Thirteen species of dinoflagellates, ranging in size from 16 to 48 pm, were tested for particle rejection behavior in the copepods Calanuspacificusand Paracalanusparvus. Five dinoflagellates were rejected as food: Gonyaulax tamarensis (429). G. tamarensis (Ipswich), Ptychodiscus brev~s, Scrippsiella trochoidea and Protoceratium reticulatum. The response of copepods to P. reticulatum was examined in detail. Starved copepods could not b e induced to feed on this species; C. pacificus maintained in bloom concentrations ceased reproduction and had high mortality. Both the cells themselves and filtrate from the cell culture suppressed feeding on normally edible dinoflagellates.Direct observations showed that P. reticulatum cells cause reverse peristalsis and regurgitation, and that Ptychodiscus brevis cells cause elevated heart rate and loss of motor control by C. pacificus We conclude the particle rejection behaviour we observed is chemically mediated, and that it may b e a n important factor in formation and maintenance of monospecific dinoflagellate blooms. We suggest chemicals produced by the dinoflagellate cause an acute physiological response which renders the herbivore incapable of ingesting more than required for its respiratory needs. Secondary effects of starvation, suspended reproduction and mortality further reduce the predation pressure of both present and future generations of predators. These factors aid in development of the bloom. Chemical defense may confer a competitive advantage, but rejected dinoflagellates pay the price in the form of reduced growth rates.

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.

Egeria densa Planchon is considered an invasive species in continental aquatic systems in Chile. Its original geographical distribution was limited to the subtropical regions of Brazil, Argentina and Uruguay. Its fast growth and dense canopy-forming habit are associated with the clear water state of shallow water continental ecosystems. As a dominant species in many of the systems in which it occurs, E. densa behaves as an ecosystem engineer by preventing the re-suspension of sediments and controlling the growth of phytoplankton by removing nutrients from the water column. At the same time, this invasive species produces unwanted effects such as: (1) clogging waterways and hydroelectric turbines, (2) out-competing native vegetation, and (3) negatively affecting the sediment seed bank. Given its importance in the recent (2004) change of state of the Río Cruces wetland, Chile, we felt it appropriate to undertake a comprehensive literature review of this species. We conclude by discussing the role of E. densa as an ecosystem engineer while cautioning against the impression that E. densa is always a highly competitive invader.Key words: Egeria densa, ecosystem engineer, invasive species. RESUMENEgeria densa Planchon es considerada una especie invasora en los sistemas acuáticos de Chile. Su distribución geográfica original estaba limitada a regiones subtropicales de Brasil, Argentina y Uruguay. Su crecimiento rápido y su tendencia a formar densas coberturas suelen estar asociadas con el estado de aguas claras en ecosistemas acuáticos continentales de aguas someras. Su dominancia sugiere que E. densa puede comportarse como una especie ingeniera, previniendo la resuspensión de sedimentos y controlando el crecimiento del fitoplancton a través de la utilización de los nutrientes. Sin embargo, al mismo tiempo, como especie invasora produce efectos no deseados tales como: (1) no permitir el crecimiento de otras macrófitas nativas, (2) disminuir la diversidad en el banco de semillas, y (3) interferir con la navegación y generación de electricidad por turbina. Dada su importancia en el reciente (2004) cambio de estado del humedal del Río Cruces, nos ha parecido oportuno realizar una recopilación de artículos científicos disponibles en la literatura sobre esta macrófita. Concluimos con una discusión 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.Palabras clave: Egeria densa, especie ingeniera, especie invasora.

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