Presencia de la gramínea exótica invasora spartina densiflora en el Parque Nacional de Doñanainvasión, dispersión, producción y descomposición

Résumé

The biodiversity does not only mean a variety of ecosystems, species and genes, but also the natural capital from the ecosystem to our economy. The biodiversity loss and decline is one of the major environmental risks and threaten ecological processes, habitats, species richness, and consequently, the employment associated with ecosystems, our economical capacity as well as our own welfare state. Currently, invasive alien species are a serious threat to the biodiversity and unless some measures to control their introduction and establishment are firmly adopted at every level the problem will become very complicated in the future, or even, insurmountable. The current thesis is framed within the project "Ecological implications of the presence of the invasive alien cordgrass Spartina densiflora (Brongn.) in the Doñana National Park and its surroundings. Prevention, control and eradication" of the Organismo Autónomo de Parques Nacionales (ref. OAPN 042/2007) that aims to define the mechanics of introduction and the early detection of this exotic species throughout this highly protected natural area, to evaluate the ecological effects of its presence and to study several techniqu es focused on its control and eradication. In the first chapter and as an introduction to the project, a brief review of the alien species definition and their mechanics of introduction are done. Special attention is paid to the recent International Conventions and legislation in terms of invasive species and how they were progressively adopted in Spain until the publication of the Spanish Catalog of Invasive Alien Species. In the same way, it is described the colonization process of S. densiflora in the coastal marshes of the Golfo de Cádiz coming from South America and its current regional distribution. Both crucial aspects will be considered for the establishment of the main objectives of this thesis. Chapter 2 is focused on the environmental characterization of the S. densiflora populations in several locations within Doñana National Park in order to delimit its potential habitat. This species presents a high plasticity, similar to what has been described in other coastal marshes close to our study site. In relation to this, some of the studied environmental factors could have restricted the colonization of S. densiflora towards the core of the National Park depending on the season. On one hand, basic pH levels and more negative redox potential associated with longer flooding periods and lower elevational limits may play an important role during the wet season. On the other hand, higher compaction and soil hardness, as a consequence of dry conditions, become more significant in summer. However, this situation could be altered after the Action nº8 from the Doñana 2005 Project which aims the partial removal of the embankment Montaña del Río. Chapters 3 addresses the assessment of the ecological extent to which the S. densiflora invasion takes place by quantifying the total biomass (B), the net aboveground primary productivity (NAPP), the ratio NAPP/B and the retention time of dead leaves and tillers in vegetation stands, making comparisons between these values and those for native species from Doñana National Park as Arthrocnemum macrostachyum (Moric.) C. Koch, Juncus subulatus Forssk., Scirpus litoralis Schrad. [=Schoenoplectus litoralis (Schrad.) Palla], Scirpus maritimus [=Bolboschoenus maritimus (L.) Palla]. In summary, S. densiflora has not only reached higher biomass and primary productivity values in comparison with these other species (especially in places without tidal influence), but also higher renovation rates, which means longer remaining time of its dead leaves and tillers. Moreover, this chapter deals with the regeneration capacity of S. densiflora after fire, obtaining as a result that this species is able to recover its values of biomass and productivity in three years. In Chapter 4, the leaves leaching rate and the biomass decomposition rate have been experimentally studied in the laboratory and in the field, respectively, for S. densiflora and the four native species considered (A. macrostachyum, J. subulatus, S. litoralis and S. maritimus). According to the results, although the alien species has not showed relevant differences in terms of nutrients retention during the leaching in comparison with the native species, its biomass loss rate was relatively lower with different treatments (with and without azide). Regarding to the decomposition rates, S. densiflora is not the one that takes the longest time, which is the native species J. subulatus. By contrast, S. litoralis was the one with the highest decomposition rate. Finally, Chapter 5 is focused on the experimental determination of the potential for long-distance dispersal of the alien species S. densiflora by two abundant waterfowl: mallards (Anas platyrhynchos) and greylag geese (Anser anser), in comparison with the long-distance dispersal of the native species A. macrostachyum and Suaeda vera and another exotic species, Ludwigia grandiflora. Intact seeds were retrieved from faeces for up to 4 days after ingestion and the proportion of seeds retrieved intact varied significantly between plant, but not bird, species. The retrieval proportion was the lowest for S. densiflora (<3%), although its germination time was reduced by gut passage in waterfowl and increased with retention time. No S. densiflora seeds germinated after retention for over 8 hours in geese and mallards. At the end of this thesis, general conclusions of the whole study are exposed.