Víctor H. Marín

The ecology of Egeria densa Planchón (Liliopsida: Alismatales): A wetland ecosystem engineer?

Egeria densa Planchon es considerada una especie invasora en los sistemas acuaticos de Chile. Su distribucion geografica original estaba limitada a regiones subtropicales de Brasil, Argentina y Uruguay. Su crecimiento rapido y su tendencia a formar densas coberturas suelen estar asociadas con el estado de aguas claras en ecosistemas acuaticos continentales de aguas someras. Su dominancia sugiere que E. densa puede comportarse como una especie ingeniera, previniendo la resuspension de sedimentos y controlando el crecimiento del fitoplancton a traves de la utilizacion de los nutrientes. Sin embargo, al mismo tiempo, como especie invasora produce efectos no deseados tales como: (1) no permitir el crecimiento de otras macrofitas nativas, (2) disminuir la diversidad en el banco de semillas, y (3) interferir con la navegacion y generacion de electricidad por turbina. Dada su importancia en el reciente (2004) cambio de estado del humedal del Rio Cruces, nos ha parecido oportuno realizar una recopilacion de articulos cientificos disponibles en la literatura sobre esta macrofita. Concluimos con una discusion sobre el rol de E. densa como especie ingeniera; a la vez que indicamos que la especie tiene ciertas vulnerabilidades y que no cumple siempre con la idea que es una especie invasora muy competitiva.

Toward Conceptual Cohesiveness: a Historical Analysis of the Theory and Utility of Ecological Boundaries and Transition Zones

Ecological transition zones are increasingly recognized as systems that play a critical role in controlling or modifying flows of organisms, materials, and energy across landscapes. Many concepts describing transitional areas have been proposed over the years, such as the prevalent and durable ecotone concept. Confusion among ecologists and land managers about transition zone concepts and the isolation of studies that use only one transition concept can hinder unified progress in understanding these key systems. Currently, a movement toward conceptual synthesis under the umbrella concept of ‘ecological boundary’ is underway. Here we examine the history and theoretical baggage of the ecotone, riparian zone, and several other concepts. Subsequently, we present a conceptual cluster analysis, which facilitates a better understanding of the similarities and differences between boundary and transition concepts. We emphasize the hierarchical nature of these concepts: higher-level synthetic concepts can be used in the development of theory, whereas lower-level concepts allow more specificity and the formulation of operational definitions. Finally, we look briefly at the utility and future use of boundary and transition zone concepts.

Upwelling shadows at Mejillones Bay (northern Chilean coast): a remote sensing in situ analysis

Upwelling shadows are high temperature regions within coastal upwelling systems. In this article its dynamics is described at Mejillones Bay, in the northern Chilean coast, using satellite images and in situ current data. A conceptual model relating the generation and breakdown of the shadow to the bifurcated upwelling occurring outside the bay is proposed.

Effects of sampling completeness on the structure of plant—pollinator networks

Plant-animal interaction networks provide important information on community organization. One of the most critical assumptions of network analysis is that the observed interaction patterns constitute an adequate sample of the set of interactions present in plant-animal communities. In spite of its importance, few studies have evaluated this assumption, and in consequence, there is no consensus on the sensitivity of network metrics to sampling methodological shortcomings. In this study we examined how variation in sampling completeness influences the estimation of six network metrics frequently used in the literature (connectance, nestedness, modularity, robustness to species loss, path length, and centralization). We analyzed data of 186 flowering plants and 336 pollinator species in 10 networks from a forest-fragmented system in central Chile. Using species-based accumulation curves, we estimated the deviation of network metrics in undersampled communities with respect to exhaustively sampled communities and the effect of network size and sampling evenness on network metrics. Our results indicate that: (1) most metrics were affected by sampling completeness but differed in their sensitivity to sampling effort; (2) nestedness, modularity, and robustness to species loss were less influenced by insufficient sampling than connectance, path length, and centralization; (3) robustness was mildly influenced by sampling evenness. These results caution studies that summarize information from databases with high, or unknown, heterogeneity in sampling effort per species and should stimulate researchers to report sampling intensity to standardize its effects in the search for broad patterns in plant-pollinator networks.

Does human-induced habitat transformation modify pollinator-mediated selection? A case study in Viola portalesia (Violaceae)

Pollinator-mediated selection is one of the most important factors driving adaptation in flowering plants. However, as ecological conditions change through habitat loss and fragmentation, the interactions among species may evolve in new and unexpected directions. Human-induced environmental variation is likely to affect selection regimes, but as yet no empirical examples have been reported. In the study reported here, we examined the influence of human-induced habitat transformation on the composition of pollinator assemblages and, hence, pollinator-mediated selection on the flower phenotype of Viola portalesia (Violaceae). Our results indicate that pollinator assemblages differed substantially in terms of species composition and visitation rate between nearby native and transformed habitats. Similarly, the insect species that contributed most to visitation rates differed between plant populations. While the magnitude and sign of pollinator-mediated selection on flower length and width did not differ between sites, selection for flower number lost significance in the transformed habitat, and a significant pattern of disruptive selection for flower shape, undetected in the native habitat, was present in the transformed one. Overall, the results of this study suggest that human-induced habitat change may not only modify the species composition of pollinator assemblages, relaxing the selection process on some flower characters, but they may also create new opportunities for fitness-trait covariation not present in pristine conditions.

The influence of coastal upwelling on the distribution of Calanus chilensis in the Mejillones Peninsula (northern Chile): implications for its population dynamics

A field experiment was carried out in October 1998 during active upwelling in a coastal area off the Mejillones Peninsula (23°S). Zooplankton was sampled at day and night, during two subsequent days at 4 stations inside and outside of the upwelling plume. Three depth strata were sampled: 0–20 m, 20–80 m and 80–200 m. Oceanographic data were obtained in a grid of 23 stations using a CTDO, a fluorometer and a Doppler current meter. Calanus chilensis was mostly represented by late stages, i.e. copepodid C5 and adult males and females. There were no day/night effects on vertical distribution, and abundance was significantly higher inside the upwelling plume in the upper 20-m layer at nearly 14 ind. m−3, compared to ca. 5 ind. m−3 outside the upwelling plume. Temperature at 10 m depth and biomass, estimated from stage numbers and their mean dry weights, were used to estimate growth and daily production of Calanus at temperature-dependent rates. The potential loss of biomass from the upwelling center because of advection in the upwelling plume was estimated from current data in the Ekman layer and biomass density. The mean cross-shelf component of the current was estimated at 10.4km d−1 within the upwelling plume. This yielded a loss of biomass of 9.7 mg dry weight m−2. Production, estimated by a temperature-dependent approach, ranged between 44 and 35 mg dry weight m−2 d−1, at mean temperatures of 14.6°C and 15.8°C inside and outside of the upwelling plume respectively. Within the plume, as much as 22% of daily production may be advected offshore. However, a higher concentration of biomass in the upwelling plume allowed a greater production compared to surrounding areas. A mass balance approach suggests that advective losses may not have a major impact on the C. chilensis population, because of very high daily production at temperature-dependent rates.

Short-term responses of phytoplankton to nutrient enrichment and planktivorous fish predation in a temperate South American mesotrophic reservoir

Top-down and bottom-up mechanisms have been proposed as potential regulators of the phytoplankton biomass of aquatic ecosystems. We evaluated the effect of nutrient enrichment and planktivorous fish predation through a mesocosm experiment conducted at a polimictic system (Peñuelas reservoir, Chile). Results show that phytoplankton biomass increased, while diversity decreased, in nutrient-enriched treatments. Planktivorous fish predation did not have a significant effect on phytoplankton biomass but led to a reduction of bigger zooplankton abundance. These results are an indication that these reservoir systems would be preferentially influenced by resource availability in short-term manipulations.

Morphometric study of Calanus chilensis males along the Chilean coast

Two species of the Calanus helgolandicus species-group, C. chilensis and C. australis, have been reported for Chilean waters since their description by Brodsky (1959). Morphological differences between both species are slight and have been found only in males. We analyzed the morphometry of male copepods of C. chilensis from the Chilean and Peruvian coast and compared it with that of C. australis off New Zealand and Australia. Our results show that only C. chilensis occurs in the Pacific Coastal waters of South America between 10° S and 42° S.

A Management Tool for Assessing Aquaculture Environmental Impacts in Chilean Patagonian Fjords: Integrating Hydrodynamic and Pellets Dispersion Models

This article introduces a management tool for salmon farming, with a scope in the local sustainability of salmon aquaculture of the Aysen Fjord, Chilean Patagonia. Based on Integrated Coastal Zone Management (ICZM) principles, the tool combines a large 3-level nested hydrodynamic model, a particle tracking module and a GIS application into an assessment tool for particulate waste dispersal of salmon farming activities. The model offers an open source alternative to particulate waste modeling and evaluation, contributing with valuable information for local decision makers in the process of locating new facilities and monitoring stations.

A simple-biology, stage-structured population model of the spring dynamics of Calanus chilensis at Mejillones del sur Bay, Chile

The population dynamics of Calanus chilensis (Copepoda: Crustacea) was analyzed using a mixture of field data and mathematical models. Field data were obtained in the Humboldt Current upwelling area, at Mejillones del Sur Bay (northern Chilean coast) during the spring of 1990 and 1991. Those data were used to set the parameter values and functions of a stage-structured population model (SSPM). The model was built and run with STELLA-II version 3.07, an interactive, iconographic modeling software. The results show that C. chilensis has a generation time of 20 days, and that its population dynamics in the spring is controlled by upwelling events, which affect both its growth rate (food dependence) and its local population size (advection).