Variability in stream macroinvertebrates at multiple spatial scales

1. We intensively sampled 16 western Oregon streams to characterize: (1) the variability
in macroinvertebrate assemblages at seven spatial scales; and (2) the change in
taxon richness with increasing sampling effort. An analysis of variance (ANOVA)
model calculated spatial variance components for taxon richness, total density, percent
individuals of Ephemeroptera, Plecoptera and Trichoptera (EPT), percent dominance
and Shannon diversity.
2. At the landscape level, ecoregion and among-streams components dominated variance
for most metrics, accounting for 43–72% of total variance. However, ecoregion
accounted for very little variance in total density and 36% of the variance was
attributable to differences between streams. For other metrics, variance components
were more evenly divided between stream and ecoregion effects.
3. Within streams, approximately 70% of variance was associated with unstructured
local spatial variation and not associated with habitat type or transect position. The
remaining variance was typically split about evenly between habitat and transect.
Sample position within a transect (left, centre or right) accounted for virtually none of
the variance for any metric.
4. New taxa per stream increased rapidly with sampling effort with the first four to
eight Surber samples (500–1000 individuals counted), then increased more gradually.
After counting more than 50 samples, new taxa continued to be added in stream
reaches that were 80 times as long as their mean wetted width. Thus taxon richness
was highly dependent on sampling effort, and comparisons between sites or streams
must be normalized for sampling effort.
5. Characterization of spatial variance structure is fundamental to designing sampling
programmes where spatial comparisons range from local to regional scales. Differences
in metric responses across spatial scales demonstrate the importance of designing
sampling strategies and analyses capable of discerning differences at the scale of
interest.
Keywords: invertebrates, metrics, sampling, spatial scale, streams