Contrasting diel activity and feeding patterns of four instars of Rhyacophila dorsalis (Trichoptera)

1. Ontogenetic shifts in prey choice and predator behaviour can affect food‐web structure. Therefore, it is important to establish if the diet and feeding activity differ between life‐stages of the same species. This hypothesis was tested for second, third, fourth and fifth larval instars of Rhyacophila dorsalis by comparing their diel activity and feeding patterns. Second to fifth instars collected from two streams were used either for gut analyses or for observations of their activity and feeding patterns in three stream tanks. Food was provided in excess; being organisms living in bryophytes on top of a large stone in each tank, augmented by different‐sized larvae of Ephemeroptera, Simuliidae and Chironomidae. As few first instars for gut analyses were found in the field, the diet of first instars reared in the laboratory was also studied. 2. Larvae for gut analyses were taken 1 h before dusk or dawn (n = 50 larvae per instar for each day or night sample). First and second instars fed on the smaller food items with no significant day‐night differences in diet. Gut contents indicated a progressive trend from feeding chiefly at night in third instars to almost exclusively at night in fifth instars. Fourth and fifth instars fed on the larger food items, whilst the diet of the third instar larvae overlapped with that of both the earlier and later instars. 3. Diel activity patterns of single larvae differed between instars but not within each instar (n = 20 larvae per instar). Second instars were active throughout the 24 h, with peaks at dusk, around midnight, dawn and around midday. A similar pattern was shown by third instars but the peak of activity at midday was less than the other three peaks. Prey were captured only during these peaks for both instars. Fourth and fifth instars were most active, and fed only, at night. They used an ambush strategy to capture more active prey at dusk and dawn (e.g. Baetis, Gammarus), and a searching strategy to capture more sedentary prey during the night (e.g. chironomids, simuliids). These experiments provided support for the hypothesis under test. If competition and/or interference occur between instars, then it could be reduced between earlier and later instars because of differences in their diet and diel pattern of feeding activity.     

Maximum likelihood estimation of the heterogeneity of substitution rate among nucleotide sites

This paper presents a maximum likelihood approach to estimating the variation of substitution rate among nucleotide sites. We assume that the rate varies among sites according to an invariant+gamma distribution, which has two parameters: the gamma parameter alpha and the proportion of invariable sites theta. Theoretical treatments on three, four, and five sequences have been conducted, and computer program have been developed. It is shown that rho = (1 + theta alpha)/(1 + alpha) is a good measure for the rate heterogeneity among sites. Extensive simulations show that (1) if the proportion of invariable sites is negligible, i.e., theta = 0, the gamma parameter alpha can be satisfactorily estimated, even with three sequences; (2) if the proportion of invariable sites is not negligible, the heterogeneity rho can still be suitably estimated with four or more sequences; and (3) the distances estimated by the proposed method are almost unbiased and are robust against violation of the assumption of the invariant + gamma distribution.      

A comparative study of three air-lift samplers used for sampling benthic macro-invertebrates in rivers

SUMMARY. From 16 air-lift samplers described in the literature, three were selected for operation from a small boat: Mackey, Pearson et al. and Verollet & Tachet samplers. Random samples (number of sampling units n= 10) were taken in a large tank with a known number of dried peas, representing invertebrates, amongst stones of uniform size. Separate experiments were performed with three sizes of stones (modal sizes 2–4, 16–20 and 32–36 mm). Stratified-random samples (usually n= 10) were taken in rivers at lour sites with modal particle sizes of < 0.1 2–4. 32–128 and 64–128 mm. The samplers were compared with a Ponar grab in the tank experiments and the field, and with a Naturalist's dredge in the field. Rates of pumping at different air-flows were established for each sampler. Water-How increased with increasing air-How and increasing submergence of the riser-pipe. The Mackey sampler achieved the highest water-flow and lifted more substratum and larger stones (64–128 mm) than the other samplers. The Verollet & Tachet sampler had the lowest pumping rate and failed to lift stones > 8 mm long, whilst the Pearson et al. sampler, with an intermediate performance, lifted mud and stones < 32 mm long. In the tank experiments, the area from which the Mackey and Pearson et al. samplers collected peas varied with air-flow, operating time and type of substratum. The Maekey sampler over-sampled and the Pearson et al. sampler usually under-sampled their respective sampling areas, except the latter sampler at high air-flows gave approximately quantitative results comparable to those of the Ponar grab for peas on the surface of stones 16–20 mm. No sampler collected much at a depth of 3 cm on larger stones (32–36 mm), and the Verollet & Tachet sampler failed completely on all substrata. In field trials, the Mackey and Pearson et al. samplers provided good qualitative samples except on large stones (32–128mm). The Verollet & Tachet sampler lifted little material. The relative abundance of taxa often differed between samplers. Estimates of the number of invertebrates per m² differed widely between samplers except for the Pearson et al. and Ponar grab which gave similar quantitative results at three sites. Values were very low for the Verollet & Tachet sampler and Naturalist's dredge and always very high for the Mackey sampler which often grossly over-sampled its sampling area. Therefore, these three samplers cannot be considered as quantitative. The relationship between the variances and the means of samples of peas in the tank and most invertebrates in the field followed a power law with values of the exponent b in the range 1.13–2.51. The present study completes the evaluation of 14 samplers used to sample benthic macro-invertebrates in deep rivers, and the more important conclusions from the comparative studies of seven grabs, four dredges and three air-lift samplers are summarized in a table.     

Predicting the spatial dominance of phytoplankton in a light limited and incompletely mixed eutrophic water column using the PROTECH model

1. A phytoplankton community model [Phytoplankton RespOnses To Environmental CHange (PROTECH)] was used to examine the effect of a wide range of varied light intensities and mixed depths upon simulated phytoplankton populations. Two different column lengths of the simulated water body were examined (the upper 5 m and the whole 14.5 m water column) for each scenario.
2. The hypotheses tested were that: (i) under low light intensity and/or deep mixing the simulated community will be dominated by a phytoplankter with a low critical light intensity; (ii) at high light intensity and shallow mixing the simulated community will be dominated by small, fast-growing phytoplankters; (iii) under all conditions, except deep mixing, the largest proportion of phytoplankton biomass will be found near the surface.
3. It was found under most conditions that, although there was a bloom in the upper column (dominated by algae such as Chlorella , Ceratium or Rhodomonas ), the largest phytoplankton biomass in the water column was located 9 m below the surface and consisted of solely Asterionella . This bloom was missed by the 5-m samples. Thus, using the whole column sample lengths, hypothesis (i) was not rejected but hypotheses (ii) and (iii) were refuted.
4. The inclusion of specific movement characteristics of phytoplankton in the model allowed the possibility of the dominance of multiple spaces within the water column and should be included in any model-based investigation of this topic. Further, the results from the model suggest that a reduced depth of mixing creates greater environmental heterogeneity, allowing more species to persist.     

Interstitial Protozoa and Algae of Louisiana Salt Marshes

SYNOPSIS. The majority of Louisiana salt marshes consists of stagnant pools among a vegetation of Spartina spp. and Distichlis spicata. The sediments constitute a "sulphide biome,'with Eh values reaching to –350 mV. The 200 interstitial ciliate species had definite stratification. Generally, algivores and omnivores occurred in the upper, and bactivores in the lower layers of the sediments. Compaction of particles prevented the flexible and large ciliates from inhabiting the lower layers, while Eh below –200 mV limited the distribution of some species but favored certain bactivores. The pH/Eh range and the abundance of nutrients in the sulfide biome provide habitable conditions for all the major groups of microflora. These, in turn, support nutritionally diverse predatory ciliate populations.     

Rates of gastric evacuation in brown trout, Salmo trutta L.

Brown trout of similar length and weight were fed a standard meal which contained a known number of food organisms of the same size-group and taxon (seven taxa were used). The weight of digestible organic matter in a trout stomach decreased exponentially with time. i.e. at a constant relative rate. At a particular water temperature, the food organisms were either evacuated from the stomach at similar rates (Group 1: Gammarus pulex, Baetis rhodani, Chironomidae, Oligochaetes) or at progressively slower rates (Group 2: Protonemura meyeri, Hydropsyche spp., Tenebrio molitor). Rates of gastric evacuation were not significantly different for food organisms of different size groups of the same taxon, or for different sized meals, or for different sizes of trout (range 20–30 cm), or for mixed and multiple meals (three meals over 16 h). Times are given for the gastric evacuation of 50%, 75%, 90% and 99% of the digestible organic matter in a meal. Starvation periods of 1, 2, 3,4 and 5 days prior to feeding did not affect evacuation rates. The rates were slightly, but not significantly, slower for starvation periods of 6 and 7 days, and were often significantly slower for starvation periods of 10, 15 and 20 days. Evacuation rates increased exponentially with increasing water temperature. It was possible to estimate both the rate and time for the gastric evacuation of different meals at water temperatures between 3–8°C and 19·1°C.     

The action of some systemic aphicides on the nymphs of Anthocoris nemorum (L.) and A. confusus Reut

The aphicides phorate, dimethoate and menazon were compared to elucidate the different pathways by which they can affect Anthocoris nymphs and their aphid prey.
When nymphs were caged in contact with deposits on bean leaves phorate and dimethoate had contact LC 50s of 20 and 3 μg/cm² respectively to Anthocoris nemorum and 46 and 6 μg/cm² to A. confusus. When the nymphs were confined on treated leaves on the opposite surface to the deposits, neither phorate nor dimethoate killed them. Menazon did not kill anthocorids at any dosage. All three aphicides killed over 50% of Acyrthosiphon pisum (Kalt.) on bean leaves at 1.6 μg/cm² whether the aphids were on the treated or untreated surface.
Experiments with ³⁵S-labelled phorate showed that anthocorids confined on phorate-treated bean plants, with or without insect food, accumulated the insecticide or its labelled derivatives. In field experiments in which A. nemorum were caged on plants treated with phorate, many were killed on young newly treated plants but not on older plants. A. confusus was relatively unaffected.
Anthocorids were reared from 2nd-instar nymphs to adults on aphids killed systemically with phorate, dimethoate or menazon without ill effects, despite evidence that ³⁵S-labelled phorate was ingested from the aphids and excreted in the faeces.
In the field, fewer large A. nemorum nymphs were found in August in plots of tick beans treated with phorate granules at 6 lb/acre (6.7 kg/ha) when sown, than in plots treated at 1.5 lb/acre (1.7 kg/ha) with phorate or menazon or untreated plots.     

A LIKELIHOOD JUSTIFICATION OF PARSIMONY

Abstract— A connection is established between maximally parsimonious cladograms and trees of highest likelihood. The assumptions needed to prove this are derivable from the structure of evolutionary theory and are independent of the frequency of homoplasy. The bearing of this justification on alternative methods of phylogenetic inference and on Felsenstein's (1978) proof that parsimony and other phylogenetic methods can be statistically inconsistent is discussed.