Dry weight loss and colonization of plant litter by macroinvertebrates: plant species and lake types compared

Rates of dry weight loss and of colonization of litter samples of different plants by macro-invertebrates were estimated using litter bags submerged in different types of lake. Samples of Carex rostrata litter were submerged for 3 months, in summer, in two lakes, one with an acidic and nutrient-poor water and one with near-neutral and nutrient-rich water. More plant litter remained (75%) in the former lake than in the latter (51–62%, varying with the mesh size of the litter bags used). The biomass of invertebrates in the litter samples from the acid lake was about 10 times less than in those from the near-neutral. Oligochaeta and Chironomidae were the dominant taxa found in the litter bags from the acidic lake, whereas Hirudinea and Isopoda predominated in those from the near-neutral lake.Litter samples from four different plants (Carex rostrata, Typha latifolia, Triticum sp. and Phleum pratense) were immersed in litter bags in another near-neutral lake further south for 12 months. When recovered, about 45% of the Triticum litter remained compared with only 7% of the Phleum. The values for Carex and Typha fell between these two extremes. When expressed as per g dry weight of residual plant litter, the greatest biomass of invertebrates was recorded in the Phleum litter samples and the least in the Typha samples. The faunal composition of the invertebrates in the different types of litter was approximately the same, with Isopoda and Hirudinea predominating.     

Macrophytes from lakes in the eastern Pyrenees: community composition and ordination in relation to environmental factors

1. Thirty-six species of macrophytes (fourteen flowering plants, two quillworts, sixteen mosses and liverworts, and two algae) were collected in an extensive survey of 116 high mountain lakes in the eastern Pyrenees. Seventy per cent of the lakes showed macrophyte development. 2. The isoetids (Isoetes lacusiris, Isoetes setacea and Subularia aquatica) were the dominant growth form, although the natopotamid Sparganium angusifolium was the most widespread species. Potamids (Potamogeton spp., Ranunculus spp.), the alga Nitella gr. opaca and some mosses (Warnstorfia exannulata, Sphagnum denticulatum) were often present. 3. A multivariate ordination analysis (RDA-redundancy analysis) revealed that water chemistry, altitude and vegetation cover of the catchment, and nutrient availability are major environmental factors associated with macrophyte distribution along the eastern Pyrenean lakes. Isoetids prevailed in softwater oligotrophic lakes, potamids in relatively hardwater oligotrophic lakes, and Potamogeton natans and Callitriche palustris in small and eutrophic water bodies affected by the presence of cattle.     

The quillworts (Isoetes) of India: distribution, endemism and species radiation

The genus Isoetes L. in India is represented by 14species, of which eight species are recognized as being local endemics [confinedto one particular phyto-geographical division (PGD)] while the remaining sixoccur in more than one PGD and are described as endemic to a wider range. Thelocal endemic species are Isoetes dixitei,Isoetes panchganiensis and Isoetessahyadriensis in Western Ghats region; Isoetespantii, Isoetes bilaspurensis, Isoetesreticulata and Isoetes tuberculata inChotanagpur Malwa Vindhya Plateau and Isoetes debii innortheastern India. The wider endemic species are Isoetespanchananii, Isoetes sampathkumaranii,Isoetes rajasthanensis, Isoetesmahadevensis, Isoetes indica andIsoetes coromandelina. Our studies on the patterns ofendemism suggest that the radiation of quillworts advanced from dry lowlandareas to the rainy uplands and mountains. Isoetescoromandelina is the first Indian quillwort to colonize in lowlandsof the coastal zone (Coromandel), from where it spread to different parts of thesub-continent and gave rise to new species. Thus this species is a key Indianspecies which has played an important role in the radiation of quillwort in thecountry and appeared as the connecting link among the quillwort flora of variousPGDs. The centres of diversity for almost all the presently known Indianquillworts species are recognized.     

The influence of invertebrates on the breakdown ofPotamogeton pectinatus L. in a coastal marina (Zandvlei,South Africa)

The influence of invertebrates upon the decomposition ofPotamogeton pectinatus L. in a coastal Marina system was examined over 112 days using litter bags. Invertebrate inclusion bags (2 mm mesh, 5 mm holes) registered a dry mass loss of 1% d^–1, while exclusion litter bags (80 µm mesh) produced a 0.4% mass loss d^–1 (a 2.5 fold difference). Losses of ash and N from inclusion bags were greater than those from exclusion bags (p < 0.05). There was a three fold difference between the two treatments in the time taken for litter to breakdown to half the initial stock: T_1/2 for inclusion bags = 43 d, exclusion bags = 130 d. In both treatments, minerals showed an expected rapid loss, due to leaching, with a subsequent slow increase relative to the dry material remaining. A total of nine invertebrate taxa was recorded from inclusion bags, with a peak biomass of 64 mg g^–1 dry massPotamogeton bag^–1 reached at 64 days after immersion. Grazing amphipods,Melita zeylanica Stebbing andAustrochiltonia subtenuis (Barnard), numerically dominated the litter bag fauna, whileM. zeylanica and nymphs of the zygopteran predatorIschnura senegalensis (Rambur) formed most of the biomass. Scanning Electron Microscopy indicated heavy grazing of micro-organisms by invertebrates, with major qualitative differences occurring 112 days after immersion. Invertebrates significantly accelerated the rate of litter breakdown through their feeding activities, assisting fragmentation and thus contributing to plant losses and also by increasing the surface area for microbial colonisation and attack.     

Evaluation of the faster initial decomposition of tropical floating leaves ofNymphaea elegans Hook

Contributions of abiotic and biotic processes to the decomposition of floating leaves ofNymphaea elegans were separately evaluated by comparing the rate obtained from anin situ experiment of submerging dry leaf material in a lake, and that from a laboratory experiment of submerging dry leaf material in lake water with a bio-fixing reagent. It took 8 days to decompose 79.4% of the initial dry weight of the floating leaf ofN. elegans in a tropical lake. Of the dry weight loss, 32.9% and 67.1% were atributed to abiotic and biotic decomposition, respectively. The relationship between decomposition rate and the mesh size of the leaf litter bags was examined by the application of a mathematical model. A reasonable value of decomposition loss at an early stage could be obtained using a bag with a mesh opening of 9.9 mm². The decomposition rate of floating leaves is faster than that of other aquatic plants. Rapid decomposition ofN. elegans leaves may be attributed to the fact that the plant has a low carbon to nitrogen ratio.     

Activities of Carboxylation Enzymes in Freshwater Macrophytes

Fifteen species of freshwater macrophytes, mainly from cool,temperate waters, were assayed for ribulose bisphosphate carboxylase-oxygenase(RuBPCase) and phosphoenolpyruvate carboxylase (PEPCase) activities.In extracts from all the species RuBPCase was the most activecarboxylation enzyme, and the RuBPCase/PEPCase ratio was atleast 2·0, even for the submersed species Isoetes lacustrisL. and Littorella unifiora (L.) Aschers. which have been reportedto show Crassulacean Acid Metabolism (CAM) activity. The PEPCaseactivity in I.lacustris was lower than that found in some non-CAM-likespecies. In this respect, I.lacustris and L unifiora differfrom most terrestrial CAM plants. However, these two species,along with Potamogeton praelongus Wulf. and Juncus bulbosusvar.fluitans L., had the lowest RuBPCASE/PEPCase ratios, lowerthan found in terrestrial C₃ species; suggesting that the potentialfor substantial photosynthetic metabolism of C₄ acids existsin some temperate, submersed plants. In the three amphibiousspecies (Potamogeton polygonifolius Pourr., Mentha aquaticaL., and Hippuris vulgaris L.) examined, the aerial leaves exhibitedhigher RuBPCase activities than the submersed leaves. Key words: Ribulose bisphosphate carboxylase-oxygenase, phosphoenolpruvate carboxylase, freshwater macrophytes     

半干旱草地植物枯落物碳、氮和磷元素释放对放牧和封育管理的响应

在半干旱草地,关于放牧和封育管理对草地植物枯落物分解及其与土壤动物互作关系的研究一直是该区域生态系统物质循环与生态恢复过程研究的重要科学问题。以放牧和封育样地中赖草(Leymus secalinus)、牛枝子(Lespedeza potaninii)及其混合枯落物为研究对象。于2017年9月、2018年5月和9月、2019年5月和9月,采用2种不同孔径(30目和250目)网袋量化中小型土壤动物的作用,调查了宁夏半干旱草地枯落物碳、氮和磷元素分解与土壤动物群落分布及其对放牧和封育样地的响应规律。结果显示：(1)封育样地中,枯落物碳元素最终残留率均表现为3种枯落物处理间无显著差异;但250目网孔中枯落物氮、磷元素最终残留率和30目网孔中氮元素最终残留率均表现为牛枝子显著高于赖草和混合物,而后两者间无显著差异;仅30目网孔中磷元素最终残留率表现为牛枝子和混合物显著高于赖草,而前两者间无显著差异。放牧样地中,250目网孔的碳、磷元素最终残留率和30目网孔中的氮、磷元素最终残留率均表现为牛枝子显著高于混合物,而赖草居中;氮元素最终残留率表现为牛枝子显著高于赖草和混合物,后两者间无显著差异。(2)...     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

The effects of species immigrations and changing conditions on isoetid communities

Historical data from the 1930s were compared with new data gathered during the 2000s to evaluate the effects of increased numbers of larger stature submersed species (both elodeids and characeans) on resident isoetid communities. The cover and species richness of submersed species were assessed in 23 seepage lakes in northwestern Wisconsin, USA, using randomly located 1 m × 1 m plots. Water clarity, conductivity and residential land use were determined on a whole-lake basis and the sediment type and water depth were recorded at each plot. The probability of elodeids or characeans occurring in isoetid plots increased with the number of elodeids and characeans gained by a lake since the 1930s, with additions ranging from two to 15 species per lake. However, not all species were equally likely to co-occur with isoetids. Six elodeid species (Najas flexilis, Najas gracillima, Potamogeton gramineus, Potamogeton pusillus, Potamogeton spirillus and Vallisneria americana) along with Chara spp. were the most frequent isoetid associates, while other species that were common in the lakes, such as Elodea canadensis and Potamogeton robbinsii, were less frequent in isoetid plots. The lake-wide proportion of isoetid plots colonized by elodeids or characeans ranged from 5% to 100%, with increasing conductivity and total elodeids (plus Chara spp. and Nitella spp.) the strongest predictors of this colonization. Approximately half (49.6%) of all isoetid plots sampled had elodeids or characeans present (39.2% elodeids, 7.4% elodeids and characeans, 3.0% characeans), and isoetid cover and species richness were lower when these larger stature species were present. The risks this colonization poses for the long-term viability of isoetid species appeared to depend on multiple factors, including whole-lake characteristics, opportunities for refuge, and connections among regional isoetid populations. There was evidence of a time lag between the introduction of elodeid or characean species to a lake and invasion of isoetid plots within the lake, a process that deserves further study.     

Litter processing in diffuse and conduit springs

We used bedrock geology and prior water chemistry data to classify and choose 4 conduit and 4 diffuse flow springs in a karst region of southeast Minnesota. Decomposition processes and chemical/physical conditions were compared between spring classes during two seasons (May and September, 1987).Although large storms and conduit run-in events did not occur during our sampling, baseline water chemistry supported our a priori spring classification. Baseline nitrate and atrazine levels were significantly higher in conduit than in diffuse springs. During a minor run-in event, atrazine levels increased significantly to at least 1.2 µg L^–1 in all conduit systems, but remained unchanged in diffuse systems.Decay rates of the two predominant litter sources in the springs (watercress [Nasturtium officinale R.Br.] in May and box elder [Acer negundo L.] in September) were used to assess differences in biological activity between the spring classes. During May, watercress decayed (1 mm coarse mesh bags) at approximately the same rate in conduit and diffuse systems, k = 0.094 and 0.099 respectively. This result was unexpected since shredder colonization was much greater on litter bags in diffuse systems. In September, boxelder decay rate was significantly higher in diffuse (k = 0.018) than in conduit (k = 0.013) springs; and decay rates seemed to reflect significantly higher shredder colonization on bags in diffuse systems. Microbial activity on watercress and boxelder litter, measured as electron transport system (ETS) activity, was seldom significantly different between spring classes. Also, both watercress and boxelder litter decay rates for fine mesh bags (240µm) were similar between conduit and diffuse springs, suggesting that microbial processing did not differ greatly between spring classes.In conduit springs, low shredder colonization was apparently the result of low benthic shredder densities. Shredders and other macroinvertebrates may be adversely affected by discharge fluctuations from conduit run-in events. We suspect that, during times of low flow, watercress standing stock was also an important influence on shredder abundance.     

Temporal changes and vertical distribution of macrophytes in Lake Kawaguchi

 The distribution of macrophytes in Lake Kawaguchi, Japan, was surveyed in August 1999 using a sampling anchor from a boat. The survey revealed that the present aquatic vegetation was composed of 17 submerged species, and no floating-leaved plants were present. The diversity of submerged plants and their vertical growth limits decreased progressively from the west to the east end of the lake. At the time of the survey, the dominant species was Elodea nuttallii (Planch.) St. John, which had once grown explosively in the mid-1980s in the lake, and except for Vallisneria asiatica Miki, all of the species commonly observed in the lake seem to have diminished their growth greatly. Among them, Potamogeton compressus L., which was the most dominant species in the lake before the invasion of Elodea nuttallii, had decreased most severely. It is suggested that the present state of aquatic vegetation in this lake was much influenced by the persistent growth of Elodea nuttallii as well as by environmental factors. Received: August 7, 2001 / Accepted: February 9, 2002     

Litter breakdown and mineralization in a central African rain forest dominated by ectomycorrhizal trees

Based on litter mass and litterfall data, decomposition rates for leaveswere found to be fast (k = 3.3) and the turnover times short (3.6mo) on the low-nutrient sandy soils of Korup. Leaf litteroffour ectomycorrhizal tree species (Berlinia bracteosa, Didelotiaafricana, Microberlinia bisulcata and Tetraberliniabifoliolata) and of three non-ectomycorrhizal species(Cola verticillata, Oubanguia alata andStrephonema pseudocola) from Korup were left to decomposein 2-mm mesh bags on the forest floor in three plots ofeachof two forest types forest of low (LEM) and high (HEM) abundance ofectomycorrhizal (caesalp) trees. The litter of the ectomycorrhizal speciesdecayed at a significantly slower rate than that of thenon-ectomycorrhizal species, although the former were richer in P and Nconcentrations of the start. Disappearance rates of the litter layer showed asimilar trend. Ectomycorrhizal species immobilized less N, but mineralized moreP, than non-ectomycorrhizal species. Differences between species groupsin K, Mg and Ca mineralization were negligible. Effect of forest type was clearonly for Mg: mineralization of Mg was faster in the HEM than LEM plots, apattern repeated across all species. This difference was attributed to a muchmore prolific fine root mat in the HEM than LEM forest. The relatively fastrelease of P from the litter of the ectomycorrhizal species suggests that thematmust allow an efficient uptake to maintain P in the forest ecosystem.     

Ecosystem development in different types of littoral enclosures

Macrophyte growth was studied in two enclosure types (gauze and polythene) in a homogeneousPotamogeton pectinatus bed in Lake Veluwe (The Netherlands). The gauze was expected to allow for sufficient exchange with the lake to maintain similar seston densities, the polythene was expected to exclude fish activity and most water exchange. Polythene enclosures held higher totalP. pectinatus biomass (ash-free dry weight, AFDW) than the lake, gauze enclosures were intermediate. The enclosures had a higher abundance of other macrophyte species (Chara sp.,Potamogeton pusillus) than the lake. Seston ash content was not but seston AFDW, periphyton ash content and AFDW were lower in polythene than in gauze enclosures. The difference in plant biomass between gauze and polythene may be attributed to a difference in periphyton density and in seston AFDW due to zooplankton grazing (Rotatoria andDaphnia densities were higher in polythene enclosures). Since seston and periphyton AFDW and ash content were similar in lake and gauze enclosures, the intermediate macrophyte biomass in the gauze enclosures may be explained by reduced wave action and mechanical stress. Alternatively, phytoplankton inhibition by allelopathic excretions from the macrophytes may have caused the high macrophyte biomass in the polythene, and an absence of sediment-disturbing fish the intermediate biomass in the gauze enclosures. Creation of sheltered areas may favour macrophyte growth through both mechanisms and we conclude that this can be an important tool in littoral biomanipulation.     

Does decomposition of standard materials differ among grassland patches maintained by livestock?

Grazing can modify vegetation structure and species composition through selective consumption, modifying plant litter quality and hence decomposability. In most grasslands, moderate stocking rates maintain a mosaic of high‐quality patches, preferentially used by herbivores (‘grazing lawns’), and low‐quality tall patches, which are avoided. In grazing lawns decomposition rates can be accelerated because of the higher litter quality of its component species and, besides, through the indirect effect of increased nutrient availability in soil. We aimed at testing this indirect effect using standard materials, comparing their decomposition in grazing lawns, open and closed tall tussock grasslands. We selected 10 patches of each type and sampled floristic composition, soil variables and cattle dung deposition. Standard materials were filter paper and Poa stuckertii litter. We prepared litterbags of 0.3 mm (thin mesh) and 1 mm mesh size (coarse mesh). Samples were incubated for 65 days in two ways: above‐ground (thin and coarse mesh) and below‐ground (only thin mesh), aiming at analysing the conditions for decomposition for surface litter and buried litter or dead roots, respectively. Physical and chemical soil variables did not differ among patch types, despite the differences in species composition. Closed tussock grasslands showed the lowest dung deposition, confirming the less intense use of these patches. Soil nitrogen availability (N‐NO₃^‐ and N‐NH₄⁺) was not significantly different among patch types. Each standard material followed a different decomposition pattern across patch types. For above‐ground incubated samples, Poa litter decomposed significantly faster in lawns, and slower in open tussock grasslands. Filter paper decomposed significantly faster in closed tussock grasslands than in the other two patch types. Decomposition of below‐ground incubated samples did not significantly differ among patch types, in line with results for soil variables. Above‐ground differences in decomposition may be associated with differences in microclimatic conditions resulting from differences in vegetation structure.     

Decomposition and macroinvertebrate colonization of aquatic and terrestrial leaf material in alkaline and acid still water

• 1 Decomposition of the leaves of Nymphaea alba L. and Betula pubescens Ehrh. was studied in two ponds of contrasting pH, buffering capacity and trophic level. Rates of carbon loss, concentrations of nitrogen, protein and phenolics and colonization of the leaf material by macroinvertebrates were studied.
• 2 Decomposition of B. pubescens leaves was slower than that of N. alba in both ponds. Protein and phenolic concentrations were higher in the decomposing N. alba material, whereas no significant difference was found for nitrogen concentrations.
• 3 Decomposition of both species was slower in the acid water. The effect of acid water on decomposition rate was similar for both species. Both nitrogen and protein concentrations of N. alba remained higher under acid conditions, whereas no effect of study site was observed on the protein and nitrogen concentrations in B. pubescens litter. The concentration of phenolic compounds was higher in N. alba litter from the acid pond; no such difference was observed for B. pubescens litter. The high concentrations of phenolic compounds in N. alba litter from the acid pond probably induced chemical immobilization (tanning reaction), yielding high concentrations of nitrogen and protein in the litter.
• 4 Diversity of the detritivorous macroinvertebrate fauna on the litter of both species was extremely low in the acid pond, while a relatively high diversity occurred in the alkaline water. Exclusion of large macrofauna inhibited decomposition in the alkaline water, while there was no comparable influence in the acid water.
• 5 The effects of low pH on the decomposition of both terrestrial and aquatic macrophyte litter appeared to be caused by interactive effects of low pH on the microbial decomposition, on the detritivorous macroinvertebrates and, in the case of N. alba, on the resource quality of the detritus.

     

Nitrogen losses from perennial grass species

Nitrogen losses from plants may occur through a variety of pathways, but so far, most studies have only quantified losses of nutrients by above-ground litter production. We used ¹⁵N pulse labelling to quantify total nitrogen losses from above- and below-ground plant parts. Using this method we were able to include also pathways other than above-ground litter production. To test the hypothesis that species from nutrient-poor habitats lose less nitrogen than species from more fertile soils, six perennial grasses from habitats with a wide range of nutrient availability were investigated: Lolium perenne, Arrhenatherum elatius, Anthoxanthum odoratum, Festuca rubra, F. ovina and Molinia caerulea. The results of an experiment carried out in pots in a green-house at two fertility levels show that statistically significant losses occur through pathways other than above-ground litter production. In the low fertility treatment, most (70%) losses from L. perenne occurred by litter production, but in Ar. elatius, F. rubra, F. ovina and M. caerulea, more than 50% of labelled N losses took place by root turn-over, leaching or exudation from roots. When nutrient supply increased, the ¹⁵N losses in above-ground dead material increased in all species and in Ar. elatius, A. odoratum and F. rubra the ¹⁵N losses via other pathways decreased. Ranked according to decreasing turnover coefficient the sequence of species was: L. perenne, A. odoratum, F. rubra, F. ovina, Ar. elatius, M. caerulea. These results suggest that species adapted to sites with low availability of nutrients lose less nitrogen (including above- and below-ground losses) than species adapted to more fertile soils.     

Inter-relationships between Najas marina L. and three other species of aquatic macrophytes

The ability of Najas marina L. to thrive in the presence of the submerged hydrophytes (Myriophyllum spicatum L. and Potamogeton lucens L.) and of an emerged hygrophyte (Scirpus litoralis Schard.) was investigated in Tel Aviv, Israel. Najas plants were unaffected by the presence of Potamogeton lucens and Scirpus litoralis, but were significantly suppressed by Myriophyllum. Bilateral negative relationships exist between Najas and Myriophyllum and these seem to be of an allelophatic nature, depending more on the nature of the accompanying species rather than on their mass.     

A Comparative Study of Leaf Breakdown of Three Native Tree Species in a Slowly‐Flowing Headwater Stream in the Colombian Andes

The dynamics of leaf breakdown in a headwater Colombian stream were evaluated for the native tree species Myrsine guianensis, Cupania latifolia and Nectandra lineatifolia using coarse and fine mesh litter bags. Ten bags of each species (five of each mesh size) were retrieved from the stream at 1, 8, 15, 30, 60 and 120 days. k values ranged from 0.0008 to 0.0058 day^–1 and density of macroinvertebrates from 35 to 55 individuals per leaf bag, peaking at day 8. Myrsine guianensis degraded more rapidly than the other species for both coarse and fine mesh bags. This species and Nectandra lineatifolia presented differences in k values between coarse and fine mesh bags, suggesting that macroinvertebrates influenced the decay rate. Despite the low densities of macroinvertebrates found, shredders represented 12.7% of individuals and 50 to 68% of the invertebrate biomass in bags, indicating that this functional feeding group was an important component of fauna associated with litter breakdown in this first order tropical stream. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of different litter bag designs on the breakdown of leaf material in a small mountain stream

Leaf breakdown of two riparian tree species, Cunonia capensis L. and Ilex mitis (L.) Radlk. was investigated in vitro at Window Stream, Table Mountain, using three different designs of litter bag. Breakdown of Cunonia and Ilex in coarse-mesh (5 mm) litter bags was very rapid (respectively 14.79 and 13.93% loss d^–1), and was significantly greater than the loss of leaf material of 1% d^–1 for both species from fine-mesh bags (180 µm). Differences recorded between fine-mesh and composite-mesh bags (180 µm mesh with 5 mm mesh top) represented macro-invertebrate ingestion, and at t = 28 d, amounted to 67.57% material loss in Cunonia and 62.58% in Ilex. The losses due to microbial activity and leaching, 31.28% in Cunonia and 29.17% in Ilex were not significantly different. Weight loss of Cunonia in coarse-mesh bags (14.79% loss d^–1) and in composite-mesh bags (13.93% loss d^–1) did not differ, but this was not the case for Ilex, where a significantly higher rate of loss in coarse-mesh bags (13.93% loss d^–1) than in composite-mesh bags (7.69% loss d^–1) was observed. This difference was used to quantify fragmentation losses. It was concluded that future leaf breakdown experiments in mountain streams must take cognisance of differential fragmentation losses before inferences can be made as to both invertebrate feeding preferences and biological decomposition of leaves.     

Riparian land use and the relationship between the benthos and litter decomposition in tropical montane streams

1. Although stream–catchment interactions have been analysed in some detail in temperate environments, little is known about the effects of land‐use changes in the tropics. Here, we analyse differences in benthic communities (macroinvertebrates and fungi) under two contrasting land uses (mature secondary forest and pasture) in montane streams in north‐western Ecuador and their influence on the rates of litter processing. 2. Between 2005 and 2006, we used a combination of coarse and fine mesh bags to study the relative contribution of macroinvertebrates and fungi to processing of two types of litter, Alnus acuminata and Inga spectabilis, in three‐first‐order streams running through mature secondary montane forests and adjacent downstream reaches running through pastures. At the same time, we characterised the assemblages of shreddering macroinvertebrates and fungi communities and the litter processing rates in stream reaches under both vegetation types. 3. Litter processing rates attributable to invertebrate feeding (coarse mesh bags) were significantly slower in streams running through pastures. Nevertheless, shredder diversity and richness were similar between pasture and forest sections, while shredder abundance was significantly higher in forest streams (mainly Phylloicus sp. :Trichoptera). Fungal reproductive activity and litter processing rates were low (fine mesh bags) and did not differ significantly between pasture and forest stream reaches. 4. Phylloicus sp. abundance was the best predictor of the percentage of litter remaining in coarse mesh bags across pasture and forest sites. Neither shredder diversity nor their species richness was a significant predictor of mass loss, as most of the decomposition was performed by a single keystone species. Although litter decomposition by microbial decomposers was low, fungal biomass (but not diversity) was the best variable explaining the percentage of litter remaining in fine mesh bags. 5. Our data suggest that, in these Neotropical montane streams, land use can have a significant impact on the rates of critical ecosystem processes, such as litter decomposition. In this study, this effect was not mediated by a major shift in the structure of the benthos, but by a decrease in the abundance and relative representation of a single species whose life history makes it critical to litter processing. 6. This study highlights the significant role that macroinvertebrate fauna can have in the processing of litter in Neotropical streams and the predominant role that single species can have in terms of controlling stream ecosystem‐level processes. Understanding the extent to which these patterns affect the long‐term and large‐scale functioning of stream ecosystems still needs further research and will become increasingly important in terms of managing lotic ecosystems in the context of rapid land‐use change.