested mainly leaf fragments, and this, rather than the size of particles in the gut, is a more useful basis for the shredder designation. A. auriculata was the most opportunistic feeder, and items in its diet additional to fine detritus varied seasonally and in the various biotopes. Of the shredders, Afronemoura spp. and D. ensifer were more varied in their diet, augmenting the staple intake of leaf material with other items. G. caffrariae was the most specialised feeder, being exclusively a shredder, regardless of biotope or season. Despite criticisms of the applicability of the FFG concept in the literature, we conclude that these taxa can reasonably be accommodated in functional feeding classes, and that the results are useful in describing the functions performed by the organisms in the river. The relationship between feeding function and river process is emphasised: we suggest that collectors contribute primarily to organic particle retention, while shredders facilitate organic particle size reduction and mobilisation, and the enhancement of substrates for microbial colonisation. An emphasis on river function is a useful context within which to view the FFG concept.     

The Influence of Pack Size and Position,Leaf Type,and Shredder Access on the Processing Rate of Atherosperma moschatum Leaves in an Australian Cool Temperate Rainforest Stream

Factors affecting the processing rate of packs of southern sassafras (Atherosperma moschatum) leaves were investigated in an Australian cool temperate rainforest stream pool. Processing rate was strongly influenced by pack weight fitting a linear inverse relationship. Processing was not significantly (p>0.05) affected by whether the packs were placed on bricks or free on the stream bed. Free packs, but not packs on bricks, were processed more rapidly near the bank than in midstream (p = 0.050). Packs placed in 300 μm mesh bags, with one side unsealed were processed at the same rate as packs attached to bricks or free on the stream bed, but packs in sealed 300 μm mesh bags were processed significantly more slowly (p<10⁻⁴). Abscissed leaves were processed significantly more slowly than fresh leaves (p<10⁻⁵) and at a rate not significantly different to leaves sealed in mesh bags (p = 0.197). There was no consistent pattern of difference between processing rates in summer and winter. The results indicate that the size of litter packs may be a more important factor in influencing processing rate estimates than the method of attachment of the packs. The slow rate of processing of abscissed leaves compared with fresh leaves indicates that they were at most, slightly influenced by stream shredders. The absence of a consistent pattern of difference between summer and winter processing rates is consistent with the results of previous litter processing studies from southeastern Australia although no clear causal factor can be identified.     

Palatability of Leaves Conditioned in Streams Affected by Mine Drainage: A Feeding Experiment with Gammarus Pulex (L.)

Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.     

Effects of diversity loss on ecosystem function across trophic levels and ecosystems: A test in a detritus‐based tropical food web

Abstract We investigated the effects of biodiversity loss across trophic levels and across ecosystems (terrestrial to aquatic) on ecosystem function, in a detritus‐based tropical food web. Diversities of consumers (stream shredders) and resources (leaf litter) were experimentally manipulated by varying the number of species from 3 to 1, using different species combinations, and the effects on leaf breakdown rates were examined. In single‐species shredder treatments, leaf diversity loss affected breakdown rates, but the effect depended on the identity of the leaves remaining in the system: they increased when the most preferred leaf species remained, but decreased when this species was lost (leaf preferences were the same for all shredders). In multi‐species shredder assemblages, breakdown rates were lower than expected from single‐species treatments, suggesting an important role of interspecific competition. This pattern was also evident when oneleaf species was available but not with higher leaf diversity, suggesting that lowered leaf diversity promotes competitive interactions among shredders. The influence of diversity and identity of species across trophic levels and ecosystems on stream functioning points to complex interactions that may well be reflected in other types of ecosystem.     

Ecosystem Linkages between Southern Appalachian Headwater Streams and Their Banks: Leaf Litter Breakdown and Invertebrate Assemblages

We examined red maple (Acer rubrum L.) leaf litter breakdown in streams and riparian zones at two sites in the southern Appalachian Mountains to understand how differences in abiotic and biotic factors influence leaf breakdown rates. Litterbags were placed in three riparian habitats differing in litter layer moisture: stream > bank > upland. Invertebrates colonizing litterbags at one site were also examined to determine how variations in community and functional structure affect breakdown rates. Leaves broke down fastest in streams and slowest in upland habitats, whereas bank habitats were intermediate and characterized by high variability. Faster leaf breakdown rates in streams appeared to be a function of greater moisture availability, a more stable thermal regime, and a higher biomass of leaf-shredding invertebrates, especially the stonefly Tallaperla. In addition, patterns of leaf breakdown and invertebrate community structure provided evidence for a stronger than expected ecological connection between the stream and the bank. Overall, detritus processing within this narrow riparian ecosystem varied considerably depending on the availability of moisture. Results from this study show that stream channel–floodplain interactions in riparian ecosystems of steep forested mountains are analogous to ones in larger downstream or low-gradient systems. Riparian zones throughout a river network display a remarkable heterogeneity in their ability to process organic matter, which is ultimately driven by changes in hydrological conditions. Received 6 March 2001; accepted 3 July 2001.     

Diet and growth of a leaf-shredding caddisfly in southern Appalachian streams of contrasting disturbance history

Diet and growth of leaf-shredding caddisfly larvae, Pycnopsyche spp.,were examined in streams draining a reference catchment and a 16-year-oldclear-cut (disturbed) catchment at Coweeta Hydrologic Laboratory insouthwestern North Carolina, USA. The objective was to explain why shredderproduction is higher in the disturbed streams despite the larvae having lessfood (i.e., leaves) available. We predicted larvae would grow faster onfast-decaying leaf material representative of the disturbed streams. Larvaeconsumed mostly leaf detritus in three streams draining each catchment overthree seasons (fall, winter, and spring), which showed larvae did notconsume higher quality foods (e.g., algae and animal material) in disturbedstreams. When fed 2-month-old conditioned black birch (Betula lenta L.) (afast-decaying leaf species) and white oak (Quercus alba L.) (a slow-decayingleaf species) leaves in the laboratory, larvae grew significantly faster onthe birch leaves. However, when larvae were fed the same leaf types after3-months conditioning, larvae grew significantly faster on oak leaves. Afield growth experiment conducted for 42 d using mixed-species leaf dietsrepresentative of each catchment and initially conditioned for 2 monthsfound that Pycnopsyche grew significantly better on the diet representativeof the reference catchment. The reference diet contained more oak leaveswhich apparently became a more acceptable food as the experiment proceeded.High shredder production in the disturbed streams could not be explained byhigh Pycnopsyche growth rates on fast-decaying leaves. Instead, larvae grewbetter on leaves that were apparently conditioned optimally regardless ofconditioning rate.