Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 45

Report describing systematic classification of ecological thresholds for cold shallow lakes

There are seldom two locations in nature with the same species composition. However, local assemblages rarely appear as completely random selections from a regional pool of species. Certain patterns in species distribution and tendencies to co−occurrence or segregation are usually distinguishable. The lack of randomness can be due to purely historical reasons or to distinct screening processes such as spatial metapopulation dynamics, previous shaping of the species niche by evolution and active competitive dynamics. This deliverable takes the form of a manuscript submitted to Freshwater Biology. The manuscript addresses the case of species distribution in alpine lakes in relation to environment. The term ?alpine lake? can be considered synonymous of ?high−mountain lake? and refers to lakes located in or above the alpine zone of plant phytosociology. Therefore, the term does not have a geographical meaning but includes all lakes located above the tree line in any mountain region throughout the world. The name also does not indicate a type of lake by itself. In fact, even in phytosociology the term “alpine” has not a unique bioclimatic meaning, because in each case it depends on the particular biological limits of the species determining the tree line. Nevertheless, alpine lakes do tend to have certain features in common, particularly outside tropical zones of the globe: i) they are usually young systems, because most of them originated during the last glacial period due to the action of ice upon crystalline or limestone bedrocks; and ii) they are extreme environments, because cold, oligotrophic and diluted conditions occur in most of them. Alpine lakes offer an interesting case to investigate to what extent distinct groups of phylogenetically related organisms are similarly conditioned in the distribution of their species by the environment. A number of climatic and chemical gradients can give rise to distinct patterns in the distribution of the species. The main aims of our study were: 1) to analyse the modes of species assembling for distinct groups of phylogenetically related species; 2) to identify environment descriptors associated with patterns of species distribution; 3) to identify ecological thresholds in the species distributions within each group, and 4) to investigate the degree of commonality in the environment partitioning among the distinct groups, which eventually could indicate general ecological thresholds in alpine lakes. In our study, we considered five groups of organisms (namely, diatoms, rotifers, benthic chydorids, crustacean plankton and chironomids). These groups markedly differ in the number of species present and the length and way of living. The study was based on the statistical study of a snap shot survey of a large number of lakes throughout the main mountain lake districts of Europe, lakes for which a large number of environmental variables were also collected. Two distinct statistical approaches that complement each other were used to highlight patterns and relationships. Constrained ordination methods were used to investigate cline mode patterns, whereas classification and non−parametric tests were applied to investigated mosaic and hierarchical structures.

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