Microscale Variations Research Articles

Microscale variation in alpine grasslands: AFLPs reveal a high level of genotypic diversity in Primula minima (Primulaceae)

The microscale variation and spatial genetic structure of the alpine plant species Primula minima L. was analysed using AFLPs. AFLP analysis based on three primer combinations and 123 fragments revealed no identical genotypes among the 86 studied samples from a 300 × 300 cm plot. Variation within the study plot was high: Nei's gene diversity was 0.22, Shannon's information index 0.33 and the percentage of polymorphic bands was 60.9. Cluster analysis revealed four main groups of genetically similar individuals and mapping these individuals resulted in a clear spatial pattern, with samples from the same group often located close together. The observed microscale structure was corroborated using a Mantel test, which revealed significant correlation of genetic and spatial distances, and by the results of a spatial autocorrelation analysis that indicated a high level of similarity between adjacent samples. An analysis of molecular variance revealed clear differentiation (18%) between the spatial groups. Overall gene flow within the plot was 1.11 and ranged from 0.33 between the spatially most distant groups to 2.33 between directly neighbouring groups. The extraordinary level of diversity detected in this study indicates an unexpectedly strong relevance of reproduction by seed for the species P. minima in alpine grasslands. The strong microscale variation suggests, however, that there is limited dispersal of seeds. Clonal reproduction is of subsidiary importance to sexual reproduction and seems to occur only over very small distances. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155, 549–556.

Use of Geostatistical Analysis to Characterize the Spatial Distribution ofLobesia botrana(Lepidoptera: Tortricidae) Larvae in Northern Greece

Gaining a better understanding of the spatial variation of pests is a key to their more effective mapping to improve our knowledge of their overall population dynamics and optimize our control strategies. Geostatistical methods were used in this study to characterize spatial variability in Lobesia botrana Denis and Schiffermüller (Lepidoptera: Tortricidae) larval infestation, injury, and damage patterns in six locations over 2 yr. Regression was used to detect and separate macroscale trends from the microscale variation. The presence of macroscale variation indicated a significant edge effect with insect population moving into vineyards and ovipositing mainly along the field edges. Similarities in the patterns of spatial variability occur between the second and third generations and also between infestation, injury, and damage in all fields. The microscale variation was studied using semivariograms for all fields. Semivariograms strongly indicate that the spatial structures of L. botrana larvae were aggregated with dependency down a row of vineyards differing from that across rows of the crop, leading to anisotropy. The average range across the vine rows was consistently shorter than along rows, suggesting that migration occurs more easily down rows than across them. The anisotropic analysis further indicates that sampling programs can deploy sample locations at greater distances along the rows than across them, and interpolation algorithms that use this information may produce better map estimates. These results have implications for developing sampling plans for management of L. botrana and for site-specific agriculture.

Seasonal changes in pollinator activity influence pollen dispersal and seed production of the alpine shrub Rhododendron aureum (Ericaceae)

In alpine ecosystems, microscale variation in snowmelt timing often causes different flowering phenology of the same plant species and seasonal changes in pollinator activity. We compared the variations in insect visitation, pollen dispersal, mating patterns, and sexual reproduction of Rhododendron aureum early and late in the flowering season using five microsatellites. Insects visiting the flowers were rare early in the flowering season (mid-June), when major pollinators were bumblebee queens and flies. In contrast, frequent visitations by bumblebee workers were observed late in the season (late July). Two-generation analysis of pollen pool structure demonstrated that quality of pollen-mediated gene flow was more diverse late in the season in parallel with the high pollinator activity. The effective number of pollen donors per fruit (N(ep)) increased late in the season (N(ep) = 2.2-2.7 early, 3.4-4.4 late). However, both the outcrossing rate (t(m)) and seed-set ratio per fruit were smaller late in the season (t(m) = 0.89 and 0.71, seed-set ratio = 0.52 and 0.18, early and late in the season, respectively). In addition, biparental inbreeding occurred only late in the season. We conclude that R. aureum shows contrasting patterns of pollen movement and seed production between early and late season: in early season, seed production can be high but genetically less diverse and, during late season, be reduced, possibly due to higher inbreeding and inbreeding depression, but have greater genetic diversity. Thus, more pollinator activity does not always mean more pollen movement.

Vertical Variation of Bark Water Storage Capacity of Pinus strobus L. (Eastern White Pine) in Southern Illinois

Abstract Knowledge of the vertical variation of bark water storage capacity (BWSC) is essential in understanding the processes of throughfall and stemflow yield in forest ecosystems. Determination of vertical variation of BWSC is also necessary to understand microscale variations in moisture availability that affect the diversity and distribution of corticolous lichen and bryophyte communities on tree boles. This field note reports that BWSC of a tree bole of Pinus strobus L. (eastern white pine; 28.4 cm dbh, 22.5 m tall) differs substantially as a function of height above ground. BWSC of the bottom section of the tree bole (5.6 L m−2) was approximately twice as much as the top section (2.5 L m−2). Total BWSC for this tree's bole was 52.5 L. Further research of the interactions among bark water storage capacity, stemflow yield, and the diversity and distribution of corticolous lichens and bryophytes will improve our understanding of the canopy water balance and the functional ecology of forest ecosystems.

Effect of ultraviolet radiation on alkaline phosphatase activity and planktonic phosphorus acquisition in Canadian boreal shield lakes

We examined how ultraviolet radiation (UVR) affects the activity of alkaline phosphatase (APase), a common extracellular enzyme. APase activity declined up to 57% under UVR exposure and decreased more often under ultraviolet A than ultraviolet B exposure, indicating that most of the observed decrease did not occur through direct enzyme inactivation. Enzyme activity in the particulate fraction ( .0.22 mm) was less susceptible to UVR than activity in the total or dissolved fractions, suggesting that attachment to the cell surface may convey some protection against UVR-induced inactivation. Samples that were 0.22-mm filtered before being subjected to radiation treatments often showed increased enzyme activity, especially in the photosynthetically active radiation‐only treatment, indicating reactivation of APase in the absence of de novo production of the enzyme. Decreases in APase can be severe near the lake surface, suggesting that UVR-induced variations in APase activity might contribute to microscale variations in nutrient availability and community composition.

Microscale variations in the mating system and heterospecific incompatibility mediated by pollination competition in alpine snowbed plants

AbstractThe effects of pollination competition between related species on pollen limitation, selfing ability and heterospecific incompatibility were studied in the alpine snowbed shrubs, Phyllodoce caerulea and Phyllodoce aleutica (Ericaceae) along local snowmelt gradients. The flowering season of these species highly overlapped and they shared the same method of pollination (via bumblebees). The floral density and nectar production were higher in P. caerulea in the early snowmelt habitat, but they were higher in P. aleutica in the late snowmelt habitat, reflecting the species‐specific sensitivity to a short growing season. Bumblebee preference shifted from P. caerulea to P. aleutica along the snowmelt gradient, reflecting the changes in the relative reward level between the species. The extent of pollen limitation of P. caerulea was severe in the late snowmelt habitat, whereas that of P. aleutica was obvious in the early snowmelt habitat. A pollination experiment revealed that P. caerulea showed a consistently low selfing ability, whereas P. aleutica showed a high selfing ability in the early snowmelt habitat and shifted to an obligate outcrosser in the late snowmelt habitat. The frequency of interspecific bumblebee movement for P. caerulea from P. aleutica increased in the late snowmelt habitat. To the contrary, the frequency of interspecific movement for P. aleutica from P. caerulea decreased along the snowmelt gradient. P. caerulea did not indicate heterospecific incompatibility to P. aleutica pollen. In contrast, P. aleutica indicated heterospecific incompatibility to P. caerulea pollen, reflecting the high risk of receiving heterospecific pollen as a result of interspecific bumblebee movement. However, heterospecific compatibility of P. aleutica disappeared in the late snowmelt habitat, where the risk of heterospecific pollen receipt was low. Therefore, changes in interspecific bumblebee movement might have been responsible for the variation in heterospecific incompatibility among P. aleutica populations. The local snowmelt gradient creates diverse biological interactions and mating system evolution in the alpine ecosystem.

Moving average fields, macro-scale response measures, and homogenizing micro-scale variation

There are two critical issues when deriving a macro-scale prediction model starting from a more complete, underlying model. The first is the precise relationship of the fields predicted by the more complete model and the fields predicted by the macro-scale model. The second is the manner of solving a closure problem that is invariably encountered in all such derivations. The understanding that moving averages of the fields predicted by the more complete model are the fields predicted by the macro-scale model is challenged on the grounds that accomplishing a moving average does not eliminate micro-scale variation, it only appears to do so in one representation of the moving average field. The solution of a closure problem by assumption is challenged on the grounds that the most common assumptions are demonstrably invalid, even while leading to prediction models that can provide reasonable estimates of the macro-scale response in some scenarios. In presenting the challenges, it is further shown how a multiresolution analysis by an orthogonal wavelet system provides a framework for both precisely defining macro-scale response fields, i.e., fields from which all micro-scale variation has been eliminated, and presenting a formally exact solution for a precisely described closure problem.

Eagles as indicators of ecosystem health: Is the distribution of Martial Eagle nests in the Karoo, South Africa, influenced by variations in land-use and rangeland quality?

We examined the relationship between the distribution of Martial Eagles Polemaetus bellicosus (and other large eagle species) breeding on electricity transmission pylons in the central and southwestern Karoo, South Africa, and the general environmental health of commercially managed farmland in that region. Eighty-three eagle territories were located along 1400 km of surveyed pylon line. The influence of habitat quality on eagle distributions was assessed in terms of (i) patterns of dispersion and spacing between territories, (ii) macro-scale differences between the habitat composition of eagle territories and an equivalent array of random plots, and (iii) micro-scale differences in habitat quality assessed at points within and outside of eagle territories. Territories were generally irregularly distributed along the pylon line ( p < 0.025 ). At the landscape scale, Martial Eagles significantly favoured habitats with lower and/or less predictable winter rainfall and lower primary productivity, while Tawny Eagles Aquila rapax preferred habitats with higher and/or more predictable summer rainfall and higher primary productivity. All eagles avoided areas of highest primary productivity and were absent from cultivated areas. On a local scale, habitat qualities within eagle territories were generally not significantly different to those recorded outside territories. However, eagle densities in areas stocked with indigenous game were significantly higher than in areas supporting only domestic stock (nearest neighbour distances, 7 km vs. 12 km, p = 0.025 ). This study shows that (i) large eagle distributions in the Karoo are significantly influenced by both macro- and micro-scale variation in habitat quality, and by the nature of land management, and (ii) that the Martial Eagle is potentially suitable as an indicator of ecosystem quality and functionality in the Karoo.

Monte Carlo Maximum Likelihood in Model-Based Geostatistics

When using a model-based approach to geostatistical problems, often, due to the complexity of the models, inference relies on Markov chain Monte Carlo methods. This article focuses on the generalized linear spatial models, and demonstrates that parameter estimation and model selection using Markov chain Monte Carlo maximum likelihood is a feasible and very useful technique. A dataset of radionuclide concentrations on Rongelap Island is used to illustrate the techniques. For this dataset we demonstrate that the log-link function is not a good choice, and that there exists additional nonspatial variation which cannot be attributed to the Poisson error distribution. We also show that the interpretation of this additional variation as either micro-scale variation or measurement error has a significant impact on predictions. The techniques presented in this article would also be useful for other types of geostatistical models.

In situ transmission electron microscopy studies of directionally solidified Ni–Mn–Ga ferromagnetic shape memory alloys

Transmission electron microscopy has been used to examine the microstructure and transformation behaviour of four non-stoichiometric Ni-Mn-Ga oriented polycrystalline alloys grown by seedless Bridgman solidification. Despite similar average compositions, the observed microstructures and transformation temperatures of these alloys varied significantly. For alloy A with nominal composition Ni 50.1 Mn 28.2 Ga 21.7 (at.%), the Heusler ordered (L2 1 ) austenite phase was observed to exhibit characteristic tweed strain contrast at room temperature. On cooling, sharp satellite reflections associated with the formation of an intermediate phase appeared at ∼200 K, prior to a martensitic transformation at ∼190 K which generated a microstructure consisting predominately of seven-layer martensite with regions of five-layer martensite. For alloys B (nominal Ni 49.3 Mn 28.6 Ga 22.1 ) and C (nominal Ni 49.3 Mn 28.8 Ga 21.9 ), the martensitic transformation occurred slightly below room temperature and the TEM images revealed a fine-scale intermixture of austenite with five-layer, seven-layer and non-modulated martensite. The microstructure of alloy D (nominal Ni 49.3 Mn 28.8 Ga 21.9 ), with a transformation temperature somewhat above room temperature was composed predominately of seven-layer martensite. The results indicate that small microscale variations in composition and/or parent phase grain size have a significant effect on the transformation behaviour of directionally solidified Ni-Mn-Ga alloys.

Microscale variations of δ13C and N content within a natural diamond with mixed-habit growth

The variations of carbon isotope composition and nitrogen content in a natural diamond with mixed octahedral and cuboid habit have been studied with the ion microprobe technique (SIMS). The systematic variations of δ 13C and Nppm in continuous growth zones belonging to both octahedral and cubic sectors have been measured. In general the octahedral growth sector in comparison with the cuboid one is richer in nitrogen by 90–300 ppm and has a slightly heavier δ 13C by 0.3–1.5‰. These data show that simultaneous mixed growth of cuboid and octahedral sectors in natural diamond entails little fractionation of carbon isotopes. The observed variations are most likely due to the different nature and kinetics of growth of natural cuboid and octahedral diamonds.

A Preliminary Investigation of Ozone Concentrations on the Lizard Peninsula Cornwall, U.K. using Continuous Monitoring and Diffusion Tubes

The Lizard Peninsula is an isolated area of unique and sensitive ecology and the occurrence of damaging ozone concentrations over the peninsula in spring challenges the successful sustainable management of the area. Ozone concentrations over Goonhilly Downs on the Lizard Peninsula in Cornwall were measured in spring 2002 using continuous monitoring and spatial mapping (diffusion tubes). Data obtained using a continuous ozone monitor (1/03/02 to 31/03/02) showed that the AOT40 5-day values were in a range from 2 to 800 μg m-3 hr (AOT40 is the sum of the differences between the measured hourly ozone concentration (μg m-3) and 80 μg m-3 (when the concentration exceeds 80 μg m-3) for the hours when global radiation exceeds 50 Wm-2). Contour mapping of ozone concentrations measured with diffusion tubes suggested that over a 2 km2 sampling area significant microscale variations were occurring with ozone concentrations ranging between 48–129 μg m-3 for one two-week period and 62–210 μg m-3 for the other.Unusually high diurnal variations in ozone concentrations were also observed at Goonhilly. It is thought that temperature inversions caused the entrapment of ozone at the surface level.

Microscale variation in Aat (aspartate aminotransferase) is supported by activity differences between upper and lower shore allozymes of Littorina saxatilis

The role of natural selection in maintaining inherited variation in morphological characters is indisputable. In contrast, genetic variation in allozyme loci is mostly assumed neutral or under weak selection, although enzyme functions are critical to central metabolic processes. A locus coding for aspartate aminotransferase (AAT) is polymorphic with two alleles in the rocky shore gastropod Littorina saxatilis. One allele (Aat 120 ) is most frequent (0.8–0.9) in upper shore subpopulations, while the other (Aat 100 ) dominates (0.7–0.8) lower shore subpopulations only meters below. Earlier studies show this variation to be maintained by divergent selection, but the mechanism is unknown. We measured enzyme activities in AAT extracts from individuals of known Aat genotype in populations from two islands and at three different temperatures. In all treatments lower shore homozygotes (Aat 100/100 ) had higher enzyme activity than upper shore homozygotes (Aat 120/120 ), with heterozygotes having intermediate levels of activity. AAT has a central role in anaerobic energy production in marine mollusks during low-tide periods of emergence, and it might appear surprising that upper shore snails, which are most likely to be emerged, have lower enzyme activity. However, upper shore snails are forced to survive long periods of desiccation, and a low metabolic rate will help to conserve limited resources of energy and nutrients and reduce accumulation of waste products.

Sulfur isotope microanalysis of sphalerite by SIMS: constraints on the genesis of Mississippi valley-type mineralization, from the Mascot-Jefferson City district, East Tennessee

The Mascot-Jefferson City (MJC) district is the most productive zinc district in East Tennessee. The deposits are of Mississippi Valley-type (MVT), hosted by carbonate rocks and dominated by sphalerite mineralization in strata-bound breccia bodies. We have utilized the high spatial resolution (20–30 μm) of the ion microprobe to obtain in situ sulfur isotopic analyses from discrete growth zones of sphalerite and analyses of associated pyrite. Two types of pyrite were noted: pre-sphalerite, diagenetic pyrite ( δ 34S of −16.1‰ and −20.0‰) and syn-sphalerite pyrite that is intergrown with sphalerite ( δ 34S of 31.3‰ to 33.7‰). Two textural varieties of sphalerite mineralization (banded and non-banded) were characterized. Banded sphalerite exhibits fine (μm to cm) banding that has grown around a carbonate substrate. Banded sphalerite has δ 34S values from 27.8‰ to 51.0‰, high Cd contents (up to 0.96 wt.%) and dark areas that are likely due to minute inclusions of organic carbon. The non-banded sphalerite has δ 34S values from 20.2‰ to 39.5‰, high Fe content and no organic inclusions. Regardless of the textural variety of sphalerite mineralization, our results show that the sulfur isotopic composition within a single polished thin section is heterogeneous and can vary by as much as 15‰. The δ 34S values recorded in this study are among the heaviest ever reported for MVT deposits. The microscale δ 34S variations and presence of high δ 34S values have been previously undocumented for East Tennessee. The data presented here suggest multiple sulfur sources and sulfide precipitation by fluid mixing. The most probable scenario involves significant sulfur input from a sulfate- and metal-bearing fluid of variable δ 34S composition mixing with a gas cap containing H 2S of relatively homogeneous δ 34S composition. The gas cap provided lesser amounts of sulfur to the system. Mixing of two isotopically different sulfur sources of variable proportions can account for the observed microscale variation in δ 34S.

Hydrogeological aspects of fault zones on various scales in theRoer Valley Rift System

The impact of faults on the groundwater flow system in the Roer Valley Rift System (RVRS) is demonstrated with examples from outcrop scale to regional scale. Faults in the RVRS can form strong barriers to horizontal groundwater flow as well as enhanced vertical groundwater flow paths at the same location. The strongly anisotropic hydraulic conductivity distribution within fault zones has important implications for the modeling of groundwater flow in sedimentary aquifer systems that are cut by faults. In this study, the hydraulic behavior of fault zones is studied at different scales. An outcrop study over the Geleen Fault zone shows deformation mechanisms as particulate flow and clay smearing in great detail. Qualitative and quantitative image analysis allows for an estimate of the micro-scale variation of the hydraulic properties within a fault zone. Additional core-plug measurements indicate that the damage zone around fault zones may form preferential flow paths. On a larger scale, observations over the Peel Boundary fault near the village of Uden also indicate that vertical groundwater flow close to the fault is enhanced, which results in a discharge of the underlying aquifers at the location of the fault zone. Finally, on a regional scale, hydraulic head patterns around the lignite mining areas in Germany show the importance of faults and the variation of their hydraulic properties to regional groundwater flow patterns.

A new method for the characterisation of micro-scale stress variations of press felts

A new method for the characterisation of micro-scale stress variations of press felts

A new deep-sea probe for in situ pH measurement in the environment of hydrothermal vent biological communities

In situ measurements have been shown to be the most relevant means to characterise the chemical properties of the highly dynamic medium surrounding hydrothermal vent organisms. However, few instrumental devices can perform such measurements at great depth, and there is great need to extend the range of chemical parameters that can be determined in situ. To investigate the spatial and temporal variations of pH within the habitats of vent organisms, a deep-sea probe was developed and successfully tested during the HOPE’99 diving cruise (13°N, East Pacific Rise). In situ measurements allowed us to quantify the pH range over different biological communities, highlighting differences between the habitats, as well as micro-scale variations. As pH is expected to play a significant role in major biogeochemical processes occurring in the seawater/fluid-mixing zone, this probe should be of great utility for the study of interactions between vent communities and their environment.

The N-factor in Natural Landscapes: Variability of Air and Soil-Surface Temperatures, Kuparuk River Basin, Alaska, U.S.A.

The n-factor, or ratio of the seasonal degree-day sum at the soil surface to that in the air at standard screen height, has been used for more than 40 yr in engineering studies to parameterize the temperature regime at the ground surface. Conceptually, this index represents the complex energy balance at the surface as a single dimensionless number and has applications in ecology, climatology, and geocryology. Although the n-factor has been used theoretically to represent the thermal regime of undisturbed natural surfaces, lack of empirical data has hindered its widespread implementation. Nine ground and one air temperature series (1995–97) from each of ten 1-ha plots on the coastal plain and Brooks Range foothills of north-central Alaska were converted to thawing (summer) n-factors and analyzed to address within- and between-plot spatial and temporal variability. Although substantial microscale variation exists, n-factors corresponding to natural vegetation/soil classes are discernable. Incorporation of n-factor values in a standard solution for the depth of thaw resulted in significantly improved estimates of active-layer thickness throughout the study area. The n-factor has considerable potential for addressing problems involving near-surface climate dynamics over extensive regions and long time periods.

Spatial and Probability Distribution of Mexican Bean Beetle (Coleoptera: Coccinellidae) Egg Mass Populations in Dry Bean

The spatial pattern and probability distribution of Mexican bean beetle, Epilachna varivestis Mulsant, egg masses were studied in four dry bean, Phaseolus vulgaris L., fields near Scottsbluff, NE, during the 1994 and 1995 growing seasons. Sampling was conducted in a regular and uniform grid to guarantee coverage of the field. The experimental unit in all fields was 0.30 row-m, and five measurements were made at 0.76-m intervals. A total of 12,290 locations was sampled. Geostatistics and discrete statistics were used to describe the egg mass distribution. Regression was used to detect and separate macroscale trends from the microscale variation. The presence of macroscale variation indicated a significant edge effect with ovipositing females moving into dry bean fields from their overwintering sites. The microscale variation estimated from the estimated residuals from regressions to estimate trends was studied using semivariograms for all fields. Semivariograms strongly indicate that Mexican bean beetle eggs are randomly distributed across the field and that this random distribution holds across 10-fold differences in population densities. As a result of the significant macroscale trends observed, we conclude that egg mass densities may be greater in areas of the field near edges adjacent to beetle overwintering sites. The probability distribution that best fit the data was the negative binomial. Our results illustrate the importance of scale in discussing and characterizing distribution. Although there is evidence of edge effect at the field level, there is no evidence of spatial dependence between egg samples at the sampling region level. At an even lower level, the egg mass itself, eggs are aggregated.

Environmental and social factors influence chorusing behaviour in a tropical frog: examining various temporal and spatial scales

Many animals use conspicuous display to attract mates, and there should be selection for displays to occur at times and places that maximise the probability of mating, while minimising energetic costs and predator attraction. To select the best times for display, individuals may use environmental cues, the presence of other individuals, or both, but few studies have examined these sources of variation in display activity. In this study, we examined physical environmental and social factors triggering displays in a tropical, terrestrially breeding frog, Cophixalus ornatus. To measure the influence of physical environmental conditions on calling activity, we recorded temperature, rainfall, moon illumination/visibility, humidity, barometric pressure and intensity of calling activity throughout a breeding season at six locations along a 560-m transect. The intensity of calling varied daily, seasonally, and at a small spatial scale. Variation in calling activity from day to day was large. There was also a strong seasonal trend in calling activity: few males called at the start of the season, activity peaked shortly after the beginning of the season, and then declined linearly from the peak to the end of the season. There was also consistent variation among sites along the transect, which may have been due to variations in frog density at each site, or to consistent microscale variations in physical conditions, or both. After statistically removing consistent local variation among sites, a principal components analysis suggested that a maximum of 35.8% of the variation in calling activity among days was due to factors common to all sites, such as weather, moon illumination, or large-scale social facilitation (e.g. of choruses by other choruses). The remainder of the variation among sites (64.2%) was due to site-specific factors, such as small-scale social facilitation or unmeasured, apparently stochastic effects, such as microenvironmental physical factors that do not vary consistently over sites. Regressions of environmental variables on residual calling activity (after removing consistent effects of site and season), alone or in combination, accounted for very little of the variation in the number of calling males (maximum 10%). Thus, our data, showing strong seasonal effects and consistent variation among sites combined with large amounts of variation in the number of calling males at small spatial scales, suggest that environmental conditions, such as temperature, rainfall, moon illumination and barometric pressure, which act over large spatial scales, may determine the overall environmental envelope within which calling can occur but do not account for most of the variation in the number of calling males on a day-to-day or site-to-site basis. Similarly, variations in the number of calling males at small spatial scales suggest that social facilitation is a relatively unimportant trigger for displays on a large scale in these frogs. On the other hand, our data suggest that social facilitation may have important effects on variation in the number of calling males on a day-to-day and site-to-site basis. We used playback experiments to assess whether the sound of calling could initiate displays. We played either a taped chorus or white noise in areas where few (zero to two) males were calling. The number of calling males increased both during and after the chorus stimulus, whereas there was no increase in calling in response to white noise. These data suggest that examining variation in calling activity at small spatial scales can reveal the sources of variation for the number of calling males, and indicate that, in these frogs, males tend to use the calling of other individuals as a cue to determine when to display.