Many species of stream invertebrates gain most of their energy by the consumption
of coarse detrital materials. While most of these organisms are univoltine or
semivoltine, the biomass of assimilable detritus varies seasonally as a result of several
processes. The period of detritus input is highly seasonal, decomposition rates
are positively temperature dependent, and winter spates result in fragmentation
and flushing of detrital materials. From two years of measuring detritus inputs and
standing crops, I showed that the abundance of this resource varied by almost two
orders of magnitude seasonally. Since many consumers which rely on this resource
have generation times equal to that of the period of resource variation, individuals
and perhaps populations may be faced with periods of low food abundance. This
work addressed the consequences of seasonal food limitation of stream insects.
To test this food Limitation hypothesis, I experimentally manipulated detritus
input rates to otherwise natural communities of stream benthos using a replicated,
3 - treatment design. These experiments were conducted in experimental streams
in the University of British Columbia research forest over the course of one year.
Increasing input rates of detritus resulted in large increases in size at maturity and
growth rates for 7 of 9 common species. This was true for both summer and winter
emerging species. Increased supply of detritus also resulted in increased densities
and higher rates of colonization for some species. There was no evidence of change in
phenology for any species. The densities of the chironomid Brillia retifinis (the only
species studied that had a short generation time) underwent exponential growth
during the first 3 months of the experiment, reaching densities 10x those of the
control and natural streams. This species apparently fills the role of a "fugitive" in
this system.
One alternative hypothesis for increased densities following addition of whole
leaf detritus was a significantly altered microhabitat. To test this possibility I compared the use of real and artificial (polyester) leaf packs by stream invertebrates.
Those species which typically consume coarse detritus were almost never found on
the artificial leaf packs, while they attained high densities on the real leaves. In
contrast, fine-particle, and algae consuming species were found in similar densities
on artificial and real leaf packs, although there was a time lag in colonization of
the polyester leaves. These results suggest that microhabitat alone cannot lead to
increased densities of detritivores.
The densities of species which do not consume large particles of detritus also
were affected by whole-leaf additions. Density of total consumers of fine particles
of detritus increased when coarse detritus was supplemented and most taxa
showed this response. This result was apparently an indirect effect of diminution
of detrital particle size by larger detritivores. Predaceous species also increased
in density under detritus supplementation. Increased densities of taxa other than
large-particle-detritus feeders indicates that effects at one trophic level can affect
other trophic levels. / Science, Faculty of / Zoology, Department of / Graduate
| Identifer | oai:union.ndltd.org:UBC/oai:circle.library.ubc.ca:2429/41449 |
| Date | January 1989 |
| Creators | Richardson, John Stuart |
| Publisher | University of British Columbia |
| Source Sets | University of British Columbia |
| Language | English |
| Detected Language | English |
| Type | Text, Thesis/Dissertation |
| Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
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