Altitudinal changes in diversity of macroinvertebrates from small streams in the Ecuadorian Andes

Altitudinal patterns in diversity of macroinvertebrate families at different spatial scales (stone, stream and altitude) were studied by collecting stone samples from six streams at each of the three altitudes: lowlands (400 m), midlands (2000 m) and highlands (3800m), in the equatorial Andes of Ecuador. Stream sites were characterised by a number of physico-chemical parameters and the fauna by several indices of richness, diversity and evenness. A MDS ordination on the composition of the fauna clearly separated the streams in three groups according to altitude. The invertebrate fauna was dominated by insects, mainly Ephemeroptera, Trichoptera and Diptera at all three altitudes. Odonata and Hemiptera were relatively rich in lowland streams, scarcely present in the midland streams and absent in the highland streams (as was the case for the less diverse orders Lepidoptera and Megaloptera). Four of the five measures of stream diversity correlated significantly with altitude and temperature. In addition, seven environmental parameters were correlated with one or more of the diversity measures. Of these parameters, stream width, riparian vegetation cover and coarse detritus cover were inter-correlated with altitude and temperature. With the effect of altitude removed, the number of families, the Jackknife richness estimate and the evenness were positively correlated with an index of physical stream stability. The effect of region (altitude) explained more of total variability in family richness than that among streams within regions. The mean number of families was 8.2, 4.9 and 4.1 per stone, 26.5, 19 and 13.3 per locality (stream), and 44, 37 and 27 per region at the three altitudes, respectively. Thus, both local and regional richness decreased approximately linearly with increasing altitude. In contrast, beta diversity (taxon turnover among streams) increased with altitude. The higher richness in the lowland streams appeared at the smallest spatial scale (stone), and was therefore not due to higher beta diversity among stones within streams. Local richness was nearly linearly related to regional richness, indicating non-saturated local communities, even in lowland streams.