Sub-surface drift feeding by coho salmon (Oncorhynchus kisutch, Walbaum): a model and test

The results of a test of a mechanistic encounter model that predicts the size-frequency composition of the diet of drift-feeding coho salmon ( Oncorhynchus kisulch , Walbaum) are described. When all taxa in the drift were included in the model the predicted and actual diets differed significantly, although the model explained most of the variance between the diet and the predictions of a null model. When non-consumed taxa, including the distasteful and aposematic taxon Hydracarina, were excluded from the drift composition there was no significant difference between predicted and actual diets. The model's goodness of fit increased when it was modified to predict the biomass size-frequency diet composition, explaining 99% of the weighted sum of squared deviations between the diet and a null model. A number of alternate models are tested and a potentially useful simplification of the model is identified.     

Nocturnal drift of mayfly nymphs as a post-contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom-feeding, nocturnal predators, has rarely been tested.
2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout ( Salmo trutta , a drift-feeding fish), one with alpine bullhead ( Cottus poecilopus , a benthic fish), one with both species, and two fishless streams.
3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non-dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish.
4. In laboratory experiments, bullheads were night-active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre-contact response) and benthic fish (a post-contact response). In streams with bottom-feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.     

The propagation of a cultural or biological trait by neutral genetic drift in a subdivided population

We study fixation probabilities and times as a consequence of neutral genetic drift in subdivided populations, motivated by a model of the cultural evolutionary process of language change that is described by the same mathematics as the biological process. We focus on the growth of fixation times with the number of subpopulations, and variation of fixation probabilities and times with initial distributions of mutants. A general formula for the fixation probability for arbitrary initial condition is derived by extending a duality relation between forwards- and backwards-time properties of the model from a panmictic to a subdivided population. From this we obtain new formulae(formally exact in the limit of extremely weak migration) for the mean fixation time from an arbitrary initial condition for Wright's island model, presenting two cases as examples. For more general models of population subdivision, formulae are introduced for an arbitrary number of mutants that are randomly located, and a single mutant whose position is known. These formulae contain parameters that typically have to be obtained numerically, a procedure we follow for two contrasting clustered models. These data suggest that variation of fixation time with the initial condition is slight, but depends strongly on the nature of subdivision. In particular, we demonstrate conditions under which the fixation time remains finite even in the limit of an infinite number of demes. In many cases-except this last where fixation in a finite time is seen--the time to fixation is shown to be in precise agreement with predictions from formulae for the asymptotic effective population size.     

Analysis of microsatellite variation in Pinus radiata reveals effects of genetic drift but no recent bottlenecks

Most conifer species occur in large continuous populations, but radiata pine, Pinus radiata, occurs only in five disjunctive natural populations in California and Mexico. The Mexican island populations were presumably colonized from the mainland millions of years ago. According to Axelrod (1981), the mainland populations are relicts of an earlier much wider distribution, reduced some 8,000 years ago, whereas according to Millar (1997, 2000), the patchy metapopulation-like structure is typical of the long-term population demography of the species. We used 19 highly polymorphic microsatellite loci to describe population structure and to search for signs of the dynamics of population demography over space and time. Frequencies of null alleles at microsatellite loci were estimated using an approach based on the probability of identity by descent. Microsatellite genetic diversities were high in all populations [expected heterozygosity (H(e)) = 0.68-0.77], but the island populations had significantly lower estimates. Variation between loci in genetic differentiation (F(ST)) was high, but no locus deviated statistically significantly from the rest at an experiment wide level of 0.05. Thus, all loci were included in subsequent analysis. The average differentiation was measured as F(ST) = 0.14 (SD 0.012), comparable with earlier allozyme results. The island populations were more diverged from the other populations and from an inferred common ancestral gene pool than the mainland ones. All populations showed a deficiency of expected heterozygosity given the number of alleles, the mainland populations more so than the island ones. The results thus do not support a recent important contraction in the mainland range of radiata pine.     

Words as alleles: connecting language evolution with Bayesian learners to models of genetic drift

Scientists studying how languages change over time often make an analogy between biological and cultural evolution, with words or grammars behaving like traits subject to natural selection. Recent work has exploited this analogy by using models of biological evolution to explain the properties of languages and other cultural artefacts. However, the mechanisms of biological and cultural evolution are very different: biological traits are passed between generations by genes, while languages and concepts are transmitted through learning. Here we show that these different mechanisms can have the same results, demonstrating that the transmission of frequency distributions over variants of linguistic forms by Bayesian learners is equivalent to the Wright–Fisher model of genetic drift. This simple learning mechanism thus provides a justification for the use of models of genetic drift in studying language evolution. In addition to providing an explicit connection between biological and cultural evolution, this allows us to define a ‘neutral’ model that indicates how languages can change in the absence of selection at the level of linguistic variants. We demonstrate that this neutral model can account for three phenomena: the s-shaped curve of language change, the distribution of word frequencies, and the relationship between word frequencies and extinction rates.     

The effect of long pools on the drift of macro-invertebrates in a mountain stream

Macro-invertebrate drift was measured entering and leaving two pools on the Middle Fork of the Cosumnes River, a third order California stream. Drift rates for Baetis spp., Chironomidae, Simulium spp., Capniidae and total drift were calculated. Significant differences in the numbers of organisms entering the two pools were found for Baetis, Chironomidae, and Capniidae. Comparisons of drift rates at the upstream and downstream ends of each pool showed that the abundance of Chironomidae, Simulium, Capniidae and total drift changed in different directions across the pools. The numbers of organisms leaving the two pools, however, were not significantly different for Baetis, Simulium, Capniidae and total drift. These findings lead us to hypothesize that long pools act as barriers, not filters, to stream macro-invertebrate drift. The composition of drift leaving the pools in this experiment appeared to be controlled by the composition of the benthic habitat at the tail of the pool and not by the composition of upstream drift entering the pools.     

Modelling evolutionary processes in small populations: not as ideal as you think

Evolutionary processes are routinely modelled using ‘ideal’ Wright–Fisher populations of constant size N in which each individual has an equal expectation of reproductive success. In a hypothetical ideal population, variance in reproductive success (V_k) is binomial and effective population size (N_e) = N. However, in any actual implementation of the Wright–Fisher model (e.g., in a computer), V_k is a random variable and its realized value in any given replicate generation () only rarely equals the binomial variance. Realized effective size () thus also varies randomly in modelled ideal populations, and the consequences of this have not been adequately explored in the literature. Analytical and numerical results show that random variation in  and  can seriously distort analyses that evaluate precision or otherwise depend on the assumption that  is constant. We derive analytical expressions for Var(V_k) [4(2N – 1)(N – 1)/N³] and Var(N_e) [N(N – 1)/(2N – 1) ≈ N/2] in modelled ideal populations and show that, for a genetic metric G = f(N_e), Var(?) has two components: Var_Gene (due to variance across replicate samples of genes, given a specific ) and Var_Demo (due to variance in ). Var(?) is higher than it would be with constant N_e = N, as implicitly assumed by many standard models. We illustrate this with empirical examples based on F (standardized variance of allele frequency) and r² (a measure of linkage disequilibrium). Results demonstrate that in computer models that track multilocus genotypes, methods of replication and data analysis can strongly affect consequences of variation in . These effects are more important when sampling error is small (large numbers of individuals, loci and alleles) and with relatively small populations (frequently modelled by those interested in conservation).     

The impact of repeated insecticidal treatments on drift and benthos of a headwater stream

A small first-order, Appalachian Mountain stream received successive seasonal treatments with the insecticide, methoxychlor. Despite an application rate of 10 mg/1 methoxychlor for 4 hours, based on stream discharge, only a small fraction (1.6%) of the insecticide was exported to downstream reaches for a 31 h period during and following treatment. Most of the insecticide was incorporated into sediments of the streambed, which had residues ranging from 0.038 to 11.7 µg methoxychlor/g dry wt of sediments in June 1986 following treatments in December 1985 and March 1986. Despite low concentrations of methoxychlor measured in stream water (maximum = 128 µg/l) during the initial treatment, massive drift (> 950 000 organisms, and 70 g AFDM biomass) occurred from a stream area of about 144 m². Numerically, collector-gatherer taxa (primarily Chironomidae) dominated drift (63 %) followed by shredders and predators; however, biomass of drift was dominated by shredders (48.9%), followed by predators and collector-gatherers. Compared with pre-treatment benthic abundances, insects were reduced by 75% following the initial treatment in December 1985, and 85% following an additional treatment in March 1986. Benthic abundances of non-insect taxa showed no significant changes. Benthic abundances of shredder, collector-filterer, and scraper functional groups exhibited significant decreases in the first month following treatment. Although benthic abundances of collector-gatherer and predator taxa were reduced by 48.6 and 40.5%, respectively, the reduction was not statistically significant because of high-sample variance. Comparisons of drift composition during the initial treatment with successive quarterly treatments (March 1986 to January 1988) reflected of ongoing pesticide disturbance of the biota as the community structure shifted from one consisting of a diverse insect and non-insect fauna toward one dominated by copepods, oligochaetes, Collembola, and chironomids.     

Causes of larval drift of the fire salamander,Salamandra salamandra terrestris,and its effects on population dynamics

Summary The larval drift of the fire salamander was investigated over a period of three years in a mountain brook (Niederbergisches Land, F.R. Germany), as well in a laboratory water channel. The rate of larval drift fluctuated between 19% and 41% of the total population of larvae in a defined section of the brook during these three years. Most (83%) of the drifting larvae were hatchlings or very young stages. The drift was dependent on the strength of the current, the number of spawning females, the presence of suitable hiding places, sufficient space and adequate food. Hungry larvae drifted more often than satiated animals. The drift behaviour of hatchlings differed distinctly from that of older larvae. The significance of ecological factors on larval drift is discussed. It is evidently a more important factor in selection than has hitherto been recognized.     

Estimating Effective Population Size from Temporally Spaced Samples with a Novel,Efficient Maximum-Likelihood Algorithm

The effective population size N_e is a key parameter in population genetics and evolutionary biology, as it quantifies the expected distribution of changes in allele frequency due to genetic drift. Several methods of estimating N_e have been described, the most direct of which uses allele frequencies measured at two or more time points. A new likelihood-based estimator NB^ for contemporary effective population size using temporal data is developed in this article. The existing likelihood methods are computationally intensive and unable to handle the case when the underlying N_e is large. This article tries to work around this problem by using a hidden Markov algorithm and applying continuous approximations to allele frequencies and transition probabilities. Extensive simulations are run to evaluate the performance of the proposed estimator NB^, and the results show that it is more accurate and has lower variance than previous methods. The new estimator also reduces the computational time by at least 1000-fold and relaxes the upper bound of N_e to several million, hence allowing the estimation of larger N_e. Finally, we demonstrate how this algorithm can cope with nonconstant N_e scenarios and be used as a likelihood-ratio test to test for the equality of N_e throughout the sampling horizon. An R package “NB” is now available for download to implement the method described in this article.     

Effects of artificial freshets on substratum composition,benthic invertebrate fauna and invertebrate drift in two impounded rivers in mid-Wales

Artificial discharges of water from reservoirs caused a six-fold and three-fold increase in discharge in the R. Tywi and R. Elan respectively but did not significantly alter particle size composition (by weight) and the porosity of the substratum or the organic matter content of fine particles (<0.5 mm). Freshets in both rivers resulted in a consistent, though not significant, reduction in total densities of invertebrates and the densities of many major taxa and abundant species. During the freshet in the R. Elan, maximum concentration and total load of suspended solids were about 11 and 35 times greater than pre-release values respectively while invertebrate drift was dominated by Chironomidae (65%) and Plecoptera (25%). Total numbers and densities of drifting chironomids increased immediately in response to the flow increase; in contrast, numbers and densities of plecopterans increased later, during the night.     

Impact of mild experimental acidification on short term invertebrate drift in a sensitive British Columbia stream

We report daytime drift behavior of lotic macroinvertebrates following short term (12 h) additions of HCl or HCl plus AlCl₃ to a circumneutral softwater (alkalinity ca. 100 µeq 1^-1) mountain stream in British Columbia, Canada. Addition of HCl (pH reduced from 7.0 to 5.9) resulted in an overall tripling of invertebrate drift density with rapid (< 1 h) increases in chironomid Diptera and Trichoptera. Small Ephemeroptera also entered the drift at high densities, but were delayed about 6 h. Addition of AlCl₃ (0.71 to 0.95 mg 1^-1 total Al³⁺) in HCl (stream pH reduced to 5.9) resulted in an overall 6-fold increase in invertebrate drift, with rapid increases by Ephemeroptera and delayed responses by chironomids and Trichoptera. These results suggest that the behavior of several macroinvertebrates from low alkalinity, unacidified streams can be altered by simulations of short-term, mild acidic deposition events. Further, the magnitude and timing of entry into the drift varies among taxonomic groups with the presence or absence of low concentrations of aluminum ions.     

The effects of pitfall trap diameter on ant species richness (Hymenoptera: Formicidae) and species composition of the catch in a semi-arid eucalypt woodland

Abstract Ants play an important role in Australian biodiversity and environmental impact assessments, with pitfall-trapping being the principal sampling method. However, the relationship between trap diameter and ant species catch has not been investigated in the context of survey design. Using four different trap diameters, each at a density of one trap per 100 m2, the present study asks three questions: (i) given an equal number of traps, do traps with larger diameters catch more species than smaller-diameter traps?; (ii) do traps with small diameters bias against large or rare species?; (iii) for equal area of the trap mouth, do small but more numerous traps catch more species than fewer but large traps? A total of 84 species were sampled within the 1600 m2 study site, with numbers of species for trap diameters of: 18mm (46 species), 42mm (56 species), 86mm (62 species) and 135mm (64 species). At equal trap density, 18 mm traps caught significantly fewer species than larger traps. Traps of 86 mm and 135mm were no more efficient than 42mm traps. Only 86mm and 135mm traps caught all species > 10mm in length (6 species). For equal area of the trap mouth, small traps were more efficient than large traps. Differences in the catch of the different-sized traps were due primarily to different capture rates of the rare species (40 species): 18mm traps caught 25% of rare species, 42 mm caught 41%, 86 mm caught 44% and 135 mm caught 52%. The role of rare ant species in environmental impact studies is discussed.     

Thermal effects on acid hydrolysis of cellulose

Acid-catalysed hydrolysis of cellulose is a technically feasible process. Cellulosic biomass often requires size reduction which imparts additional cost to the hydrolysis process. A simplified heterogeneous model was developed to study the effect of non-uniform temperature distribution inside wood chips on the hydrolysis process. Increasing particle size was found to result in lower glucose yields and higher reaction times. The effect was more pronounced at higher reaction temperatures. Mathematical correlations were found to quantify this effect.     

Influence of a metal-contaminated tributary on the invertebrate drift fauna of the King River (Tasmania,Australia)

Drifting invertebrates were collected upstream of, at, and downstream of the input of metal contaminated water into the previously unpolluted King River in western Tasmania. In the upstream section the drift fauna showed characteristics similar to those observed by other workers with a marked nocturnal peak. Changes in drift rates at the site of input were observed and it is hypothesized that mayfly nymphs, and possibly other animals, respond to the change in water quality by leaving the water column and randomly searching the substrate for clean water. Most nymphs re-enter the drift in a current of clean water which was displaced towards the opposing bank by the entry of contaminated water. This re-entry of animals into the drift occurs some four hours after the main nocturnal peak in drift rates.     

The effects of genetic drift during range expansion on geographical patterns of variation: a computer simulation of the colonization of Australia by Bufo marinus

A computer simulation is performed of allele frequency changes resulting from genetic drift at eleven loci during the probable course of colonization of Australia by populations of the Giant Toad, Bufo marinus. The history of twelve populations for which allele frequency data are available is modelled. Account is taken of the likely pattern of relationship among the populations, the effective size of the populations (as indicated by the observed variance of allele frequencies) and the probable isolation of the populations from each other following their separation. In all simulations, allele frequencies at some loci show a significant association with latitude while others do not. In six of ten simulations, there is a significant association between degree of variation at a locus and the presence of a latitudinal cline of allele frequencies. There are also indications of this kind of association in simulations of a uni-directional range expansion. These results demonstrate that such associations, which were also observed in the data from the actual populations, can result from genetic drift during a range expansion, and therefore cannot be taken as evidence of the action of natural selection.     

Macroinvertebrates associated with submerged macrophytes: sample size and power to detect effects

When planning and conducting ecological experiments, it is important to consider how many samples are necessary to detect differences among treatments with acceptably high statistical power. An analysis of statistical power is especially important when studying epiphytic macroinvertebrate colonization of submerged plants because they exhibit large plant-to-plant variability. Despite this variability, many studies have suggested that epiphytic macroinvertebrates preferentially colonize plants based on plant architecture type (broad versus dissected leaves). In this study, we calculated the power and number of samples necessary to detect differences in epiphytic macroinvertebrate abundance (numbers and biomass) among five species and two architecture types of macrophytes in a lake in MI, U.S.A. Using power analysis, we found that we had very high power to detect the differences present between macroinvertebrate abundance by architecture type and by macrophyte species (power = 1.000 and 0.994; effect sizes = 0.872 and 0.646, respectively. However, to detect very small differences between the two architecture types and the five plant species, we determined that many more samples were necessary to achieve similar statistical power (effect size = 0.1–0.3, number of samples = 60–527 and 36–310, respectively; power = 0.9). Our results suggest that macroinvertebrate abundance does in fact vary predictably with plant architecture. Dissected-leaf plants harbored higher abundances of macroinvertebrates than broad-leaf plants (ANOVA, density p = 0.001, biomass p < 0.001). This knowledge should allow us to better design future studies of epiphytic macroinvertebrates.     

Within- and between-generation effects of temperature on life-history traits in a butterfly

Non-genetic parental effects may largely affect offspring phenotype, and such plasticity is potentially adaptive. Despite its potential importance, little is known about cross-generational effects of temperature, at least partly because parental effects were frequently considered a troublesome nuisance, rather than a target of experimental studies. We here investigate effects of parental, developmental and acclimation temperature on life-history traits in the butterfly Bicyclus anynana. Higher developmental temperatures reduced development times and egg size, increased egg number, but did not affect pupal mass. Between-generation temperature effects on larval time, pupal time, larval growth rate and egg size were qualitatively very similar to effects of developmental temperature, and additionally affected pupal mass but not egg number. Parental effects are important mediators of phenotypic plasticity in B. anynana, and partly yielded antagonistic effects on different components of fitness, which may constrain the evolution of cross-generational adaptive plasticity.     

Reorganisation of a long-term monitoring network using moss as biomonitor for atmospheric deposition in Germany

The determination of atmospheric deposition in forests can be accomplished using technical sampling devices (bulk samplers, wet only samplers), biomonitors or modelling. In Europe, since 1990 moss sampled every five years at up to 7300 places in up to 35 countries was used as biomonitor. In the moss specimens, heavy metals (HM), nitrogen (N, since 2005) and persistent organic pollutants (POPs, since 2010) were determined. Germany participated in all surveys with the exception of that in 2010. For the moss survey 2015, the biomonitoring network applied in the 2005 campaign should be reorganized. To this end, a complex statistically based methodology including a decision support system was developed and implemented. Its application yielded a network with a reduction of sample points from 726 to 402. By use of the data collected in 2005 the performance of the reorganized network did not reveal significant loss of statistical validity.