[en] Lake macroinvertebrates and the altitudinal gradient in the Pyrenees [Los macroinvertebrados lacustres y el gradiente altitudinal en los Pirineos]" is a thesis about freshwater macroinvertebrate communities inhabitting the littoral zone of the mountain lakes of the Pyrenees, and about the effect of altitude on the species composition, and richness, of these communities. The investigation is addressed by analysing the relative relevance of the environmental and spatial components of the altitudinal gradient on macroinvertebrate communities. This is performed by comparing different macroinvertebrate groups, which attain for different evolutionary histories and ecological idiosyncrasies, including different dispersal capabilities. The taxonomical resolution achieved in macroinvertebrate determinations is high, which allows for among-group comparisons at the species level. These comparisons can be used to test ecological theories of current usage. Mountain lakes are particularly useful to this purpose, as these ecological systems account for large environmental gradients related to altitude and, at the same time, the altitudinal ecological gradient includes spatial components that may be relevant in shaping macroinvertebrate community assemblages. The spatial component of altitude relates to the uneven distribution of lakes along altitude in mountain ranges, which implies different degrees of ecological isolation between lakes. Also, the littoral zone of mountain lakes is particularly suited for among-group comparisons because it is in the littoral zone where most freshwater macroinvertebrate biodiversity of lakes concentrates. In the Results section, the results obtained are presented first as a general overview of the distribution of main macroinvertebrate taxa across altitude. The influence of environmental variables related to altitude on distributions is explored, and the relative relevance of different environmental variables on macroinvertebrate communities is partialled out. Then, the effect of different environmental and spatial factors is analysed for each macroinvertebrate group, separately. The main taxa analysed in each of the chapters are, arranged by convenience, as follows: Oligochaeta, Chironomidae, Mollusca, Megaloptera, Trichoptera, Coleoptera, Hydracarina and Nematoda. In the Discussion section, altitudinal distributions are compared between groups, and differences observed in this regard between the main macroinvertebrate taxa are discussed. The effect of both environmental and spatial factors related to altitude is suggested. Then, species richness patterns are also compared between the main macroinvertebrate taxa. The relative contribution of environmental and spatial factors is, to some extent, disentangled. Finally, the effect of altitude on the similarity of communities is also compared between groups, de-composing this effect in environmental and spatial terms. The following conclusions are drawn from the results obtained: 1) The altitudinal distribution patterns of lake macroinvertebrates are the consequence of both the selective influence of the environment and the more stochastic nature of dispersal. 2) Environmental factors covarying with altitude are not equally relevant for all macroinvertebrate groups. 3) Mountain lakes are biogeographic islands. 4) Similarity between communities is slightly higher, and stochasticity slightly lower, at mid altitudes. 5) The stochastic influence of environmental dynamics also contributes to explain species richness patterns. 6) Large river basins do not play a relevant role in the similarity of communities, which is more dependent on the altitudinal location of lakes. 7) An altitudinal ecological threshold in community assemblages occurs at ca. 2550 m a.s.l., for all groups responding to environmental changes across altitude, but independently of their dispersal strategy. 8) The altitudinal threshold, because of its environmental nature, is expected to shift upwards under current climate warming. 9) Global changes will affect macroinvertebrate communities of mountain lakes through different components, including climate warming, habitat loss and fragmentation, and invasive species, which may also show mutual interactions.

[es] Lake macroinvertebrates and the altitudinal gradient in the Pyrenees [Los macroinvertebrados lacustres y el gradiente altitudinal en los Pirineos]" es una tesis sobre las comunidades de macroinvertebrados acuáticos que habitan la zona litoral de los lagos de montaña de los Pirineos, y sobre el efecto de la altitud en la composición y riqueza de especies de estas comunidades. La investigación se efectúa analizando la importancia relativa de los componentes ambientales y espaciales del gradiente altitudinal sobre las comunidades de macroinvertebrados. Para ello, se comparan diferentes grupos de macroinvertebrados, de distinta idiosincrasia ecológica, incluyendo diferencias en cuanto a capacidad de dispersión. La resolución taxonómica con la que se analizan los datos es elevada, lo cual permite comparaciones entre grupos a nivel de especie, útiles para contrastar teorías ecológicas de uso actual. Las conclusiones de la tesis son las siguientes: 1) Los patrones de distribución altitudinal de los macroinvertebrados lacustres son consecuencia tanto de la influencia selectiva del ambiente, como de la dispersión, un proceso de naturaleza más estocástica. 2) Los distintos factores ambientales que covarían con la altitud no son igualmente relevantes para todos los grupos de macroinvertebrados. 3) Los lagos de montaña son islas biogeográficas. 4) La similaridad entre las comunidades es ligeramente mayor, y la estocasticidad en el ensamblaje de las mismas ligeramente menor, en altitudes intermedias. 5) La influencia de la dinámica estocástica del ambiente también contribuye a explicar los patrones de riqueza de especies. 6) Las cuencas de los ríos principales no juegan un papel relevante en la similaridad de las comunidades, que es más dependiente de la altitud de los lagos. 7) Existe un umbral ecológico para las comunidades de macro-invertebrados, situado a una altitud aproximada de 2550 m, que afecta a todos los grupos que responden a los cambios ambientales asociados a la altitud, pero con independencia de su estrategia de dispersión. 8) El umbral altitudinal, debido a su naturaleza ambiental, posiblemente se desplazará hacia arriba en altitud con el cambio climático actual. 9) El cambio global afectará a las comunidades de macroinvertebrados de los lagos de montaña a través de varios de sus componentes, incluyendo cambio climático, pérdida y fragmentación de hábitat, y especies invasoras, los cuales pueden mostrar también efectos de interacción mutua.