The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores. 2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values. 5. Even at low density (approximately 2 m^–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.     

Combined effects of freshwater salinization and leaf traits on litter decomposition

Hydrobiologia - Freshwater salinization is a matter of major global concern due to its consequences on the aquatic biota and ecosystems functioning. Salt contamination negatively affects the leaf...     

Influence of conidial traits and leaf structure on attachment success of aquatic hyphomycetes on leaf litter

Attachment of conidia on leaves is a critical first step in the life cycle of aquatic hyphomycetes in streams. In a first series of microcosm experiments, attachment success of three common aquatic hyphomycete species differing in conidial shape (compact, filiform and tetraradiate) was determined on two leaf species, black alder and downy oak. Fungal species identity and leaf surface structure significantly affected conidial attachment after 24 h. The lower sides of oak leaves with extensive tufts trapped 2.4-8.8 more conidia than the upper sides of oak leaves and both sides of alder leaves. In a second experiment with seven fungal species, attachment success of two species with tetraradiate conidia was much greater than that of two other tetraradiate and three compact conidia, which all had similar attachment success. The species with the largest spores was also the most successful, but this pattern was not consistent across the size range of tested conidia. These results highlight the importance of leaf surface structure, possibly conidial shape and size and additional properties of aquatic hyphomycete conidia in determining attachment success on leaves and they point to the potential role of these factors in structuring fungal communities on decomposing leaves in streams.     

Effects of invasive European bird cherry (Prunus padus) on leaf litter processing by aquatic invertebrate shredder communities in urban Alaskan streams

European bird cherry (Prunus padus) (EBC) is an invasive ornamental tree that is spreading rapidly in riparian forests of urban Alaska. To determine how the spread of EBC affects leaf litter processing by aquatic invertebrate shredders, we conducted complementary leaf pack experiments in two streams located in Anchorage, Alaska. The first experiment contrasted invasive EBC with three native tree species—thin-leaf alder (Alnus tenuifolia), paper birch (Betula neoalaskana), and black cottonwood (Populus trichocarpa)—in one reach of Chester Creek; finding that EBC leaf litter broke down significantly faster than birch and cottonwood, but at a similar rate to alder. The second experiment contrasted EBC with alder in four reaches of Campbell and Chester creeks; finding that while EBC leaf litter broke down significantly faster than alder in Chester Creek, EBC broke down at a similar rate to alder in Campbell Creek. Although EBC sometimes supported fewer shredders by both count and mass, shredder communities did not differ significantly between EBC and native plants. Collectively, these data suggest that invasive EBC is not currently exhibiting strong negative impacts on leaf litter processing in these streams, but could if it continues to spread and further displaces native species over time.     

Relationships among leaf functional traits,litter traits,and mass loss during early phases of leaf litter decomposition in 12 woody plant species

Slow bee paralysis virus (SBPV)—previously considered an obligate honeybee disease—is now known to be prevalent in bumblebee species. SBPV is highly virulent in honeybees in association with Varroa mites, but has been considered relatively benign otherwise. However, condition-dependent pathogens can appear asymptomatic under good, resource abundant conditions, and negative impacts on host fitness may only become apparent when under stressful or resource-limited conditions. We tested whether SBPV expresses condition-dependent virulence in its bumblebee host, Bombus terrestris, by orally inoculating bees with SBPV and recording longevity under satiated and starvation conditions. SBPV infection resulted in significant virulence under starvation conditions, with infected bees 1.6 times more likely to die at any given time point (a median of 2.3 h earlier than uninfected bees), whereas there was no effect under satiated conditions. This demonstrates clear condition-dependent virulence for SBPV in B. terrestris. Infections that appear asymptomatic in non-stressful laboratory assays may nevertheless have significant impacts under natural conditions in the wild. For multi-host pathogens such as SBPV, the use of sentinel host species in laboratory assays may further lead to the underestimation of pathogen impacts on other species in nature. In this case the impact of ‘honeybee viruses’ on wild pollinators may be underestimated, with detrimental effects on conservation and food security. Our results highlight the importance of multiple assays and multiple host species when testing for virulence, in order for laboratory studies to accurately inform conservation policy and mitigate disease impacts in wild pollinators.     

Effects of agricultural development on processing of tussock leaf litter in high country New Zealand streams

1. We compare the rates and mechanisms of processing of tussock (Chionochloa spp.) leaf litter in six New Zealand streams draining grassland catchments that contrast in the extent to which they have been developed for pasture. 2. Rates of processing, measured as rate of weight loss of leaf packs and rate of leaf softening, were at the slow end of the spectrum for vascular plant processing. Processing was faster at developed sites, mediated mainly through the influence of oxidized nitrogen concentration on microbial activity. 3. Few invertebrate shredders colonized leaf packs and it is unlikely that invertebrates had an appreciable effect on leaf processing in our study streams, which do not effectively retain leaf litter. Very small headwater tributaries appear to retain leaf litter and possess a more abundant shredder community. 4. Measures of leaf processing in our six streams were significantly correlated with Petersen's (1992) RCE score of stream condition. We discuss the potential for using rate of leaf litter processing as a method of bioassessment. 5. Even the most degraded stream in our study is classed as ‘good’ using the RCE inventory system. Human impact in the Taieri River is relatively small compared with the degradation observed in some parts of the world.     

Microbial-mediated feedbacks of leaf litter on invasive plant growth and interspecific competition

Background and Aims

In line with the Stress Gradient Hypothesis, studies of facilitation have tended to focus on plant–plant interactions (biotic nurses), while the relative role of abiotic nurses has been little studied. We assessed the role of biotic and abiotic nurses, and their interaction, on soil enhancement and the consequential performance of a native annual grass, Dactyloctenium radulans.

Methods

We used a growth chamber study with two levels of water application to compare the performance of D. radulans growing in soil from foraging pits of the Short-beaked echidna (Tachyglossus aculeatus; abiotic nurse) and non-pit soil from either under tree canopies (biotic nurse) or surrounding open areas.

Results

All measures of plant performance were more pronounced under the high than the low water treatment. The greatest differences between pit and surface Microsites occurred under the low water application, reinforcing our view that facilitatory effects are greater in resource-limited environments. Despite tree canopy soil having greater N, there was no significant effect on plant performance, nor any significant interaction with Microsite.

Conclusions

Our study provides strong evidence that foraging pits enhance soil properties and this soil, in turn, facilitates plant growth; and supports previous work documenting the positive effect of nurse-protégé interactions under greater levels of abiotic stress.     

Species-specific effects of gastropods on leaf litter processing in pond mesocosms

A study of the diet of native brown trout (Salmo trutta) parr and introduced European minnow (Phoxinus phoxinus) in the subalpine lake, Øvre Heimdalsvatn, showed that the two species had considerable dietary overlap, both in the littoral zone and in the outlet of the lake. Chironomidae constituted a substantial proportion of the diet of the two species in both habitats. The results indicated that both zooplankton (Cladocera) and large macroinvertebrates (EPT-species) made up a higher proportion of the minnow diet in the early phase of the minnow establishment (1975–1977) than later, and that the significance of small macroinvertebrates (Chironomidae) as prey has increased during the same period. Dietary analysis of the sympatric brown trout and minnow population in Øvre Heimdalsvatn does not provide a definitive conclusion about the degree of competition between the two species. However, together with the findings in other studies in Øvre Heimdalsvatn that have documented reduced recruitment and individual growth of brown trout, decreased individual growth of minnows and a marked decline in the density of large crustaceans (Lepidurus arcticus and Gammarus lacustris) in the shallow littoral of the lake during the last decades indicates that competitive interactions between the two species are likely. This is probably an example of the competition between two fish species with a high degree of dietary overlap when living in sympatry, most likely caused by absence of alternative prey and alternative habitats due to the high predation risk for both brown trout parr and minnows in deeper parts and in the open waters of the lake.     

Potential impact of invasive amphipods on leaf litter recycling in aquatic ecosystems

The impact of biological invasions on local biodiversity is well established, but their impact on ecosystem functioning has only been sketchily documented. However, biological invasions may impede services provided by aquatic ecosystems, such as, for example, the decomposition of organic matter, a key process in most small streams. To address this question, we experimentally quantified the leaf litter breakdown activity of native and invasive amphipod species, which are keystone species in aquatic ecosystems. The breakdown rate of each species was used to estimate the potential leaf litter recycling in the Rhône and Meurthe Rivers in sites occupied solely by native species and sites dominated by invasive species. We found that invaders were not able to compensate for the activity of native species and that the replacement of native species led to a decrease of at least 66% in the rate of leaf litter recycling. Our approach provides empirical evidence of the functional impact of non-indigenous species on leaf litter recycling, using standard protocols and literature data.     

Effects of flooding on leaf litter decomposition in microcosms

Summary The effects of hydroperiod on decomposition rates of senescent Acer rubrum leaves were tested in microcosms in a controlled laboratory environment. Microcosm treatments included continuously flooded, continuously unflooded, and fluctuating hydroperiods. All flooding treatments promoted decomposition but variations in hydroperiod had no significant effects. A leaching experiment indicated the higher decay rates under flooded conditions were primarily due to high leaching losses from soaking. Unlike nutrient dynamics in the field, where net accumulation occurs, nitrogen and phosphorus in the litter in the microcosms exhibited net losses. The major external inputs which provide a source of nitrogen and phosphorus for immobilization in the field were lacking in the microcosms. Calcium, magnesium, and potassium exhibited net losses except for calcium in the unflooded microcosms. The microcosm results demonstrated the importance of external inputs to litter nutrient relations.     

Effects of gamma irradiation on instream leaf litter decomposition

Hydrobiologia - Leaf litter decomposition is a key process in stream ecosystems, the rates of which can vary with changes in litter quality or its colonization by microorganisms. Decomposition in...     

Effects of temperature and litter type on fungal growth and decomposition of leaf litter

The dependence of fungal decomposition of leaf litter on incubation temperature and litter types used as substrata was assessed under pure culture conditions. Isolates of Xylaria sp., a major ligninolytic fungus in cool temperate forests in Japan, were used as the fungal material. Xylaria sp. is mesophilic; maximum growth and decomposition occurred at 25°C. In the temperature test, the decomposition pattern of beech leaf litter by three isolates of Xylaria sp. changed at a threshold at 25°C. Cellulolytic activity increased with temperature from 5 to 25°C, whereas above 25°C ligninolytic activity increased at the expense of cellulolytic activity, leading to suppressed overall decomposition as a result of the higher temperature. The mass loss of leaf litter caused at 20°C by an isolate of Xylaria sp. was variable among 15 litter types and was correlated negatively with acid-unhydrolyzable residue (AUR) content and positively with total carbohydrate content for the 15 litter types. The effects of temperature and litter type on the growth and decomposition of leaf litter by Xylaria sp. may have implications for changes in fungal decomposition of leaf litter that would be predicted in response to future environmental changes.     

Effects of terrestrial isopods on the decomposition of woodland leaf litter

Summary The indirect contribution terrestrial isopods make to decomposition processes by stimulating microbial activites has been quantified in laboratory experiments. The extent to which microbial metabolism is enhanced as a result of the passage of Betula pendula leaf litter through the alimentary system of isopods was measured for both freshly fallen and decayed leaves. Faeces derived from 1 g freshly fallen litter lost 75 mg g^-1 D.W. more than did intact leaves, as a result of enhanced microbial metabolism. Faeces derived from 1 g of previously decayed leaves, which were shown to be the preferred food of isopods, lost only 17.5 mg g^-1 D.W. more than intact decaying leaves. The isopod's direct contribution to soil metabolism was calculated to be 151 mg and 138 mg g^-1 litter ingested when fed on freshly fallen and decayed leaves respectively. It is concluded that the physical and chemical changes in the leaf substrate which result from fragmentation and digestion by isopods do not necessarily accelerate the subsequent decomposition of the litter very significantly. Fungal propagule density was 3.2x and 3.6x higher in faeces derived from freshly fallen and decayed leaves respectively than in the intact litter. Numbers of viable bacteria were correspondingly 126x and 34x higher in faeces than in the freshly fallen and the decayed leaves. Levels of microbial inhibitors were lower in the faeces than in the leaves but levels of free amino acids stayed higher for longer in the faeces than they did in intact litter. In the field the physical removal of litter by the soil macrofauna from surface to deeper and moister microsites may be the most important indirect contribution that they make to decomposition processes.     

Effects of leaf litter on woody seedlings in xeric successional communities

We investigated the effect of leaf litter on the establishment of Eucalyptus incrassata, a mallee eucalypt. It has been suggested that litter accumulation may hinder seedling establishment, and that the removal of litter may be one of the mechanisms through which fire enhances recruitment. We conducted factorial experiments testing the effects of three kinds of leaf litter on E. incrassata seeds and seedlings at three contiguous sites with different land use histories. One site was an uncleared E. incrassata open mallee woodland (Mallee site), one a cleared area that had been ungrazed for about five years (Pasture site) and the third an area of mallee rolled some 40 years ago and permitted to regenerate (Regrowth site). Litter had no effect on emergence of planted E. incrassata seeds, but emergence differed between sites. Overall, the percentage of seeds that germinated and emerged was substantial (mean 35.2% ± 25.9%). Seedling shoot biomass did not differ between sites or litter treatments. Although seedlings grown in Pasture litter suffered higher mortality rates, overall mortality rates were low (mean 13.2% ± 15.5%), suggesting that leaf litter has little effect on recruitment rates during winter and spring. We conclude that leaf litter does not affect emergence or growth in young E. incrassata seedlings during winter and spring, when most establishment occurs. Our results emphasize the difficulty in predicting litter effects on recruitment.     

Trophic cascading effects of predatory fish on leaf litter processing in a Japanese stream

A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish-present and fish-absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatment (0.56 g m^–2 in dry mass on average) than that in the fish-absent treatment (1.32 g m^–2), there being no significant treatment effect evident for the second-dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish-present treatment (0.31 g week^–1 in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week^–1). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.     

UV-B辐射对杉木凋落叶分解的影响

采用分解袋法对自然和UV-B辐射滤减环境下的杉木凋落叶进行分解试验.结果表明:与对照相比,UV-B辐射滤减处理使杉木凋落叶的分解速率降低了69.6%(P<0.001),凋落叶中氮、磷和木质素的相对含量分别增加150%、83.3%和13.8%,抑制了钾和碳的释放.木质素光降解在杉木凋落叶分解过程中的作用不明显.UV-B辐射可以加快杉木凋落叶的分解,促进氮、磷、钾和碳的释放,以及杉木林凋落物层的营养周转速度,增加地表的碳通量,对杉木林的碳源或碳汇功能具有潜在的影响.     

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition.
2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem.
3. Although overall mass loss rates were similar between treatments ( k = 0.0213 day⁻¹ and 0.0206 day⁻¹), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour.
4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves.
5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching.
6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.     

Effects of chronic chlorine exposure on litter processing in outdoor experimental streams

SUMMARY 1. The effects of chlorine on litter ( Potamogeton crispus L.) processing were examined using six outdoor experimental streams. Downstream portions of two streams were dosed at c . 10 μg l⁻¹ Total Residual Chlorine (TRC), one stream at 64 μgl¹, and one stream at 230μg l⁻¹. Two control streams were not dosed; upstream riffles of each stream served as instream controls.
2. Two 35 day litter breakdown (per cent AFDW remaining) experiments indicated significantly lower decay rates in the high dose riffle. No other concentration of chlorine significantly affected decay rate.
3. A third experiment, conducted in medium and high dose streams, indicated that high dose chlorine exposure reduced litter decomposition rates significantly, and reduced microbial colonization, microbial electron transport system activity, and microbial litter decomposition after 4 days but not after 11 days of exposure. The number of amphipod shredders colonizing litter bags was also reduced significantly with high chlorine dose.
4. A fourth experiment, after dosing was terminated, provided direct evidence that amphipod shredders were important in facilitating litter decomposition: litter bags stocked with amphipods had significantly higher decomposition rates than bags which excluded shredders.
5. Overall results indicate that the high dose (c. 230 μgl⁻¹ TRC) of chlorine reduced litter processing rates partly by reducing initial microbial conditioning, but primarily by reducing the colonization of amphipod shredders.