Springs like islands: implications on richness and species density

Many hypotheses exist concerning specific factors that control plant species richness, but currently there is no consensus about the mechanisms underlying these relationships (GRIME 2001). As far as we know, works especially dedicated to investigate the diversity in spring habitats are completely lacking. Few works exist oil spring vegetation, and those are from a phytosociological point of view (e.g., ZECHMEISTER & MUCINA 1994). Spring habitats have several characteristics which make them worth to be studied and may provide interesting contributions to individuate the relative importance of different factors controlling diversity. Springs are generally distinguishable from the surrounding habitats because water is continuously present throughout the year, has minimal temperature fluctuations over the years (CANTONATI et al. 2006). and discharge usually shows only limited seasonal variations. These characteristics allow considering springs like "water islands," as proposed by WERUM (2001), both physically and theoretically. A large literature exists on island diversity (WHITTAKER et al. 2001 and references therein) showing that one of the most important factors in determining species richness is area extension. As demonstrated by LOMOLINO & WEISER (2001), confounding variation in area with variation in other environmental factors could lead to biased results. One way to control area, a fundamental step often ignored, is to use sample units of equal dimensions and calculate species density instead of species richness. We agree with the WHITTAKER et al. (200 1) distinction between diversity in richness (the number of species recorded in unstandardized way) and density (the number of plant species in standardized plots). Springs and relative biota can be correctly considered as azonal (sensu Ellenberg) because of their relative independence of climate conditions. In other words, the spring communities appear in approximately the same form because they are determined more by the presence of water than overall climate as in the zonal units. Therefore, differences among spring assemblages are mainly determined by difference in water characteristics (e.g., chemistry, current flow). The "azonalness" of spring assemblages call depend on the degree of "aquaticness" of the species living in a spring. Considering an altitudinal gradient, for example, more species variation will occur in spring assemblages as the degree of species aquaticness decreases. CANTONATI et al. (2006) reported that diatoms did not show relevant patterns along altitudinal gradients with reference to samples collected always from submersed substrates. However, because bryophytes and vascular plants grow even outside the water in spring habitats, it would be interesting to explore plant richness and density in springs along altitudinal gradients. We formulated the following hypotheses: (a) richness (intended as the summation of the bryophytes and vascular plants species per spring) depends on spring area because of the well known species-area relationship; (b) density (intended as number of sp/m(2)) is independent from spring area; (c) because the majority of plants growing in springs are not strictly aquatic, we expected both density and richness to change along altitude, analogously to other habitats.