La dinámica espaciotemporal de una población multiespecífica de lombrices en un rodal de roble.

We studied the spatial dynamics of endogeic earthworms in a Quercus robur forest. Samples were taken using a 30 point grid every three months during a year plus a summer. Growth rates were estimated for each point based on abundance data. Abundances appeared to be correlated in the direction NW-SE showing a net movement of the organisms to the SE region of the studied field. The migration speed of earthworms was 1.2 m month-1. We fitted the spatial dynamics to several growth mathematical models; the most accurate models were the logistic model of Smith, that is a model with physiological constraints, and the non-symmetric logistic equation, that considers inter-specific competition. The models revealed a significant non-hierarchic diffuse competition and a low rate of individual resource consumption. Moreover, growth rates were high at low population densities but remained roughly constant at densities above 20 individuals m-2.Se estudió la dinámica de una población multiespecífica de lombrices endogeas en un rodal de robles (Quercus robur) tomando muestras en pequeños parches sobre un mosaico. Se tomaron muestras con reposición en una grilla de 30 puntos equidistantes. La frecuencia de muestreo fue estacional (cada tres meses) durante cinco estaciones. A partir de los datos de abundancias se estimaron las tasas de crecimiento en cada punto y se correlacionaron con las abundancias en el mismo punto y en los puntos vecinos para detectar la dirección de máxima correlación que resultó ser NO-SE. La velocidad migratoria media de las lombrices en esa dirección fue de 1.2 m mes-1. Se pusieron a prueba cuatro modelos de dinámica de abundancias; dos de ellos, el modelo logístico de Smith (que incorpora los diferentes requerimientos energéticos según el tamaño individual) y la logística asimétrica (que considera competencia intraespecífica con variación no lineal con la densidad) mostraron un ajuste significativo a los datos, revelando un predominio de competencia difusa no jerárquica, una baja tasa de consumo de recursos por individuo y una tasa de crecimiento muy alta a densidades pequeñas y casi constante a densidades por encima de los 20 individuos m-2

Relación entre la abundancia de distintas especies de lombrices de tierra (Annelids, Oligochaeta) y algunos parámetros fisicoquímicos en un suelo típico de la estepa pampeana

Se tomaron muestras de lombrices de tierra en 14 fases de un suelo argiudol típico sobre una superficie total de 250 has. Se midieron 13 parámetros fisicoquímicos y se estudió su relación con la presencia de diferentes especies de lombrices. Los factores que resultaron más útiles en la predicción de las especies de lombrices presentes fueron el pH, el contenido de materia orgánica, el contenido de fósforo asimilable, el contenido de nitrógeno y la resistencia eléctrica de la pasta. De las siete especies halladas, las de los géneros Octolasion y Microscolex mostraron preferencia por los suelos con alto contenido de fósforo asimilable. Todas las especies encontradas son comunes en ambientes sometidos a erturbaciones agrícolas. Aporrectodea caliginosa aparenta ser la especie pionera. La única especie autóctona fue Microscolex dubius.Sampling of earthworms was carried out in a 250 ha field encompassing 14 soil phases belonging to the argiudol type soil. The relation between earthworm species composition and 13 physicochemical parameters was studied. The most important accounting for the presence and abundance of the dfferent endogeic mesohumic earthworms species were pH, soil organic content, assimilable phosphorus, nitrogen, and electric resistance of the paste. Octolasion and Microscolex were the most restricted to high contents of phosphorus and organic matter genera. All species found in the area are common in environments subjected to agricultural disturbance. Aporrectodea caliginosa seems to be the pioneer species. All the earthworms identified in this study were exotic species except Microscolex dubius

Population dynamics of two amphipod species and its relationship with the aquatic vegetation in a Luján river basin microenvironment (Argentina)

Se estudió la dinámica anual de las poblaciones de dos especies de anfípodos en un microambiente de agua dulce del arroyo Las Flores, afluente del río Luján. Hyalella (Mesohyallela) curvispina es un anfípodo herbívoro que se alimenta principalmente de las algas del fitobentos; Hyalella (Mesohyalella) sp. utiliza como refugio a las plantas macrófitas acuáticas y es un carnívoro depredador de Hyalella (M.) curvispina. Las comunidades vegetales del arroyo siguen una dinámica escalonada a lo largo del año, con un máximo de perifiton en junio, de fitobentos en octubre y de macrófitas entre noviembre y febrero. H. (M.) curvispina tiene su mayor densidad (1267 indi- viduos/m2) y su producción secundaria más alta (2225 mg.m-2.mes-1) en coincidencia con el pico de fitobentos de octubre, con cuya abundancia se correlaciona positivamente. Hyalella (Mesohyalella) sp. presenta su abundancia máxima (7067 individuos/m2) y su mayor producción secundaria (1018 g.m-2.mes-1) en enero; ambas cantidades se correlacionan positivamente con la biomasa de macrófitas en el ambiente.The dynamics of two amphi- pod species in a freshwater micro-environment was studied. Samples were taken monthly, during a whole year, from Las Flores stream, a Luján river tributary. Hyalella (Mesohyalella) curvispina is a herbivore and feeds mainly on phytobenthos. Hyalella (Mesohyalella) sp. is a H. (M.) curvispina predator and uses macrophytes as a refuge. Periphyton algal biomass reaches its maximum during June, the phytobenthos biomass reaches its maximum during October, and macrophyte biomass reaches its maximum between November and February. H. (M.) curvispina shows its maximum density (1267 individuals/m2) and its highest secondary production (2225 mg.m-2.month-1) in October coinciding with the phytobenthos. peak in biomass; its density correlates positively with phytobenthos abundance. H. (M.) sp. density (7067 individuals/m2) and secondary production (1018 mg.m-2.month-1) both peak in January. H. (M.) curvispina density is positively correlated with phytobenthos biomass. H. (M.) sp. density, biomass and secondary production are all positively correlated with macrophyte biomass

Effects of macroalgae loss in an Antarctic marine food web: applying extinction thresholds to food web studies

Antarctica is seriously affected by climate change, particularly at the Western Antarctic Peninsula (WAP) where a rapid regional warming is observed. Potter Cove is a WAP fjord at Shetland Islands that constitutes a biodiversity hotspot where over the last years, Potter Cove annual air temperatures averages increased by 0.66 °C, coastal glaciers declined, and suspended particulate matter increased due to ice melting. Macroalgae are the main energy source for all consumers and detritivores of Potter Cove. Some effects of climate change favor pioneer macroalgae species that exploit new ice-free areas and can also decline rates of photosynthesis and intensify competition between species due to the increase of suspended particulate matter. In this study, we evaluated possible consequences of climate change at Potter Cove food web by simulating the extinction of macroalgae and detritus using a topological approach with thresholds of extinction. Thresholds represent the minimum number of incoming links necessary for species’ survival. When we simulated the extinctions of macroalgae species at random, a threshold of extinction beyond 50% was necessary to obtain a significant number of secondary extinctions, while with a 75% threshold a real collapse of the food web occurred. Our results indicate that Potter Cove food web is relative robust to macroalgae extinction. This is dramatically different from what has been found in other food webs, where the reduction of 10% in prey intake caused a disproportionate increase of secondary extinctions. Robustness of the Potter Cove food web was mediated by omnivory and redundancy, which had an important relevance in this food web. When we eliminated larger-biomass species more secondary extinctions occurred, a similar response was observed when more connected species were deleted, yet there was no correlation between species of larger-biomass and high-degree. This similarity could be explained because both criteria involved key species that produced an emerging effect on the food web. In this way, large-biomass and high-degree species could be acting as source for species with few trophic interactions or low redundancy. Based on this work, we expect the Potter Cove food web to be robust to changes in macroalgae species caused by climate change until a high threshold of stress is reached, and then negative effects are expected to spread through the entire food web leading to its collapse

Multifractal Spatial Patterns and Diversity in an Ecological Succession

We analyzed the relationship between biodiversity and spatial biomass heterogeneity along an ecological succession developed in the laboratory. Periphyton (attached microalgae) biomass spatial patterns at several successional stages were obtained using digital image analysis and at the same time we estimated the species composition and abundance. We show that the spatial pattern was self-similar and as the community developed in an homogeneous environment the pattern is self-organized. To characterize it we estimated the multifractal spectrum of generalized dimensions Dq. Using Dq we analyze the existence of cycles of heterogeneity during succession and the use of the information dimension D1 as an index of successional stage. We did not find cycles but the values of D1 showed an increasing trend as the succession developed and the biomass was higher. D1 was also negatively correlated with Shannon's diversity. Several studies have found this relationship in different ecosystems but here we prove that the community self-organizes and generates its own spatial heterogeneity influencing diversity. If this is confirmed with more experimental and theoretical evidence D1 could be used as an index, easily calculated from remote sensing data, to detect high or low diversity areas

Analysis of trends and sudden changes in environmental long-term data from King George Island (Antarctica): Relationships between global climatic oscillations and local system response

A long-term environmental data set from King George Island, Western Antarctic Peninsula, was analysed using statistical approaches that can deal with missing values. Since the WAP region belongs to the most rapidly warming regions on earth it is vital to find methods that take into account the incompleteness of Antarctic time series, but are still applicable to analyze general patterns in the series to arrive to conclusions on long term trends and inter-annual variability in the face of the rapidly changing climate in Western Antarctica. We applied U statistics after Pettit and Buishand to detect sudden changes over time, a dynamic factor analysis as well as additive modelling to detect common trends and patterns related to climatic cycles such as the Southern Annular Mode and El Niño Southern Oscillation. Our results not only reveal sudden changes for sea surface temperature and salinity, but also clear patterns in all investigated variables (sea surface temperature, salinity, suspended particulate matter and Chlorophyll a) that can be directly be assigned to climatic cycles. Hence, our statistical analyses may prove valuable for other polar environmental data sets and contribute to a better understanding of the regional variability of climate change and its impact on coastal systems

Architecture of marine food webs: To be or not be a 'small-world'.

The search for general properties in network structure has been a central issue for food web studies in recent years. One such property is the small-world topology that combines a high clustering and a small distance between nodes of the network. This property may increase food web resilience but make them more sensitive to the extinction of connected species. Food web theory has been developed principally from freshwater and terrestrial ecosystems, largely omitting marine habitats. If theory needs to be modified to accommodate observations from marine ecosystems, based on major differences in several topological characteristics is still on debate. Here we investigated if the small-world topology is a common structural pattern in marine food webs. We developed a novel, simple and statistically rigorous method to examine the largest set of complex marine food webs to date. More than half of the analyzed marine networks exhibited a similar or lower characteristic path length than the random expectation, whereas 39% of the webs presented a significantly higher clustering than its random counterpart. Our method proved that 5 out of 28 networks fulfilled both features of the small-world topology: short path length and high clustering. This work represents the first rigorous analysis of the small-world topology and its associated features in high-quality marine networks. We conclude that such topology is a structural pattern that is not maximized in marine food webs; thus it is probably not an effective model to study robustness, stability and feasibility of marine ecosystems