Missing Shredders Can’t Cut It in Urban Streams

Aquatic research on larvae insects

This post is available in Spanish here.

Decomposition is a team sport that requires nearly every player in an ecosystem, biotic (living) and abiotic (non-living), to work together.  Think of a large leaf floating in a stream–the temperature of the water and air, chemicals present, microbes, insects, stream flow, sediment load, and stream channel shape all affect the decomposition rate of the leaf.  This rate will change if any one of those players is increased/decreased or missing.

Applied Ecology Prof. Alonso Ramirez and colleagues compared the functional diversity of streams in Puerto Rico by placing leaf bags in urban and rural streams and measuring the rate leaves decomposed.  Each bag started with 5g of ficus leaves and the bags were weighed over time. The team found that urban streams’ leaf litter bags remained the heaviest while rural stream bags became lighter as the leaves decomposed.  They also noted that streams in urban areas had fewer shredder insects present, which are larvae forms of crane flies and caddisflies.  Shredders live up to their name by breaking down organic matter into manageable nutrition chunks for other insects, microbes, and fish.  Without shredders, the rest of the ecosystem players are unable to fulfill their roles in the decomposition process.

“Those streams crossing our cities are living ecosystems that enrich our quality of life,” says Ramirez.  “Understanding how streams function is an important step in their protection.”

Urban streams that lack shredders are not able to fulfill their ecosystem service of decomposition.  When streams are unable to process and use up these materials, they end up causing issues further downstream affecting coastal ecosystems.  Prof. Ramirez and colleagues recommend that urban stream designs should improve water quality to increase the amount of functional diversity and prevent negative effects on the ecosystem services we rely on.

The paper, “Leaf litter decomposition and macroinvertebrate assemblages along an urban stream gradient in Puerto Rico,” was published in Biotropica July 24. First author of the paper is Leticia Classen‐Rodríguez of Saint Louis University. The paper was co-authored by Pablo E. Gutiérrez‐Fonseca at the Department of Biology & Center for Research in Marine Science and Limnology in the University of Costa Rica. The work was done with support from the Puerto Rico‐Louis Stokes’ Alliance for Minority Participation (AMP) and the Luquillo Long‐Term Ecological Research (LTER) program, both funded by the US National Science Foundation.

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