UBC Theses and Dissertations
Seasonal food limitation of detritivorous insects in a montane stream Richardson, John Stuart
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.
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