BIOGEOGRAPHY OF HEPTAGENIID MAYFLIES IN SASKATCHEWAN: A MULTIVARIATE ECOLOGICAL STUDY

Abstract

The distributions of 22 species of heptageniid mayflies (Ephemeroptera: Heptageniidae) in Saskatchewan and west-central Manitoba are described. The distribution patterns of 16 species are examined intensively at 35 sites in central Saskatchewan, using multivariate methods and both quantitative and qualitative data. Cluster analysis of quantitative data (using Ward's method and Euclidean distance) shows two major groups of sites or faunal zones: 1) the Saskatchewan Rivers, with seven characteristic species (Heptagenia adequata, H. elegantula, H. flavescans, Macdunnoa nipawinia, Raptoheptagenia cruentata, Rhithrogena undulata and Stenonema terminatum) and 2) boreal streams and rivers, with six characteristic species (Leucrocuta hebe, Nixe inconspicua, N. lucidipennis, N. rusticalis, N. simplicoides and Stenonema vicarium). High gradient str€ams form a distinct subgroup of boreal streams characterized by the presence of Nixe rusticalis and N. simplicoides. Ordination using detrended ,correspondence analysis shows two major community gradients in the intensive study area. Saskatchewan River and boreal sites are separated on the first and most important community gradient; high gradient and other boreal sites are separated on the second. Both gradients are discontinuous showing the distinctness of the faunal zones. Faunal differences between zones are greatest in June and early July when larvae of species with summer life cycles are present. Multivariate analyses of presence/absence data from 172 sites throughout Saskatchewan show similar faunal zonation, but with two additional boreai subzones (Shield and low gradient streams) and one additional Saskatchewan River subzone (large southern boreal and parkland streams that are similar to the Saskatchewan Rivers). Environmentally, the Saskatchewan River and boreal faunal zones differ in depth, water temperature, siltation, water clarity, and type of organic"matter. High gradient and other boreal streams differ in current velocity, water temperature, and type of organic matter. These environmental differences may all be due to differences in stream size and associated longitudinal gradation of physical parameters and organic matter characteristics. Although differences in feeding strategies among heptageniids are not known, longitudinal zonation of organic matter characteristics provides the best explanation of observed heptageniid distribution patterns. These effects are probably modified by streambank vegetation. Observed heptageniid distribution patterns do not appear to be influenced by drainage basins or by historical factors associated with the Wisconsin glaciation.