Rüdiger Wagner

Detritus processing by invertebrate shredders: a neotropical–temperate comparison

Abstract Leaf litter is a major component in the organic matter budgets of streams worldwide. Shredding invertebrates are widely considered to be of central importance for the breakdown of allochthonous organic material in temperate-zone streams. However, various authors report an absence of this group in tropical streams. Various phenomena, including hydraulic disturbance, chemical leaf quality, and biotic control through macroconsumers, may cause variable shredder performance in streams. Our paper discusses the hypothesis that biogeographic distribution and the life-cycle strategies of the shredders are additional factors affecting the contribution of invertebrates to lotic decomposition processes. As a case study, we analyzed the type of organic matter inputs, the community of benthic invertebrate shredders, and the decomposition of temperate zone leaves (alder, Alnusglutinosa) in the Breitenbach (BRB), a temperate stream in Hesse, Germany, and in the Córrego Tenente Amaral (CTA), a neotropical Cerrado stream in Mato Grosso, Brazil. Quantities of natural leaf-litter inputs were comparable in the 2 systems (CTA: 820 g m−2 y−1, BRB: 700 g m−2 y−1), but the diversity of trees varied from 60 species/ha in CTA to 5 in BRB. Several shredding species were found in CTA, but in very low abundances. Larvae of the shredding calamoceratid caddisfly Phylloicus sp. were found only in lateral pools and not in the stream channel at CTA. Earlier decomposition experiments with naturally fallen native leaves did not indicate any importance of shredders at CTA, but green leaves of A. glutinosa were rapidly consumed by mining chironomids (Stenochironomus sp.). Decomposition rates were faster in CTA (–k = 0.035 ± 0.006) than in BRB during wintertime (0.0095 ± 0.0021). We conclude that detritivorous neotropical insects may have 2 different species traits. Either they have mass occurrences during short periods when their food source is available and they must be adapted to a secondary, permanently available resource on which they can survive for the rest of the time (facultative specialists), or they are restricted to the few sites that provide a permanent food source (localized specialists). Apart from these special cases, however, the community in the neotropical stream was largely composed of omnivores.

Modelling water quality, bioindication and population dynamics in lotic ecosystems using neural networks

The assessment of properties and processes of running waters is a major issue in aquatic environmental management. Because system analysis and prediction with deterministic and stochastic models is often limited by the complexity and dynamic nature of these ecosystems, supplementary or alternative methods have to be developed. We tested the suitability of various types of artificial neural networks for system analysis and impact assessment in different fields: (1) temporal dynamics of water quality based on weather, urban storm-water run-off and waste-water effluents; (2) bioindication of chemical and hydromorphological properties using benthic macroinvertebrates; and (3) long-term population dynamics of aquatic insects. Specific pre-processing methods and neural models were developed to assess relations among complex variables with high levels of significance. For example, the diurnal variation of oxygen concentration (modelled from precipitation and oxygen of the preceding day; R 2 0.79), population dynamics of emerging aquatic insects (modelled from discharge, water temperature and abundance of the parental generation; R 2 0.93), and water quality and habitat characteristics as indicated by selected sensitive benthic organisms (e.g. R 2 0.83 for pH and R 2 0.82 for diversity of substrate, using five out of 248 species). Our results demonstrate that neural networks and modelling techniques can conveniently be applied to the above mentioned fields because of their specific features compared with classical methods. Particularly, they can be used to reduce the complexity of data sets by identifying important (functional) inter-relationships and key variables. Thus, complex systems can be reasonably simplified in clear models with low measuring and computing effort. This allows new insights about functional relationships of ecosystems with the potential to improve the assessment of complex impact factors and ecological predictions.

Modelling population dynamics of aquatic insects with artificial neural networks

We modelled the total number of individuals of selected water insects based on a 30-year data set of population dynamics and environmental variables (discharge, temperature, precipitation, abundance of parental generation) in a small stream in central Germany. For data exploration, visualisation of data, outlier detection, hypothesis generation, and to detect basic patterns in the data, we used Kohonens self organizing maps (SOM). They are comparable to statistical cluster analysis by ordinating data into groups. Based on annual abundance patterns of Ephemeroptera, Plecoptera and Trichoptera (EPT), species groups with similar ecological requirements were distinguished. Furthermore, we applied linear neural networks, general regression neural networks, modified multi-layer perceptrons, and radial basis function networks combined with a SOM (RBFSOM) and successfully predicted the annual abundance of selected species from environmental variables. Results were visualised in three-dimensional plots. Relevance detection methods were sensitivity analysis, stepwise method and Genetic Algorithms. Instead of a sliding windows approach we computed the in- and output data of fixed periods for two caddis flies. In order to assess the quality of the models we applied several reliability measures and compared the generalisation error with the long-term mean of the target variable. RBFSOMs were used to denominate and visualise local and general model accuracy. Results were interpreted on the basis of known species traits. We conclude that it is possible to predict the abundance of aquatic insects based on relevant environmental factors using artificial neural networks.

The influence of environmental variables on the abundance of aquatic insects: a comparison of ordination and artificial neural networks

Two methods to predict the abundance of the mayflies Baetis rhodani and Baetis vernus (Insecta, Ephemeroptera) in the Breitenbach (Central Germany), based on a long-term data set of species and environmental variables were compared. Statistic methods and canonical correspondence analysis (CCA) attributed abundance of emerged insects to a specific discharge pattern during their larval development. However, prediction (specimens per year) is limited to magnitudes of thousands of specimens (which is outside 25% of the mean). The application of artificial neural networks (ANN) with various methods of variable pre-selection increased the precision of the prediction. Although more than one appropriate pre-processing method or artificial neural networks was found, R2 for the best abundance prediction was 0.62 for B. rhodaniand 0.71 for B. vernus.

Global diversity of dipteran families (Insecta Diptera) in freshwater (excluding Simulidae, Culicidae, Chironomidae, Tipulidae and Tabanidae)

Today’s knowledge of worldwide species diversity of 19 families of aquatic Diptera in Continental Waters is presented. Nevertheless, we have to face for certain in most groups a restricted knowledge about distribution, ecology and systematic, particularly in the tropical environments. At the same time we realize a dramatically decline or even lack of specialists being able, having the time or the opportunity to extend or even secure the present information. The respective families with approximate numbers of aquatic species are: Blephariceridae (308), Deuterophlebiidae (14), Nyphomyiidae (7), Psychodidae (∼2.000), Scatopsidae (∼5), Tanyderidae (41), Ptychopteridae (69), Dixidae (173), Corethrellidae (97), Chaoboridae (∼50), Thaumaleidae (∼170), Ceratopogonidae (∼6.000), Stratiomyidae (∼43), Empididae (∼660), Lonchopteridae (2), Syrphidae (∼1.080), Sciomyzidae (∼190), Ephydridae (∼1.500), Muscidae (∼870). Numbers of aquatic species will surely increase with increased ecological and taxonomical efforts.

A laboratory study on the life cycle of Sericostoma personatum (Kirby & Spence), and light dark-dependent food consumption

The life cycle of Sericostoma personatum (Spence) was studied at 6 °C, 10 °C and 14 °C and at each temperature at 8 and 14 hrs daylength. Embryogenesis was not temperature dependent in the 12°–18°C range. Only 7 of 38 (app. 18%) had a direct development, the rest remained in diapause with partly developed larvae. Hatching success of single egg masses was over 95%. At 6 °C at both LDs, about 452 days are required for larval development. At 10 °C 370 days (LD 8/16), or 320 days (LD 14/10) and at 14° C 319 days (LD 8/16) and 295 days (LD 14/10) were required. Duration of instars III and IV was longer at 6 °C (both LDs), compared with all other groups. Vth instar larvae of the 14 °C (LD 14/10) group grew fastest. Instar VI larvae of the 10 °C short day group developed faster than all others. Instar VII larvae of both 14 °C groups and of the 10 °C long day group develop faster than the rest. Duration of pupal instar is only temperature dependent, regardless of light regime. The field life cycle of S. personatum may require 2–5 years. Larvae are night active. They feed on Coarse Particular Organic Material (CPOM) on the sediment surface at night. They release faeces (Fine Particular Organic Material, FPOM) into the sediment where they rest by day at a few cm depth. Their burrowing behavior thus contributes to the retention of FPOM in the stream channel. Daily food consumption at constant 10 °C is significantly dependent on night length (r2 = 0.979, p < 0.05). Two factors thus may limit food consumption: in winter, low temperatures, and in summer short nights. The species thus avoids competition by day-active shredders and predation by day-active predators.

Does Agapetus fuscipes cultivate algae in its case

The presence of algae within the cases ofAgapetus fuscipes was investigated. Cases recognised as ‘dirty’ or ‘clean’ with the naked eye had more and less algal growth, respectively. Larvae in the former survived significantly longer when starved in the laboratory. It is suggested that the presence of algae within the cases would be of ecological advantage during periods of flood.

Assessing the phylogeographic history of the montane caddisfly Thremma gallicum using mitochondrial and restriction‐site‐associated DNA (RAD) markers

Repeated Quaternary glaciations have significantly shaped the present distribution and diversity of several European species in aquatic and terrestrial habitats. To study the phylogeography of freshwater invertebrates, patterns of intraspecific variation have been examined primarily using mitochondrial DNA markers that may yield results unrepresentative of the true species history. Here, population genetic parameters were inferred for a montane aquatic caddisfly, Thremma gallicum, by sequencing a 658-bp fragment of the mitochondrial CO1 gene, and 12,514 nuclear RAD loci. T. gallicum has a highly disjunct distribution in southern and central Europe, with known populations in the Cantabrian Mountains, Pyrenees, Massif Central, and Black Forest. Both datasets represented rangewide sampling of T. gallicum. For the CO1 dataset, this included 352 specimens from 26 populations, and for the RAD dataset, 17 specimens from eight populations. We tested 20 competing phylogeographic scenarios using approximate Bayesian computation (ABC) and estimated genetic diversity patterns. Support for phylogeographic scenarios and diversity estimates differed between datasets with the RAD data favouring a southern origin of extant populations and indicating the Cantabrian Mountains and Massif Central populations to represent highly diverse populations as compared with the Pyrenees and Black Forest populations. The CO1 data supported a vicariance scenario (north–south) and yielded inconsistent diversity estimates. Permutation tests suggest that a few hundred polymorphic RAD SNPs are necessary for reliable parameter estimates. Our results highlight the potential of RAD and ABC-based hypothesis testing to complement phylogeographic studies on non-model species.

On a collection of Psychodidae (Diptera) by Dr. L. Botosaneanu from some Caribbean Islands

The following new taxa of Psychodidae (Diptera) are described and figured: Telmatoscopus caribicus sp.n., Trichomyia botosaneanui sp.n., Alepia martinicana sp.n., Pericoma hygropetrica sp.n., Maruina hoguei sp.n. (putative larva and male), Maruina tobagensis sp.n. (putative larva and male), Atrichobrunettia insularis sp.n. and Setomima stylappendiculata sp.n. Arisemus boxi Satchell, 1955 is recorded from Martinique and the description of an interesting larva is provided that cannot be determined at present. All specimens were collected during a collecting trip to the Islands of Martinique and St. Vincent by Dr. L. Botosaneanu (Amsterdam) in February and March 1989.

The influence of the diel activity pattern of the larvae of Sericostoma personatum (Kirby & Spence) (Trichoptera) on organic matter distribution in stream-bed sediments — a laboratory study

Diel patterns in mobility and feeding behaviour of the larvae of the stream-dwelling trichopteran Sericostoma personatum larvae were investigated. Larvae fed at night on coarse particulate organic matter (CPOM) at the sediment surface. In the daytime they rested a few cm below the sediment surface, during which time their defaecation activity effected a release of fine particulate organic matter (FPOM) into the sediment. The amount of faeces (mean particle size = 0.1 ± 0.044 mm, x ± SD, n = 500) introduced into the sediment by the larvae, evaluated in two experiments, was 0.4–0.56 mg day−1. This amount did not differ significantly from the organic input resulting from bacterial activity (0.36–0.64 mg day−1). The presence of S. personatum larvae increased the sediment organic content by 42.9 mg (75.8 %) and 59.8 mg (185.6%) AFDW per 16 cm3 sediment over a 90-day period, as compared with control systems containing no larvae.