Environmental constraints on oviposition limit egg supply of a stream insect at multiple scales

Species with complex life cycles pose challenges for understanding what processes regulate population densities, especially if some life stages disperse. Most studies of such animals that are thought to be recruitment limited focus on the idea that juvenile mortality limits the density of recruits (and hence population density), fewer consider the possibility that egg supply may be important. For species that oviposit on specific substrata, environmental constraints on oviposition sites may limit egg supply. Female mayflies in the genus Baetis lay egg masses on the underside of stream rocks that emerge above the water’s surface. We tested the hypothesis that egg mass densities are constrained by emergent rock densities within and between streams, by counting egg masses on emergent rocks. All emergent rocks were counted along 1-km lengths of four streams, revealing significant variation in emergent rock density within streams and a more than three-fold difference between streams. In each stream, egg mass density increased with the density of emergent rocks in 30-m stretches. We used regression equations describing these small-scale relationships, coupled with the large-scale spatial variation of emergent rocks, to estimate egg mass densities for each 1-km stream length, a scale relevant to population processes. Scaled estimates were positively associated with emergent rock density and provided better estimates than methods that ignored environmental variation. Egg mass crowding was inversely related to emergent rock density at the stream scale, a pattern consistent with the idea that oviposition substrata were in short supply in streams with few emergent rocks, but crowding did not compensate entirely for differences in emergent rock densities. The notion that egg supply, not larval mortality, may limit population density is an unusual perspective for stream insects. Environmental constraints on egg supply may be widespread among other species with specialised oviposition behaviours.     

Novel use of digital colour-infrared imagery to test hypotheses about grazing by intertidal herbivorous gastropods

Colour-infrared (CIR) imagery was evaluated for its usefulness to test hypotheses about grazing in the intertidal gastropods Nerita atramentosa (Reeve) and Bembicium nanum (Lamarck). Tiles of natural sandstone were placed onto the shore to accumulate micro-algae for a period of 6 weeks. Then, in the laboratory, three replicate tiles were assigned into each of 6 treatments (Nerita and Bembicum at densities of 3 or 6 animals, plus a control and a control for artifacts due to cages). CIR images were acquired before and three times after the animals were placed on the tiles. NIR / red ratio images, which are used to estimate micro-algal biomass, were generated from calibrated CIR data. Nerita at the greater density were more effective at removing micro-algae than at the smaller density. The latter were more effective than Bembicium at either density, which did not significantly decrease algal biomass from that in controls. These data are consistent with previous field observations and indicate that CIR imagery is an effective method for testing hypotheses about grazing by intertidal gastropods.     

Comparability of introduced tiles and natural substrates for sampling lotic bacteria, algae and macro invertebrates

SUMMARY.

• 1 Benthic microflora (bacteria and algae) and macro invertebrates on two types of introduced substrates, unglazed clay tiles and sterilized rocks, were compared quantitatively with natural rocks in a third-order stream. Big Sulphur Creek, California, U.S.A.
• 2 Exposure periods ranging from 28 to 153 days for introduced substrates indicated that tiles accurately represented bacterial density, chlorophyll a, and macro invertebrate density and species composition of natural rocks within 28 days; phaeophytin and total organic material (as ash-free dry weight) were accurately represented within 63 days. In contrast. sterilized rocks required a 63 day exposure to simulate most of the above natural-rock features.
• 3 Tiles reduced sampling variability (i.e. increased precision) when compared with either natural or sterilized rocks, especially the variability associated with algal measurements. In benthic studies where a sufficiently long exposure period is possible (1-2 months), introduced substrates can reduce the effort and cost of benthic sampling while minimizing habitat disruption.

     

CONSTANCY OF POPULATION PARAMETERS FOR LIFE-HISTORY AND FLORAL TRAITS IN RAPHANUS SATIVUS L. II. EFFECTS OF PLANTING DENSITY ON PHENOTYPE AND HERITABILITY ESTIMATES

To determine the effect of growing conditions on population parameters in wild radish, (Raphanus sativus L.: Brassicaceae), we replicated maternal and paternal half-sib families of seed across three planting densities in an experimental garden. A nested breeding design performed in the greenhouse produced 1,800 F₁ seeds sown in the garden. We recorded survivorship, measured phenotypic correlations among and estimated narrow-sense and broad-sense heritabilities (h²) of: days to germination, days to flowering, petal area, ovule number/flower, pollen production/flower, and modal pollen grain volume. Survivorship declined with increasing density, but the relative abundances of surviving families did not differ significantly among densities. Seeds in high-density plots germinated significantly faster than seeds sown in medium- or low-density plots, but they flowered significantly later. Plants in high-density plots had fewer ovules per flower than those in the other treatments. Petal area and pollen characters did not differ significantly among densities. Densities differed with respect to the number and sign of significant phenotypic correlations. Analyses of variance were conducted to detect additive genetic variance (V_a) of each trait in each density. At low density, there were significant paternal effects on flowering time and modal pollen grain volume; in medium-density plots, germination time, flowering time and ovule number exhibited significant paternal effects; in high-density plots, only pollen grain volume differed among paternal sibships. The ability to detect maternal effects on progeny phenotype also depended on density. Narrow-sense h² estimates differed markedly among density treatments for germination time, flowering time, ovule number and pollen grain volume. Maternal, paternal and error variance components were estimated for each trait and density to examine the sources of variation in narrow-sense h² across densities. Variance components did not change consistently across densities; each trait behaved differently. To provide qualitative estimates of genetic correlations between characters, correlation coefficients were estimated using paternal family means; these correlations also differed among densities. These results demonstrate that: a) planting density influences the magnitude of maternal and paternal effects on progeny phenotype, and of h² estimates, b) traits differ with respect to the density in which heritability is greatest, c) density affects the variance components that comprise heritability, but each trait behaves differently, and d) the response to selection on any target trait should result in different correlated responses of other traits, depending on density.     

Optimising settlement tiles: The effects of surface texture and energy,orientation and deployment duration upon the fouling community

The aim of this study was to produce a set of tiles for field studies on settlement and biofouling with carefully controlled surface characteristics and practical design, and test them under field conditions. Impressions of precisely defined surface textures were made in silicone. Double sided tiles in epoxy, polyester and silanised epoxy resins were cast from the impressions. Tile characteristics tested were texture (R_a = 0, 0.19, 0.62, 1.1, 2.2 mm), surface free energy (60, 52, 24 mN m^{m 2}), and surface orientation (up, down, into, away). Tiles were deployed in the Red Sea for 4 and 7 months. Measures of community cover, dominance and richness were all significantly affected by each of the factors. The tiles proved durable and robust during the 7 month deployment with no observable changes in surface characteristics and none were lost or broken. These settlement tiles have a wide applicability for both biofouling and ecological studies. The field test demonstrated the complexities of the interactions between just four surface characteristics. This study has also underlined the need for multidimensional analysis of fouling communities for applied and basic research.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

Settlement and early survival of red coral on artificial substrates in different geographic areas: some clues for demography and restoration

The red coral Corallium rubrum (L 1758) is a long-lived, slow-growing gorgonian, endemic to Mediterranean rocky bottoms. Because of its high economic value, red coral has long been harvested, and most populations have been depleted. In the present study, 54 marble tiles were placed in June 2003 within red coral populations over 3 different geographic areas (Calafuria–Livorno and Elba MPA in Italy and Medes Islets MPA, in Spain), on vertical cliffs between 25 and 35 m. In each area 2 different sites were randomly selected. Tiles were subsequently sampled photographically. Between July and August 2003 red coral recruits settled on tiles in all the geographic areas and sites, exhibiting wide variability in their density. On the basis of a 2-factors nested ANOVA a significant variability between different sites at a few hundred metres distance occurred, indicating high variations in the recruitment process within the same red coral population. Mortality, measured in June 2004, widely varied between different geographic areas.     

Effect of rock type on the recruitment and early mortality of the barnacle Chthamalus montagui

Discontinuities in distribution of sessile species due to changes in available habitat have rarely been investigated. In the English Channel, soft sedimentary rocks including chalk could potentially present a barrier to eastward range extension of the southern barnacle Chthamalus montagui. To test the effect of rock type on recruitment and mortality, prepared settlement tiles of four calcareous rocks that form important platforms close to existing limits of distribution were fixed to the shore at a site in south-west England, known to have consistently high larval settlement. Prior to fixation, the roughness parameter “R_a” and Potential Settlement Sites (PSS) index was measured. Recruitment was monitored photographically on four occasions during the settlement season and subsequent mortality for 7 months after the settlement season. By the end of the settlement season, recruitment on the hard Kimmeridge Cementstone was significantly less than on the other three rock types and attributed to low surface roughness and fewest Potential Settlement Sites (PSS), yet only 28% of variance could be attributed to PSS. Post-recruitment mortality was more variable, however after 7 months there were significant differences between rock types. Simultaneous surveys of post-recruitment mortality in natural populations showed that although recruitment on chalk can potentially be high, survivorship was relatively low. Thus rock type could be influential in setting species geographical limits.     

Genetic studies in an exotic population of corn (Zea mays L.) grown under two plant densities

Summary Progenies of a Design II [Comstock and Robinson (1948)] using random S ₁ lines from an exotic population of corn (Zea mays L.) were evaluated in a randomized incomplete block design with two replications at two plant-population densities (1 7,222 plants/ha and 68,888 plants/ha) in 1970 and 1971, at Lincoln, Nebraska. Five traits were studied i.e. grain weight, number of ears, days to flower, plant height and ear height.Under both densities the estimates of additive genetic variance were much larger than those of dominance genetic variance for all traits. The ratio of dominance to additive genetic variance estimates was less than 0.5 suggesting that for the majority of loci controlling the traits, partial to complete dominance is likely.The estimates of additive genetic x year interaction variance were high and significantly different from zero under both densities, indicating that estimates of additive genetic variance in this population obtained from experiments conducted in only one year may be seriously biased. The estimates of dominance genetic x year interaction variance were not significant and most of them were negative.Under both densities high genetic inter-relationships were indicated between grain weight and number of ears, days to flower and plant height, days to flower and ear height, and plant height and ear height.Even though there was a large difference between the two densities used in the study, the differences between the estimates of genetic parameters were not significant in all cases.The sample size of S ₁ plants representing each S₀ parent in the crossing nursery used in the present study (11.75) caused a small upward bias in the estimates of additive genetic variance, but it caused an upward bias in the estimates of dominance genetic variance of 6–7% of the total genetic variance.It is suggested that a trait such as grain weight should be expressed on a unit area basis when genetic parameters (except for correlation and the ratio between two values) obtained from experiments with different plant-population densities are to be compared.Published as Paper Number 3542, Journal Series, Nebraska Agricultural Experimental Station. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the Ph. D. degree.A. I. D. Participant.The work was supported in part by a grant from the Rockefeller Foundation.     

Using citizen‐based survey data to determine densities of large mammals: a case study from Mana Pools National Park,Zimbabwe

Since 1993, members of the national wildlife society have undertaken annual surveys of large mammals in the Zambezi alluvial woodlands of Mana Pools National Park, Zimbabwe. Data are collected along 36 systematically‐arranged transects. We provide the first thorough assessment of the data from any survey within this long‐term project. The transect data from 2011 were analysed with DISTANCE software to assess if the data were suitable for determining the densities of large mammals using distance sampling techniques. Successful application of distance sampling depended on observers using printed, large‐scale, georeferenced satellite images onto which they mapped the location of animal groups detected. The assumptions of the distance sampling were well met and thus the 2011 survey provided reliable estimates of the densities of nine species of common large mammals on the Zambezi alluvium during the late dry season. Estimated density in this dry‐season concentration area varied from 3.6 km^?2 for kudu, to 204 km^?2 for impala. The precision of the estimates ranged from a coefficient of variation of 7.9% for elephant, to 25.5% for buffalo. For elephant, warthog and baboon, the morning and afternoon densities differed significantly.     

COMMUNITY DYNAMICS AND METABOLISM OF BENTHIC ALGAE COLONIZING WOOD AND ROCK SUBSTRATA IN A FOREST STREAM

Colonization dynamics and metabolism of algal communities on wood (small pieces of Douglas fir) and artificial rock (tiles) substrata were investigated in an open reach in Mack Creek, Cascade Mountain Range, Oregon, for 42 days. Chlorophyll a concentration was not significantly different between the two algal communities (ANOVA, P = 0.119). Even though differences in cell densities were not significantly different between the two algal communities (ANOVA, P = 0.063), biovolume and diversity were greater in epixylic than in epilithic communities (ANOVA, P = 0.011 and 0.002, respectively). The same algal taxa occurred on both substrata, but some of them (e.g. Ceratoneis arcus Grun., Cymbella minuta Hilse ex Rabh., Zygnema sp.) were more numerous on wood, whereas a few others preferred tiles (e.g. Achnanthes lanceolata (Bréb.) Grun.). Scanning electron microscopy revealed a much higher surface roughness on wood, which likely led to a patchy distribution of the algae and the development of stalked diatom species. However, adnate species adapted much better to the mainly flat surfaces of tiles. Net community primary productivity (NCPP) measured in 42-day-old substrata was moderate on tiles but negative on wood. Community respiration (CR) was significantly higher on wood than on tiles. Epixylic algal communities exhibited greater nitrogen demand than epilithic communities after ammonium addition. NCPP shifted to positive values on wood but did not increase on tiles. Despite metabolic differences, the variations between the two communities are more closely related to the physical characteristics of the substrata and the attachment abilities of the algal taxa.     

Stimulation of aquatic bacterial activity by cyanobacteria

The time-course response of natural bacterial populations and isolates from lake water to various densities of the filamentous cyanobacteriaAphanizomenon flos-aquae andLyngbya birgei collected from the same lake is reported. The cyanobacteria were separated from the bacteria by dialysis membranes that allowed only dissolved cyanobacterial products to pass. Bacterial³H-thymidine incorporation and cell number were significantly (p<0.05) correlated with cyanobacterial density for both species. Estimated dissolved organic carbon (DOC) utilization, based on bacterial biomass changes over time, were usually significantly (p<0.01) correlated with cyanobacterial density and the decrease in bulk pool DOC for both species. Bacterial volume per cell increased significantly (p<0.05) in response to cyanobacterial density on day 5 of the experiments; cell volume remained unchanged on day 1. Bacterial cell numbers on outer surfaces of the tubular membrane containing the cyanobacteria (on the side exposed to the test bacteria) were significantly (p<0.01) correlated with cyanobacterial density. Statistical analysis inferred that bacteria closely associated with cyanobacteria (i.e. attached) responded more strongly to cyanobacterial products than free-living bacteria. Overall, our results indicate that cyanobacterial products have a potentially important role in regulating bacterioplankton productivity in aquatic systems.     

Genotype by environment interaction for adult body weights of shrimp Penaeus vannamei when grown at low and high densitie

Shrimp is one of few marine species cultured worldwide for which several selective breeding programs are being conducted. One environmental factor that can affect the response to selection in breeding programs is the density at which the shrimp are cultured (low-medium-high). Phenotypic plasticity in the growth response to different densities might be accompanied by a significant genotype by environment interaction, evidenced by a change in heritabilities between environments and by a genetic correlation less than one for a unique trait between environments. Our goal was to understand whether different growth densities affect estimates of those genetic parameters for adult body weight (BW) in the Pacific white shrimp (Penaeus vannamei). BW heritabilities were significantly different between environments, with the largest at high density. These differences resulted from both an increased additive genetic variance and a decreased environmental variance when grown at high density. The genetic correlation between BWs at the two environmental conditions was significantly less than one. Whereas these results might be suggestive for carrying out shrimp selective breeding for BW under high density conditions, further understanding of genetic correlations between growth and reproductive traits within a given environment is necessary, as there are indications of reduced reproductive fitness for shrimp grown at high densities.     

The effects of population density and lake characteristics on growth and size structure of brook trout Salvelinus fontinalis (Mitchill, 1815) in boreal forest lakes in Canada

This paper examines the effect of lake characteristics on population density and how this variation affects growth, mortality and population size structure of brook trout, Salvelinus fontinalis. The study was conducted on 17 recreationally fished, reproductively isolated boreal forest lakes in Newfoundland, Canada from 1993 to 2000. A standardized sampling program, the Fyke Littoral Index Netting program (FLIN) was used to collected data that describes brook trout population parameters and life history attributes. Regression analyses showed significant relationships between fish density and biomass and characteristics of the lakes. Variation in fish density and biomass was explained by lake surface area and littoral habitat area. Significant relationships were found when growth, mortality and size structure were regressed against density. The proportional stock distribution and theoretical maximum size of brook trout were negatively related to density, and natural mortality was positively related to density. The largest maximum length and highest proportional stock densities occurred at brook trout densities of less than 30 fish/ha. In general, the higher the proportion of littoral habitat area the higher the densities of brook trout, which correspondingly had important effects on growth, natural mortality and size structure of the brook trout populations. This information is critical to the development of management strategies aimed at altering size distribution to produce specific fisheries management outcomes.     

Macroinvertebrate drift in the upper Wye catchment,Wales

Drift samples collected at five sites in the upper catchment of the River Wye yielded 99 taxa (excluding Chironomidae and Simuliidae). Significantly fewer taxa were collected from a site on the impounded River Elan (W4) compared with nearby River Wye sites. Mean daily density of drifting macroinvertebrates ranged from 6.3 × 10^-2 to 782.9 × 10^-2m^-3, with greatest densities during the summer months, and Ephemeroptera, Coleoptera and Diptera generally comprised the greatest proportion of animals collected. Chironomids formed a considerable proportion of collections at W4 and the relative abundance of ephemeropterans, coleopterans and trichopterans was generally significantly less at W4 than at other sites. Estimates of the total numbers of invertebrates drifting past each site varied from 8.3 × 10³ to 1 373.1 × 10³day^-1. Total numbers drifting were significantly related to estimates of total benthic density and similar relationships were established for some major taxonomic groups and some species.     

EFFECTS OF LIGHT AND WAVE DISTURBANCE ON VERTICAL ZONATION OF ATTACHED MICROALGAGE IN A LARGE RESERVOIR1

Experimental field manipulations of artificial substrata were used to examine the mechanisms controlling attached algal zonation down the face of the dam in Lake McConaughy, a large reservoir in western Nabraska. Sets of clay tiles were incubated in the upper (2.5 m depth) and lower (8 m depth) growth zones for two weeks. Five sets tiles were then switched from the upper to the lower growth zone and vice versa. Five additional sets of tiles were switched to the lower zone and artificially disturbed. Diatom cell densities increased rapidly in both the zones; however, wind-induced turbulence caused dramatic declines (up to 61%) in densities in the upper zone. Consequently, cell densities in the upper and lower growth zones were not significantly different after four weeks, despite the 17–30% higher light levels in the upper zone. Based on cell densities and relative abundances on clay tiles and naturally occurring rocks, 26 of the 32 most common diatom taxa had a significant upper (10) or lower (16) zone preference. Of these, 15 taxa exhibited a consistent response to one or both switching manipulations, confirming a growth zone preference, and two showed a clear preference for disturbed substrata. Diatom growth form appeared to play a major role in determining the vertical zonation of attached communities, since actively motile taxa exhibited a lower zone preference and stalked forms occurred primarily in the upper zone. The present study indicates that light attenuation and wave disturbance are primary mechanisms that control the vertical zonation of freshwater epilithic algae.     

The phosphorus concentration of common rocks—a potential driver of ecosystem P status

Background

Soil phosphorus (P) availability can be an important regulator of ecosystem processes. Changes in P availability over time have long been studied, but the P concentration of soil parent materials—which determines ecosystem P concentration at the onset of soil formation—have never been systematically explored. Here we ask two questions: 1) how does P concentration vary among soil parent materials? and 2) under what range of conditions do those differences influence soil P concentration?

Methods

We used the Earthchem webportal to compile the P concentration of 263,539 rocks. We then gathered data from 62 sites (MAT ranging from 200–5,000 mm?yr^-1 and soil age from 0.3–4,100 ky) and assessed the correlation between rock and soil P concentration.

Results

We found a 30 fold difference in median P concentration among rock types, ranging from 120 ppm (several ultramafic rocks) to >3,000 ppm (several alkali basalts). Median P was significantly lower in common silica-rich rocks (e.g. granite - 436 ppm) and higher in common iron-rich rocks (e.g. andesite - 1,000 ppm). In sedimentary rocks, which make up 70 % of the ice-free land surface, median P was highest in mudstone (1,135 ppm) and decreased with increasing grainsize (siltstone-698 ppm, sandstone-500 ppm). Where soil P and parent material P were measured in the same site, parent material P explained 42 % of the variance in total soil P (n?=?62), and explanatory power was higher for sites with similar climate.

Conclusion

The variation in P concentration among common rock types is on a comparable scale to the changes in total P, and several P pools, over long-term soil development. Quantifying these differences may be an important step towards characterizing regional and global variation in soil and ecosystem P status.     

Spatial and temporal distribution of Myrophis punctatus (Ophichthidae) and associated fish fauna in a northern Brazilian intertidal mangrove forest

The mangrove forest along the northern Brazilian coast is not inundated during low tide. However, many fish species stay in the mangrove forest during this time. Tidal behaviour strategies are described for fish species that linger in the mangrove forest during low tide. The samples were taken at the end of the dry season (December 1996) and at the end of the rainy season (July 1997). Fish were captured using an ichthyotoxic plant extract (Ichthyotere cunabi). Spatial and temporal fish density and biomass were analyzed statistically. Thirty-six samples were taken with a total density of 2.8 ind m^-2 and a total biomass of 17.4 g m^-2 distributed among seven families and 14 species. Myrophis punctatus was the most important species in number (1.66 ind m^-2) and weight (12.68 g m^-2) of all catches. The total fish densities were not significantly different among areas and between months, although, total biomass differed significantly in time and space. The densities and biomass for the three most dominant species (M. punctatus, Poecilia spp. and Gobionellus smaragdus) differed significantly among species. Only the biomass of these species showed significant monthly differences. The only significant main effect on variance in the densities and biomass of M. punctatus were encountered between months. In addition, the factor area was significantly different for the variable number of species.     

Relationships between stream water chemistry and catchment geology in a mountainous region in Nagano Prefecture, Japan II: mid to southern part along the Median Tectonic Line

Major ion concentrations were determined for a large number of stream water samples collected from the mountainous area around the Median Tectonic Line (MTL) in the mid to southern part of Nagano Prefecture, Japan. The MTL, a major fault in the accretionary sedimentary complex to Japan Arc, extending linearly in the east–west direction in southwest Japan, bends in this area to south–north along the Akaishi Mountain chain. To the east of the MTL are, in order, Jurassic metamorphic rocks, including black/green schists and ultra-basic serpentinite (Sambagawa-Mikabu Belt); Jurassic mudstone with voluminous chert and calcite (Chichibu Belt); and Cretaceous sandstone (Shimanto Belt). On the other hand, on the west side of the MTL, Cretaceous granitic rocks (including tonalite and quartzdiorite) intruded and significantly metamorphosed the Jurassic sedimentary systems (Ryoke-Belt). In this area, more than 200 water samples were collected from small streams. Ion concentrations were analyzed by ion chromatography, whereby higher ion concentrations relatively rich in Mg²⁺ and Ca²⁺ and lower concentrations relatively rich in Na⁺ were obvious for the streams in Sambagawa-Mikabu, Chichibu, and Ryoke Belts, respectively. These tendencies could be explained by whole rock analysis on XRF (X-ray fluorescence spectrometry) as well as by a preliminary leaching experiment of the powdered rock samples with water. The results strongly suggested that the analytical data of the major ions in the stream waters might be compiled into a hydrogeochemical map, corresponding to the geological map. Received: October 25, 1999 / Accepted: March 23, 2000     

Comparison of survey methods for wintering grassland birds

ABSTRACT Although investigators have evaluated the efficacy of survey methods for assessing densities of breeding birds, few comparisons have been made of survey methods for wintering birds, especially in grasslands. In winter, social behavior and spatial distributions often differ from those in the breeding season. We evaluated the degree of correspondence between density estimates based on different survey methods. Surveys were conducted during two winters (2001–2002 and 2002–2003) on 16 grassland sites in southwestern Oklahoma. Line‐transect (using a detection function to account for birds present but not detected) and area‐search (where density was based on the total count within a given area) methods were employed. Observations on line transects were also analyzed as strip transects, where density was based on total count within a given strip width and no detection function was used. Savannah Sparrows (Passerculus sandwichensis), LeConte's Sparrows (Ammodramus leconteii), Song Sparrows (Melospiza melodia), Smith's Longspurs (Calcarius pictus), Chestnut‐collared Longspurs (C. ornatus), and Eastern Meadowlarks (Sturnella magna) were sufficiently abundant to allow comparison. Area‐search density estimates tended to be higher than line‐transect estimates for Savannah Sparrows, Song Sparrows, and Eastern Meadowlarks, suggesting that some individuals initially located close to the transect line were not detected on line transects. The area‐search and line‐transect methods gave similar density estimates for Chestnut‐collared and Smith's longspurs. Area‐search estimates of Eastern Meadowlarks were significantly higher in the second year of the study only. For this species, area‐search estimates did not differ from those of strip transects covering an equal area, so the reason for the differing meadowlark estimates is not clear. Higher density estimates using the area‐search method likely resulted from: (1) birds that might escape detection by hiding were more likely detected (flushed) during area searches because of the repeated passes through the area, and (2) birds close to the line in line transects escape detection by hiding, biasing those estimates low. We also evaluated the correspondence of density rankings for the six species as determined by the different survey methods and for the same species across survey sites. Correlations among the six species of the area‐search results with those of line transects and strip transects generally were high, increasing in 2002–2003 when densities of birds were greater. All three methods provided similar density rankings among species. Density rankings within species across sites for the four non‐longspur species generally were concordant for the three methods, suggesting that any of them will adequately reflect among‐site differences, especially when densities vary greatly across sites. Further research is needed to determine the extent to which grassland birds are missed on line transects. We suggest that workers using line transects to study these species give careful consideration and make additional efforts to satisfy the distance‐sampling assumption that all birds on or near the line are detected. If density is measured as a total count in a fixed area, we recommend that observers pass within <10 m of all points in the area.