|Abstract or Summary
- Comparative ecological studies of Sialis californica Banks and
Sialis rotunda Banks were made in Western Oregon from 1966 to 1968.
Field collections for S. rotunda were made in fish holding pond at the
Oak Creek laboratories, 5 miles west of Corvallis, Benton County,
and for S. californica in enriched sections of Berry Creek experimental
stream, 13 miles north of Corvallis.
Life histories were determined in aquaria, in laboratory
streams and the natural habitats. S. rotunda, predominantly a pond
species, completed its life cycle in one year in all situations. S.
californica, commonly a stream species, took one or two years
depending on oviposition time and food availability. Both species
were reared successfully in the laboratory through all stages of their
life cycle. There are ten larval instars. The larvae are carnivorous
and feed on insects and other small benthic organisms.
Biweekly or monthly samples were collected from the two locations
for density, biomass, growth rate and production studies.
Density and biomass of S. rotunda in the pond were much higher than
for S. californica in the stream. Enrichment with sucrose and urea
resulted in differences in density and biomass of S. californica
between the four experimental sections at Berry Creek. The
unenriched section usually had a high density but low or similar
biomass compared with the enriched sections which had few individuals.
The lower density in the enriched sections was associated with
unfavorable substrate conditions for early-instar larvae because of
dense mats of the filamentous bacterium, Sphaerotilus natans.
Larger larvae were not hindered by the S. natans filaments and had
rapid growth because food was abundant.
Production of both species was calculated by two methods:
(1) Ricker's (1946) method, the product of growth rate and mean
biomass; and (2) Allen's (1951) graphical method, using numbers
per m² and mean individual weight at each sampling date, The
results from both methods were comparable, with the graphical
method usually providing a slightly higher estimate of production.
Annual production of S. rotunda in the pond was 13.18 Kilocalories/
m². This was more than double the production of S. californica in
any of the sections of Berry Creek. In Berry Creek, the population
in the unenriched section had low growth rate but the density of larvae resulted in high biomass and a comparable production to that in the
enriched sections. The accumulated annual production was:
unenriched, 4.94 Kilocalories; enriched, 6, 34 Kilocalories.
The relationship between larval growth and food consumption
rate was established by experiments in laboratory streams during
summer and winter. These data were used to estimate food consumption
by field populations. In the pond, S. rotunda larvae consumed
137 gms/m²/year. In Berry Creek, individuals in enriched
sections consumed one and half to two times more food than did
individuals in the unenriched section.