Effects of reduced detritus on invertebrate community structure in Costa Rican headwater streams.

摘要

Stream invertebrate community composition is structured by seasonal changes in temperature and precipitation as well as physical variables such as altitude, sediment type, stream size and availability of detritus. Small, headwater streams that are heavily shaded from riparian vegetation have little in-stream primary productivity, therefore, the community relies allochthonous detritus (terrestrial leaf litter) and secondary producers for energy. Stream hydrology in tropical systems is greatly affected by seasonal changes in heavy rain events that cause spates. Spates change the community structure by causing mortality or displacement of individuals, alter the stream's physical structure by moving benthic substrates, and reducing detritus availability. In order to examine the effects of benthic detritus on the structure of invertebrate communities, I manipulated the amount of detritus in first order tributary streams of the San Lorencito River. The removal of detritus closely mimicked the effects of spates by increasing invertebrate diversity while reducing abundance. Additionally, removal of detritus in the dry season had significant and long lasting effects that were not mitigated by a natural recharge of detrital standing stocks with fresh litter. These long lasting effects could be due to low colonization rates by invertebrates in the dry season. Low stream flow would cause low invertebrate drift into the site from upstream and it is possible that life history traits of tropical invertebrates may lead them to ovoposit during the dry season less than the rainy season. Additionally it is possible that the quality of the fresh litter entering the stream was not the same as the letter that was removed. So while there was litter entering the stream it may not have had enough time for microbes and fungi to colonize it and break it down to a point where it could be consumed by invertebrates. Finally, the process of removing CPOM likely also washed out FPOM from the sites. Most invertebrates in these sites were feeding on FPOM and due to low flow there may have been low recharge of FPOM levels. Because most invertebrates in these streams feed on FPOM this would have had lasting impacts on the community as a whole.