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Small-scale Variations (small-scale + variation)
Selected AbstractsSmall-scale variation in growing season length affects size structure of scarlet monkeyflowerOIKOS, Issue 1 2004Jennifer L. Williams Growing season length can control plant size over altitudinal and biogeographic scales, but its effect at the scale of meters is largely unexplored. Within the riparian zone of a northern California river, scarlet monkeyflower, Mimulus cardinalis, grows significantly larger at sites high in the channel as compared to sites low in the channel, and even larger where tributaries meet the main stem of the river. We explored the hypothesis that markedly different growing season length controls this size variation. Due to the very gradual retreat of the water level following winter flooding, emergence time is three months longer for plants growing at tributary confluences than for plants growing at low elevations in the channel. Consistent with the growing season length hypothesis, we found no difference in transplant growth between river and tributary confluence sites in an experiment where we equalized growing season length at these locations. Moreover, a second experiment showed that individuals planted earlier in the year gain a distinct size advantage over those planted later, even though growing conditions are less ideal. These results suggest that emergence time may be a key determinant of plant size structure along rivers, an important result considering forecasted variation in water flows with climate change. [source] Small-scale variability in surface moisture on a fine-grained beach: implications for modeling aeolian transportEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2009Brandon L. Edwards Abstract Small-scale variations in surface moisture content were measured on a fine-grained beach using a Delta-T Theta probe. The resulting data set was used to examine the implications of small-scale variability for estimating aeolian transport potential. Surface moisture measurements were collected on a 40 cm × 40 cm grid at 10 cm intervals, providing a total of 25 measurements for each grid data set. A total of 44 grid data sets were obtained from a representative set of beach sub-environments. Measured moisture contents ranged from about 0% (dry) to 25% (saturated), by weight. The moisture content range within a grid data set was found to vary from less than 1% to almost 15%. The magnitude of within-grid variability varied consistently with the mean moisture content of the grid sets, following an approximately normal distribution. Both very wet and very dry grid data sets exhibited little internal variability in moisture content, while intermediate moisture contents were associated with higher levels of variability. Thus, at intermediate moisture contents it was apparent that some portions of the beach surface could be dry enough to allow aeolian transport (i.e. moisture content is below the critical threshold), while adjacent portions are too wet for transport to occur. To examine the implications of this finding, cumulative distribution functions were calculated to model the relative proportions of beach surface area expected to be above or below specified threshold moisture levels (4%, 7%, and 14%). It was found that the implicit inclusion of small-scale variability in surface moisture levels typically resulted in changes of less than 1% in the beach area available for transport, suggesting that this parameter can be ignored at larger spatial scales. Copyright © 2009 John Wiley & Sons, Ltd. [source] Mammal community structure in relation to disturbance and resource gradients in southern AfricaAFRICAN JOURNAL OF ECOLOGY, Issue 1 2009Märtha Wallgren Abstract The arrangements of mammals differ along environmental gradients, such as of disturbance and resources. We examined how mammal community composition in the Kalahari, Botswana, varied in relation to disturbance and resource gradients. We predicted that livestock-keeping villages are disturbances and pans are resources for wildlife, that the responses of mammals to disturbance and resources depend on their functional types and that increased disturbance over time has reduced the numbers and distributions of wildlife. The methods involved road-side counts of mammals >0.2 kg and trapping of small mammals, <0.2 kg. The disturbance gradient was more important than the resource gradient for explaining the distribution of mammals >0.2 kg. Communities in low disturbance-high resource areas were most diverse regarding species and functional types. Small mammal species richness and abundances were unaffected by villages, but increased with distance from pans. Villages were particularly deterring to large wild herbivores, functionally similar to livestock. Most large wild herbivores had decreased since 1975,1983. We conclude that large and medium-sized mammals are highly affected by large-scale disturbance gradients, while small mammals are most dependent on small-scale variation in resources, probably shelter and food. Increased disturbance over time leads to decreasing ranges and numbers of the large wild herbivores. Résumé Les associations de mammifères diffèrent selon des gradients environnementaux tels que ceux des perturbations et des ressources. Nous avons examiné comment la composition de la communauté de mammifères variait dans le Kalahari, au Botswana, en fonction des gradients de perturbations et de ressources. Nous avions prédit que les villages qui gardent du bétail sont des perturbations, et les pans des ressources pour la faune sauvage, que les réponses des mammifères à ces deux facteurs dépendaient de leur type de fonctionnement, et que les perturbations croissant avec le temps avaient réduit l'abondance et la distribution de la faune sauvage. Les méthodes employées comprenaient les comptages le long de la route de tous les mammifères de plus de 0,2 kg et le piégeage des petits mammifères de moins de 0,2 kg. Le gradient de perturbation était plus important que celui des ressources pour expliquer la distribution des mammifères >0,2 kg. Les communautés vivant dans les zones de faibles perturbations et d'abondantes ressources étaient plus diverses au point de vue du nombre d'espèces et de types fonctionnels. La richesse en espèces de petits mammifères et leur abondance n'étaient pas affectées par les villages mais augmentaient avec la distance par rapport au pan. Les villages étaient particulièrement dissuasifs pour les grands herbivores sauvages qui étaient, fonctionnellement, semblables au bétail. La plupart des grands herbivores sauvages ont diminué depuis 1975,1983. Nous concluons que les herbivores grands et moyens sont fort affectés par les gradients de perturbations à grande échelle alors que les petits mammifères sont surtout dépendants des variations à petite échelle des ressources, probablement les abris et la nourriture. Les perturbations croissant avec le temps entraînent une réduction des domaines vitaux et du nombre des grands herbivores sauvages. [source] Grazing effect on diversity of annual plant communities in a semi-arid rangeland: interactions with small-scale spatial and temporal variation in primary productivityJOURNAL OF ECOLOGY, Issue 6 2002Yagil Osem Summary 1The interactive effect of grazing and small-scale variation in primary productivity on the diversity of an annual plant community was studied in a semiarid Mediterranean rangeland in Israel over 4 years. The response of the community to protection from sheep grazing by fenced exclosures was compared in four neighbouring topographic sites (south- and north-facing slopes, hilltop and wadi (dry stream) shoulders), differing in vegetation, physical characteristics and soil resources. The herbaceous annual vegetation was highly diverse, including 128 species. Average small-scale species richness of annuals ranged between 5 and 16 species within a 20 × 20 cm quadrat, and was strongly affected by year and site. 2Above-ground potential productivity at peak season (i.e. in fenced subplots) was typical of semiarid ecosystems (10,200 g m,2), except on wadi shoulders (up to 700 g m,2), where it reached the range of subhumid grassland ecosystems. Grazing increased richness in the high productivity site (i.e. wadi), but did not affect, or reduced, it in the low productivity sites (south- and north-facing slopes, hilltop). Under grazing, species richness was positively and linearly related to potential productivity along the whole range of productivity. Without grazing, this relationship was observed only at low productivity (< 200 g m,2). 3The effect of grazing along the productivity gradient on different components of richness was analysed. At low productivity, number of abundant, common and rare species all tended to increase with productivity, both with and without grazing. Rare species increased three times compared with common and abundant species. At high productivity, only rare species continued to increase with productivity under grazing, while in the absence of grazing species number in the different abundance groups was not related to productivity. 4In this semiarid Mediterranean rangeland, diversity of the annual plant community is determined by the interaction between grazing and small-scale spatial and temporal variation in primary productivity, operating mainly on the less abundant species in the community. [source] Species Composition of Neotropical Understory Bird Communities: Local Versus Regional Perspectives Based on Capture DataBIOTROPICA, Issue 1 2009John G. Blake ABSTRACT Species richness and composition of bird assemblages found in the understory of undisturbed Neotropical forests vary at local and regional scales but the extent of that variation has not been well documented. Yet, such variation can be important for understanding patterns of diversity and for conservation. Here, we use capture data from two ca 100-ha study plots (sampled from March 2001 through March 2005) to compare understory assemblages at a local scale; nets on the plots were separated by approximately 1.7 km at the closest point. A total of 157 species (133 per plot) was represented in 6023 captures of 4001 individuals. After eliminating species not likely to be well-sampled with nets, there were 122 species total with 110 and 113 on the two plots, respectively. Species-accumulation curves and abundance-rank relationships were almost identical on both plots. Capture rates were high (53 and 56 birds captured per 100 mist-net-hours, mnh) on both plots. Distributions of species and individuals among families and genera were similar on the two plots but numbers of captures of some common species differed between plots in response to small-scale variation in environmental features. Indicator-species analyses selected seven species as more characteristic of one plot and nine as more characteristic of the other. At the regional level, understory assemblages were most similar to a site in Peru but differed from sites in Brazil, Venezuela, and, especially, Costa Rica. Dissimilarity in species richness per family was related to geographic distance only when Costa Rica was included in the analysis. RESUMEN La riqueza de especies y la composición de ensambles de aves de sotobosque en bosques neotropicales varían a escala local y regional, pero el grado de variación no ha sido bien documentado. Sin embargo, esta variación puede ser importante para entender los patrones de diversidad y para conservación. En este trabajo usamos datos de captura de dos parcelas de ca 100-ha (muestreados entre Marzo 2001 y Marzo 2005) para comparar los ensambles de aves de sotobosque a escala local; las redes en las dos parcelas estuvieron ubicadas a una distancia mínima aproximada de 1.7 km. Un total de 157 especies (133 por parcela) estuvieron representadas por 6023 capturas correspondientes a 4001 individuos. Una vez eliminadas aquellas especies que no son factibles de ser muestreadas adecuadamente con redes, el total de especies fue de 122, con 110 y 113 en cada una de los parcelas, respectivamente. Las curvas de acumulación de especies y las relaciones abundancia-rango fueron casi idénticas en ambas parcelas. Las tasas de captura fueron altas (53 y 56 aves capturadas por 100 horas red). La distribución de especies e individuos entre familias y géneros fueron similares en las dos parcelas, pero los números de captura de algunas especies comunes difirieron entre las parcelas en respuesta a la variación de características ambientales a pequeña escala. Los análisis de especies indicadoras seleccionaron siete especies como las más características de una de las parcelas y nueve como las más características de la otra. A nivel regional los ensambles de aves de sotobosque fueron más similares a aquellos presentes en un sitio en Perú, pero fueron diferentes a los presentes en sitios de Brasil, Venezuela y especialmente Costa Rica. La diferencia en riqueza de especies por familia estuvo relacionada con la distancia geográfica sólo cuando Costa Rica fue incluida en el análisis. [source] A spatially explicit study of prey,predator interactions in larval fish: assessing the influence of food and predator abundance on larval growth and survivalFISHERIES OCEANOGRAPHY, Issue 1 2003P. Pepin Abstract We apply a coupled biophysical model to reconstruct the environmental history of larval radiated shanny in Conception Bay, Newfoundland. Data on the larvae, their prey and predators were collected during a 2-week period. Our goal was to determine whether environmentally explicit information could be used to infer the characteristics of individual larvae that are most likely to survive. Backward drift reconstruction was used to assess the influence of variations in the feeding environment on changes in the growth rates of individual larvae. Forward drift projections were used to assess the impact of predators on mortality rates as well as the cumulative density distribution of growth rates in the population of larvae in different areas of the bay. There was relatively little influence of current feeding conditions on increment widths. Patterns of selective mortality indicate that fast-growing individuals suffered higher mortality rates, suggesting they were growing into a predator's prey field. However, the mortality rates appeared to increase with decreasing predator abundance, based on the drift reconstructions. The relationship of growth and mortality with environmental conditions suggests that short-term, small-scale variations in environmental history may be difficult to describe accurately in this relatively small system (,1000 km2). [source] Seasonal and inter-stream variations in the population dynamics, growth and secondary production of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) in monsoonal Hong KongFRESHWATER BIOLOGY, Issue 9 2009GRACE Y. YANG Summary 1.,Balitorid loaches are widespread and highly diverse in Asian streams, yet their life history and ecology have received little attention. We investigated seasonal (wet versus dry season) and spatial variation in populations of algivorous Pseudogastromyzon myersi in Hong Kong, and estimated the magnitude of secondary production by this fish in pools in four streams (two shaded and two unshaded) over a 15-month period. 2.,Mean population densities of P. myersi ranged from 6.0 to 23.2 individuals m,2, constituting more than half (and typically >70%) of benthic fishes censused. Abundance was c. 25% greater in the wet season, when recruitment occurred. Significant density differences among streams were not related to shading conditions and were evident despite small-scale variations in P. myersi abundance among pools. Mean biomass varied among streams from 0.85 to 3.87 g ash-free dry weight (AFDW) m,2. Spatial and seasonal patterns in biomass and density were similar, apart from some minor disparities attributable to differences in mean body size among populations. 3.,All four P. myersi populations bred once a year in June and July, and life spans varied from 24 to 26 months. Populations consisted of three cohorts immediately after recruitment but, for most of the study period, only two cohorts were evident. Cohort-specific growth rates did not differ significantly among streams but, in all streams, younger cohorts had higher cohort-specific growth rates. 4.,Secondary production of P. myersi estimated by the size-frequency (SF) method was 2.7,11.5 g AFDW m,2 year,1 and almost twice that calculated by the increment-summation (IS) method (1.2,6.6 g AFDW m,2 year,1). Annual P/B ratios were 1.17,2.16 year,1 (IS) and 2.73,3.22 year,1 (SF). Highest production was recorded in an unshaded stream and the lowest in a shaded stream, but site rankings by production did not otherwise match shading conditions. Wet-season production was six times greater than dry-season production, and daily production fell to almost zero during January and February. Cool temperatures (<17 °C) may have limited fish activity and influenced detectability during some dry-season censuses. Estimates of abundance and annual production by P. myersi are therefore conservative. 5.,Comparisons with the literature indicate that the abundance and production of P. myersi in Hong Kong was high relative to other benthic fishes in tropical Asia, or their temperate counterparts in small streams. Manipulative experiments are needed to determine the influence of P. myersi, and algivorous balitorids in general, on periphyton dynamics and energy flow in Asian streams. [source] A physical, mechanistic and fully coupled hillslope hydrology modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 11 2005David A. Raff Abstract We present the mathematical development and numerical solution of a new model of flow processes on an infiltrating hillslope. We also present validation and sample applications. The model is a distributed, mechanistic, physically based hillslope hydrologic model. The model describes the small-scale processes associated with overland flow, erosion, and sediment transport on an infiltrating surface and is capable of capturing small-scale variations in flow depth, flow velocities, interactive infiltration, erosion rates, and sediment transport. The model couples the fully two-dimensional hydrodynamic equations for overland flow, the one-dimensional Richards equation for infiltration, and a sediment detachment and transport model. Two simulations are presented highlighting the model's ability to capture and describe the interaction between precipitation, overland flow, erosion and infiltration at very small scales. Results of the two-dimensional simulations indicate the system of equations produces hillslopes possessing characteristics of self-organization as observed in real world systems. Copyright © 2005 John Wiley & Sons, Ltd. [source] The influence of multi-scale environmental variables on the distribution of terricolous lichens in a fog desertJOURNAL OF VEGETATION SCIENCE, Issue 6 2006Jennifer S. Lalley Abstract Question: How do environmental variables in a hyper-arid fog desert influence the distribution patterns of terricolous lichens on both macro- and micro-scales? Location: Namib Desert, Namibia. Methods: Sites with varying lichen species cover were sampled for environmental variables on a macro-scale (elevation, slope degree, aspect, proximity to river channels, and fog deposition) and on a micro-scale (soil structure and chemistry). Macro-scale and micro-scale variables were analysed separately for associations with lichen species cover using constrained ordination (DCCA) and unconstrained ordination (DCA). Explanatory variables that dominated the first two axes of the constrained ordinations were tested against a lichen cover gradient. Results: Elevation and proximity to river channels were the most significant drivers of lichen species cover in the macro-scale DCCA, but results of the DCA suggest that a considerable percentage of variation in lichen species cover is unexplained by these variables. On a micro-scale, sediment particle size explained a majority of lichen community variations, followed by soil pH. When both macro and micro-scale variables were tested along a lichen cover gradient, soil pH was the only variable to show a significant relationship to lichen cover. Conclusion: The findings suggest that landscape variables contribute to variations in lichen species cover, but that stronger links occur between lichen growth and small-scale variations in soil characteristics, supporting the need for multi-scale approaches in the management of threatened biological soil crust communities and related ecosystem functions. [source] | |||||||||||||