Contrasting patterns in local and zonal family richness of stream invertebrates along an Andean altitudinal gradient

1. Describing and understanding patterns in biological diversity along major geographical gradients is an important topic in ecology. Samples collected from a large number of physically and chemically comparable stream sites along a 4000 m gradient of altitude in the Andes of Ecuador served to characterise patterns of family richness of aquatic macroinvertebrates at the scale of the stream site (local) and at that of discrete altitudinal zones. 2. Both mean local and zonal family richness decreased by about 50% from sea level to 4000 m a.s.l. Local richness declined linearly, while zonal richness remained constant from sea level up to a threshold altitude of about 1800 m, whereafter it decreased. 3. From sea level to 1800 m few families were lost from zonal richness and few were gained. From 1800 to 3800 m the decrease in the number of families was accounted for by a loss of families present in lowland streams, with few new families gained. Hence, there was relatively little turnover of families along the entire gradient. 4. The diverging pattern of local and zonal richness was caused by sporadically occurring families inflating zonal richness at mid-altitudes. If the sporadic families were represented by the same species found commonly in the lowlands, then the mid-altitudinal zonal richness would be maintained by a 'rescue effect'. More probably, however, the sporadically occurring families found at mid-altitudes are each represented by new species replacing each other along the gradient, the families progressively diminishing in species richness and occurrence as the overall temperature tolerance of the family is approached. 5. This study demonstrates that spatial scale affects altitudinal patterns in the taxonomic richness of stream invertebrates. It also showed that family-level identification can facilitate interpretation of sources and sinks of biodiversity along geographic gradients.